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Course Notes

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BIOL 373
Heidi Engelhardt

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BIOLOGY 373 COURSE NOTES Table of Contents Page Introduction 1 Lecture #1: Communication between Cells, and the Endocrine System 2 Lecture #2: Metabolism, Maintenance of Body Temperature, and Short-term control of Energy Reserves by Hormones 8 Lecture #3: Homeostasis of the Body's Energy Content, and Hormonal Controls of the Metabolic Rate, Growth, and the Plasma Concentration of Calcium 14 Lecture #4: The Central Nervous System, Principles of Sensory Physiology, Skin Sensors and Taste 20 Lecture #5: Olfaction, Hearing, Equilibrium, and Vision 27 Lecture #6: Diffusion and Osmosis, Body Fluids, Homeostasis, Control Systems, and Epithelia 36 Lecture #7: Kidney Anatomy, and Processes in the Glomerulus and Proximal Tubule 39 Lecture #8: Processes in the Loop of Henle, Distal Tubule and Collecting Duct, Measurement of Kidney Function, and Water Balance 42 Lecture #9: Maintenance of Homeostasis in the Extracellular Fluid by the Kidneys 47 Lecture #10: Nutrients, and an Overview of the Gastrointestinal Tract 52 Lecture #11: Processes in the Gastrointestinal System: Mouth and Pharynx, Esophagus, Stomach and Liver 55 Lecture #12: Processes in the GastrointestinalSystem: Pancreas and Small Intestine 60 Lecture #13: Processes in the Gastrointestinal System: the Large Intestine, Co-ordination of Gastrointestinal Functions, and the Fate of Absorbed Nutrients 65 Lecture #14: Sexual Maturation, Production of Sperm Cells, Production of Egg Cells, and the Female Reproductive Cycle 70 Lecture #15: Fertilization of the Egg Cell, Embryonic and Fetal Development, Birth, and Nursing of the Baby 78 BIOL 373 COURSE NOTES 1 Introduction These notes are intended to be used in conjunction with your textbook; neither of these written sources, nor the lectures alone, will give you all the information you need. The course notes summarize, and in a few cases, restate the contents of the recorded lectures. Figures have been included that cannot be found in your textbook. BIOL 373 COURSE NOTES 2 Lecture #1 Communication between Cells, and the Endocrine System Communication between Cells Cells can signal each other electrically or chemically. ‒ Gap junctions are structures in cell membranes that allow electrical pulses to pass from one cell to its neighbour. ‒ Chemical messenger molecules can be released by a cell into the interstitial fluid surrounding it. The cell which receives the message has areceptor within it or on its surface that specifically fits the shape of the messenger. ‒ The binding of a chemical messenger to its receptor causes events to occur within the target cell. ‒ Chemical messenger molecules can be classified according to their chemical structures. Chemical Class Example amino acid or thyroid hormones amino acid derivative epinephrine peptide vasopressin steroid estrogen fatty acid derivative prostaglandin other acetylcholine Messengers classified by chemical structure ‒ Chemical messenger molecules can also be classified by the distance between the cell that releases the signal and the target cell. ‒ Autocrines affect the same cell that secreted them. ‒ Paracrines affect cells that are close to the secreting cell. ‒ Autocrines and paracrines travel from the source cell to the target cell in the interstitial fluid. ‒ Hormones affect cells that may be distant from the cell that secretes them. ‒ Hormones normally travel in the blood. BIOL 373 COURSE NOTES 3 ‒ Cytokines influence cells thatmay be close to their sources or distant from them. ‒ They may either circulate in the blood or diffuse through interstitial fluid. ‒ Cytokines are produced by all cells that have nuclei, and can affect a broad range of target cells. A ligandis anything that binds with a receptor. If the binding produces the normal response, the ligand is an agonist. If the binding does not result in the normal effect, the ligand is an antagonist. ‒ Pharmaceuticals that bind to receptors and mimic the action of their natural chemical messengers are agonists. ‒ Antagonists are molecules that prevent natural chemical messengers from having their effects by binding to their receptors. Hydrophobic chemical messengers are able to enter their target cells, and bind to receptors in their cytoplasms or nuclei. ‒ Steroid hormones are hydrophobic messengers that bind to receptors within their target cells. The complex formed when a messenger binds to its receptor induces the target cell to increase or decrease the rate of synthesis of one or more proteins. Hydrophilic chemical messengers are usually unable to diffuse through the membranes of their target cells and enter their cytoplasms. They bind to receptors that are incorporated into the membranes of the target cells.Asingle messenger may bind to more than one type of receptor on the target cell, and so have more than one effect.After binding, the receptor-messenger complex may be brought into the cell by endocytosis. The messenger can then be removed and inactivated, and the receptor re-inserted into the membrane. ‒ Because they bring the message only as far as the target cell’s surface, hydrophilic chemical signallers are called first messengers. ‒ The act of binding to the messenger alters the shape of the receptor molecule, allowing it to make some kind of change in the cytoplasm. There are a number of possible changes. ‒ The receptor may function as a transmembrane channel. Binding of the messenger to the receptor can affect the membrane’s permeability to some other ion or molecule. ‒ The resulting increase or decrease in the concentration of that ion or molecule in the cell’s cytoplasm may change its transmembrane potential, or bring about some other effect. BIOL 373 COURSE NOTES 4 ‒ The portion of the receptor that faces the target cell’s interior may have an enzymatic function, and catalyze a reaction that alters a certain kind of cellular component. ‒ An example is the enzymatic addition of a phosphate group to the amino acid tyrosine which is part of an intracellular protein. The structural alteration in the protein changes its functional ability, allowing it to bring about the cellular response. In this case the receptor is calleda tyrosine kinase. ‒ Another possibility is that the binding of a receptor to its messenger activates a G protein in the target cell’s membrane. ‒ The active G protein can either influence a channel protein, or modify an intracellular enzyme. ‒ The enzyme may be a kinase, or it could be adenylyl cyclase, which converts ATP molecules to cyclic AMP. ‒ Molecules of cyclicAMP, like calcium ions, serve as multi-purpose signals inside cells. The effect of these second messengersdepends on the capabilities of the target cell. ‒ Acellular enzyme that is activated by the binding of messenger and receptor could activate several molecules of a second enzyme, each of which activates several molecules of a third, etc. ‒ The resulting cascade of activations allows a single messenger molecule to have a very large effect in its target cell. ‒ The rapid deactivation or removal of second messenger molecules prevents the cellular effect from lasting much longer than the time that the messenger is bound to the receptor. Atarget cell may adjust the level of its response to the chemical messenger by changing the number of receptors it has, or by altering their affinity for the messenger. ‒ Down-regulation is a reduction in the number of receptors, or in their ability to bind to their agonists. It is often a response to a prolonged period in which the concentration of the messenger is at a higher level than normal. ‒ Up-regulation is the opposite. It allows a target cell to respond to a messenger that is in low concentration. BIOL 373 COURSE NOTES 5 The Endocrine System Hormones are difficult to study for several reasons. ‒ They are often found at very low concentrations in the blood. ‒ Each endocrine gland usually produces more than one type of hormone, and a single hormone may be secreted by more than one type of gland. ‒ Asingle hormone may have multiple effects on different targets ‒ Hormones often interact. ‒ Two hormones are synergistic if the effect they have in combination is greater than the sum of the effects they have separately. ‒ If a hormone has its full effect only when another is present, the second hormone is said to be permissive for the first. ‒ Antagonistic hormones have opposite effects on a target. Hormones have effects that are slower and longer-lasting than those of the nervous system. Based on their chemical structures, there are three classes of hormones. ‒ Peptide hormones are linear linkages of amino acids. The largest of these are called protein hormones. They are hydrophilic. ‒ Steroidhormones are hydrophobic molecules derived from cholesterol. ‒ The aminehormones include the two thyroid hormones, and the three catecholamines (epinephrine, norepinephrine and dopamine). The thyroid hormones are hydrophobic and the catecholamines are hydrophilic. The chemical differences between the hydrophobic and hydrophilic hormones affect the way they are released and how they travel to their targets. These differences influence their effective concentrations in the blood. BIOL 373 COURSE NOTES 6 Hydrophilic hormones dissolve easily in Because of its size, the carrier-hormone aqueous solutions such as interstitial fluid and complex cannot easily be removed from the plasma. blood as it passes through the kidneys. The carrier also helps to protect the hormone from enzymes in the circulatory system that could inactivate the hormone. These actions allow hydrophobic hormones to