Chapter 45 Hormones and Endocrine System Key Concepts

Concept 45.2 Hormones and other chemical signals bind to target cell receptors, initiating pathways that culminate in specific cell responses

Hormones convey information via the bloodstream to target cells throughout the body. Other chemical signals—local regulators—transmit information to target cells near the secreting cells. Pheromones carry messages to different individuals of a species. Three major classes of molecules function as hormones in vertebrates: proteins and peptides, amines, and steroids. Most protein/peptides and amine hormones are water-soluble, unlike steroid hormones. Signaling by all hormones involves three key events: reception, signal transduction, and response. Reception of the signal occurs when the signal molecule binds to a specific receptor protein in or on the target cell. Binding of a signal molecule to a receptor protein triggers signal transduction within the target cell that results in a response, a change in the cell's behavior. Cells that lack receptors for a particular chemical signal are unresponsive to that signal. Water-soluble hormones have cell-surface receptors. The receptors for water-soluble hormones are embedded in the plasma membrane. Binding of a hormone to its receptor initiates a signal transduction pathway, a series of changes in cellular proteins that converts an extracellular chemical signal to a specific intracellular response. The response may be the activation of an enzyme, a change in uptake or secretion of specific molecules, or rearrangement of the cytoskeleton. Signal transduction from some cell-surface receptors activates proteins in the cytoplasm that move into the nucleus and directly or indirectly regulate gene transcription. An example of the role of cell-surface receptors involves changes in a frog's skin color, an adaptation that helps camouflage the frog in changing light. Skin cells called melanocytes contain the dark pigment melanin in cytoplasmic organelles called melanosomes. The frog's skin appears light when melanosomes cluster tightly around the cell nuclei and darker when they spread out in the cytoplasm. A peptide hormone called melanocyte-stimulating hormone controls the arrangement of melanosomes and, thus, skin color. Adding melanocyte-stimulating hormone to the interstitial fluid containing the pigment-containing cells causes the melanosomes to disperse. However, direct microinjection of melanocyte-stimulating hormone into individual melanocytes has no effect. This provides evidence that interaction between the hormone and a surface receptor is required for hormone action. A particular hormone may cause diverse responses in target cells having different receptors for the hormone, different signal transduction pathways, and/or different proteins for carrying out the response. Lipid-soluble hormones have intracellular receptors. Evidence for intracellular receptors for steroid hormones came in the 1960s. Researchers demonstrated that estrogen and progesterone accumulate within the nuclei of cells in the reproductive tract of female rats but not in the nuclei of cells in tissues that do not respond to estrogen. These observations led to the hypothesis that cells sensitive to steroid hormones contain internal receptor molecules that bind the hormones. Researchers have since identified the intracellular protein receptors for steroid hormones, thyroid hormones, and the hormonal form of vitamin D. All these hormones are small, nonpolar molecules that diffuse through the phospholipid interior of cell membranes. Intracellular receptors usually perform the entire task of transducing the signal within the target cell. The chemical signal activates the receptor, which directly triggers the cell's response. In almost every case, the intracellular receptor activated by a lipid-soluble hormone is a transcription factor, and the response is a change in gene expression. Most intracellular receptors are located in the nucleus. The hormone-receptor complexes bind to specific sites in the cell's DNA and stimulate the transcription of specific genes. Some steroid hormone receptors are trapped in the cytoplasm when no hormone is present. Binding of a steroid hormone to its cytoplasmic receptor forms a hormone-receptor complex that can move into the nucleus and stimulate transcription of specific genes. In both cases, mRNA produced in response to hormone stimulation