CH-32 Digestion and Nutrition – Integrated Principles of Zoology : Chapter Notes

This document contains Chapters 32 to 33 CHAPTER 32 DIGESTION AND NUTRITION CHAPTER OUTLINE 32.1. Trophic Levels and Routing A. Classification 1. Sunlight provides the ultimate source of energy for life. 2. Green plants are autotrophic organisms; most are chlorophyll-bearing phototrophs. 3. Some gain energy from inorganic chemical reactions and are chemotrophs. 4. Almost all animals are heterotrophic organisms that depend on compounds already synthesized. 5. Herbivorous animals mainly feed on plant life. 6. Carnivorous animals feed on herbivores and other carnivores. 7. Omnivorous animals feed on both plants and animals. 8. Saprophagous animals feed on decaying organic matter. B. Routing 1. Ingestion of foods and their reduction by digestion only begins the steps in nutrition. 2. Foods reduced by digestion are absorbed into the circulatory system. 3. Foods are transported to the tissues of the body. 4. They are assimilated into the structure of cells. 5. Oxygen is also transported to tissues where food products are oxidized to yield energy and heat. 6. Food not immediately used is stored for future use. 7. Wastes produced by oxidation must be excreted. 8. Food products unsuitable for digestion are egested in the form of feces. 32.2. Feeding Mechanisms A. Feeding on Particulate Matter (Figures 32.1, 32.2) 1. Ocean drifting microscopic particles consist of plankton and organic debris. 2. The richest zones occur in estuaries and upwellings and feed many larger animals. 3. Suspension Feeders a. Suspension feeders use ciliated surfaces to draw drifting food particles into their mouths. b. Many trap particulate food on mucous sheets that convey food to the digestive tract. c. Others use sweeping movements to convey particles to their mouths. d. Suspension or filter-feeding has evolved many times among crustaceans, sharks, whales, etc. 4. Deposit Feeding a. This variation of particulate feeding extracts organic material or detritus from substrate. b. Some annelids and hemichordates pass the substrate through their bodies and remove nutrients. c. Scaphopods and some bivalve molluscs use appendages to gather in organic deposits. B. Feeding on Food Masses (Figures 32.3–32.6) 1. Some animals that eat solid food are heavily adapted for this task. 2. Predators locate, capture, hold and swallow prey. 3. Some carnivores seize food and swallow it intact; some may employ toxins. 4. Although invertebrates lack true teeth, some have beaks or tooth-like structures to bite and hold. 5. The polychaete Nereis seizes food with jaws on a muscular pharynx. 6. Fish, amphibians and nonavian reptiles use teeth to grip prey until it is swallowed. 7. Many invertebrates can reduce food by shredding devices. 8. True mastication or chewing or crushing is only found among mammals where there are four types of teeth. a. Incisors bite, cut and strip. b. Canines seize, pierce and tear. c. Premolars and molars are for grinding and crushing. 9. Variations in teeth reveal the specialized food habits of animals. a. Herbivores have suppressed canines and well-developed molars. b. Rodents have well-developed and self-sharpening incisors that must be constantly worn away. c. An elephant’s tusk is a modified upper incisor used for defense, attack and rooting. 10. Some invertebrates have specialized scraping mouthparts, such as the radula of the snail. 11. Horses are herbivorous mammals that have corrugated molars for grinding plant tissue. C. Feeding on Fluids 1. Fluid feeding is especially characteristic of parasites. 2. Some internal parasites simply absorb the nutrients around them. 3. Some bite or rasp the tissues of the host to suck the blood or feed on intestinal contents. 4. Leeches, lampreys, mosquitoes, sucking lice, bedbugs, ticks and mites and fleas are a few of the organisms that feed on blood or other body fluids and some may vector disease agents. 5. Parasites that feed on blood, such as the mosquito, use an anticoagulant to keep blood from clotting. 32.3. Digestion A. Overview (Figure 32.7) 1. Digestion mechanically and chemically breaks food into small units for absorption. 2. Food solids contain carbohydrates, proteins and fats that must be reduced to simpler molecules. 3. An animal must then reassemble the digested and absorbed units into the animal’s own compounds. 4. Digestion in sponges and protozoa is entirely intracellular; food particles are phagocytized. 5. Intracellular digestion is limited in the size of food particles that can be utilized. 6. The invention of the alimentary system allowed extracellular digestion to take place. 7. This allowed cells lining the lumen of the alimentary canal to specialize for digestion or absorption. 8. Development of mouth-to-anus flow-through systems allowed regional specialization of digestion. B. Action of Digestive Enzymes 1. Mechanical processes of cutting and grinding are important but limited to reducing size of foods. 2. Reduction of molecules to absorbable size relies on chemical breakdown by enzymes. 3. Digestive enzymes are hydrolytic enzymes or hydrolases; molecules are split by adding water. 4. Proteins must be split into hundreds or thousands of small amino acid molecules. 5. Carbohydrates must be reduced to simple sugars. 6. Fats are reduced to glycerol and fatty acids although some are absorbed without being hydrolyzed. 7. Specific enzymes form an “enzyme chain” so one may complete what another has started. C. Motility in the Alimentary Canal (Figure 32.8) 1. Food moves through the digestive tract by cilia, specialized musculature or both. 2. Acoelomate and pseudocoelomate animals lack mesodermally derived gut musculature and use cilia. 3. Most molluscs also use cilia; the coelom is weakly developed. 4. In coelomic animals, the gut is lined with circular and longitudinal layers of smooth muscle. 5. Gut movements cause segmentation, alternate constriction of rings of muscle to divide gut contents; this mixes food but does not move it through the gut. 6. Peristalsis, or waves of contractions, moves food down the gut. 32.4. Organization and Regional Function of the Alimentary Canal (Figures 32.9, 32.10) A. Receiving Region 1. Mouthparts may include mandibles, jaws, teeth, radula or bills. 2. The buccal cavity and pharynx are inner chambers. 3. Most metazoans, other than suspension feeders, have salivary glands to produce lubricating secretions. 4. Salivary Glands a. Specialized saliva may contain toxins to quiet struggling prey. b. Leech saliva contains an anaesthetic and enzymes to prevent blood coagulation and increase flow. c. Salivary amylase is found in herbivorous molluscs, insects and primate mammals. d. Salivary amylase breaks starch into two-glucose fragments of maltose. 5. Tongue a. Only vertebrates evolved a tongue, usually attached to the floor of the mouth. b. It assists in food manipulation and swallowing. 6. As food is moved toward the pharynx, the nasal cavity reflexively raises the soft palate. 7. The epiglottis keeps food from entering the trachea. 8. Food in the esophagus is smoothly moved by peristalsis to the stomach. 9. The top one-third of the esophagus is surrounded by skeletal muscle as well as smooth muscle. 10. The act of swallowing is voluntary until the food has traveled past this region. B. Conduction and Storage Region 1. The esophagus of vertebrates and many invertebrates moves food to the digestive system. 2. In annelids, insects and octopods, the esophagus is expanded into a crop, a food storage area. 3. Among vertebrates, only birds have a crop; it softens grain and allows mild fermentation. C. Region of Grinding and Early Digestion (Figures 32.9, 32.11) 1. The stomach is a region for initial digestion and storage of food in vertebrates and some invertebrates. 2. Herbivorous animals often continue the grinding and crushing of plants in the stomach. 3. Swallowed stones and grit assist the muscular gizzard of oligochaete worms and birds. 4. The insect proventriculus has chitinous teeth, and crustaceans have a gastric mill. 5. Digestive diverticula are blind tubules or pouches that supplement the stomach and secrete enzymes and/or absorb nutrients. 6. The Problem with Cellulose a. The cellulose that encloses plant cells is a very abundant molecule. b. Only the enzyme cellulase can break down the cellulose molecule. c. No metazoan animal can produce cellulase for direct digestion of cellulose. d. Many herbivorous animals harbor bacteria and protozoa in their gut that do produce cellulase. e. These microorganisms ferment cellulose under anaerobic conditions of the gut, producing fatty acids and sugars. f. Ruminant ungulates harbor these organisms in a multi-chambered stomach. g. Other animals harbor the microorganisms in the intestine or the cecum. 7. Acidity of the stomach is probably an adaptation for killing prey and halting bacterial activity. 8. A cardiac sphincter opens to allow food to enter from the esophagus; it closes to prevent regurgitation. 9. In humans, peristaltic waves churn the stomach at about three waves per minute. 10. Food is released into the intestine by the pyloric sphincter. 11. Gastric Glands a. In humans, tubular glands in the stomach wall secrete about 2 liters of gastric juice a day. b. Chief cells secrete pepsin, a protease that only acts in an acid medium (pH 1.6 to 2.4). c. The pepsin breaks down only specific peptide bonds; other proteases will split all peptide bonds. d. Pepsin is present in the stomachs of nearly all vertebrates. e. Parietal cells secrete hydrochloric acid. 12. Rennin is a milk-curdling enzyme found in the stomach of ruminants and other mammals. 13. Secretion of gastric juices increases at the sight of food, food in the stomach or during anxiety. 14. Classic experiments were made in 1825–1833 by Army surgeon William Beaumont who observed stomach action through a permanent gunshot wound in a patient. 15. The use of a permanent opening or fistula is common in animal digestive research. D. Region of Terminal Digestion and Absorption: Intestine (Figures 32.12, 32.13) 1. Absorptive Structures a. In invertebrates with digestive diverticula, the intestine may serve only to carry wastes away. b. In invertebrates with simple stomachs and in vertebrates, intestines digest and absorb nutrients. c. One method to increase digestive surface is to increase the length of the intestine. d. A coiled intestine is rare in invertebrates but may be eight times body length in some mammals. e. Invertebrates may use infolding to increase surface area as in the typhlosole in oligochaetes. f. Lampreys and sharks have longitudinal or spiral folds in their intestines. g. Villi are minute finger-like projections that increase the surface area of some vertebrate intestines. h. Each cell likewise has short microvilli that, along with villi, increase surface area a million times. 2. Digestion in the Vertebrate Small Intestine (Figure 32.14) a. The pyloric sphincter releases acidic food into the small intestine. b. The initial segment is the duodenum where pancreatic juice and bile are also added. c. Both have high bicarbonate content, which neutralizes the stomach acid. d. The liquified food mass is now liquid chyme; its pH rises from 1.5 to 7. e. All intestinal enzymes function near this neutral pH. f. Cells of the intestinal mucosa are constantly being shed in large numbers. g. Bile i. The liver secretes bile into the bile duct that drains into the duodenum. ii. Between meals, the bile collects in the gallbladder that responds to fat in the duodenum. iii. Bile contains water, bile salts, and pigments but no enzymes. iv. Bile salts reduce fat droplets to smaller size to allow increased enzyme action. v. Bile contains pigments from hemoglobin breakdown and gives feces its dark color. h. Pancreatic Juice 1) Trypsin and chymotrypsin are proteases that split apart peptide bonds in protein molecules. 