remain in the blood for a longer time than the hydrophilic hormones. The molecules of hydrophilic hormones are The hydrophobic hormones are relatively small and can readily be filtered out of the insoluble in water, and must travel in the blood at the kidneys’glomeruli. bloodstream bound to a protein carrier molecule. The effective concentration of a hydrophilic Not all of the molecules of a hydrophobic hormone is equal to its total concentration in the hormone are bound to carriers.Asmall minority blood. are dissolved in the plasma. The effective concentration of a hydrophobic hormone depends on the fraction of molecules that are free and able to diffuse into their target cells. Hydrophilic hormones are often stored within The rates of inactivation and removal of vesicles in the cells where they have been hydrophilic hormones from the blood are often synthesized so high that the hormone can have its effect only as long as it is being actively secreted. Exocytosis of the vesicles when the hormone is When a hydrophobic hormones diffuses out of released causes its effective concentration in the the cell where it was synthesized and into the blood to rise sharply. blood, most of the molecules immediately bind to carrier proteins. The effective concentration Molecules of the hydrophobic hormones, such of the hormone therefore rises only by the small as the steroids, cannot be stored in vesicles, fraction of new hormone molecules that remain because they are able to diffuse through free membranes. They must be synthesized when they are needed. As a consequence their rate of release is much lower than that of the hydrophilic hormones. The signals for the release of all neurohormones (hormones secreted by nerve cells), and many other hormones as well, originate in the central nervous system (CNS). ‒ The cue for hormone release may be environmental. Sensory cells transmit information about the environment to the CNS. ‒ Electrical signals in the nervous system stimulate neuroendocrine cells to secrete the hormone. BIOL 373 COURSE NOTES 7 Oxytocin and vasopressin are peptide neurohormones synthesized in the brain’s hypothalamus and secreted by the posterior pituitary gland. ‒ Vasopressin controls the reabsorption of water in the kidneys. ‒ Oxytocin stimulates smooth muscle contraction in the uterus and the breasts. Epinephrine and norepinephrine are amine neurohormones secreted by the medullas of the adrenal glands. Their secretory cells are postganglionic neurons of the sympathetic nervous system. ‒ These hormones bring about the fight-or flight response. The hypothalamus secretes 7 peptide neurohormones that all have the same target: the endocrine anterior pituitary gland. They are called trophic hormones because each controls the secretion of another hormone. ‒ The hormones enter capillaries within the hypothalamus and are carried by a portal system to another set of capillaries in the anterior pituitary. The small size of this portal system provides two advantages: ‒ It allows the hypothalamic hormones to contact the anterior pituitary very quickly, since the blood carrying them has to travel only a few millimetres; and ‒ because the volume of blood in the system is small, only a tiny amount of hormone has to be secreted in order to reach an effective concentration. Five of the 6 anterior pituitary hormones for which the hypothalamic hormones are trophic are themselves trophic. For the relationships between the hypothalamic hormones and the anterior pituitary hormones, please see figure 7.9 on page 222 of your textbook. The secretion of many hormones is controlled by negative feedback. The linking of one trophic hormone to another affords multiple opportunities for feedback control. ‒ Short-loop feedback is found where a high plasma concentration of a hormone from the anterior pituitary inhibits the secretion of its hypothalamic releasing hormone. ‒ Long-loop feedback has one more step: a high concentration in the blood of a hormone (the thyroid hormone T 3 for example) whose production is stimulated by an anterior pituitary hormone (thyrotropin) inhibits the secretion of both the pituitary hormone and the hypothalamic hormone (thyrotropin-releasing hormone) that is responsible for initiating the entire reflex. ‒ Inhibition of hypothalamic hormones is possible because circulating hormones are able to pass out of the blood vessels in the hypothalamus through their relatively permeable walls. BIOL 373 COURSE NOTES 8 Lecture #2 Metabolism, Maintenance of Body Temperature, and Short-term Control of Energy Reserves by Hormones. Metabolism Almost all of the energy incorporated into the food ingested in the diet is converted into heat or work. There arethree types of work: 1. Transport moves ions and molecules across biological membranes against their concentration gradients. 2. Mechanical work results in movement of objects ― for example, body motions, pumping of the blood, or movement of cellular components. 3. Chemicalwork is used for synthesis and growth. Heat is a byproduct of all the biochemical pathways in the body. It is used to keep the organs in the centre of the body at an average temperature of about 38 C. ‒ Adaily or circadian cycle has its peak in the late afternoon or early evening and the lowest temperature just before the normal time of waking. The difference between the highest and lowest points in the cycle is normally about one Celsius degree. ‒ In women the average body temperature is about 0.5 Celsius degree higher in the second half of the menstrual cycle than in the first half. The environment in which most people live is cooler than body temperature. Heat energy is transferred between the body and its surroundings through 4 processes. 1. Radiation. Electromagnetic waves are given off by the body surface. It also receives radiant energy from objects that are warmer than body temperature. 2. Conduction. Heat is directly transferred to (or from) anything that is in contact with the body. 3. Convection. This is a special form of conduction, in which heat flows into or out of a fluid medium (air or water) that is in movement across the body surface. 4. Evaporation.Alarge amount of energy is needed to convert liquid water into its gaseous form. BIOL 373 COURSE NOTES 9 Peripheral thermoreceptors in the skin monitor its temperature. Central thermoreceptors in the head measure the body’s internal temperature. The sensory information provided by both sets of thermoreceptors is received and integrated in the brain’s hypothalamus. ‒ If there is a risk of excessive heat loss, the rate at which heat energy is generated can be raised, and the insulating ability of the body surface can be improved. ‒ Signals originating in the brain cause short-duration contractions in the large skeletal muscles. Shivering results in heat production, but no overall body motion. ‒ Infants have a tissue specialized for providing heat. In brown fat, high energy electrons derived from the metabolic breakdown of nutrients or stored fat are used exclusively to generate heat energy. ‒ Signals sent through the sympathetic nervous system cause the small blood vessels in the skin to constrict. The skin loses less heat to the environment when less blood is flowing through it. ‒ Behavioural responses, such as moving to a warmer location or putting on extra clothing reduce heat loss. ‒ When more heat is being generated than lost, measures are taken to stabilize body temperature. ‒ Dilation of the skin’s blood vessels allows extra heat to be given off by conduction and convection at the body surface. ‒ Glands in the skin secrete sweat. Its evaporation removes heat energy from the skin. ‒ The generation of metabolic heat can be reduced by loss of appetite in hot weather and unwillingness to exercise strenuously. ‒ Behavioural responses can also increase the rate of heat loss. Atemperature in the body’s core which is higher than normal can be tolerated for short periods of time, such as at the beginning of a period of heavy exertion. ‒ Immune cells respond to certain types of infection by secreting pyrogens. These cytokines cause the hypothalamic thermostat to be adjusted to a higher setting. The result is a fever. The degree of activity of the body’s biochemical processes, and therefore its rate of heat production can be influenced by several factors. ‒ exercise ‒ gender ‒ age ‒ relative amounts of muscle and adipose tissues in the body ‒ the ingestion of food BIOL 373 COURSE NOTES 10 The total amount of energy used while the body is at rest is called the Basal Metabolic Rate (BMR). ‒ By convention, the BMR is measured when the subject has been at rest for at least 30 minutes in an environment in which neither heat loss nor heat gain is significant, and at least 12 hours have passed since (s)he has eaten. There are two possible procedures for evaluating the BMR: ‒ The subject is sealed into an insulated chamber, and the amount of heat produced per unit of time is measured directly; or ‒ the subject respires through a closed system so that the rate of oxygen consumption can be monitored. ‒ In the second procedure, the amount of oxygen used is converted to a figure that represents the amount of energy used. This method can also be used to measure metabolic rates that are not basal, such as during exercise. Short-term Control of Energy Reserves by Hormones. The body incorporates deposits of stored energy that can be used when absorption from the digestive tract is not taking place, and during longer periods when energy requirements exceed energy intake. Energy can be stored within the body in three forms. 