is translated into new protein in the cytoplasm. For example, estrogen induces cells in the reproductive system of a female bird to synthesize large amount of ovalbumin, the main protein of egg white. As with hormones that bind to cell-surface receptors, hormones that bind to intracellular receptors may exert different effects on different target cells. A variety of local regulators affect neighboring target cells. Local regulators convey messages between neighboring cells, a process referred to as paracrine signaling. Local regulators can act on nearby target cells within seconds or milliseconds, eliciting responses more quickly than hormones can. Some local regulators have cell-surface receptors; others have intracellular receptors. Binding of local regulators to their receptors triggers events within target cells similar to those elicited by hormones. Several types of chemical compounds function as local regulators. Among peptide/protein local regulators are cytokines, which play a role in immune responses, and most growth factors, which stimulate cell proliferation and differentiation. Another important local regulator is the gas nitric oxide (NO). When blood oxygen level falls, endothelial cells synthesize and release NO. NO activates an enzyme that relaxes neighboring smooth muscle cells, dilating the walls of blood vessels and improving blood flow to tissues. Nitric oxide also plays a role in male sexual function, increasing blood flow to the penis to produce an erection. Highly reactive and potentially toxic, NO usually triggers changes in the target cell within a few seconds of contact and then breaks down. Viagra sustains an erection by interfering with the breakdown of NO. When secreted by neurons, NO acts as a neurotransmitter. When secreted by white blood cells, it kills bacteria and cancer cells. Local regulators called prostaglandins (PGs) are modified fatty acids derived from lipids in the plasma membrane. Released by most types of cells into interstitial fluids, prostaglandins regulate nearby cells in various ways, depending on the tissue. In semen that reaches the female reproductive tract, prostaglandins trigger the contraction of the smooth muscles of the uterine wall, helping sperm to reach the egg. PGs secreted by the placenta cause the uterine muscles to become more excitable, helping to induce uterine contractions during childbirth. Other PGs help induce fever and inflammation, and intensify the sensation of pain. These responses contribute to the body's defense. The anti-inflammatory effects of aspirin and ibuprofen are due to the drugs' inhibition of prostaglandin synthesis. Prostaglandins also help regulate the aggregation of platelets, an early stage in the formation of blood clots. This is why people at risk for a heart attack may take daily low doses of aspirin. In the respiratory system, two prostaglandins have opposite effects on the smooth muscle cells in the walls of blood vessels serving the lungs. Prostaglandin E signals the muscle cells to relax, dilating the blood vessels and promoting oxygenation of the blood. Prostaglandin F signals the muscle cells to contract, constricting the vessels and reducing blood flow through the lungs.

Concept 45.3 The hypothalamus and pituitary integrate many functions of the vertebrate endocrine system

The hypothalamus integrates vertebrate endocrine and nervous function. This region of the lower brain receives information from nerves throughout the body and from other parts of the brain then initiates endocrine signals appropriate to environmental conditions. The hypothalamus contains two sets of neurosecretory cells whose hormonal secretions are stored in or regulate the activity of the pituitary gland, located at the base of the hypothalamus. The posterior pituitary (neurohypophysis) stores and secretes two hormones produced by the hypothalamus. The long axons of these cells carry the hormones to the posterior pituitary. The anterior pituitary (adenohypophysis) consists of endocrine cells that synthesize and secrete at least six different hormones directly into the blood. Several of these hormones have other endocrine glands as their targets. Hormones that regulate the function of endocrine glands are called tropic hormones. They are particularly important in coordinating endocrine signaling throughout the body. The anterior pituitary itself is regulated by tropic hormones produced by a set of