2) Carboxypeptidase removes the amino acids from carboxyl ends of polypeptides. 3) Pancreatic lipase hydrolyzes fats into fatty acids and glycerol. 4) Pancreatic amylase is a starch-splitting enzyme identical to salivary amylase. 5) Nucleases degrade RNA and DNA to nucleotides. i. Membrane Enzymes i. Cell lining the intestine have digestive enzymes embedded in their surface membrane. ii. Aminopeptidase splits terminal amino acids from the amino end of short peptides. iii. Disaccharidases split 12-carbon sugar molecules into 6-carbon units; this includes maltase, sucrase and lactase. iv. Alkaline phosphatase is an enzyme that attacks several phosphate compounds. v. Nucleotidases and nucleosidases continue the breakdown of nucleotides into nucleosides and finally ribose and deoxyribose sugars, purines, and pyrimidines. 3. Absorption a. Little food is absorbed in the stomach; digestion is not complete and absorptive surface is limited. b. Most digested food is absorbed by the villi of the small intestine. c. Sugars are absorbed as monosaccharides; the intestine is impermeable to polysaccharides. d. Proteins are absorbed as amino acids or small protein or peptide fragments. e. Both active and passive processes transfer sugars and amino acids across intestinal epithelium. f. Passive transfer would only occur after meals when intestinal concentrations were highest. g. Glucose, galactose and most amino acids are carried by protein transporters. h. Fat droplets are emulsified by bile salts and digested by pancreatic lipase. i. Micelles of resulting monoglycerides and fatty acids are absorbed across villi by simple diffusion. j. The endoplasmic reticulum of absorptive cells resynthesizes them into triglycerides and passes them into lacteals that transfer them to the lymph system and blood. E. Region of Water Absorption and Concentration of Solids 1. The large intestine consolidates the undigested material as semisolid feces. 2. Reabsorption of water is the main function and is critical in desert species. 3. Some animals have specialized rectal glands to absorb water and ions, leaving nearly dry fecal pellets. 4. In nonavian reptiles and birds, most of the water is reabsorbed in the cloaca leaving a white paste-like feces. 5. In adult humans, about one-third of the dry weight of feces is bacteria. 6. Bacteria play an important role in degrading organic wastes and providing some vitamins. 32.5. Regulation of Food Intake A. Intake Factors 1. Most animals unconsciously adjust intake of food to balance energy expenditure. 2. A hunger center in the hypothalamus regulates the intake of food. 3. A drop in blood glucose levels stimulates a craving for food. 4. In humans, obesity appears to be a genetic predisposition to gain weight on a high-fat diet and a reduced ability to burn excess calories by “nonshivering thermogenesis.” 5. There are direct correlationgs between the consumption of dietary fructose and obesity and Type II diabetes. 6. Brown Fat a. Placental mammals have a dark adipose tissue called brown fat, specialized for heat generation. b. Newborn animals have more than adults; it is located in the chest, upper back and near kidneys. c. Their abundant mitochondria contain a protein called uncoupling protein. d. Thermogenin acts to uncouple production of ATP during oxidative phosphorylation. e. Thermogenesis in brown fat is stimulated by excess food and by cold temperatures. f. People of normal weight dissipate excess energy as heat; obese people do not. g. Brown fat is especially well developed in hibernating species of bats and rodents. h. The Pima Indians of Arizona have low sympathetic nervous system activity and this may contribute to high levels of obesity. i. Fat stores are supervised by the hypothalamus, which has a set point. j. A high setting can be somewhat lowered by exercise, but the body varies little from the set point. k. A hormone produced by fat cells and called leptin was discovered in 1995. l. If fat levels are high, leptin is released and diminishes appetite and increases thermogenesis. m. White adipose tissue comprises the bulk of body fat and is derived from surplus carbohydrates and fats. B. Regulation of Digestion (Figure 32.15) 1. The gastrointestinal (GI) tract is the body’s most diffuse endocrine tissue. 2. Because of their diffuse origins, the GI hormones have been difficult to isolate and study. 3. Gastrin a. Gastrin is a small polypeptide hormone produced by endocrine cells in the pyloric stomach. b. It is secreted on stimulation by parasympathetic nerve endings or when protein enters the stomach. c. Its main action is to stimulate hydrochloric acid secretion and increase gastric motility. d. Gastrin is unusual in that the stomach produces it, and the stomach is also the target tissue. 4. Cholecystokinin (CCK) a. CCK is secreted by endocrine cells in the walls of the upper small intestine in response to fatty acids and amino acids in the duodenum. b. It stimulates gallbladder contraction and increases flow of bile salts into the intestine. c. CCK stimulates an enzyme-rich secretion from the pancreas. d. It also acts on the brain to contribute a feeling of satiety, especially after a meal rich in fats. 5. Secretin a. Secretin was first hormone to be discovered; is produced by endocrine cells in the duodenal wall. b. It is secreted in response to food and strong acid in the stomach and intestine. c. It mainly stimulates the release of an alkaline pancreatic fluid that neutralizes stomach acid. d. It also aids fat digestion by inhibiting gastric motility and increasing bile secretion from the liver. 6. Other GI hormones are being isolated and some appear to play neurotransmitter roles in the brain. 32.6. Nutritional Requirements A. Food Categories and Vitamins (Figures 32.16, 32.17, Table 32.1) 1. Carbohydrates and fats are required as fuels for energy and for synthesis of various substances. 2. The amino acid units of proteins are needed for synthesis of species-specific proteins and other nitrogen compounds. 3. Water is a critical solvent for body chemistry. 4. Inorganic salts are required for anions and cations of body fluids and to form structural components. 5. Vitamins are accessory factors from food that are often built into the structure of many enzymes. 6. Vitamins a. A vitamin is a simple compound that is not a carbohydrate, fat, protein or mineral. b. Vitamins are needed in very small amounts for some specific cellular function. c. Vitamins are not themselves sources of energy, but may be associated with metabolic enzymes. d. Animals have lost ability to synthesize these needed chemicals and must secure them from food. e. Vitamins are classified as either fat-soluble or water-soluble. f. Water-soluble vitamins include the B complex and vitamin C. g. Almost all animals need B vitamins and they are considered “universal” vitamins. h. Dietary need for vitamin C and fat-soluble vitamins A, D, E and K is restricted to vertebrates. i. A rabbit does not require vitamin C, but guinea pigs and humans do. j. Some songbirds require vitamin A and others do not. B. Essential Nutrients and Malnutrition 1. It has long been recognized that lack of certain nutrients resulted in dietary deficiency diseases. 2. Essential nutrients are needed for normal growth and maintenance and must be supplied in a diet. 3. Nearly 30 organic chemicals (vitamins and amino acids) and 21 elements are essential for humans. 4. This is a short list compared to the thousands of organic compounds in the body. 5. Most compounds can be synthesized in the animal cell. 6. Lipids are needed to provide energy; three fatty acids are needed because we cannot synthesize them. 7. A human diet that is high in saturated lipids may be associated with atherosclerosis. 8. Of the 20 amino acids commonly found in proteins, eight are essential to humans. 9. All eight essential amino acids must be present for protein synthesis. 10. Amino acids cannot be stored and they are soon broken down for energy. 11. Use of several plants in a diet will probably provide the full spectrum of needed amino acids. 12. Animal proteins are a rich source of amino acids. 13. Undernourishment includes marasmus in infants on a low-calorie-low-protein diet, and kwashiorkor that occurs in infants also lacking an adequate diet. 14. Malnutrition in late stages of pregnancy can lead to a child with uncorrectable brain damage. 15. World population growth is a major force driving the global environmental crisis. Lecture Enrichment 1. While a carnivore must reduce food molecules to basic units (e.g., amino acids, fatty acids, etc.) just as a herbivore does, the assortment of molecules is a closer match to what it needs. Meat, as muscle tissue, can be more fully utilized by an animal than plant tissue that contains much cellulose and other molecules. Thus, in general, herbivores must consume much more tissue than carnivores consume, and likewise defecate more. 2. The value of saliva and mucin in swallowing can be imagined by thinking about trying to swallow a spoonful of dry cracker crumbs or blotted cole slaw with no saliva present; it cannot be done without time for more saliva to be secreted to stick it all together. This mental illustration usually makes sense to most students. 3. Students can also try to mentally visualize swallowing without using the tongue. They will actually go through the oral motions in “thinking this out.” If the tongue has to be surgically removed, or as was the case in early history as a punishment for treason, etc., a person must resort to throwing the head backwards to swallow. 