Hormones 1: The neuroendocrine system.

Question 1

Essay Plan:

Answer 1

• Introduction: Define the Endocrine system • Define the Neuroendrocrine system • Endocrine Vs Nervous System • Define Hormones • Define Hormonal signalling: general mechanism • Hypothalamus • Anterior & posterior Pituitary gland • From hypothalamus to pituitary gland • Hormones in pituitary gland and their function • LINK BACK TO ESSAY QUESTION

Question 2

Endocrine system:

Answer 2

The endocrine system is the collection of glands that produce hormones that regulate metabolism, growth and development, tissue function, sexual function, reproduction, sleep, and mood, among other things. The endocrine system is made up of the pituitary gland, thyroid gland, parathyroid glands, adrenal glands, pancreas, ovaries (in females) and testicles (in males). The endocrine system is a major (chemical) communication system of the body; it includes glands and hormones. A gland is a structure which secretes a specific chemical substance or substances. Glands are the target for signals, often coming from the brain. A hormone is the chemical messenger released in response to these signals. The endocrine system controls several physiological and behavioural activities in an organism, maintains a constant internal environment in the body (homeostasis) and regulates growth and development.

Question 3

Neuroendocrine system

Answer 3

The neuroendocrine system is the interaction between the nervous system and the endocrine system, including the biological features of the cells involved, and how they communicate. The nervous and endocrine systems often act together in a process called neuroendocrine integration, to regulate the physiological processes of the human body. The neuroendocrine system is the mechanism by which the hypothalamus maintains homeostasis, regulating reproduction, metabolism, eating and drinking behaviour, energy utilisation, osmolarity, and blood pressure.

Question 4

Endocrine Vs Nervous System

Answer 4

The endocrine system acts with nervous system to coordinate the body’s activities. Both systems enable cells to communicate with others by using chemical messengers. The endocrine system uses chemical messengers called hormones that are transported by the circulatory system (blood). They act on target cells that may be anywhere in the body. The endocrine system is slower than the nervous system because hormones must travel through the circulatory system to reach their target. Target cells have receptors that are specific to the signaling molecules. The binding of hormones to the receptors on or within the target cell produces a response by the target cell. The chemical messengers used by the nervous system are neurotransmitters. Neurotransmitters travel across a narrow space (the synaptic cleft) and bind to receptors on the target cell. The nervous system conducts signals much quicker than the endocrine system.

Question 5

Hormones:

Answer 5

Hormones are any member of a class of signaling molecules produced by glands in multicellular organisms that are transported by the circulatory system to target distant organs to regulate physiology and behaviour. The glands that secrete hormones comprise the endocrine signalling system. Hormones are regulatory biochemicals produced in specific parts of the organism and able to travel some distance to target tissue. The properties of hormones are produced and released in secreting cells. They have an effect at a site different from where they are made, called the target cell; hormones travel in the bloodstream and are effective in low concentrations. Properties of hormones: They fit precisely into receptor molecules in the target like a key in a lock, hormones are therefore specific for a particular target, Hormones are involved in motivated behaviour by communicating between organs and tissues to regulate a variety of physiological and behavioural activities, e.g. digestion, metabolism, growth, reproduction and mood by travelling the bloodstream to relevant organs and tissues. Types of motivated behaviour: Regulatory behaviour: Behaviour whose motivation is to meet the survival needs of an animal behaviour: Internal body temperature, eating and drinking, salt consumption and waste elimination. Non-regulatory behaviour: Sex, parenting, aggression, food preference and curiosity/reading.

Question 6

Hormonal signalling: general mechanism

Answer 6

Classical hormones are produced by endocrine cells, circulate in the bloodstream and exert their action at more distant sites. Neuro-hormones are produced in neurosecretory cells of the hypothalamus (rather than in the endocrine system) in order to: a) Stimulate the secretion of hormones originating in the anterior pituitary gland- i.e. releasing hormones (e.g. Corticotropin releasing factor, CRF); b) Act on distant organs via the posterior pituitary gland (with vasopressin and oxytocin). Neuro-hormones exert their action locally & do not circulate in bloodstream.