neurosecretory cells in the hypothalamus. Some hypothalamic tropic hormones (releasing hormones) stimulate the anterior pituitary to release its hormones. Others (inhibiting hormones) inhibit hormone secretion. Hypothalamic releasing and inhibiting hormones are secreted into capillaries at the base of the hypothalamus. The capillaries drain into portal vessels that subdivide into a second capillary bed within the anterior pituitary. Every anterior pituitary hormone is controlled by at least one releasing hormone. Some have both a releasing hormone and an inhibiting hormone. The posterior pituitary releases two hormones, oxytocin and antidiuretic hormone. Both are peptides made by neurosecretory cells in the hypothalamus and, thus, are neurohormones. Oxytocin induces contraction of the uterus during childbirth and causes mammary glands to eject milk during nursing. Oxytocin signaling in both cases exhibits positive feedback. Antidiuretic hormone (ADH) promotes retention of water by the kidneys, decreasing urine volume. ADH helps regulate osmolarity of the blood via negative feedback. Secretion is regulated by water/salt balance. The anterior pituitary produces many different hormones. Four function as tropic hormones, stimulating the synthesis and release of hormones from the thyroid gland, adrenal glands, and gonads. Several others exert only direct, nontropic effects on nonendocrine organs. One, growth hormone, has both tropic and nontropic actions. Three of the tropic hormones secreted by the anterior pituitary are closely related in their chemical structures. Follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH) are similar glycoproteins. FSH and LH are also called gonadotropins because they stimulate the activities of the gonads. TSH promotes normal development of the thyroid gland and the production of thyroid hormones. Adrenocorticotropic hormone (ACTH) is a peptide hormone that stimulates the production and secretion of steroid hormones by the adrenal cortex. All four anterior pituitary tropic hormones participate in complex neuroendocrine pathways. In each pathway, signals to the brain stimulate release of an anterior pituitary tropic hormone. The tropic hormone then acts on its target endocrine tissue, stimulating secretion of a hormone that exerts systemic metabolic or developmental effects. Nontropic hormones produced by the anterior pituitary include prolactin, melanocyte-stimulating hormone (MSH), and ß-endorphin. These peptide/protein hormones, whose secretion is controlled by hypothalamic hormones, function in simple neuroendocrine pathways. Prolactin (PRL) stimulates mammary gland growth and milk production and secretion in mammals. It regulates fat metabolism and reproduction in birds, delays metamorphosis in amphibians (where it may also function as a larval growth hormone), and regulates salt and water balance in freshwater fishes. This suggests that prolactin is an ancient hormone whose functions have diversified during the evolution of various vertebrate groups. Secretion is regulated by hypothalamic hormones. Melanocyte-stimulating hormone (MSH) regulates the activity of pigment-containing cells in the skin of some fishes, amphibians, and reptiles. In mammals, MSH acts on neurons in the brain, inhibiting hunger. ß-endorphin belongs to a class of chemical signals called endorphins. All the endorphins bind to receptors in the brain and dull the perception of pain. Both MSH and ß-endorphin are formed by cleavage of the same precursor protein that gives rise to ACTH. Growth hormone (GH) is so similar structurally to prolactin that scientists hypothesize the genes directing their production evolved from the same ancestral gene. GH acts on a wide variety of target tissues with both tropic and nontropic effects. Its major tropic action is to signal the liver to release insulin-like growth factors (IGFs), which circulate in the blood and directly stimulate bone and cartilage growth. In the absence of GH, the skeleton of an immature animal stops growing. GH also exerts diverse metabolic effects that raise blood glucose, opposing the effects of insulin. Abnormal production of GH can produce several disorders. Gigantism is caused by excessive GH production during development. Acromegaly is caused by excessive GH production during adulthood. Pituitary dwarfism is caused by childhood GH deficiency, and can be treated by therapy with genetically engineered GH.