4. All body tubes are collapsed unless something is in them, with the exception of the trachea, etc. where rings of cartilage hold them open. Peristalsis is the “milking” of a bolus of food through an otherwise collapsed tube. 5. The complexity of the liver, both in structure and function, is reflected in the fact that it was one of the last organs to be successfully transplanted. There is no “liver machine” parallel to a dialysis unit or “kidney machine.” Commentary/Lesson Plan Background: Students who have cared for pet or farm animal’s may have an awareness of animal’s varying nutritional requirements; however, cat and dog food and some farm feeds can be fairly artificial and distant from their natural food. Misconceptions: The extent to which humans modify food collection and preparation, masks our underlying biological adaptations to hunting, gathering and scavenging that molded our digestive biology. Therefore, many students will not recognize our physical and chemical adaptation to an omnivorous diet and may hold beliefs in vegetarianism, herbalism, etc. that do not match our biological heritage. There are a wide array of erroneous beliefs about cholesterol, fat, sugar, vitamins, calories, obesity, etc. due to commercial and cultural values and modern myths. Schedule: HOUR 1 32.1. Trophic Levels and Routing A. Classification B. Routing 32.2. Feeding Mechanisms A. Feeding on Particulate Matter B. Feeding on Food Masses C. Feeding on Fluids 32.3. Digestion A. Overview B. Action of Digestive Enzymes C. Motility in the Alimentary Canal 32.4. Organization and Regional Function of the Alimentary Canal A. Receiving Region B. Conduction and Storage Region HOUR 2 C. Region of Grinding and Early Digestion D. Region of Terminal Digestion and Absorption: The Intestine E. Region of Water Absorption and Concentration of Solids 32.5. Regulation of Food Intake A. Intake Factors B. Regulation of Digestion 32.6. Nutritional Requirements A. Food Categories and Vitamins B. Essential Nutrients and Malnutrition ADVANCED CLASS QUESTIONS: 1. Why do the demands of feeding have such a profound effect on the morphology and behavior of animals compared to the requirement for reproduction? Answer:The demands of feeding often have a more profound effect on the morphology and behavior of animals compared to the requirement for reproduction due to several reasons: 1. Immediate Survival: Feeding is essential for an organism's immediate survival. Without food, an organism cannot obtain the energy, nutrients, and resources needed to sustain basic physiological functions, maintain bodily structures, and support vital activities such as movement and growth. Therefore, the morphology and behavior of animals are heavily influenced by the need to efficiently obtain and process food in order to survive from day to day. 2. Energy Allocation: Animals must allocate a significant portion of their energy budget to feeding in order to meet their metabolic needs. This energy allocation affects various aspects of an animal's biology, including its growth, development, and overall fitness. Consequently, the morphology and behavior of animals often evolve in ways that optimize their ability to acquire and utilize food resources effectively. 3. Ecological Interactions: Feeding interactions with other organisms, including predators, prey, competitors, and symbionts, shape the morphology and behavior of animals in profound ways. Evolutionary pressures associated with feeding interactions drive adaptations such as specialized feeding structures, hunting strategies, foraging behaviors, and dietary preferences. These adaptations are crucial for exploiting available food resources, avoiding predation, and competing for resources within ecosystems. 4. Environmental Variation: The availability and distribution of food resources vary spatially and temporally in natural environments. Animals must adapt to these fluctuations by adjusting their feeding behaviors, dietary preferences, and foraging strategies accordingly. This flexibility in feeding-related traits allows animals to exploit diverse food sources and habitats, enhancing their ability to survive and reproduce in dynamic environments. In contrast, while reproduction is essential for the long-term persistence of a species, its immediate impact on an individual's morphology and behavior may be less pronounced compared to feeding. Reproduction typically involves discrete events such as mating, gestation, and offspring care, which may occur intermittently or seasonally depending on environmental conditions. While reproduction undoubtedly influences the morphology and behavior of animals over evolutionary time scales, the day-to-day demands of feeding often exert a more immediate and direct influence on an organism's biology. 2. Are the stomach contents of an animal actually “inside” the animal proper, in the sense that blood or lymph is “in” the animal? If a baby swallows a dime, is the dime “inside” the baby? Answer:The stomach contents of an animal, including ingested food and other materials, are indeed considered to be "inside" the animal, but in a different sense than blood or lymph. When we refer to something being "inside" the body in the context of blood or lymph, we're typically describing substances that are contained within the circulatory or lymphatic systems, which are internal networks of vessels that transport fluids throughout the body. Blood and lymph play crucial roles in transporting nutrients, oxygen, hormones, immune cells, and waste products to and from various tissues and organs. On the other hand, the contents of the stomach are located within the gastrointestinal (GI) tract, which is a tubular structure that runs from the mouth to the anus. The GI tract is considered part of the body's internal environment, and its contents are contained within the body proper. However, the GI tract is technically a hollow tube that is open at both ends, allowing materials to pass through it. Therefore, while the stomach contents are "inside" the body in the sense that they are located within the GI tract, they are not contained within a closed system like the circulatory or lymphatic systems. If a baby swallows a dime, the dime would indeed be considered to be "inside" the baby, but again, in the context of being within the GI tract rather than within the bloodstream or lymphatic system. However, the presence of foreign objects such as a dime in the GI tract can pose potential risks, including obstruction or injury, and may require medical attention to ensure the object is safely passed or removed. 3. Why are stomach contents but not intestinal contents regurgitated? Answer: Stomach contents are more likely to be regurgitated compared to intestinal contents due to differences in the digestive processes and anatomical structures of the stomach and intestines. 1. Digestive Processes: The stomach primarily functions as a storage organ for ingested food and initiates the process of digestion by mixing food with gastric juices, which contain hydrochloric acid and digestive enzymes. These gastric juices help break down food into smaller particles and begin the chemical digestion of proteins. However, the stomach does not perform extensive absorption of nutrients. Instead, it gradually releases partially digested food into the small intestine for further digestion and absorption. If the stomach becomes overly full or if the digestive process is disrupted (e.g., due to overeating, rapid consumption of food, or certain medical conditions), there is a risk of regurgitation, where stomach contents are expelled back up the esophagus and out of the mouth. This regurgitation can occur relatively easily because the stomach is located high in the abdominal cavity, and the lower esophageal sphincter, which normally prevents reflux of stomach contents, may become relaxed or weakened under certain circumstances. 2. Anatomical Differences: The small intestine, in contrast to the stomach, is primarily involved in the absorption of nutrients from digested food rather than storage or mixing. It is composed of a long, coiled tube with a much larger surface area for absorption compared to the stomach. Additionally, the small intestine has a series of valves called the ileocecal valve and several sphincters along its length that help control the movement of intestinal contents and prevent backflow. Because the small intestine is optimized for nutrient absorption and has efficient mechanisms for propelling contents in one direction (towards the large intestine), regurgitation of intestinal contents is much less common than regurgitation of stomach contents. The tight closure of the ileocecal valve and sphincters along the intestinal tract helps maintain the one-way flow of contents through the digestive system, reducing the likelihood of retrograde movement and regurgitation. Overall, the differences in digestive processes and anatomical structures between the stomach and intestines contribute to the greater likelihood of regurgitation of stomach contents compared to intestinal contents. 4. Owls regurgitate the fur, bones, and feathers of prey items as “owl pellets” commonly found on the ground around their roosts. Would the owl pellet be laden with bacteria similar to feces? Why? Answer:Owl pellets, which consist of undigested parts of prey items such as fur, bones, feathers, and other indigestible materials, are not typically laden with bacteria to the same extent as feces. There are several reasons for this: 1. Digestive Process: Owl pellets are formed through the process of regurgitation rather than digestion. When an owl consumes prey, the prey items enter the owl's digestive system, where they are initially broken down by the mechanical action of the gizzard and partially digested by enzymes in the stomach. However, unlike feces, which undergo extensive digestion and fermentation in the intestines, the contents of an owl pellet are not subjected to the same level of microbial activity. 2. Sterile Environment: The environment within the owl's digestive tract, particularly the stomach, is relatively acidic due to the presence of gastric juices containing hydrochloric acid. This acidic environment helps to inhibit the growth of bacteria and other microorganisms. As a result, the undigested materials regurgitated as owl pellets are less likely to harbor large populations of bacteria compared to feces, which are produced in the intestines where bacterial fermentation occurs. 3. Rapid Passage: Owl pellets are typically regurgitated relatively soon after the owl consumes its prey, often within a few hours to a day. This rapid passage through the digestive system limits the opportunity for bacterial colonization and growth within the undigested materials. While owl pellets may contain some bacteria, especially those present on the surfaces of prey items prior to ingestion, they are generally not as heavily laden with bacteria as feces. Owl pellets are commonly used by scientists and educators for studying the diet and ecology of owls and other birds of prey, and they are typically handled with appropriate precautions to minimize the risk of exposure to any potential bacteria present. 5. Would the crop of an earthworm and a bird be considered homologous structures? Answer: The crop of an earthworm and the crop of a bird are not considered homologous structures. Homologous structures are traits that are inherited from a common ancestor and share a similar underlying structure, even if they may have different functions in different organisms. In other words, homologous structures are derived from the same ancestral structure but may have undergone modifications over evolutionary time to adapt to different functions or environments. The crop of an earthworm and the crop of a bird serve similar functions—they are both storage organs that temporarily hold food before it is further processed or digested. However, their anatomical structures are different: 1. Earthworm Crop: In earthworms, the crop is a part of the digestive system located near the mouth. It is a thin-walled, expandable pouch where ingested soil and organic matter are temporarily stored before being passed into the gizzard for further grinding and digestion. 2. Bird Crop: In birds, the crop is an enlargement of the esophagus located in the neck region. It serves as a temporary storage organ for food before it moves into the stomach for digestion. The bird's crop is often well-developed and can expand to accommodate large amounts of food, particularly in species that need to ingest food quickly and then digest it later in a safe location. While both the earthworm crop and the bird crop serve a similar function of temporary food storage, they have evolved independently in these different groups of organisms and do not share a common evolutionary origin. Therefore, they are not considered homologous structures. 