Question 7

Pulsatile hormonal release (Armstrong et al, 2009)

Answer 7

Hormones tend to be released in pulses, they are discharged several times a day in large surges which typically last no more than a few minutes. Hormone levels in the blood are regulated by the changes in the frequency and duration of the hormone pulses. One consequence of pulsatile hormonal release is that there are often large minute to minute fluctuation in the levels of circulating hormones. Accordingly when the pattern of human male gonadal hormone release is referred to as “steady” it means that there are no systematic changes in the circulating gonadal hormone levels from day to day, not that the levels never vary.

Question 8

Hypothalamus

Answer 8

The hypothalamus is a small structure at the base of the brain which maintains the body’s internal balance (homeostasis) and it regulates many body functions such as the heart rate and blood pressure, body temperature, fluid and electrolyte balance, including thirst, appetite and body weight, glandular secretions of stomach & intestines, production of neuro-hormones that influence the pituitary gland to release hormones and sleep cycles by producing ‘releasing’ and ‘inhibiting’ hormones which either start or stop the production of other hormones in the body. The hypothalamus regulates the pituitary gland and is the link between the endocrine and the nervous systems.

Question 9

Negative Feedback Inhibition

Answer 9

Hormone secretions by glands that are under the control of the hypothalamus are controlled by negative feedback. When the hormone levels are high, they inhibit the hypothalamus and anterior pituitary, resulting in a decline in their levels.

Question 10

Pituitary gland

Answer 10

Pituitary gland: Very small (pea size) 0.5 to 1 gram in weight called the ‘master gland’ because it controls the functioning of several other glands and organs. Hypothalamus Pituitary Gland is connected with the hypothalamus with whom it forms the ‘neuroendocrine system’ which is the link between the nervous system and many endocrine glands. The pituitary gland is divided into anterior and posterior lobe.

Question 11

Anterior lobe:

Answer 11

Represents about 80% of the pituitary gland and regulates growth, stress, metabolism, and reproduction through the hormones that it produces and that act on target organs. The production of these hormones is stimulated or inhibited by chemical messages from the hypothalamus.

Question 12

Anterior pituitary gland:

Answer 12

Six major hormones are produced:1.Prolactin (PRL) 2.Growth hormone (GH) 3.Adrenocorticotropin hormone (ACTH) 4.Luteinizing hormone (LH) 5.Follicle-stimulating hormone (FSH) 6.Thyroid-stimulating hormone (TSH). These two body parts are affected by hormones produced by the posterior pituitary.

Question 13

Control of the Anterior Pituitary:

Answer 13

The hypothalamus produces hormones that travel in blood vessels to the anterior pituitary, stimulating it to produce other hormones. The hormones produced by the hypothalamus are called hypothalamic-releasing hormones. The anterior pituitary produces at least six different hormones. Each one is produced in response to a specific hypothalamic-releasing hormone. The blood vessel that carries hypothalamic-releasing hormones from the hypothalamus to the pituitary is called a portal vein because it connects two capillary beds. One capillary bed is in the hypothalamus and the other is in the anterior pituitary. Release-inhibiting hormones produced by the hypothalamus inhibit the pituitary from secreting its hormones. Example: The pituitary is stimulated to release growth hormone (GH) by growth hormone releasing hormone (GHRH) produced in the hypothalamus. It is inhibited from releasing growth hormone by growth hormone release-inhibiting hormone (GHRIH), also produced by the hypothalamus. Six different hormones produced by the anterior lobe will be studied here. Three of these have direct effects on the body, the other three control other glands.

Question 14

Posterior lobe:

Answer 14

A neurosecretory organ is made of same neuronal tissue as the hypothalamus (Wheater, Burkitt & Daniels, 1987). It is directly controlled by neurons in the hypothalamus (Marieb, 2004). It does not synthesise any hormones but stores and releases two hormones: vasopressin & oxytocin. These hormones are stored in the pituitary axons and they are released when the hypothalamus sends messages to pituitary gland through nerve cells.