Concept 45.4 Nonpituitary hormones help regulate metabolism, homeostasis, development, and behavior

The thyroid gland of mammals consists of two lobes located on the ventral surface of the trachea. The thyroid gland produces two very similar hormones derived from the amino acid tyrosine: triiodothyronine (T3), which contains three iodine atoms, and thyroxin (T4), which contains four iodine atoms. In mammals, the thyroid secretes mainly T4, but target cells convert most of it to T3 by removing one iodine atom. Although the same receptor molecule in the cell nucleus binds both hormones, the receptor has greater affinity for T3 than for T4. It is primarily T3 that brings about responses in target cells. This process involves a complex neuroendocrine pathway with two negative feedback loops. The thyroid plays a crucial role in vertebrate development and maturation. Thyroid controls metamorphosis of a tadpole into a frog, which involves massive reorganization of many different tissues. The thyroid is equally important in human development. Cretinism, an inherited condition of thyroid deficiency, retards skeletal growth and mental development. The thyroid gland has important homeostatic functions. In adult mammals, thyroid hormones help to maintain normal blood pressure, heart rate, muscle tone, digestion, and reproductive functions. Throughout the body, T3 and T4 are important in bioenergetics, increasing the rate of oxygen consumption and cellular metabolism. Too much or too little of these hormones can cause serious metabolic disorders. Hyperthyroidism is the excessive secretion of thyroid hormones, leading to high body temperature, profuse sweating, weight loss, irritability, and high blood pressure. An insufficient amount of thyroid hormones is known as hypothyroidism. This condition can cause cretinism in infants. Adult symptoms include weight gain, lethargy, and cold intolerance. A deficiency of iodine in the diet can result in goiter, an enlargement of the thyroid gland. Without sufficient iodine, the thyroid gland cannot synthesize adequate amounts of T3 and T4. The resulting low blood levels of these hormones cannot exert negative feedback on the hypothalamus and anterior pituitary. The pituitary continues to secrete TSH, elevating TSH levels and enlarging the thyroid. In addition to cells that produce T3 and T4, the mammalian thyroid gland produces calcitonin. This hormone acts in conjunction with parathyroid hormone to maintain calcium homeostasis. Parathyroid hormone and calcitonin balance blood calcium. Rigorous homeostatic control of blood calcium level is critical because calcium ions (Ca2+) are essential to the normal functioning of all cells. If blood Ca2+ falls substantially, skeletal muscles begin to contract convulsively, a potentially fatal condition called tetany. In mammals, parathyroid hormone and calcitonin play a major role in maintaining blood Ca2+ near a set point of about 10 mg/100 mL. When blood Ca2+ falls below the set point, parathyroid hormone (PTH) is released from four small structures, the parathyroid glands, embedded on the surface of the thyroid. PTH raises the level of blood Ca2+ by direct and indirect effects. In bone, PTH induces specialized cells called osteoclasts to decompose the mineralized matrix of bone and release Ca2+ into the blood. In the kidneys, it promotes the conversion of vitamin D to its active hormonal form. An inactive form of vitamin D is obtained from food or synthesized in the skin. The active form of vitamin D acts directly on the intestines, stimulating the uptake of Ca2+ from food. A rise in blood Ca2+ above the set point promotes release of calcitonin from the thyroid gland. Calcitonin exerts effects on bone and kidneys opposite those of PTH and thus lowers blood Ca2+ levels. The regulation of blood Ca2+ levels illustrates how two hormones with opposite effects (PTH and calcitonin) balance each other, exerting tight regulation and maintaining homeostasis. Each hormone functions in a simple endocrine pathway in which the hormone-secreting cells themselves monitor the variable being regulated. In classic feedback, the response to one hormone triggers release of the antagonistic hormone, minimizing fluctuations in the concentration of Ca2+ levels in the blood. Endocrine tissues of the pancreas secrete insulin and glucagon, antagonistic hormones that regulate blood glucose. The pancreas has both endocrine and exocrine functions. Its exocrine function is the secretion of bicarbonate ions and digestive enzymes, which are released into small ducts and carried to the small intestine via the pancreatic duct. Tissues and glands that discharge secretions into ducts are described as exocrine. Clusters of endocrine cells, the islets of Langerhans, are scattered throughout the exocrine tissues of the pancreas. Each islet