1. Proteinis used for this purpose to a very limited extent because of two disadvantages. ‒ The metabolism of protein requires the elimination of waste nitrogen that is derived from the amino group of each amino acid; and ‒ Use of protein for its energy content results in the destruction of essential amino acids that can be replaced only through dietary intake. 2. Carbohydrate, in the form of the polymer glycogen, acts as a reservoir for blood glucose. Its main disadvantage is that it must have water deposited along with it in order to maintain isosmotic conditions within the cell. The water makes glycogen deposits heavy. 3. Lipid, since it is not dissolved in the cytoplasms of the cells where it is deposited, needs no water to accompany it. The metabolism of lipid also yields more than twice as many calories per gram as protein or glycogen. These features make lipid the most favoured means of storing energy. During the part of the digestive cycle in which nutrients are being absorbed into the blood (the absorptive or “fed” state), the products of digestion are available to all the body’s cells. ‒ The hormone insulin facilitates the uptake of glucose and amino acids into muscle and liver cells. BIOL 373 COURSE NOTES 11 ‒ insulin is secreted by the β cells of the endocrine portion of the pancreas, the Islets of Langerhans, in response to 3 different stimuli: 1. glucose-dependent insulinotropic peptide, which is secreted by the small intestine; 2. plasma concentrations of glucose and amino acids that are typical of the absorptive state; and 3. signals from the brain carried by parasympathetic nerves. ‒ Since it is a small peptide, insulin is quickly removed from the blood. Its actions last only as long as it is being secreted. ‒ Insulin’s effects: ‒ It accelerates the entry of glucose into two kinds of cells, those of adipose and skeletal muscle. It does this by bringing about the incorporation of GLUT 4 transporters into their membranes. The transporters act as channels, allowing glucose molecules to diffuse across the membranes into the cells. ‒ Insulin stimulates liver and skeletal muscle cells to increase the rate at which they convert glucose into glycogen, and to enhance the process of glycolysis which is the first step in the metabolic breakdown of glucose. These actions keep the cytoplasmic concentrations of glucose low in the cells, and maintain the gradient that promotes its inward diffusion. ‒ It enhances the formation of triacyglycerides from surplus glucose in the liver. These lipid molecules can be exported into the bloodstream in the form of VLDL particles, and contribute to deposition of fat in adipose cells. ‒ Insulin inhibits the enzymes of gluconeogenesis and glycogenolysis, pathways that produce glucose within cells. It also suppresses the breakdown of triacyglycerol molecules in adipose cells. ‒ Protein synthesis is stimulated by insulin. Conversion of amino acids into protein molecules favours the continued entry of amino acids into liver and skeletal muscle cells. During the part of the digestive cycle in which nutrients are no longer being absorbed into the blood (the postabsorptive or “fasted” state), the concentrations of nutrients in the blood fall. ‒ For most cells in the body, glucose is the preferred fuel; this is especially true of nerve cells, which are able to use alternatives only under extreme conditions of glucose shortage. For this reason, regulatory processes exist to ensure that the blood concentration of glucose does not fall below a minimum level. BIOL 373 COURSE NOTES 12 ‒ Glucagon, a peptide hormone produced by the α cells of the pancreas, helps provide a continuous supply of glucose to the cells that need it. In most ways glucagon’s effects are antagonistic to those of insulin. Its rate of release is relatively stable, rising only in response to very low concentrations of glucose in the plasma, or a very high concentration of amino acids. ‒ The effects of insulin and glucagon depend more on the ratio between their levels in the blood than on their absolute concentrations. ‒ The insulin-to-glucagon ratio is high during the absorptive phase when insulin is being released. But when absorption comes to an end, the secretion of insulin stops while glucagon release continues. The insulin-to-glucagon ratio falls and the following effects appear. ‒ The rate of breakdown of triacylglycerol molecules in adipose tissue exceeds their rate of formation. ‒ This results in the release of fatty acids into the blood. The heart and skeletal muscle can use fatty acids as a fuel source instead of glucose. ‒ Glycerol molecules are also produced when triacylglycerols are hydrolyzed. Glycerol is one of the compounds that can be converted to glucose in the liver by gluconeogenesis, a pathway which is also stimulated under these conditions. ‒ Glucose is released when the liver’s stores of glycogen are mobilized. Glycogenolysis is promoted when the insulin-to-glucagon ratio is low. ‒ Glucose produced by the two processes of gluconeogenesis and glycogenolysis diffuses out of the liver cells into the blood. ‒ The breakdown of protein in muscle tissue makes amino acids available. They too can enter the gluconeogenesispathway for glucose production. ‒ Fatty acid molecules that enter liver cells cannot be fully metabolized, but are converted to ketone bodies. These organic molecules are released into the blood, and can serve as energy sources for muscle and nerve cellswhen the supply of glucose is low. Liver production of ketone bodies is favoured by a low insulin- to-glucagon ratio. ‒ The stimulation of gluconeogenesis and glycogenolysis in these conditions ensures that glucose levels in the blood will not fall to danger levels following a meal that contains protein but very little carbohydrate. The high concentrations of amino acids that would result from the absorption of this meal would stimulate the release of insulin, causing glucose uptake from the blood. BIOL 373 COURSE NOTES 13 – Simultaneous release of glucagon promotes glycogenolysis and gluconeogenesis by the liver, allowing glucose to be exported into the blood Malfunctions that involve insulin release or the responsiveness of its target cells can present serious risks to health. ‒ People who suffer from type 1 (insulin-dependent) diabetes mellitus cannot secrete normal quantities of insulin. ‒ Their plasma glucose levels are high because of the inability of their adipose and muscle cells to raise the rates of glucose uptake in the absorptive state.At the same time, the lack of conversion of glucose to glycogen in the liver retards glucose entry into its cells. ‒ With little insulin being secreted, the insulin-to-glucagon ratio is low, stimulating production of glucose (by gluconeogenesis and glycogenolysis) and its release into the bloodstream. ‒ The concentration of glucose in the blood rises above its renal threshold. Glucose that cannot be reabsorbed by the kidneys is lost in the urine. ‒ The high osmotic concentration of the urine caused by the glucose it contains limits the kidneys’ability to reclaim water. Dehydration reduces the volume of the extracellular fluid, and blood pressure declines to the point where blood cannot circulate normally. ‒ The rate of production of ketone bodies, raised by the low insulin-to-glucagon ratio, exceeds the ability of the body to utilize them. The surplus of ketone bodies in the blood causes acidosis. ‒ Treatment for type 1 diabetes mellitus involves regular administration of insulin. ‒ Type 2 (or non-insulin-dependent) diabetes mellitus involves normal secretion of insulin, but reduced responses of the target organs. ‒ The symptoms of this disease are similar to, but often less severe than, those of type 1 diabetes mellitus ‒ The incidence of type 2 diabetes mellitus appears to be related to aging and to obesity. There is no known disease caused by insufficient rates of glucagon secretion or lack of response by its target organs. ‒ Many of the effects of glucagon can be accomplished by epinephrine, or by signals carried directly to its target organs through the sympathetic nervous system. BIOL 373 COURSE NOTES 14 Lecture #3 Homeostasis of the Body’s Energy Content, and Hormonal Controls of the Metabolic Rate, Growth, and the Plasma Concentration of Calcium Homeostasis of the Body’s Energy Content The amount of energy used metabolically is normally matched by the amount of energy in the food consumed during the same period. ‒ As a consequence, the amount of stored energy in the body tends to remain constant. ‒ The balancing is accomplished by regulation of the amount of food eaten. ‒ The feeding centre in the hypothalamus is responsible for appetite and eating behaviour. ‒ The satiety centre in the same area of the brain generates signals of fullness that override the signals continuously sent out by the feeding centre. ‒ There are several theories concerning the regulation of food intake. 1. When the plasma concentration of glucose is low (in the “fasted” state), the feeding centre’s signals are stronger than those of the satiety centre, resulting in an urge to find food and eat it. When the plasma concentration of glucose is high (in the “fed” state), the satiety centre’s signals predominate,and the motivation for eating disappears. ‒ The strength of the signals from the satiety centre depends on the rate of glucose utilization occurring in it. Insulin increases the permeability of its cells’membranes to glucose, allowing glucose to enter and be metabolized. 2. The hormone CCK stimulates the satiety centre to increase the strength of its signals. CCK is secreted by endocrine cells in the duodenal lining when chyme arrives from the stomach, and is also released by brain cells. 3. Appetite may depend on the total amount of lipid stored in adipose tissue, or the amount in each adipose cell. The relationship is inverse: the higher the level of storage, the less appetite. 