Question 15

Posterior pituitary:

Answer 15

Hormones (neuro-hormones) are produced in specific groups of nerve cells (nuclei) in the hypothalamus; they are carried through the axons in the stalk and end in the posterior pituitary. In response to nerve signals, hormones at the axon terminals in posterior pituitary are extruded into a capillary network and then into the bloodstream.

Question 16

From hypothalamus to pituitary gland

Answer 16

Releasing hormones (neurohormones) are synthetized here and secreted into capillaries that carry them to the anterior pituitary. Releasing hormones leave the capillary and act on hormone-secreting anterior pituitary cells.

Question 17

Hormones in pituitary gland and their function

Answer 17

• Thyroid Stimulating Hormone (TSH): Stimulates the thyroid gland to release thyroid hormones., controls basal metabolic rate and is important for growth & maturation. • Growth Hormone (GH): Principal hormone that regulates growth. • Adrenocorticotropic Hormone (ACTH): Triggers the adrenal glands, which regulate stress response with the release of hormones such as cortisol and aldosterone. • Luteinizing Hormone (LH) & Follicle Stimulating Hormone (FSH): Control reproduction. • Prolactin (PRL): Stimulates secretion of breast milk. • Oxytocin: released in large amounts after distension of the cervix and uterus during labour after stimulation of the nipples, facilitating birth and breastfeeding. • Vasopressin, also called anti-diuretic hormone (ADH): Promotes water retention.

Question 18

Disorders of the pituitary gland Common causes:

Answer 18

• Tumor: While most are benign, they can produce excessive amounts or deficiency of a specific pituitary hormone, and compress surrounding tissues. • Congenital (present at birth): could be due to insufficient development, or other genetic abnormalities. • Physical: traumatic brain injury, subarachnoid hemorrhage, neurosurgery, surgical or radiation treatment of a previous pituitary disorder. • Vascular: impaired blood supply. • Infection: infections of the brain (brain abscess, meningitis, encephalitis) or of the gland itself. [This can be due to some body tissues not responding normally to an hormone, e.g. the receptors are abnormal, so that hormones cannot act normally on cells and bring about their usual effects].

Question 19

Disorders of the pituitary gland: Consequences:

Answer 19

• Anterior pituitary: overproduction or underproduction of a pituitary hormone will affect the respective end-organ or tissue; for example deficiency in growth hormone will manifest in growth failure and short stature. • Posterior pituitary: mostly disordered water homeostasis. • Blood vessels and the optic nerves are in close proximity to the pituitary gland. Pressure from a tumour can cause headaches, visual disturbances, loss of vision, fatigue, weakness, and seizures, as well as a host of signs and symptoms related to diminished hormone production.

Question 20

Conclusion

Answer 20

The endocrine system is a major (chemical) communication system of the body that consists of glands and hormones and that controls several physiological and behavioural activities, including maintaining a stable internal environment (homeostasis). The hypothalamus and pituitary gland (neuro-endocrine system) are the most important elements of the endocrine system. Hormones can be produced by endocrine or neurosecretory cells and are distributed to specific distant organs or glands via the bloodstream. Disorders to the endocrine system, which can affect the hypothalamus, the pituitary gland or other glands typically cause severe hormonal disfunctions, which depend on the specific gland that is affected, and may have consequences on other organs.