6. Would the gizzard of an earthworm and a bird be considered homologous structures? Answer: The gizzard of an earthworm and the gizzard of a bird are not considered homologous structures. Homologous structures are traits that are inherited from a common ancestor and share a similar underlying structure, even if they may have different functions in different organisms. In other words, homologous structures are derived from the same ancestral structure but may have undergone modifications over evolutionary time to adapt to different functions or environments. While both the earthworm's gizzard and the bird's gizzard serve a similar function of mechanically breaking down food, their anatomical structures and evolutionary origins are different: 1. Earthworm Gizzard: In earthworms, the gizzard is a specialized muscular organ located in the digestive system, near the posterior end of the body. It is responsible for grinding ingested soil and organic matter, using strong muscles and abrasive particles within its lumen to aid in digestion. The earthworm's gizzard is lined with a tough, chitinous layer that helps to pulverize food. 2. Bird Gizzard: In birds, the gizzard is also a muscular organ located in the digestive system, between the stomach and the intestines. It serves a similar function to the earthworm's gizzard, mechanically breaking down food particles. However, the bird's gizzard is lined with a tough, keratinous lining, often containing small stones or grit. Birds swallow these stones, which are stored in the gizzard and used to aid in the mechanical breakdown of food. While both the earthworm's gizzard and the bird's gizzard perform a similar function and are located within the digestive system, they have evolved independently in these different groups of organisms and do not share a common evolutionary origin. Therefore, they are not considered homologous structures. 7. What evolutionary reasons can you suggest for brown fat being especially well developed in hibernating species of bats and rodents? Answer: Brown fat, or brown adipose tissue (BAT), is a specialized type of fat tissue that plays a crucial role in thermogenesis, the production of heat. While white adipose tissue (WAT) primarily stores energy in the form of triglycerides, brown adipose tissue is rich in mitochondria and contains a protein called uncoupling protein 1 (UCP1), which allows it to generate heat by dissipating the energy produced during mitochondrial respiration. In hibernating species of bats and rodents, brown fat is especially well-developed for several evolutionary reasons: 1. Energy Conservation: During hibernation, animals enter a state of torpor characterized by significantly reduced metabolic rates and body temperatures. By activating brown fat and increasing thermogenesis, hibernating animals can maintain their body temperature within a narrow range despite the cold ambient temperatures encountered during winter. This allows them to conserve energy and reduce the need for food intake during periods when food availability may be limited. 2. Survival in Cold Environments: Hibernating species of bats and rodents often inhabit regions with cold climates where temperatures can drop below freezing. Brown fat provides these animals with a crucial mechanism for thermoregulation, allowing them to withstand cold temperatures without the need for external heat sources. The ability to maintain body temperature during hibernation increases their chances of survival in harsh winter conditions. 3. Adaptation to Seasonal Changes: Hibernating species typically undergo seasonal changes in metabolic and physiological processes to cope with variations in food availability and environmental conditions. Brown fat allows these animals to quickly adapt to changes in temperature and energy requirements by efficiently generating heat when needed. The ability to regulate body temperature through thermogenesis helps synchronize their metabolic activity with seasonal changes and facilitates the transition into and out of hibernation. 4. Evolutionary Heritage: Brown fat is believed to have evolved early in mammalian evolution as a mechanism for thermoregulation in small, endothermic (warm-blooded) animals. Hibernating species have retained and further specialized this tissue as an adaptation to their specific hibernation strategy. The development of brown fat in hibernating bats and rodents reflects the evolutionary pressures associated with surviving and thriving in cold, seasonal environments. Overall, the well-developed brown fat in hibernating species of bats and rodents serves as a crucial adaptation for coping with cold temperatures, conserving energy, and surviving seasonal fluctuations in food availability. It represents an evolutionary solution to the challenges posed by hibernation and has contributed to the success of these animals in their respective habitats. 8. A tobacco hornworm caterpillar can eat tobacco but dies if fed milkweed. A monarch butterfly caterpillar eats milkweed but dies if fed tobacco. Both insects are in the order Lepidoptera and both plants are dicotyledons. What is the difference in these two insects from the digestive-system-enzyme perspective? Why can’t all herbivorous insects eat all green plants, and of what importance is this to farmers and gardeners? Answer: The difference in the ability of the tobacco hornworm caterpillar and the monarch butterfly caterpillar to digest and utilize different host plants, such as tobacco and milkweed, respectively, can be attributed to several factors, including differences in their digestive enzyme profiles, physiological adaptations, and co-evolutionary relationships with their host plants. 1. Digestive Enzyme Specificity: Herbivorous insects have evolved specialized digestive enzymes that allow them to break down and utilize the specific types of plant material found in their preferred host plants. These enzymes may include proteases (for digesting proteins), carbohydrases (for digesting carbohydrates), and lipases (for digesting lipids). The composition and activity of these enzymes can vary among insect species, reflecting their adaptation to different dietary preferences. 2. Co-evolutionary Relationships: Over time, herbivorous insects and their host plants have co-evolved complex chemical defenses and counter-adaptations. Host plants often produce secondary metabolites such as alkaloids, terpenoids, and phenolic compounds that can deter herbivory by inhibiting digestion, impairing nutrient absorption, or acting as toxins. In response, herbivorous insects have evolved mechanisms to detoxify or tolerate these plant defenses, allowing them to feed on specific host plants without adverse effects. 3. Physiological Adaptations: Herbivorous insects may possess specialized physiological adaptations that facilitate their ability to detoxify or sequester plant toxins. For example, some insects have evolved detoxification enzymes, such as cytochrome P450 monooxygenases, esterases, and glutathione S-transferases, which enable them to metabolize and excrete plant toxins more effectively. Additionally, certain insect species may possess symbiotic gut microbes that assist in the digestion and detoxification of plant material. The inability of all herbivorous insects to eat all green plants is of great importance to farmers and gardeners for several reasons: 1. Pest Management: Understanding the host plant specificity of herbivorous insects can help farmers and gardeners develop targeted pest management strategies. By identifying the preferred host plants of insect pests and their natural enemies, farmers can implement measures such as crop rotation, intercropping, and the use of resistant plant varieties to reduce pest damage and minimize the need for chemical insecticides. 2. Crop Selection: Farmers may choose to cultivate plant species or varieties that are less preferred by insect pests in order to reduce crop damage and improve yields. Plant breeding programs can also focus on developing crop varieties with enhanced resistance to herbivory through mechanisms such as the production of natural toxins or the induction of plant defenses. 3. Conservation of Beneficial Insects: Understanding the host plant specificity of herbivorous insects can help promote the conservation of beneficial insect species, such as pollinators and natural enemies of pests. By providing suitable habitat and food resources for these insects, farmers and gardeners can enhance biological control services and promote ecosystem resilience. Overall, the host plant specificity of herbivorous insects reflects complex interactions between insect physiology, plant chemistry, and ecological relationships. By recognizing and managing these interactions, farmers and gardeners can optimize crop production while minimizing the negative impacts of insect pests. CHAPTER 33 NERVOUS COORDINATION: NERVOUS SYSTEM AND SENSE ORGANS CHAPTER OUTLINE 33.1. Neuron: Functional Unit of the Nervous System A. Structure (Figures 33.1–33.3) 1. The neuron may assume many shapes depending on function and location. 2. The nucleated cell body has two types of cytoplasmic process. 3. One or often many more dendrites are the nerve cell’s receptive apparatus. 4. A single axon is often very long and carries impulses away from the cell body. 5. In vertebrates and some complex invertebrates, an insulating sheath of myelin covers the axon. 6. Neurons are afferent or sensory, efferent or motor, and interneurons that interconnect neurons. 7. Afferent and efferent neurons lie mostly outside the central nervous system (CNS) in the peripheral nervous system 8. Interneurons, which in humans make up 99% of all neurons, lie entirely within the CNS. 9. Afferent neurons are connected to receptors that convert stimuli to nerve impulses. 10. Only in the central nervous system are impulses perceived as conscious sensation. 11. When impulses move to efferent neurons, effectors carry them to muscles or glands. 12. A nerve is a bundle of nerve processes, usually axons, wrapped in connective tissue. 13. Cell bodies of the nerve processes are either in the CNS or in ganglia, bundles of nerve cell bodies outside the CNS. 14. Neuroglial cells surround neurons; in the vertebrate brain, they outnumber neurons by 10 to 1. 15. Schwann cells form the insulating myelin sheath by laying down concentric rings. 16. Oligodendrocytes form the myelin sheath in the CNS. 17. Star-shaped astrocytes serve as nutrient and ion-reservoirs for neurons and as a scaffold during brain development so neurons can grow to a certain destination. 18. Astrocytes and microglial cells are essential in regenerating tissue after brain injury. B. Nature of the Nerve Action Potential 1. All nerve signals are the same type of common electro-chemical message of neurons. 2. An action potential is an “all-or-none” effect; either the fiber is conducting an impulse or it is not. 3. The frequency of a signal is the only variation a nerve fiber can accomplish. 4. Nerve signals may vary from a few per second to nearly 1000 per second. 