4. The degree of stretching of the stomach may influence the appetite. BIOL 373 COURSE NOTES 15 Hormonal Control of the Metabolic Rate The thyroid gland secretes two closely related homones, tetraiodothyronine (thyroxine or T ) and 4 triioidothyroninine (T 3. Like steroid hormones, T & 4 are 3ydrophobic and travel in the blood bound to carrier proteins. ‒ The production of T &4T beg3ns with a large glycoprotein called thyroglobulin. Following its synthesis within follicular cells of the thyroid gland, thyroglobulin is secreted, forming a reservoir of material called colloidwithin each follicle. ‒ Among the amino acids that make up the thyroglobulin molecule are several tyrosine residues. While it is being stored in the colloid, thyroglobulin molecules are acted upon by enzymes that change its tyrosine residues to T & 4 whil3 they are still joined by peptide bonds to adjacent amino acids. ‒ When the thyroid gland receives the signal for release of its hormones, the follicular cells phagocytose some of the colloid material and digest it with proteinase enzymes. The molecules of T & 4 tha3 are released diffuse out of the cells. ‒ The bond between T and4its carrier protein is stronger than that between T and i3s carrier.As a result, T4is degraded less by enzymes in the circulatory system and less of it is removed by the kidneys. ‒ Like the molecules of other hydrophobic hormones, those of T & T di4fuse3into their target cells. Inside the cells, 4 is enzymatically converted to T ,3which has a stronger effect in promoting protein synthesis. Under fasting conditions, the rate of this conversion falls below normal. The hypothalamus is the integrating centre for information concerning body temperature. Based on information it receives from internal and peripheral thermoreceptors, it releases the trophic hormone TRH (thyrotropin-releasing hormone) which signals the anterior pituitary gland to secrete thyrotropin (thyroid-stimulating hormone). ‒ Thyrotropin stimulates the thyroid gland to release T & 4 . 3 ‒ The major effect of T &4T in 3dults is to raise the metabolic rate in many target organs. More heat energy is generated, helping to maintain the proper body temperature. ‒ T & T are permissive for epinephrine and norepinephrine, are able to raise cardiac 4 3 output, and cause dilation of blood vessels in the skin. ‒ The most important role for T4 & T3 is in the fetus and newborn child, when the nervous system is still developing. The thyroid hormones allow nerve cells to make the right synaptic connections with other nerve cells, and promote myelinization of axons. The synthesis of T &4T re3uires iodine. If the diet does not provide this mineral in adequate amounts, the plasma concentrations of thyroid hormones will be too low, leading to fatigue, the sensation of cold, and other effects. BIOL 373 COURSE NOTES 16 ‒ Low blood levels of the thyroid hormones also prevent them from providing the negative feedback which normally inhibits the release of TRH and thyrotropin. Under the influence of extra thyrotropin, the thyroid gland becomes enlarged; this condition is called goiter. The adrenal cortexesproduce 3 classes of steroid hormones: ‒ aldosterone regulates the reabsorption of NaCl in the kidneys; ‒ the female and male sex hormones are secreted in small amounts; and ‒ cortisol has a number of effects, most of which are metabolic. ‒ It promotes gluconeogenesis in the liver. Some of the newly-formed glucose diffuses out into the blood, and the rest is converted to glycogen. ‒ It mobilizes energy reserves, causing the release of their breakdown products into the blood: ‒ amino acids are produced by the hydrolysis of protein in skeletal muscles, and ‒ fatty acids and glycerol are freed by the mobilization of lipid in adipose cells. ‒ It stimulates the adrenal medullas to synthesize epinephrine, and it is permissive for both epinephrine and norepinephrine. The plasma concentration of cortisol has a circadian pattern, higher in the morning and lower in the evening. Its release is also influenced by the anterior pituitary hormone, corticotropin. ‒ Cortisol is necessary to enable the body to withstand stress. Hormonal Control of Growth Growth hormone (GH) is secreted by the anterior pituitary. Unusual for a peptide hormone, it has a carrier protein which binds to about 50% of the circulating GH. ‒ The release of growth hormone has a circadian pattern, and is influenced by 2 antagonistic hypothalamic hormones ─ growth hormone-releasing hormone (GHRH), which is trophic for it, and growth hormone-inhibiting hormone (GHIH or somatostatin). ‒ It has many targets. Growth hormone is trophic for a set of peptides produced by the liver called somatomedins or insulin-like growth factors. The somatomedins and growth hormone affect many other tissues. BIOL 373 COURSE NOTES 17 ‒ Some of growth hormone’s effects are metabolic: ‒ mobilization of lipid reserves in adipose tissue; ‒ inhibition of glucose uptake into muscle cells; ‒ stimulation of gluconeogenesis; and ‒ enhancement of the uptake of amino acids into muscle cells and the formation of protein in muscle and liver. ‒ Other effects are not metabolic: ‒ stimulation of the growth of bones, and ‒ enhancement of RNAsynthesis in liver. ‒ The somatomedins stimulate bone growth, and the synthesis of RNAand protein. During fetal growth, the components of the skeleton are first formed as cartilage. Between this time and adulthood, the skeletal elements gradually enlarge and bone tissue takes the place of cartilage. ‒ The replacement of cartilage by bone begins near the centre of each element and spreads slowly toward the two ends. ‒ Cells called osteoblasts invade the cartilage and secrete collagenousmatrix, the organic part of bone. – Matrix becomes bone as crystals of the mineral calcium phosphate (in the form of hydroxyapatite) are deposited in it. ‒ The mineralization of bone isolates the osteoblast cells, which become osteocytes. ‒ The structure of bone is not permanent; after it has been formed, it is continuously remodelled. Old bone is reabsorbed by cells called osteoclasts, and new matrix laid down by the osteocytes. Remodelling of bone is directed by several factors, including age and the amount of stress placed upon the skeleton. ‒ Around the time of birth, replacement of cartilage by bone begins to occur at the ends of each skeletal element as well as at the centre. ‒ By late childhood, the ends of each element and the centre portion are fully-formed bone. The two remaining areas in which matrix is being secreted are called the epiphyseal plates. BIOL 373 COURSE NOTES 18 ‒ The skeletal elements grow in length (and in diameter) by addition of matrix at these locations. ‒ Growth hormone and the somatomedins stimulate the cells of the epiphyseal plates until puberty, when the sex hormones begin to reduce their ability to respond to the peptide hormones. This “closing” of the epiphyseal plates marks the end of bone lengthening. Hormonal Control of the Plasma Concentration of Calcium Ninety-nine percent of the calcium contained in the body is deposited in the bones, 0.9% is located within the cells, and the remaining 0.1% is dissolved in the extracellular fluid. ‒ Ca ++ions act as signallers when they enter certain kinds of cells, are essential in the process of blood clotting, and affect the functioning of nerve cells. The concentration of Ca ions in extracellular fluid is very closely controlled. ‒ Around 15% of the calcium taken in with the diet is absorbed into the extracellular fluid. ‒ About half of the Ca++ ions in the plasma are bound to protein, but the other half can be filtered by the kidneys. ++ ‒ On average, 90% of the filtered Ca ions are reabsorbed in the proximal tubules and loops of Henle, and another 9% in the distal tubules. The remaining 1% is excreted in the urine. ++ Three hormones are involved in controlling the plasma concentration of Ca ions — parathyroid hormone, calcitriol (or calcitrol), and calcitonin. ‒ Parathyroid hormone, a peptide with a short life span in the blood, is produced by the 4 parathyroid glands on the dorsal surface of the thyroid gland. ‒ Secretion occurs when the level of Ca ++ ions in the plasma falls below a threshold. The hormone raises the level in 3 ways. ++ 1. It enhances the rate of reabsorption of Ca ions in the kidneys’distal tubules. 