Question 21

Endocrine Vs Nervous System: Extra

Answer 21

The endocrine system acts with nervous system to coordinate the body’s activities. Both systems enable cells to communicate with others by using chemical messengers. The endocrine system uses chemical messengers called hormones that are transported by the circulatory system (blood). They act on target cells that may be anywhere in the body. The endocrine system is slower than the nervous system because hormones must travel through the circulatory system to reach their target. Target cells have receptors that are specific to the signaling molecules. The binding of hormones to the receptors on or within the target cell produces a response by the target cell. The chemical messengers used by the nervous system are neurotransmitters. Neurotransmitters travel across a narrow space (the synaptic cleft) and bind to receptors on the target cell. The nervous system conducts signals much quicker than the endocrine system. Endocrine Vs Exocrine glands Endocrine glands do not have ducts. Exocrine glands have ducts that carry their secretions to specific locations. Two Kinds of Hormones Peptide Hormones Peptide hormones are composed of amino acids. A peptide hormone binds to a cell-surface receptor, it does not enter the cell. The resulting complex activates an enzyme that catalyzes the synthesis of cyclic AMP from ATP. Cyclic AMP activates other enzymes that are inactive. Cyclic AMP is a second messenger; the hormone is the first messenger. Other second messengers have been discovered. Steroid Hormones Steroid hormones enter the cell and bind to receptors in the cytoplasm. The hormone-receptor complex enters the nucleus where it binds with chromatin and activates specific genes. Genes (DNA) contain information to produce protein as diagrammed below. When genes are active, protein is produced. Steroid hormones act more slowly than peptide hormones because of the time required to produce new proteins as opposed to activating proteins that are already present. Hypothalamus The hypothalamus is part of the brain. It maintains homeostasis (constant internal conditions) by regulating the internal environment (examples: heart rate, body temperature, water balance, and the secretions of the pituitary gland). Pituitary Gland The pituitary contains two lobes. Hormones released by the posterior lobe are synthesized by neurons in the hypothalamus. Unlike the posterior lobe, the anterior lobe produces the hormones that it releases. Refer to the diagram below as you read about the hypothalamus, pituitary, and each of the glands they control. Posterior pituitary The posterior pituitary contains axons of neurons that extend from the hypothalamus. Hormones are stored in and released from axon endings in the posterior lobe of the pituitary. Oxytocin Oxytocin stimulates the uterine contractions of labor that are needed to move the child out through the birth canal. The hormone stimulates the release of milk from the mammary glands by causing surrounding cells to contract. After birth, stimulation of the breast by the infant feeding stimulates the posterior pituitary to produce oxyticin. Antidiuretic Hormone (ADH) Antidiuretic hormone increases the permeability of the distal convoluted tubule and collecting duct of the kidney nephron resulting in less water in the urine. The urine becomes more concentrated as water is conserved. The secretion of ADH is controlled by a negative feedback mechanism as follows: concentrated blood (too little water) –> hypothalamus –> ADH –> kidney –> reabsorbs water, makes blood more dilute Below: Within the kidney, fluid and dissolved substances are filtered from the blood and pass through tubules where some of the water and dissolved substances are reabsorbed. The remaining liquid and wastes form urine. Details of this process are discussed in the chapter on the excretory system. The presence of too much blood in the circulatory system stimulates the heart to produce a hormone called atrial natriuretic factor (ANF). This hormone inhibits the release of ADH by the posterior pituitary causing the kidneys to excrete excess water. Alcohol inhibits the release of ADH, causing the kidneys to produce dilute urine. Control of the Anterior Pituitary The hypothalamus produces hormones that travel in blood vessels to the anterior pituitary, stimulating it to produce other hormones. The hormones produced by the hypothalamus are called hypothalamic-releasing hormones. The anterior pituitary produces at least six different hormones. Each one is produced in response to a specific hypothalamic-releasing hormone. The blood vessel that carries hypothalamic-releasing hormones from the hypothalamus to the pituitary is called a portal vein because it connects two capillary beds. One capillary bed is in the hypothalamus and the other is in the anterior pituitary. Release-inhibiting hormones produced by the hypothalamus inhibit the pituitary from secreting its hormones. Example The pituitary is stimulated to release growth hormone (GH) by growth hromone releasing hormone (GHRH) produced in the hypothalamus. It is inhibited from releasing growth hormone by growth hormone release-inhibiting hormone(GHRIH), also produced by the hypothalamus. Six different hormones produced by the anterior lobe will be studied