has a population of alpha cells, which produce the hormone glucagon, and a population of beta cells, which produce the hormone insulin. Both hormones are secreted directly into the circulatory system. Insulin and glucagon are antagonistic hormones that regulate the concentration of glucose in the blood. This is a critical bioenergetic and homeostatic function, because glucose is a major fuel for cellular respiration and a key source of carbon skeletons for the synthesis of other organic compounds. Metabolic balance depends on maintaining blood glucose concentrations near a set point of about 90 mg/100 mL in humans. When blood glucose exceeds this level, insulin is released and lowers blood glucose. When blood glucose falls below this level, glucagon is released and its effects increase blood glucose concentration. Each hormone operates in a simple endocrine pathway regulated by negative feedback. Insulin lowers blood glucose levels by stimulating all body cells (except brain cells) to take up glucose from the blood . Brain cells can take up glucose without insulin and, thus, have access to circulating fuel at all times. Insulin also decreases blood glucose by slowing glycogen breakdown in the liver and inhibiting the conversion of amino acids and glycerol to glucose. The liver, skeletal muscles, and adipose tissues store large amounts of fuel and are especially important in bioenergetics. The liver and muscles store sugar as glycogen, whereas adipose tissue cells convert sugars to fats. The liver is a key fuel-processing center because only liver cells are sensitive to glucagon. The antagonistic effects of glucagon and insulin are vital to glucose homeostasis and regulation of fuel storage and fuel consumption by body cells. The liver's ability to perform its vital roles in glucose homeostasis results from the metabolic versatility of its cells and its access to absorbed nutrients via the hepatic portal vein. Diabetes mellitus is perhaps the best-known endocrine disorder. It is caused by a deficiency of insulin or a depressed response to insulin in target tissues. There are two types of diabetes mellitus with very different causes, but each is marked by high blood glucose. In people with diabetes, elevated blood glucose exceeds the reabsorption capacity of the kidneys, causing them to excrete glucose. As glucose is concentrated in the urine, more water is excreted with it, resulting in excessive volumes of water and persistent thirst. Without sufficient glucose to meet the needs of most body cells, fat becomes the main substrate for cellular respiration. In severe cases of diabetes, acidic metabolites formed during fat breakdown accumulate in the blood, threatening life by lowering blood pH. Type I diabetes mellitus (insulin-dependent diabetes) is an autoimmune disorder in which the immune system destroys the beta cells of the pancreas. Type I diabetes usually appears in childhood and destroys the person's ability to produce insulin. The treatment is insulin injections, usually several times a day. Human insulin is available from genetically engineered bacteria. Type II diabetes mellitus (non-insulin-dependent diabetes) is characterized by deficiency of insulin or, more commonly, by a decreased responsiveness to insulin in target cells, due to some change in insulin receptors. This form of diabetes occurs after age 40, and the risk increases with age. Although heredity can play a role in type II diabetes, excess body weight and lack of exercise significantly increase the risk. Type II diabetes accounts for more than 90% of diabetes cases. Many type II diabetics can manage their blood glucose with regular exercise and a healthful diet, although some require insulin injections. The adrenal medulla and adrenal cortex help the body manage stress. The adrenal glands are located adjacent to the kidneys. In mammals, each adrenal gland is actually made up of two glands with different cell types, functions, and embryonic origins. The adrenal cortex is the outer portion, and the adrenal medulla is the central portion. Like the pituitary, the adrenal gland is a fused endocrine and neuroendocrine gland. The adrenal cortex consists of true endocrine cells, while the secretory cells of the adrenal medulla derive from the neural crest during embryonic development. The adrenal medulla produces two hormones, epinephrine (adrenaline) and norepinephrine (noradrenaline). These hormones are members of a class of hormones, the catecholamines, amines that are synthesized from the amino acid tyrosine. Both are also neurotransmitters in the nervous system. Either positive or negative stress stimulates secretion of epinephrine and norepinephrine from the adrenal medulla. These hormones act directly on several target tissues to give the body a rapid bioenergetic boost . They increase the rate