2. It signals the kidneys to increase the rate of synthesis of calcitriol. ++ 3. It indirectly stimulates the osteoclast cells to break down bone tissue. The Ca and phosphate ions that are released enter the blood. ‒ This situation presents a risk. If calcium and phosphate ions are both present in abundance in the blood, they can spontaneously form crystals of hydroxyapatite. BIOL 373 COURSE NOTES 19 ‒ Parathyroid hormone reduces this risk by inhibiting the reabsorption of phosphate ions in the kidneys.As less phosphate is reabsorbed, more is excreted and its plasma concentration falls. ‒ Vitamin D can be synthesized in the skin when the sun shines on it. For many people, this 3 process does not provide enough of the vitamin, and so it must be part of the diet. ‒ Vitamin D i3 converted to the hormone calcitriol (chemical name: 1,25 dihydroxy- cholecalciferol) by 2 chemical reactions. The first takes place in the liver, the second in the kidneys. ‒ Calcitriol is a steroid that travels in the blood bound to a protein carrier molecule. It has 3 effects: 1. it stimulates the epithelial cells of the small intestine to increase their rates of absorption of calcium and phosphate; 2. along with parathyroid hormone, it enhances the rate of bone breakdown by osteoclast cells; and ++ 3. it helps parathyroid hormone to increase the rate of reabsorption of Ca ions in the kidneys. ‒ Calcitonin is a peptide hormone that has the potential to act antagonistically to parathyroid hormone and calcitriol. When the plasma concentration of Ca ++ ions rises above a threshold, cells in the thyroid gland that are outside the follicles secrete this peptide. ‒ It reduces the rate of bone breakdown by osteoclasts, and slows calcium reabsorption in the kidneys. ‒ Although these effec++ can be measured, in adults they contribute little to reducing the concentration of Ca ions in the blood. BIOL 373 COURSE NOTES 20 Lecture #4 The Central Nervous System, Principles of Sensory Physiology, Skin Sensors and Taste The Central Nervous System (CNS) The CNS, made up of the brain and spinal cord, receives input from sensors that provide (afferent) information about internal and external conditions – these signals are integrated and used to determine outgoing (efferent) signals – myelinated (white) components of the CNS carry signals quickly over long distances – its unmyelinated (grey) components process the signals – neurons make up about 10% of the cells in the CNS; the rest are glial cells – physical protection of the CNS is provided by 1. the bone that surrounds it (skull and vertebrae) 2. three membranes – the dura mater on the outside is tough but flexible – the central arachnoid layer covers the brain’s blood vessels – the pia mater is in direct contact with the nervous tissue 3. the cerebrospinal fluid in the subarachnoid space (between the arachnoid layer and the pia mater) that acts as a cushion – the rate of exchange between the blood in the capillaries of the CNS and the surrounding ISF is exceptionally low. Selective transfer of materials out of the blood offers the CNS protection from dissolved substances in the blood that might adversely affect it Nerves connect to the spinal cord on each side at the level of every vertebra between the skull and the lower back – each spinal nerve’s ventral root carries signals to muscles and other effector organs – its dorsal root carries afferent signals from sensors to the CNS The white matter in the spinal cord is made up of myelinated axons – ascending tracts conduct sensory information up to the brain – descending tracts bring signals from the brain to the effector organs BIOL 373 COURSE NOTES 21 The grey matter in the spinal cord includes groupings of interneurons that process signals: 1. the sensory nuclei deal with incoming information; 2. the somatic motor nuclei pass impulses to skeletal muscles; 3. the autonomic efferent nuclei contain the cell bodies of preganglionic neurons of the autonomic division Spinal reflexes, such as the knee-jerk reflex, show that the spinal cord is capable of receiving sensory signals, integrating them, and sending out appropriate responses Cerebrospinal fluid (CSF) is formed by choroid plexuses inside the ventricles of the brain – the ciliated inner walls of the ventricles propel CSF down to the spinal cord – it flows back up to the brain, and is reabsorbed into the blood through the arachnoid villi – the composition is CSF is different from that of ISF, but the two fluids exchange materials The brain’s medulla oblongata is continuous with the upper end of the spinal cord – ascending tracts pass through it, carrying sensory signals to the brain – its descending tracts conduct signals which originate in the brain to the lower body – many of these tracts switch sides as they pass through the medulla, from right to left and vice versa – chemical synapses in the medulla allow integration of signals to occur there – homeostasis of blood pressure, heartrate and other functions is controlled in the medulla The brain structure above the medulla is the pons, through which pass communications between the cerebellum and the cerebrum Above the pons is the midbrain, which helps to control the muscles that direct the eyes The medulla, pons and midbrain together form the brain stem – eleven of the twelve pairs of cranial nervesattach to it – it also contains the reticular formation, which is involved in the waking/sleeping cycle, and the focussing of attention BIOL 373 COURSE NOTES 22 Dorsal to the brain stem is the cerebellum – it receives sensory information and creates signals that contribute to balance, posture, and body movement At the top of the brain stem is the diencephalon, made up of the thalamus and hypothalamus – the thalamus transmits and modifies signals travelling to the cortex from many sensory sources and from the cerebellum – the hypothalamus has three main functions: 1. it synthesizes two neurohormones which are released from the posterior pituitary gland, an outgrowth of brain tissue on its ventral side – attached to the front of the posterior pituitary is the anterior pituitary, a true endocrine gland which releases a variety of hormones, as directed by the hypothalamus 2. it maintains homeostatic control of body temperature, the concentration of body fluids and the intake and expenditure of energy 3. it influences the release of the sympathetic hormone epinephrine from the adrenal glands The cerebrum is the largest component of the brain – on its surface is the cortex, formed of grey matter, with many ridges and grooves – beneath the cortex is white matter – the corpus callosumis a channel which allows the two sides (hemispheres) of the cerebrum to communicate with each other The cerebral cortex has many small locations, each of which is dedicated to a specific function – areas in the sensory cortexreceive information from receptors – for example, the visual cortex processes the signals sent from the eyes – neighbouring areas in the cortex receive signals from adjacent areas in the body – the motorcortex produces the signals that control individual skeletal muscles – the cortex displays plasticity – the areas that are devoted to certain functions may expand or shrink The basal nuclei are areas of grey matter embedded in the white matter of the cerebrum whose output controls skeletal muscles, contributing to posture and effective motion BIOL 373 COURSE NOTES 23 The limbic system, another area of grey matter buried deep in the cerebrum’s white matter, links memory, emotion and reasoning Sensory Physiology Asensory receptor is a cell or group of cells that can create an electrical signal when a physical condition changes, a process called transduction – receptor cells may be neurons, or other types of cells – each receptor responds only when the change in the physical condition (the stimulus) exceeds the threshold – each receptor is most sensitive to one type of stimulus, its adequate stimulus – thermoreceptors are most sensitive to changes in temperature – chemoreceptors respond to the presence of certain molecules – mechanoreceptors detect changes in pressure – a receptor can respond to a stimulus that is not its adequate stimulus if it is strong enough – the brain still regards the signal from this receptor as originating from the adequate stimulus – receptor cells generate graded potentials by opening or closing membrane ion channels when the stimulus surpasses the threshold – the graded potential in a receptor cell is called a receptor potential An afferent pathway connects a receptor to the central nervous system – the first neuron in the afferent pathway is the primary sensory neuron – the next neuron in line, the secondary sensory neuron, has its cell body within the CNS and conducts a series of action potentials BIOL 373 COURSE NOTES 24 The physical area in which all stimuli create signals in a single sensory neuron is called that neuron’s receptive field – if signals from several primary sensory neurons converge on a single secondary sensory neuron, its receptive field may be large – in the most sensitive sensory systems, only one primary sensory neuron is connected to a secondary sensory neuron, and as a result its receptive field is small The central nervous system can distinguish between weak and strong stimuli by two means: 1. a stronger stimulus creates a sequence of action potentials that are spaced more closely together in time (frequency coding) 2. a stronger stimulus activates a larger number of receptors, and the resulting action potentials are carried to the CNS through multiple afferent pathways (population coding) – population coding depends on a set of receptors having a range of different thresholds Lateral inhibition helps to localize the origin of a stimulus in an afferent pathway in which there is convergence – if a single stimulus causes signals to be sent through a number of secondary sensory neurons, the neuron with the strongest signal inhibits the other neurons, weakening the signals they carry The brain is able to determine in which approximate direction a source of sound lies by computing the lag between the time the sound reaches one ear and the time it reaches the other Receptors may be classified according to their responses to a prolonged stimulus – in a tonic (slowly adapting) receptor, the graded potential slowly loses its amplitude, leading to the creation in the primary sensory neuron of a signal in which the intervals between action potentials gradually grow longer – the graded potential of aphasic (rapidly-adapting) receptor lasts for only a brief period and then disappears, even though the stimulus persists – the creation of action potentials in its primary sensory neuron likewise stops before the stimulus – some phasic receptors may produce a new graded potential when the stimulus comes to an end BIOL 373 COURSE NOTES 25 Acomparison of the human sensory systems to others in the animal kingdom shows that the human are not the most sensitive, nor do they have the widest ranges The somatic senses are those of the skin, and the