5. Resting Membrane Potential (Figure 33.4) a. Neuron membranes have a permeability that creates ionic imbalances. b. Interstitial fluid on the outside of neurons has high sodium (Na+) and chloride (Cl-) ion concentrations and low potassium ion (K+) levels. c. Inside the neuron membrane, there is a low Na+ and Cl- ion concentration and high K+ levels. d. At rest, the membrane of a neuron is selectively permeable to K+ but permeability to Na+ is nearly zero since the Na+ channels are closed. e. K+ diffuse outward until the positive charge outside repels any more K+ from exiting. f. When the electrical gradient balances the concentration gradient (~90 millivolts), the membrane concentrations for K+ are at equilibrium (equilibrium potential). g. Resting potential is usually –70 millivolts (inside of the membrane negative to the outside). 6. Sodium Pump a. Sodium pumps are a complex of protein subunits embedded in the membrane of the axon. b. Each sodium pump uses energy stored as ATP to transport sodium from the inside to outside. c. Astrocytes help maintain balance of ions surrounding neurons by sweeping up excess K+. 7. Action Potential (Figure 33.5) a. A nerve action potential is a rapidly moving change in electrical membrane potential. b. This action potential is a very rapid and brief depolarization of the membrane. c. During the action potential, the membrane potential reverses for an instant with the inside positive compared to the outside being negative. d. The nerve action potential is self-propagating; it moves along the nerve fiber on its own. e. When the action potential arrives at a given point, changes in membrane potential cause voltage-gated Na+ channels to open. f. A flood of Na+ diffuse into the axon from the outside. g. The Na+ voltage-gated channels remain open for less than a millisecond. h. The change in the polarity resulting from the influx of Na+ ions trigger K+ voltage-gated channels to open, allowing K+ ions to move out of the neuron during repolarization. i. Because K+ voltage-gated channels are slow to close, the potential within the neuron falls below that of the resting potential called the hyperpolarization phase. j. After K+voltage-gated channels finally close, Na+ permeability is restored to normal and K+ permeability briefly increases above resting level. k. Action potentials begin at the axon hillock and end at the axon terminals. 8. High-Speed Conduction (Figure 33.6) a. Speed varies from 0.1 m/sec in sea anemones to 120 m/sec in some mammal motor axons. b. Speed of conduction is related to diameter of the axon. c. Small axons conduct slowly because internal resistance to current flow is high. d. In most invertebrates, axon diameters are large to facilitate fast conduction for fast responses; for example, the giant axon of a squid is 1 mm in diameter and carries impulses 10 times faster than most axons. e Vertebrates do not possess giant axons but achieve high speed by using the myelin sheath. f. Nodes of Ranvier interrupt insulating myelin sheaths where the surface is exposed. g. Myelin insulation prevents depolarization, which therefore only occurs at the nodes. h. Saltatory conduction describes this action potential that leaps from node to node. i. A frog myelinated axon 12 µm in diameter conducts nerve impulses as fast as a squid axon 350 µm in diameter. j. Temperature also regulates conduction velocity: endotherms have a high conduction velocity since they maintain a constant temperature, whereas ectotherms conduction velocity fluctuates with environmental temperatures. 33.2. Synapses: Junctions between Nerves (Figures 33.7, 33.8) A. Function 1. An action potential passing down an axon must cross a small gap, the synapse. 2. Electrical Synapses a. Electrical synapses are uncommon, but occur in both invertebrates and vertebrates. b. Ionic currents flow directly across a narrow gap junction from one neuron to another. c. They show no time lag and are important in escape reactions. d. Signals are bidirectional at many synapses, but unidirectional ones occur in crustacea. e. They also are an important method of communication between cardiac muscle cells of the heart. 3. Chemical Synapses a. Neurons bringing action potentials to the gap are presynaptic neurons. b. Neurons carrying action potentials away are postsynaptic neurons. c. The synaptic cleft or gap between the neuron tips is about 20 nanometers wide. d. The presynaptic knobs of axons contain packets of chemicals called neurotransmitters. e. Many axon terminals may input on the thousands of dendrites of one neuron. f. The fluid-filled gap between presynaptic and postsynaptic membranes prevents the action potential from continuing. g. The presynaptic knobs secrete a neurotransmitter; one of the most common is acetylcholine. h. The neurotransmitter such as acetylcholine is packaged inside tiny synaptic vesicles. i. The action potential causes an inflow of Ca+ ions, which induces exocytosis of synaptic vesicles. j. The acetylcholine molecules diffuse across the gap in a fraction of a millisecond. k. The acetylcholine binds to receptor molecules on ion channels in the postsynaptic membrane. l. This causes a voltage change in the postsynaptic membrane. m. If the voltage change is large enough, a new action potential is generated. n. Voltage change depends on the number of molecules released and channels opened. o. The enzyme acetylcholinesterase rapidly converts acetylcholine into acetate and choline; this prevents the acetylcholine from continuing to stimulate the postsynaptic membrane. p. Organophosphate insecticides kill by blocking acetylcholinesterase. q. Choline is eventually reabsorbed into the presynaptic terminal and resynthesized into acetylcholine. r. Excitatory synapses occur where neurotransmitters such as acetylcholine, norepinephrine and glutamate depolarize the postsynaptic membrane. s. Inhibitory synapses occur where neurotransmitters such as gamma aminobutyric acid hyperpolarize the postsynaptic membrane and stabilize it against depolarization. t. The net balance of all excitatory and inhibitory inputs determines if a neuron will transmit an action potential. u. The synapse and this summation process are the decision-making equipment of the CNS. 33.3. Evolution of the Nervous System A. Invertebrates: Development of Centralized Nervous Systems (Figure 33.9A–C) 1. Protozoa are unicellular and lack nerves. 2. Nerve Net a. The simplest pattern of nervous system found in sea anemones, jellyfish, hydra and comb jellies. b. The nerve net is an extensive network in and under the epidermis. c. Signals are conducted in all directions; synapses do not direct one-way signals. d. There are no sensory, motor or interneurons. e. Nerve net survives in advanced animals as a nerve plexus that governs intestinal movement. 3. Bilateral Nervous Systems a. Flatworms represent the simplest bilateral nervous system. b. They have two anterior ganglia leading to two main nerve trunks that run posteriorly. c. Lateral branches form a ladder appearance. d. This is the simplest system to have a peripheral nervous system extending to all parts of the body, and a central nervous system concentrating nerve cell bodies. e. Annelids have advanced to segmented ganglia and distinct afferent and efferent neurons. f. Segmental ganglia are relay stations for coordinating regional activity. 4. Molluscan Nervous Systems a. The basic plan centers on three pairs of well-defined ganglia. b. In cephalopods, the ganglia have burgeoned into nervous centers of over 160 million cells. 5. Arthropod Nervous System a. The ganglia are larger than those found in annelids. b. Sense organs are generally better developed. c. Some social behavior is elaborate, but some examples of learning have been documented. B. Vertebrates: Fruition of Encephalization 1. Encephalization is the increase and elaboration in size of the brain. 2. The Spinal Cord (Figure 33.10) a. The brain and spinal cord compose the central nervous system. b. Both begin as an ectodermal neural groove that folds into a long, hollow neural tube. c. The cephalic end enlarges into the brain vesicles and the rest becomes spinal cord. d. Unlike invertebrate nerve cord, segmental nerves of spinal cord are separated into dorsal sensory roots and ventral motor roots. e. Both meet to form a mixed spinal nerve. f. The spinal cord is enclosed in the spinal canal and wrapped in three layers of meninges. g. In cross section, the spinal cord has an inner gray zone containing the cell bodies of motor neurons. h. The outer white zone contains bundles of axons and dendrites that link with other regions and the brain. 3. Reflex Arc (Figure 33.11) a. Many neurons work in groups called reflex arcs of at least two neurons and often more. b. This seems to be the fundamental unit of neural operation. c. Parts of a Reflex Arc 1) A receptor is a sense organ in the skin, muscle, or other organ. 2) An afferent or sensory neuron carries impulses toward the CNS. 3) The central nervous system makes synaptic connections between sensory and interneurons. 4) The efferent or motor neuron makes a synaptic connection with the interneuron and carries impulses from the CNS. 5) An effector is a muscle, gland, ciliated cell, electric organ, or pigmented cell that responds. d. The simplest reflex arc may only have a sensory and motor neuron, as in the “knee-jerk” example. e. Usually interneurons are interposed between sensory and motor neurons. f. Reflex arcs may be complex with many inputs or outputs, and may be modified by motor neurons. g. A reflex act is the involuntary response due to a reflex arc. h. Most are vital processes and are innate, but some are acquired through learning. 4. Brain (Figures 33.12--33.14) a. While the spinal cord has changed little in vertebrate evolution, the brain has changed dramatically. b. The ancestral vertebrate brain of fishes has become a deeply fissured intricate brain of mammals. c. The human brain contains 35 billion nerve cells; each cell may receive tens of thousands of synapses. d. The ratio of weight of brain to spinal cord provides a scale of intelligence. 1) In fishes and amphibians, the brain: spinal cord ratio is about 1:1. 2) In humans, this ratio is 55:1. e. The brain of early vertebrate fishes has three principal divisions. 1) The prosencephalon, or forebrain, dealt with the sense of smell. 2) The mesencephalon, or midbrain, dealt with vision. 3) The rhombencephalon, or hindbrain, perceived hearing and balance. f. Hindbrain (Figure 33.14) 1) The medulla oblongata is the most posterior division of the brain and is a continuation of the spinal cord. 2) Together with the anterior midbrain, the medulla constitutes the “brain stem” that controls heartbeat, respiration, vascular tone, gastric secretions and swallowing. 3) The pons contains a thick bundle of fibers that carry impulses to either side of the cerebellum and connects the medulla and cerebellum to other regions. g. Cerebellum 1) This lies dorsal to the medulla. 2) The cerebellum controls equilibrium, posture, and movement. 3) It is more developed in agile bony fish and weakly developed in amphibians and nonavian reptiles. 4) It is most developed in birds and mammals. 5) It does not initiate movement but is a precision error-control center to program movement. 6) Movements initiated in the motor cortex of the cerebrum are programmed here. 7) Cerebellar coordination can result in simultaneous contraction of hundreds of muscles. h. Midbrain 1) This consists of a tectum that contains nuclei and serves as centers for visual and auditory reflexes. 