of glycogen breakdown in the liver and skeletal muscles, promote glucose release into the blood by liver cells, and stimulate the release of fatty acids from fat cells. The released glucose and fatty acids circulate in the blood and can be used by the body as fuel. Epinephrine and norepinephrine also exert profound effects on the cardiovascular and respiratory systems. They increase heart rate and stroke volume of the heartbeat and dilate the bronchioles in the lungs to increase the rate of oxygen delivery to body cells. Catecholamines also act to shunt blood away from skin, digestive organs, and kidneys, and increase blood supply to the heart, brain, and skeletal muscles. Epinephrine generally has a greater effect on heart and metabolic rates, while the primary role of norepinephrine is in sustaining blood pressure. Secretion of these hormones by the adrenal medulla is stimulated by nerve signals carried from the brain via the sympathetic division of the autonomic nervous system. In response to a stressful situation, nerve impulses from the hypothalamus travel to the adrenal medulla, where they trigger the release of epinephrine. Norepinephrine is released independently. The adrenal medulla hormones act in a simple neurohormone pathway. The neurosecretory cells are modified peripheral nerve cells. Hormones from the adrenal cortex also function in the body's response to stress. Stressful stimuli cause the hypothalamus to secrete a releasing hormone that stimulates the anterior pituitary to release the tropic hormone ACTH. When ACTH reaches the adrenal cortex via the bloodstream, it stimulates the endocrine cells to synthesize and secrete a family of steroids called corticosteroids. Elevated levels of corticosteroids in the blood suppress the secretion of ACTH. The two main types of corticosteroids in humans are the glucocorticoids, such as cortisol, and the mineralocorticoids, such as aldosterone. Both hormones help maintain homeostasis when stress is experienced over a long period of time. The primary effect of glucocorticoids is on bioenergetics, specifically on glucose metabolism. Glucocorticoids make more glucose available as fuel. They act on skeletal muscle, causing a breakdown of muscle proteins. The synthesis of glucose from muscle proteins is a homeostatic mechanism providing circulating fuel when body activities require more than the liver can metabolize from its metabolic stores. Cortisol and other glucocorticoids also suppress certain components of the body's immune system. Because of their anti-inflammatory effect, glucocorticoids have been used to treat inflammatory diseases such as arthritis. However, long-term use of these hormones can have serious side effects due to their metabolic actions and can also increase susceptibility to infection. Mineralocorticoids act principally on salt and water balance. Aldosterone stimulates cells in the kidneys to reabsorb Na+ and water from filtrate, raising blood pressure and volume. Aldosterone secretion is stimulated primarily by angiotensin II, as part of the regulatory pathway that controls the kidney's ability to maintain ion and water homeostasis of the blood. When an individual is under severe stress, the resulting rise in blood ACTH levels can increase the rate at which the adrenal cortex secretes aldosterone as well as glucocorticoids. A third group of corticosteroids is composed of sex hormones. All the steroid hormones are secreted from cholesterol, and their structures differ in minor ways. However, these differences are associated with major differences in their effects. The sex hormones produced by the adrenal cortex are mainly male hormones (androgens) with small amounts of female hormones (estrogens and progestins) Androgens secreted by the adrenal cortex may account for the female sex drive. Gonadal steroids regulate growth, development, reproductive cycles, and sexual behavior. The gonads are the primary source of the sex hormones. The gonads produce and secrete three major categories of steroid hormones: androgens, estrogens, and progestins. All three types are found in males and females but in different proportions. Sex hormones affect growth and development and regulate reproductive cycles and sexual behavior. The testes primarily synthesize androgens, the main one being testosterone. Androgens promote development and maintenance of male sex characteristics. Androgens produced early in development determine whether a fetus develops as a male or a female. At puberty, high levels of androgens are responsible for the development of male secondary sex characteristics, including male patterns of hair growth, a low voice, and increased muscle mass and bone mass typical of males. The muscle-building action of testosterone and other anabolic steroids has led some athletes to take them as supplements. Abuse