proprioceptors that monitor the position of the joints and the tension in the muscles The other senses are collectively called the special senses Sensors in the Skin There are a variety of types of touch sensorsin the skin, capable of distinguishing between steady pressure, stretching, and vibration. – the receptors for touch sensors are all neurons whose endings have been modified to act as mechanoreceptors – the most sensitive areas of the skin have many receptors, and a correspondingly large area of the sensory cerebral cortex which receives their input Separate types of thermoreceptorssense heat and cold in the skin Nociceptors respond to chemicals that are produced when cells are damaged – signals sent through their afferent pathways carry information of pain or irritation Taste Taste buds are barrel-shaped structures recessed into the lining of the mouth – a pore allows liquid to enter and come into contact with the receptor cells that surround the central cavity – the receptor cells are modified epithelial cells which are replaced about every 10 days – microvilli form the apical surface of each receptor cell – some types of receptor cells have proteins attached to the membrane surface of the microvilli, which can bind to specific types of molecules dissolved in saliva – this binding leads to an increase in the concentration of Caions in the receptor cell’s cytoplasm BIOL 373 COURSE NOTES 26 – the calcium “signal” causes the release of transmitter molecules across the cell’s basal membrane – transmitter causes an electrical signal to arise in the receptor cell’s primary sensory neuron Each gustatory (taste) receptor is in one of 5 categories: – the surface proteins of “sweet” receptorsbind to molecules of glucose, some other sugars, and certain molecules whose structures are similar to those of sugars – “umami” receptors have membrane proteins which bind to some amino acids – “bitter” receptorshave many different types of membrane proteins, and so can bind to a variety of different molecules – in place of proteins that bind to molecules at the cell surface, the membranes of “sour” + receptor cells contain channels which allow H ions to enter the cell and depolarize it, leading to the creation of the calcium signal – “salty” receptors are similar to sour receptors, but their membrane channels allow Na ions to diffuse into the cell By comparing the strengths of the signals from the 5 types of receptor, the brain can create complex perceptions of taste – the smell of food greatly influences our impression of its taste BIOL 373 COURSE NOTES 27 Lecture #5 Olfaction, Hearing, Equilibrium, and Vision Olfaction Receptor cells that transduce smell are concentrated in the olfactory epitheliumlocated in the upper part of the nasal cavity – olfactory receptors are neurons which are replaced about every 60 days – most neurons elsewhere in the body are not replaced when they die – forceful inhalation increases the perception of smell by bring more incoming air into contact with the olfactory cells Each receptor cell sends an axon to the olfactory bulb above it, where it has a synapse with its secondary sensory neuron – the secondary sensory neurons can send signals directly to the limbic system – all other sensory afferent pathways pass through the thalamus before entering the cerebrum Olfactory receptor cells have cilia which lie in the mucus that lines the nasal cavity 1. the membranes of the cilia contain protein receptor molecules which can bind to odorant molecules dissolved in the mucus – the binding causes a graded potential in the olfactory cell – olfactory cells have thousands of different types of membrane proteins, each type capable of binding to a single kind of odorant – the human brain can distinguish about 10,000 different olfactory sensations (smells) – the olfactory sensory system rapidly adapts to prolonged exposure, but at the level of the brain, not the individual sensory cells BIOL 373 COURSE NOTES 28 Hearing Sound is made of a number of waves of compression (and rarification) in the air – the degree of compression is equivalent to the amplitude of the wave, or the loudness of the sound – the number of waves per second is the sound’s frequency, perceived as its pitch – the ear can detect sound at any frequency between 20 Hz and 20,000 Hz The outer ear collects sound waves and conducts them down to the eardrum – the eardrum moves in and out at the same frequency as the sound waves which reach it – inside the eardrum is the middle ear, an air-filled chamber, connected to the pharynx by the eustachian (auditory) tube. The middle ear contains 3 small bones: – the malleus is attached to the eardrum – the movement of the malleus is transferred to the second bone, the incus – the stapes articulates with the incus, and moves in harmony with it – transferal of vibrations through the 3 bones is very efficient – muscles that attach to the bones can restrict their motion if the sound is too loud The fluid inside the cochlea of the inner ear is rhythmically compressed by the motion of the stapes bone – the inner ear contains 3 longitudinal compartments – the vestibular duct, the tympanic duct and, between them, the cochlear duct – the vestibular and tympanic ducts are filled with perilymph, whose composition is similar to interstitial fluid + + – the cochlear duct is filled with endolymph which has more K and less Na than the perilymph – the stapes bone is attached to the oval windowin the base of the vestibular duct – as the bone moves, it creates waves of compression (and rarification) in the perilymph inside the duct BIOL 373 COURSE NOTES 29 – these waves pass up the length of the vestibular duct, then through the helicotrema at the apex of the cochlea and into the tympanic duct – any waves of compression that are strong enough to reach the end of the tympanic duct are dissipated by the movement of the round window at its base – almost all of the energy in the eardrum’s vibrations is transferred to the oval window, which has a much smaller surface area Within the cochlear duct, the organ of Corti is attached to the surface of the basilar membrane which separates the cochlear and tympanic ducts – the hair cells that transduce the waves of compression are in the organ of Corti 1. each hair cell has as many as 100 stereociliaon its apical surface – the stereocilia are graded in length, with the tallest at one side of the cell and the shortest at the other – each stereocilium is attached to those closest to it by protein filaments – every protein filament connects at one end to an ion channel – bending of a stereocilium causes the opening or closing (depending on the direction of bending) of ion channels in the nearby stereocilia – the resulting depolarization or hyperpolarization of the hair cell’s resting membrane potential affects the rate of release of transmitter molecules at its basal end – a change in the rate of transmitter release from each hair cell changes the pattern of action potentials in its primary sensory neuron The tips of the longest stereocilia are embedded in the tectorial membrane which lies over the organ of Corti – the tectorial membrane and the basilar membrane both move as compression waves pass down the length of the cochlea, but they do not move together. – the relative motion of the two membranes causes bending of the hair cells’ stereocilia BIOL 373 COURSE NOTES 30 – the flexibility of the basilar membrane increases progressively, from most flexible at the end nearest the helicotrema to stiffest at the end closest to the oval window – the location of its greatest movement depends on the frequency of the compression waves – compression waves of a single frequency will be transduced at one point along the length of the organ of Corti – the point of origin of the transduced signals allows the brain to identify the frequency of the sound Equilibrium The vestibular apparatus senses acceleration and gravity – attached to the cochlear duct, it contains both endolymph and hair cells – the hair cells of the vestibular apparatus have, in addition to the regular set of stereocilia, a kinocilium, which is larger than other stereocilia – three semicircular canalsand two otolith organs form the vestibular apparatus The utricle is the otolith organ that transduces linear acceleration in the horizontal plane – the tips of its hair cells’stereocilia are embedded in a small mass of jelly-like material that contains crystals of calcium carbonate (the otoliths) – when the head is positioned so that the eyes are straight ahead, on the horizon, the patch of hair cells in the utricle is horizontal, and their stereocilia vertical – initiation of any movement in the horizontal plane, or any inclination of the head, shifts the jelly-like mass and causes the stereocilia to bend The saccule is similar in function to the utricle, but its patch of hair cells are vertical – the saccule responds to vertical acceleration – by integrating the signals from the two otolith organs, the brain can determine the angle of any linear acceleration, or any inclination of the head The three semicircular canals are mutually perpendicular – each has its own set of hair cells BIOL 373 COURSE NOTES 31 – their stereocilia of each canal are embedded in a cupula, a small amount of gelatinous material that can move when endolymph pushes on it – as the head rotates, the walls of the semicircular canal move with it, but the endolymph it contains moves more slowly – the inertia of the endolymph causes it to shift the cupula, resulting in bending of stereocilia – rotational acceleration in any plane can be determined by the brain as it evaluates and compares the signals received from the three semicircular canals Vision The eye is a