2) In this usage, “nuclei” are small aggregations of nerve cell bodies within the CNS. 3) It mediates the complex behavior of fishes and amphibians using visual, tactile and auditory input. 4) However, these functions have been assumed by the forebrain in amniotes. 5) In mammals, the midbrain is a relay center for information going to higher brain centers. i. Forebrain 1) The thalamus and hypothalamus are the most posterior elements of the forebrain. 2) The egg-shaped thalamus analyzes and passes sensory information to higher brain centers. 3) The hypothalamus regulates body temperature, water balance, appetite and thirst. 4) Neurosecretory cells in the hypothalamus produce several neurohormones. 5) The hypothalamus has centers regulating reproductive function, sexual behavior and emotions. 6) The anterior forebrain is the cerebrum. 7) The cerebrum is divided into the paleocortex and the neocortex. 8) The paleocortex is known as the limbic system and mediates behaviors relating to feeding and sex, functions that evolutionarily have depended on olfaction. 9) One region, the hippocampus, is involved with spatial learning and memory. 10) Neurons do not divide in adults except in the hippocampus. 11) The neocortex is the cerebral cortex and envelops the forebrain and midbrain. 12) The cortex contains discrete motor and sensory areas. 13) Motor areas control voluntary muscle movements. 14) The sensory cortex is the center of conscious perception. 15) Large “silent regions” called association areas are concerned with memory, judgment, reasoning and other integrative functions, but not directly connected to sense organs or muscles. j. In mammals and especially humans, separate areas mediate conscious and unconscious functions. k. The brain is an endocrine gland that regulates and receives feedback from the endocrine system. l. The unconscious mind is all of the brain except the cerebral cortex. m. The conscious mind in the cerebral cortex is the site of higher mental activities. n. Memory appears to transcend all parts of the brain rather than being a property of any one part. o. The corpus callosum is a neural connection bridging the right and left hemispheres. p. In humans, the two hemispheres specialize for different functions. 1) The left hemisphere handles language, mathematical and learning capabilities. 2) The right hemisphere handles spatial, artistic, musical, intuitive and perceptual activities. q. Birds, likewise, have one side of the brain specialized for song production. 5. Peripheral Nervous System a. The peripheral nervous system (PNS) includes all nervous tissue outside of the CNS. b. The sensory or afferent division brings sensory information to the CNS. c. The motor or efferent division conveys major commands to muscles and glands. d. The efferent division has two components: somatic nervous system and autonomic nervous system. e. Autonomic Nervous System (Figures 33.16, 33.17) 1) System governs involuntary, internal functions that do not ordinarily affect consciousness. 2) It controls movements of the alimentary canal and heart, smooth muscle of blood vessels, urinary bladder, iris of the eye, and secretions of various glands. 3) Autonomic nerves originate in the brain or spinal cord but consist of two motor neurons. 4) They synapse once after leaving the cord and before reaching the effector organ. 5) Synapse in ganglia outside spinal cord is between preganglionic and postganglionic fibers. 6) The autonomic system is subdivided into parasympathetic and sympathetic systems. 7) Most organs are innervated by both systems and they both work to control activity. 8) The parasympathetic neurons emerge from the CNS from the brain-stem or pelvic region. 9) The sympathetic nervous system nerve cell bodies of preganglionic fibers are located in the thoracic and upper lumbar areas; their fibers pass out through ventral roots of the spinal nerves and form a chain along the spine. 10) Ganglia are often remote from the effector organ in the sympathetic nervous system. 11) They are often embedded in tissue layers close to effector organs in the parasympathetic nervous system. 12) All preganglionic fibers in both systems release acetylcholine at the pre/postganglionic synapse. 13) Parasympathetic postganglionic fibers release acetylcholine at their endings, while sympathetic postganglionic fibers usually release norepinephrine. 14) Generally, the parasympathetic division is active during resting conditions. 15) The sympathetic division is active under conditions of physical activity and stress and during resting conditions in maintaining normal blood pressure and body temperature. 33.4. Sense Organs A. Stimuli 1. Sense organs are specialized receptors designed to detect environmental status and change. 2. Sense organs are the first level of perception and are channels for bringing information to the brain. 3. A stimulus is some form of energy: electrical, mechanical, chemical or radiant. 4. The sense organ must transform the energy form of the stimulus into action potentials that can be transmitted to the CNS. 5. Sensory receptors are biological transducers and usually respond to only one kind of stimulus. 6. In the 1830s, Johannes Muller detected that animals perceived different sensations only because impulses originating from one sense organ arrive at a particular sensory area: the “law of specific nerve energies.” B. Classification of Receptors 1. Receptors near the external surface are exteroceptors; those in internal organs are interoceptors. 2. Muscles, tendons and joints have proprioceptors sensitive to changes in the tension of muscles and providing a sense of body position. 3. Receptors are classified by the form of energy to which they respond: chemical, light, thermal, etc. C. Chemoreception (Figures 33.18–33.20) 1. This is the oldest and most universal sense in the animal kingdom. 2. Protozoa use contact chemical receptors to locate food and oxygenated water, and to avoid harmful substances. 3. Chemotaxis is an orientation behavior toward or away from a chemical source. 4. Most metazoans have specialized and sensitive distance chemical receptors or a “sense of smell.” 5. Olfaction is useful to guide feeding behavior, locate sexual mates, mark territories and elicit alarm. 7. Taste Receptors a. The sense of taste is more restricted in response and less sensitive than the sense of smell. b. CNS centers for taste and smell are located in different parts of the brain. c. Insect chemoreceptors are located in sensory hairs called sensilla. d. Pheromones 1) Social insects, many mammals and others produce species-specific chemicals to communicate. 2) An animal releases a pheromone to affect the behavior of another member of the same species. 3) Information about territory, social hierarchy, sex, and reproductive state are transmitted in this way. 4) Ants have many glands to produce many chemical signals. 5) Releaser pheromones include alarm and trail pheromones. 6) Primer pheromones alter endocrine and reproductive systems of different castes in a colony. e. In vertebrates, taste receptors are found in the mouth cavity and on the tongue surface. f. A taste bud is a cluster of several receptor cells surrounded by supporting cells. g. They are slender sensory cells that project through a small external pore. h. Molecules being tasted combine with specific receptor sites on microvilli of receptor cells. i. The correct chemical will depolarize the specific sensitive cell and generate an action potential. j. Subjected to wear and tear, taste buds have a short life and are continually replaced. k. Each of the four tastes detected by humans has a different taste bud: sweet, sour, salty and bitter. l. Many potentially dangerous materials are bitter, and this sense is most sensitive. m. Taste sensations are categorized as sweet, salty, acid, bitter, and possibly umami (“savory”). n. Contrary to what was originally thought, taste receptors can respond to different types of taste categories, although they may respond more strongly to one particular type. o. Taste discrimination depends on assessment by the brain of the relative activity of many different taste receptors. 8. Sense of Smell a. Olfaction is most highly developed in mammals, much more so in dogs than in humans. b. Olfactory endings are located in nasal cavity epithelium and are covered with thin film of mucus. c. Millions of olfactory neurons lie in the epithelium, each with several hair-like cilia protruding. d. Odor molecules bind to receptor proteins in the cilia; this generates the signal to the olfactory lobe of the brain. e. Odor information is then sent on to the olfactory cortex where odors are analyzed. f. Odor information is projected to higher brain centers and affects emotions, thoughts and behavior. g. Cloning and molecular techniques have located genes in mammals that code for odor reception. h. About 70 genes from the same family have been identified in fruit flies and some in nematodes. i. Each of 500 to 1000 genes encodes a separate type of odor receptor. j. Since mammals detect at least 20,000 different odors, each receptor must respond to several molecules. k. Each olfactory neuron projects to a characteristic olfactory bulb; brain “maps” active receptors. l. Odors rise from the mouth to form the complex tastes beyond the four basic tastes. m. Many terrestrial vertebrates possess an additional olfactory organ, the vomeronasal organ (Jacobson’s Organ). D. Mechanoreception (Figure 33.21) 1. These receptors respond to touch, pressure, stretching, sound, vibration and gravity, and all forms of motion. 2. Touch a. Insects have tactile hairs sensitive to both touch and vibration. b. Most vertebrate touch receptors are gathered in “sensitive” areas—a redundant statement. c. Each hair follicle has receptors sensitive to touch. d. Pacinian corpuscles are large mechanoreceptors for deep touch in mammalian skin. 1) Each has a nerve terminus with a capsule of onion-like layers of connective tissue. 2) Pressure at any point on the capsule distorts the nerve ending, producing a graded receptor potential. 3) Progressively stronger stimuli lead to stronger receptor potentials until a threshold current is produced. 4) If pressure is sustained, the corpuscle adjusts to the new shape and no longer responds. 3. Pain and Pleasure a. Pain receptors are unspecialized free nerve endings that respond to mechanical and heat stimuli. b. Pain fibers respond to small peptides, substance P and bradykinins, released by injured cells. c. Chemical responses cause a slow pain response; a pin prick or burn causes a fast pain response. d. Pain helps us avoid damage; pleasure is a stimulus to reinforce needed behaviors. 4. Lateral Line System of Fish and Amphibians (Figure 33.22) a. The lateral line detects wave vibrations and currents in water; it is a distant touch reception system. b. Neuromasts are receptor cells found in aquatic amphibians and some fishes. c. They often occur in canals under the epidermis and opening at intervals to the surface. d. Each neuromast has a collection of hair cells with the cilia embedded in a gelatinous cupula. e. The cupula projects into the lateral line canal and bends in response to water disturbance. f. The lateral line system is a major guide to fishes in locating predators and prey, and schooling. g. The lateral line system may also function in the reception of bioelectric signals. h. Electroreceptor cells are found in pores closely associated with the lateral line system. i. Some fish generate weak or strong electric fields with electric organs. 