of these hormones carries many health risks, and they are banned in most competitive sports. Estrogens, the most important of which is estradiol, are responsible for the development and maintenance of the female reproductive system and the development of female secondary sex characteristics. In mammals, progestins, which include progesterone, are involved in promoting uterine lining growth to support the growth and development of an embryo. Both estrogens and androgens are components of complex neuroendocrine pathways. Their secretion is controlled by gonadotropins (FSH and LH) from the anterior pituitary gland. FSH and LH production is controlled by a releasing hormone from the hypothalamus, GnRH (gonadotropin-releasing hormone). The pineal gland is involved in biorhythms. The pineal gland is a small mass of tissue near the center of the mammalian brain. The pineal gland synthesizes and secretes the hormone melatonin, an amine. Depending on the species, the pineal gland contains light-sensitive cells or has nervous connections from the eyes that control its secretory activity. Melatonin regulates functions related to light and to seasons marked by changes in day length. Its primary functions are related to biological rhythms associated with reproduction. Melatonin secretion is regulated by light/dark cycles. Melatonin is secreted at night, and the amount secreted depends on the length of the night. Thus, melatonin production is a link between a biological clock and daily or seasonal activities such as reproduction. Recent evidence suggests that the main target cells of melatonin are the part of the brain called the suprachiasmatic nucleus (SCN), which functions as a biological clock. Melatonin seems to decrease the activity of neurons in the SCN, and this may be related to its role in mediating rhythms. Much remains to be learned about the precise role of melatonin and about biological clocks in general.

45.1 The endocrine system and the nervous system act individually and together in regulating an animal's physiology

Animals have two systems of internal communication and regulation, the nervous system and the endocrine system. Collectively, all of an animal's hormone-secreting cells constitute its endocrine system. Hormones coordinate slow but long-acting responses to stimuli such as stress, dehydration, and low blood glucose levels. Hormones also regulate long-term developmental processes such as growth and development of primary and secondary sexual characteristics. Hormone-secreting organs called endocrine glands secrete hormones directly into the extracellular fluid, where they diffuse into the blood. The nervous and endocrine systems overlap to some extent. Certain specialized nerve cells known as neurosecretory cells release hormones into the blood. The hormones produced by these cells are sometimes called neurohormones. Chemicals such as epinephrine serve as both hormones of the endocrine system and neurotransmitters in the nervous system. The nervous system plays a role in certain sustained responses—controlling day/night cycles and reproductive cycles in many animals, for example—often by increasing or decreasing secretions from endocrine glands. The fundamental concepts of biological control systems are important in regulation by hormones. A receptor, or sensor, detects a stimulus and sends information to a control center. After comparing the incoming information to a set point, the control center sends out a signal that directs an effector to respond. In endocrine and neuroendocrine pathways, this outgoing signal, called an efferent system, is a hormone or neurohormone, which acts on particular effector tissues and elicits specific physiological or developmental changes. The three types of simple hormonal pathways (simple endocrine pathway, simple neurohormone pathway, and simple neuroendocrine pathway) include these basic functional components. A common feature of control pathways is a feedback loop connecting the response to the initial stimulus. In negative feedback, the effector response reduces the initial stimulus, and eventually the response ceases. This prevents overreaction by the system. Negative feedback regulates many endocrine and nervous mechanisms. Positive feedback reinforces the stimulus and leads to an even greater response. The neurohormone pathway that regulates the release of milk by a nursing mother is an example of positive feedback. Suckling stimulates sensory nerve cells in the nipples, which send nervous signals that reach the hypothalamus, the control center. The hypothalamus triggers the release of the neurohormone oxytocin from the posterior pituitary gland. Oxytocin causes the mammary glands to secrete milk. The release of milk in turn leads to more suckling and stimulation of the pathway, until the baby is satisfied.