complex structure, mostly contained within the orbit or eye socket – the front of the eye is kept clean and lubricated by tears, and protected by the eyelid and eyelashes – six muscles rotate the eye within the socket, allowing it to be aimed The sclera is the tough outer surface of the eye, modified at the front to form the transparent cornea – the front chamber of the eye, filled with aqueous humour, lies between the cornea and the lens – since blood vessels are absent in these structures, the aqueous humour supplies the cornea and lens with oxygen and nutrients and removes waste products – behind the lens is the eye’s rear chamber, lined by the retinaand filled with gelatinous vitreous body – between the retina and the sclera is the well-vascularized choroid layer The iris surrounds the pupil, and controls the amount of light that enters the eye’s rear chamber – its radially-aligned smooth muscle cells contract when they receive sympathetic stimulation, increasing the diameter of the pupil and allowing more light to enter – parasympathetic stimulation causes contraction of the circumferentially-aligned smooth muscle cells, reducing the size of the pupil and limiting light entry – testing the reflexive constriction of both pupils when a bright light shines into one eye is a one way to evaluate basic brain function BIOL 373 COURSE NOTES 32 The lens and cornea bend, or refract, light rays inward so that they can form a sharp image on the retina – the cornea contributes more than the lens to the refraction of incoming light rays because the difference in optical density between the air and the aqueous humour is greater than the difference in optical density between the lens and the material that surrounds it – but the lens has an ability that the cornea lacks – it can change is refractive power so that a sharply defined image on the retina can be produced even if the object is close to the eye – when the eye is focussed on a distant object (i.e more than 6 metres away), suspensory ligaments pull outward on the rim of the lens – the suspensory ligaments are attached to the ciliary body, a ring-like structure in whose centre the lens is mounted – when the eye’s focus shifts to an object closer than 6 metres, circumferentially- aligned smooth muscle cells in the ciliary body contract, reducing its diameter, and relieving the tension in the ligaments – without the outward pull, the lens elastically changes shape, becoming slightly thicker, and more convex on its front and rear surfaces – the thicker, more convex lens is able to form a sharp image on the retina of the close object – as the lens’s elasticity declines with age, it can no longer provide good focussing of close objects – the ciliary body’s smooth muscle cells are signalled to contract by parasympathetic output, and to relax by sympathetic output The retina has four layers of cells 1. on the outside, next to the choroid layer, is the pigment epithelium – melanin particles in its cells limit the reflection of light rays inside the eye 2. the rod and cone cells of the second layer are the photoreceptors that transduce light 3. bipolar neurons are synaptically connected to the rods and cones BIOL 373 COURSE NOTES 33 4. the innermost layer is made up of ganglion cells, which receive signals from the bipolar neurons – their axons collectively form the optic nerve, which carries the action potentials that result from phototransduction to the brain – the optic nerve exits the retina at the optic disk(blind spot), where no rod or cone cells are located – in the central fovea are cone cells with no overlying bipolar neurons or ganglion cells – to form the sharpest possible image of an object, the eye muscles aim the eye so that its image falls on the fovea – surrounding the fovea is the macula, which contains fewer bipolar neurons or ganglion cells than in the rest of the retina Each rod cell has three regions 1. the outer segment, touching the pigmented epithelium, has its membrane folded many times so that it resembles a stack of coins (or rod) 2. the inner segmenthouses the cell’s nucleus and organelles 3. at the synaptic terminal the rod cell releases neurotransmitter which affects its bipolar neuron The membrane of a rod cell contains K leak channels and ligand-gated Na channels + – in the dark, the Na channels are open because they are bound to the abundant cyclic GMP in the cytoplasm + + – in this condition++Na influx exceeds K efflux, and so the cell is depolarized, and voltage-gated Ca channels in its membrane stay open – the Ca++ ions that enter the cell through these channels cause neurotransmitter to be released continuously – the amount of neurotransmitter that is released determines the graded potential of the bipolar cell BIOL 373 COURSE NOTES 34 Bound to the membrane of the outer segment of a rod cell are light-sensitive rhodopsin molecules – light causes a rhodopsin molecule to dissociate into two parts: 1. the protein opsin remains attached to the membrane 2. retinalseparates from it and diffuses away – the splitting of rhodopsin is a signal to increase the rate of destruction of cyclic GMP molecules inside the cell – as the cytoplasmic concentration of cGMPfalls, some of the Na channels close + + – if the rate of outflow of K ions exceeds the rate of Na inflow, the cell hyperpolarizes – some voltage-gated Ca channels close, and fewer neurotransmitter molecules are released – the bipolar cell’s graded potential is altered, affecting the sequence of action potentials produced in its ganglion cell – the type of rhodopsin found in rod cells responds to a relatively broad spectrum of light – as a result, signals that originate with rod cells are perceived as shades of grey There is a great deal of convergence in the afferent pathways of rod cells – many rods release transmitter to a single bipolar cell – many bipolar cells are synapticallyconnected to a single ganglion cell – rod cells can detect very low levels of light, but their resolution is not good Cone cells function in much the same way as rod cells, but for two major differences: 1. their afferent pathways have much less convergence than those of rods – reduced convergence makes cone cells less sensitive to light, but gives them better ability to resolve BIOL 373 COURSE NOTES 35 2. there are three different types of cone cells, each with a type of rhodopsinthat responds most strongly to green, blue and red light. – this selective sensitivity provides colour vision – structurally, the inner segment of a cone cell is tapered instead of cylindrical Aganglion cell’s visual (receptive) field is circular, and divided into two components, a central circle and a surrounding ring – ganglion cells respond most strongly when the difference in light levels between the two components is high – the ability to respond to contrast allows the outline of objects to be determined, even in faint light conditions Output from each retina is split at the optic chiasm on its way to the visual cortex – some of the signals from the left side of the right eye’s retina are sent to the right side of the brain, and vice-versa – in this way, the two perspectives (i.e. one from each eye) of the visual field are processed together – the brain is able to create a three-dimensional impression of the objects in view BIOL 373 COURSE NOTES 36 Lecture#6 Diffusion and Osmosis, Body Fluids, Control Systems, and Epithelia Diffusion and Osmosis Diffusion is the movement of particles in a solution from an area of higher concentration to an area of lower concentration. Osmosis is the diffusion of water down its own concentration gradient. – If two solutions of differing concentrations are separated by a membrane that allows the passage of water but not solutes, the net movement of water will be from the more dilute solution into the more concentrated. – Osmolarity is a measure of the total number of dissolved particles per unit of volume of a solution, regardless of their size, charge or chemical nature. – Differences in osmolarity determine the direction and force of osmotic movement of water. Solutions of equal osmolarity are called isosmotic. – The osmolarity of body fluids is 290 milliosmolar, most often rounded off to 300 milliosmolar. Tonicity terms describe the effect of a solution on cells exposed to it: – a hypotonic solution causes the cells to swell; – a hypertonic solution causes the cells to shrink; – if the cells maintain their original size, the solution is isotonic. Body Fluids and the Distribution of Water and Solutes in the Body The fluid in the body is divided into three main classes: – intracellular fluid is contained within all of the body’s cells (~ 67% of the total fluid); – plasmais the liquid part of the blood (~ 8% of the total fluid); and – interstitial fluid (ISF) is outside the cells, but not included within the circulatory system (~25% of the total fluid). – Plasma and ISF together make up the extracellular fluid(ECF). BIOL 373 COURSE NOTES 37 Each type of body fluid is separated from the others by a barrier which prevents some of its solutes from diffusing into the other compartments. ‒ Cell membranes restrict the intracellular fluid to the inside of the cells. They also contain active transport systems that pump ions and molecules across the membranes. ‒ The walls of the circulatory system isolate the plasma from the ISF. – As a result, the concentrations of solutes and electrical charges are unequal among the three compartments. The barriers are permeable to water, which distributes itself evenly throughout the three types of body fluid. Control Systems Body processes can be controlled in two ways. ‒ Local or intrinsic controls are contained within the organ. They are usually mediated by