5. Hearing (Figures 33.23–33.27) a. The ear is a specialized receptor for detecting sound waves. b. Most invertebrates inhabit a silent world; only some crustaceans, spiders and insects can hear. c. The specialized tympanic membrane of locusts, cicadas, crickets, grasshoppers and moths allow them to detect a mate, rival male or predator. d. Certain nocturnal moths have ears specialized for distant and high-intensity bat sounds. e. The vertebrate ear evolved as a balancing organ, the labyrinth. f. The labyrinth has two chambers, the saccule and utricle, and three semicircular canals. g. In fish, the base of the saccule extended into a tiny pocket, the lagena. h. In evolution, the fingerlike lagena evolved into the cochlea. i. The outer ear collects sound waves and directs them down the auditory canal to the tympanum. j. The middle ear is air-filled and contains three ossicles, the malleus, incus and stapes. These bones conduct and amplify sound waves from tympanum to the oval window of the inner ear. k. The eustachian tube permits air pressure to equalize on both sides of the tympanic membrane. l. The inner ear organ is the cochlea; it is coiled in mammals. m. The cochlea is divided into three tubular canals running parallel with each other. n. The vestibular canal has the oval window at its base. o. The tympanic canal has its base closed by the round window. p. The cochlear duct runs between these canals and has the organ of Corti, the sensory apparatus. q. The organ of Corti has fine rows of hair cells from the base to the tip; > 24,000 in a human ear. r. Each hair cell is connected with neurons of the auditory nerve. s. Hair cells rest on the basilar membrane and are covered by the tectorial membrane. t. Sound waves are transmitted from tympanum to oval window to basilar membrane. u. Place Hypothesis of Pitch Discrimination (Figure 33.27) 1) Georg von Bekesy proposed different areas of basilar membrane respond to different frequencies. 2) There are specific hair cells located on the basilar membrane that respond to certain acoustic frequencies; impulses from specific fibers are interpreted in the hearing center of the brain as particular tones. 3) The amplitude of a tone depends on the number of hair cells stimulated. 4) The quality of a tone is produced by the pattern of hair cells stimulated by sympathetic vibrations. 6. Equilibrium (Figures 33.28, 33.29) a. Statocysts 1) Invertebrates often detect gravity and low-frequency vibrations with statocysts. 2) Each statocyst is a sac lined with hair cells and containing a statolith. 3) Hairlike filaments of the sensory cells are activated by the shifting position of the statolith. 4) Statocysts are similar design among many invertebrates. b. Labyrinth 1) The labyrinth is the vertebrate organ of equilibrium. 2) It has two chambers, the saccule and utricle, and three semicircular canals. 3) The utricle and saccule are static balance organs that function like statocysts. 4) The semicircular canals are designed to respond to rotational but not linear acceleration. 5) Each canal has an ampulla with hair cells embedded in a gelatinous membrane. 6) Rotation causes the hair cell to be pressed back in the stable fluid and this distortion produces the sensation of movement. 7) With three canals in different planes, acceleration in any direction provides a combination of stimulations from each ampulla. E. Photoreception (Figures 33.30–33.33) 1. Photoreceptors are light-sensitive, and range from simple cells randomly scattered on the body surface to the complex eye of vertebrates. 2. Arthropod Compound Eyes a. These are composed of many independent units called ommatidia. b. The eye of a bee contains about 15,000 and each views a separate narrow visual field. c. This allows them to detect motion, but they do not have good resolution to see objects sharply. d. Many insects have color vision; honeybees can use ultraviolet light to see nectar guides and many flying insects detect polarized light for navigation. 3. Single-Lens Camera-Type Eye a. The eyes of some annelids, molluscs and all vertebrates are built like a camera. b. An image is focused on a light sensitive surface at back of light-tight chamber. c. The eyeball has three layers. 1) A tough outer white sclera provides support and protection. 2) A middle choroid coat has blood vessels for nourishment. 3) The light-sensitive retina lines the inside. d. The cornea is a transparent anterior part of the sclera and it focuses the light that enters. e. Light passes through the cornea, the pupil formed by the iris, and is further focused by the lens. f. The lens is an oval disc that can be flattened by ciliary muscles to focus the image on the retina. g. Watery fluid between the cornea and lens is the aqueous humor. h. Vitreous humor fills the larger inner chamber between the lens and retina. i. Retina 1) The retina has several cell layers. 2) The outermost layer is closest to the sclera and contains pigment cells. 3) The next layer contains rods and cones, the photoreceptor cells. 4) About 125 million rods and 1 million cones are in each human eye. 5) Rods provide colorless vision in dim light; cones detect colors in ample light. 6) Next is a network of intermediate neurons that process and relay visual information to ganglion cells that form the optic nerve. 7) Information from several hundred rods may converge on a single ganglion cell. 8) Cones, however, show little convergence. 9) Fovea centralis is a region of keenest vision near the center of retina; contains only cones. 10) Visual acuity depends on the density of cones in the fovea. 11) Many birds have eight times the density of cones in this area, and far greater visual acuity. 4. Chemistry of Vision a. Both rods and cones contain light-sensitive pigments known as rhodopsins. b. Rhodopsin consists of a large protein enzyme, opsin and a small carotenoid molecule, retinal. c. When rhodopsin absorbs light, retinal changes shape. d. Enzyme activity of opsin induces an excitatory cascade generating an action potential. e. The amount of intact rhodopsin in the retina depends on the intensity of light reaching the eye; a dark-adapted eye contains much rhodopsin and is sensitive to weak light, and in a light-adapted eye, much rhodopsin is broken down into retinal and opsin. 5. Color Vision (Figure 33.34) a. Cones require 50–100 times more light for stimulation than rods to perceive color. b. Nocturnal animals have pure rod retinas. c. Purely diurnal animals, such as squirrels and some birds, have only cones and are blind at night. d. In 1802, an English physician proposed color vision was due to three kinds of photoreceptors; in the 1960s, researchers confirmed the function of red, green and blue photoreceptors. f. Blue cones absorb light at 430 nm, green cones at 540 nm, and red cones at 575 nm wavelength. g. Variation in structure of opsin produces different visual pigments found in the rods and cones. h. Bony fishes and birds have very good color vision; amphibians lack color vision. i. Most mammals are mostly color blind except for primates and squirrels. Lecture Enrichment 1. The concept of a nerve impulse as a change in polarity, but not the movement of ions as the impulse, can be demonstrated by setting up a row of dominoes. In this analogy, as the dominoes fall the impulse moves quite rapidly, but the individual dominoes stay in position. Setting the dominoes back up represents the sodium pump. 2. For students who have not had physics, the difference between the positive and negative pole of a battery may be useful in representing a potential. 3. The introductory illustration of the impossibility of being able to perceive as another animal perceives becomes clearly understandable as the various sensory systems are described. 4. The property of Pacinian corpuscle physiological “adaptation” can also be recognized by students who are sitting in classroom chairs and are now unaware of the many pressure points they have with the hard chair edge, the belt buckle, the seam in their jeans, etc. 5. Most students should have experienced and remembered elementary school recess where they spun around until they became dizzy and fell down, and the world “felt” like it continued spinning. This can be used during explication of the semicircular canal function. Commentary/Lesson Plan Background: Most students have sprayed an insect and watched it die with tremors representing the action described in the text for inactivation of the enzyme acetylcholinesterase. Students are conscious of the various human sensory systems, and examples can be readily drawn from their experiences. Misconceptions: The “wave of negativity” that results in a nerve impulse is often visualized as the sodium ions actually moving this distance themselves. Humans are very visual-oriented and students will often have great difficulty understanding that many animals primarily orient by odors and live in a “world of smells.” We see a mouse running a maze as a learning experience of turning left or right, but unless the track is cleaned each time, the mouse is mainly cueing on its previous scent. Of their many senses, students will state “I can live without pain.” This fails to recognize the critical role pain plays in providing humans and other animals with a sense of self, as is seen when the sense of pain is lost in leprosy and a person wears away tissues. Schedule: HOUR 1 33.1. Neuron: Functional Unit of the Nervous System A. Structure B. Nature of the Nerve Action Potential 33.2. Synapses: Junction Points between Nerves A. Function 33.3. Evolution of the Nervous System A. Invertebrates: Development of Centralized Nervous Systems B. Vertebrates: Fruition of Encephalization HOUR 2 33.4. Sense Organs A. Stimuli B. Classification of Receptors C. Chemoreception D. Mechanoreception E. Photoreception ADVANCED CLASS QUESTIONS: 1. Would a species of insect ever produce a pheromone, but not have any ability to detect the pheromone? Answer: It's theoretically possible for a species of insect to produce a pheromone without having the ability to detect it themselves, although it might seem counterintuitive at first glance. Pheromones are chemical signals used for communication among members of the same species. Here's a scenario where this could happen: Imagine an insect species that undergoes rapid evolution. At one point in its evolutionary history, some individuals develop the ability to produce a pheromone as a means of communication or signaling. This pheromone might confer some advantage, such as attracting mates or warning others of danger. However, the ability to detect this pheromone may not have evolved in all individuals of the species yet. Over time, individuals who can produce the pheromone might still benefit from its effects, even if they themselves cannot perceive it. For example, they might attract mates more successfully or deter predators. As a result, the genes responsible for producing the pheromone could become widespread in the population even before the ability to detect it evolves. However, it's important to note that this would likely be a temporary situation in evolutionary terms. Over time, natural selection would likely favor individuals who can both produce and detect the pheromone, as this would provide a more precise and efficient means of communication. So while it's possible for a species to produce a pheromone without being able to detect it, it's unlikely to persist in the long term. 2. What are the physiological and the evolutionary reasons that taste sensations are not all equal? Answer: Taste sensations are not all equal due to a combination of physiological and evolutionary factors. Physiologically, taste sensations are mediated by taste receptors on the tongue and other parts of the oral cavity. These receptors are specialized to detect specific types of molecules in food, such as sweet, sour, salty, bitter, and umami (savory). The differences in taste sensations arise from the activation of different receptors by different types of molecules. For example, sweet taste receptors are activated by sugars, while bitter taste receptors are activated by certain alkaloids. Evolutionarily, taste sensations have likely evolved to serve specific functions that are important for survival and reproduction. For example: 1. Sweetness: The ability to taste sweetness likely evolved to help animals identify and consume energy-rich foods, such as ripe fruits, which are important for providing the energy needed for survival and reproduction. 