chemical messengers. ‒ An example is arteriolar dilation in response to oxygen starvation in a muscle. ‒ Extrinsic control depends on a reflex that is not totally contained within the affected organ. It may use either chemical messengers, the nervous system or a combination of the two. A generalized reflex has seven elements: 1. a stimulus, either external or internal, elicits a signal from 2. a receptor, which passes the signal through 3. an afferent pathway to 4. the integrating centre. The integrating centre sends a signal through 5. the efferent pathwayto 6. the effector, which produces 7. the response. Not all seven elements must be present in all reflexes. BIOL 373 COURSE NOTES 38 The stimulus, which must be above threshold level in order to initiate the reflex, may be an indication that the value of an internal variable is different from its setpoint, or outside the desired range. If the reflex is used to maintain homeostasis in a system, the response resets the variable back to or near its setpoint. This eliminates the stimulus or reduces it to below threshold, forming a negative feedback loop. Positive feedback loops increase the size of the stimulus. They are not used to maintain homeostasis. Feedforward controlsanticipate a change in a system, and act to correct or accommodate the change. Epithelia Epithelial layers serve as boundaries between the body’s interior and the external environment, and as the lining of the circulatory and lymphatic systems. Epithelial tissue can also be secretory. ‒ Secretions into the external environment are exocrine. ‒ Secretions into the interstitial fluid are endocrine. Epithelial layers are attached to a basal lamina of connective tissue. Epithelia that act as boundaries can prevent the passage of materials from one side to the other by tight junctions. Each cell in a tight epithelium is sealed to all of its neighbours by tight junctions. Tight epithelia can selectively transfer materials through active or passive transport. Leaky epithelia can promote the passage of materials through unsealed gaps between their cells. Ciliated epithelial cells move mucus out of the body cavities or channels that they line. Keratinized epithelium can withstand abrasion. BIOL 373 COURSE NOTES 39 Lecture #7 KidneyAnatomy, and Processes in the Glomerulus and Proximal Tubule The kidneys have several functions: ‒ elimination of metabolic wastes ‒ elimination of excess water ‒ control of the volume of the extracellular fluid ‒ control of the osmotic concentration of the extracellular fluid ‒ control of the pH of the extracellular fluid ‒ secretion of hormones ‒ synthesis of glucose (if necessary) Extracellular fluid serves as an environment for the body’s cells. Constancy of this environment allows the cells to function properly. Anatomy of the Kidney Each kidney receives blood from a renal artery.After flowing through the kidney, blood is drained through a renal vein. Auretertakes urine from the kidney to the urinary bladder. Each kidney is a solid mass of tissue divided into two layers: an outer cortex, and an inner medulla. The functional unit of the kidneys is the nephron, whose walls are made up of a single layer of epithelial cells. Filtered plasma enters the nephron by Bowman’s capsule which has an inner wall and an outer wall, separated by Bowman’s space. Filtrate flows from Bowman’s space into the first section of the tubule, called the proximal tubule. The proximal tubule and Bowman’s capsule are both located in the kidney’s cortex.After passing through the proximal tubule, the filtrate enters the descending limb of the loop of Henle, flowing down into the kidney’s medulla. The second half of the loop of Henle is the ascending limb, which carries the filtrate back into the cortex. The last section of the nephron is the distal tubule, which conducts filtrate to a collecting duct. The collecting duct takes the filtrate from the kidney’s cortex down through the medulla, where it flows into the renal pelvis. The interstitial fluid within the kidney’s medulla has a very high osmotic concentration.As filtrate inside the collecting duct flows through the medulla, some of the water in it can diffuse across the duct’s walls, drawn by the high osmotic concentration of the ISF. Reabsorption of water concentrates the filtrate. Blood brought to the kidney by the renal artery flows into the Bowman’s capsule of each nephron through an afferent renal arteriole. It then enters a network of capillaries called the glomerulu. The glomerulus is drained by an efferent renal arteriole, which carries the blood to the peritubular capillaries encircling the nephron’s tubule. Peritubular capillaries return to the bloodstream materials that are reabsorbed from the filtrate, and bring to the nephron oxygen, nutrients, and materials that are secreted into the filtrate. BIOL 373 COURSE NOTES 40 The ascending limb of each nephron’s loop of Henle is located so that it is in contact with both the afferent and efferent renal arterioles just outside Bowman’s capsule. The grouping of ascending limb, afferent arteriole and efferent arteriole is called the juxtaglomerular apparatus. Kidney Processes 1. Filtration in the Glomerulus Blood pressure forces some of the plasma out of the glomerulus, across the inner wall of Bowman’s capsule, into Bowman’s space. The pressure of the blood in the glomerulus (about 55 mm Hg) is higher than in other capillaries because of the large diameter of the afferent arteriole and the small diameter of the efferent arteriole. The forces that are opposed to the blood pressure that presses liquid out of the glomerulus are the pressure of the filtrate within Bowman’s space (~ 15 mm Hg) and the osmotic pressure due to the high molecular weight solutes in the plasma that remain within the glomerulus (~ 30 mm Hg). The net force favouring filtration therefore is (55 mm Hg − [15 mm Hg + 30 mm Hg] ), or 10 mm Hg. The walls of the glomerulus are leakier that those of other capillaries because of spaces or fenestrations between the endothelial cells, each with a width of about 50 nm - 100 nm (nanometres or millionths of a millimetre). Surrounding each glomerular capillary is the basal lamina, a non-living layer of structural fibres. Its negative charges repel the negatively-charged plasma proteins, and hamper their passage into Bowman’s space.Around the outside of the layer of basal lamina are the podocytes, epithelial cells with long extensions that interdigitate. The gaps between the podocytes’extensions are the filtration slits, through which the filtrate must pass. The width of the filtration slits averages about 20 nm , but can be adjusted by contractile mesangial cells, which are in contact with the podocytes. The glomerular filtration rate (or GFR) is the total rate at which liquid leaves the circulatory system and enters the Bowman’s spaces in all of the kidneys’2 million nephrons. It averages 180 litres per day or 125 millilitres per minute, equivalent to 2.5% of the cardiac output. GFR does not fluctuate with changes in blood pressure for two reasons: a. The myogenic response of smooth muscle in the walls of the renal arterioles, especially the afferent, compensates for rising and falling blood pressure. b. Tubuloglomerular feedback.Arate of flow of filtrate within the distal tubule that is higher than normal results in reflexive constriction of the afferent renal arteriole. Arapid drop in blood pressure below 80 mm Hg can cause sympathetic constriction of all the afferent arterioles, reducing the GFR until normal blood pressure can be restored. BIOL 373 COURSE NOTES 41 2. Reabsorption in the Proximal Tubule The proximal tubule and its peritubular capillaries are specialized for the reclamation from the filtrate of water and valuable plasma solutes such as glucose, amino acids, water -soluble vitamins and a variety of ions. + + ‒ The Na /K ATPase pump in the basolateral membranes of the p+oximal tubule’s epithelial cells keeps the intracellular concentration of Na ions low. This allows them to move from the filtrate into the ISF surrounding the tubule. Cl ions follow.About ⅔ of the NaCl found in the original filtrate is reabsorbed in the proximal tubule. ‒ The peritubular capillaries’low blood pressure and high osmotic pressure favour the bulk flow of ISF into the bloodstream. ‒ Tight junctions fuse a band of membrane of each epithelial cell to the membranes of all the cells that touch it. Material that is reabsorbed is forced to pass through the cells rather than between them. This pathway promotes selective reabsorption. Normally, glucose is totally reabsorbed from the filtrate by secondary active transport. Each glucose molecule together with a Na ion passes across an epithelial cell’s apical membrane. Glucose crosses the cell’s basolateral membrane, moving into the ISF and then into the blood contained within the peritubular capillaries. Amino acids, citric acid, malic acid and ascorbic acid (vitamin C) are reabsorbed the same way as glucose. Proteins in the filtrate are reabsorbed by endocytosis into the epithelial cells, followed either by digestion to amino acids, or exocytosis. Reabsorption of solutes from the filtrate causes an osmotic imbalance between it and the ISF that surrounds the proximal tubule.As a result, water is reabsorbed passively, until about 65% of the filtered water is returned to the bloodstream. Urea also diffuses down its concentration gradient from filtrate to ISF, and into the peritubular capillaries. 3. Secretion in the Proximal Tubule Acetylcholine, prostaglandins, penicillin, and some toxins are actively transported from the blood within the peritubular capillaries into the filtrate. BIOL 373
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