2. Sourness: Sour taste is often associated with acidity, which can indicate the presence of unripe or spoiled foods. Avoiding sour or acidic foods may help prevent ingesting potentially harmful substances. 3. Saltiness: Salt taste is important for identifying and consuming sodium-rich foods, which are essential for maintaining proper electrolyte balance in the body. In environments where sodium is scarce, the ability to detect and consume salty foods would confer a survival advantage. 4. Bitterness: Bitter taste is often associated with toxins and other harmful substances in nature. The ability to detect bitterness helps animals avoid ingesting potentially poisonous or harmful foods. 5. Umami: Umami taste is associated with the presence of amino acids and other protein-rich compounds. The ability to taste umami may help animals identify and consume protein-rich foods, which are important for growth, development, and reproduction. Overall, the differences in taste sensations reflect adaptations that have evolved to help animals identify and consume foods that are beneficial for survival and reproduction, while avoiding potentially harmful substances. 3. A Pacinian corpuscle quickly adjusts to a new shape and no longer responds, a property called “adaptation.” How is this physiological “adaptation” also an evolutionary adaptation? Answer: The physiological adaptation of Pacinian corpuscles to quickly adjust to a new shape and no longer respond is also an evolutionary adaptation because it enhances the organism's ability to detect relevant stimuli in its environment efficiently. Evolutionary adaptation refers to the process by which organisms evolve traits that increase their fitness or survival in their environment over generations. In the case of Pacinian corpuscles, their ability to rapidly adapt to changes in pressure or vibration is advantageous because it allows them to detect new stimuli effectively while ignoring constant or repetitive stimuli. From an evolutionary perspective, organisms that possess sensory receptors capable of adaptation have a competitive advantage. For example, consider a scenario where an animal needs to detect the presence of a predator in its environment. Pacinian corpuscles, by quickly adapting to the constant pressure of the ground or the animal's own movements, can focus on detecting sudden changes in pressure that might indicate the presence of a predator. This ability to detect new or changing stimuli while filtering out irrelevant background information increases the organism's chances of survival. Over generations, organisms with sensory systems that include adaptation mechanisms are more likely to survive and reproduce, passing on their adaptive traits to future generations. Thus, the physiological adaptation of Pacinian corpuscles to rapidly adjust to new stimuli is not only a short-term response to sensory input but also a long-term evolutionary adaptation that enhances the organism's fitness and survival in its environment. 4. Why do ctenophores that drift in black ocean depths need statocysts? Answer: Ctenophores, commonly known as comb jellies, are gelatinous marine animals that inhabit various ocean environments, including deep, dark depths. Despite the absence of light in these environments, ctenophores still need mechanisms to orient themselves and maintain their position in the water column. Statocysts play a crucial role in fulfilling this need. Statocysts are sensory organs found in many invertebrates, including ctenophores. They typically consist of a small sac lined with sensory cells and containing a mineralized mass called a statolith. The statolith is denser than the surrounding fluid and moves in response to gravity, providing the organism with information about its orientation in space. In the case of ctenophores drifting in black ocean depths, statocysts are essential for several reasons: 1. Orientation: Ctenophores use statocysts to sense the direction of gravity, allowing them to maintain a vertical orientation in the water column. This is important for positioning themselves at the optimal depth for feeding or avoiding predators. 2. Vertical Migration: Many deep-sea organisms, including ctenophores, engage in vertical migration, moving up to shallower depths at night to feed and descending to deeper depths during the day to avoid predation. Statocysts help ctenophores navigate these vertical movements by providing information about changes in depth. 3. Balancing: Statocysts also help ctenophores maintain balance and stability while drifting in the water column. By sensing changes in orientation, they can adjust their position and movements to remain stable despite currents and other environmental factors. Overall, statocysts provide ctenophores with essential sensory information that enables them to navigate and survive in their dark, deep-sea habitat. They help these organisms maintain orientation, regulate vertical movement, and stabilize their position in the water column, contributing to their overall fitness and survival in their environment. 5. Why do nighttime street scenes, except near the streetlights, actually appear to us to be shades of gray when we know the objects in daytime have bright colors? Answer: At night, when street scenes appear to be shades of gray except near the streetlights, several factors contribute to this perception: 1. Limited Light: At night, the primary source of illumination is often artificial lighting, such as streetlights or building lights. Compared to natural daylight, artificial lighting tends to be dimmer and less evenly distributed. As a result, the overall amount of light available for vision is reduced, leading to decreased color perception. 2. Diminished Color Sensitivity: The human eye has two types of photoreceptor cells responsible for detecting light: rods and cones. Rods are more sensitive to low light levels but do not perceive color, while cones are responsible for color vision but require higher light levels to function effectively. In low-light conditions, the rods become dominant, leading to decreased color sensitivity. This shift towards rod-based vision reduces the ability to perceive colors accurately, resulting in a grayscale appearance of the scene. 3. Color Temperature of Light Sources: Different light sources emit light with varying color temperatures, which can influence color perception. Artificial lighting sources commonly used at night, such as streetlights, often have a cooler color temperature, which tends to suppress warm colors (reds, oranges, yellows) and enhance cooler colors (blues, greens). This can further contribute to the perception of a scene as predominantly grayscale, especially when compared to the vibrant colors seen under daylight. 4. Adaptation to Low Light: The human visual system undergoes adaptation to low light conditions over time, a process known as dark adaptation. During dark adaptation, the sensitivity of the visual system increases, allowing for better detection of dim light. However, this heightened sensitivity comes at the cost of decreased color discrimination, further contributing to the grayscale appearance of nighttime scenes. Overall, the combination of reduced light levels, diminished color sensitivity in low light, the influence of artificial lighting color temperatures, and dark adaptation of the visual system collectively result in nighttime street scenes appearing as shades of gray, with colors being less vibrant and prominent compared to daytime scenes. 6. The lens adjustment to light and dark is relatively fast and yet, your ability to see when you enter a dimly lit room adjusts and improves over a half-hour. What is the physiological basis of this? Answer: The physiological basis of the adjustment of vision in dimly lit conditions over a period of time involves several mechanisms, including both rapid and more gradual processes: 1. Pupil Dilation: When entering a dimly lit room, the pupil of the eye dilates in response to the low light levels. This allows more light to enter the eye, increasing the amount of light that reaches the retina. Pupil dilation is a rapid process that occurs within seconds to minutes and helps improve sensitivity to dim light. 2. Rhodopsin Regeneration: In the retina, there are photoreceptor cells called rods, which are responsible for vision in low-light conditions. Rhodopsin, a light-sensitive pigment in rods, is bleached when exposed to light and requires time to regenerate in the dark. When entering a dimly lit environment, the regeneration of rhodopsin gradually increases sensitivity to dim light over time. This process occurs over the course of minutes to hours and contributes to the continued improvement in vision in dim light. 3. Dark Adaptation: Dark adaptation is the process by which the eyes adjust to low-light conditions after being exposed to bright light. It involves both physiological and biochemical changes in the retina and visual pathways. Dark adaptation typically occurs over a period of several minutes to half an hour or longer, depending on the intensity of the previous light exposure. During this time, the sensitivity of the rods increases, and visual acuity in dim light improves significantly. 4. Cone Adaptation: While rods are primarily responsible for vision in low-light conditions, cones, the other type of photoreceptor cells in the retina, also contribute to vision in dimly lit environments. Cone adaptation to low light occurs more slowly compared to rods but still plays a role in improving visual perception in dim light over time. Overall, the adjustment and improvement in vision in dimly lit conditions over a half-hour period involve a combination of rapid physiological processes such as pupil dilation and gradual biochemical processes such as rhodopsin regeneration and dark adaptation. These mechanisms work together to enhance sensitivity to dim light and improve visual perception in low-light environments. Instructor Manual for Integrated Principles of Zoology Cleveland Hickman, Jr., Susan Keen, Allan Larson, David Eisenhour, Helen I'Anson, Larry Roberts 9780073524214