here. Three of these have direct effects on the body, the other three control other glands. Anterior Pituitary Hormones that Directly Affect the Body Growth Hormone (GH or Somatotropic Hormone) Growth hormone stimulates body cells to grow. If too little hormone is produced, pituitary dwarfism results. The secretion of too much hormone results in a pituitary giant. Acromegaly is a genetic disease in which growth hormone is produced throughout a persons lifetime. Prolactin Prolactin is produced in quantity after childbirth. It stimulates the development of the mammary glands and the production of milk. It is also involved in the metabolism of fats and carbohydrates. Melanocyte-Stimulating Hormone (MSH) This hormone causes skin color changes in some fishes, amphibians, and reptiles. In humans, it stimulates the melanocytes to synthesize melanin. Anterior pituitary hormones that regulate other glands The pituitary also controls other glands and is often referred to as the “master gland”. Three kinds of pituitary hormones that regulate other glands are discussed below. The glands that they regulate will be discussed in the following section. Thyroid Stimulating Hormone (TSH) –>thyroid –> thyroxin Adrenocorticotropic Hormone (ACTH) –> adrenal cortex –> cortisol Gonadotropic Hormones (FSH and LH) –> ovaries and testes –> sex hormones; controls gamete production Negative Feedback Inhibition Hormone secretions by glands that are under the control of the hypothalamus are controlled by negative feedback. When the hormone levels are high, they inhibit the hypothalamus and anterior pituitary, resulting in a decline in their levels. Thyroid gland The thyroid produces thyroxin (also called T4 because it contains 4 iodine atoms) and triiodothyronine (also called T3 because it contains 3 iodine atoms). Both T4 and T3 have similar effects on target cells. In most target tissues, T4 is converted to T3. They influence metabolic rate, growth, and development. Thyroxin production is regulated by a negative feedback mechanism in which it inhibits the hypothalamus from stimulating the thyroid. Hypothyroidism occurs when the thyroids produce too little hormone. In adults, it results in lethargy and weight gain. In infants, it causes cretinism, which is characterized by dwarfism, mental retardation, and lack of sexual maturity. Administering thyroid hormones treats these affects. Too much T3 and T4 (hyperthyroidism) increases heart rate and blood pressure, and causes weight loss. Iodine is needed to manufacture thyroid hormones. A deficiency in iodine prevents the synthesis of thyroid hormones which, in turn, results in an excess of thyroid stimulating hormone being produced by the anterior pituitary. A goiter results when constant stimulation of the thyroid causes it to enlarge. Calcitonin The thyroid gland also secretes calcitonin, which stimulates calcium deposition in the bones. This is the opposite of the action of parathyroid hormone (see below). Calcitonin production is not regulated by the anterior pituitary. It’s secretion is stimulated by high calcium levels in the blood. Parathyroid glands The parathyroid glands are 4 small glands embedded in posterior surface of the thyroid gland. They secrete parathyroid hormone (PTH), which increases blood levels of Ca++. Bone tissue acts as a storage reservoir for calcium; PTH stimulates the removal of calcium from the bone to increase levels in the blood. PTH also increases the kidney’s reabsorption of Ca++ so that less is lost in urine and it activates vitamin D which enhances Ca++ absorption from food in the gut. Secretion is regulated by the Ca++ level in the blood, (not hypothalamic or pituitary hormones). Adrenal Cortex The outer layer of an adrenal gland is the adrenal cortex. It produces three kinds of steroid hormones. These are glucocorticoids, mineralocorticoids, and small amounts of sex hormones. The major glucocorticoid is cortisol and the major mineralocorticoid is aldosterone. Cortisol (A Glucocorticoid) Glucocorticoids are produced in response to stress. Cortisol raises the level of glucose in the blood by stimulating the liver to produce glucose from stored non-carbohydrate sources such as proteins and lipids and to release it into the blood. Cortisol reduces swelling by inhibiting the immune system. Swelling of tissues due to injury or infection is discussed in the chapter on the immune system. The drug prednisone, derived from cortisol, is used to treat inflammation. Negative feedback control of cortisol level is diagrammed below. Aldosterone (A Mineralocorticoid) Aldosterone secretion is not under the control of the anterior pituitary. It acts primarily on the kidney to promote absorption of sodium and excretion of potassium. Increased sodium levels contributes to the retention of water and thus increased blood volume. In the absence of aldosterone, sodium is excreted and the lower sodium levels result in decreased blood volume and lower blood pressure. The presence of too much blood in the circulatory system stimulates the heart to produce atrial natriuretic factor. This hormone inhibits the release of aldosterone by the adrenal cortex and ADH by the posterior pituitary causing the kidneys to excrete excess water. The loss of water and sodium contribute to lowering the blood volume. Adrenal Medulla The adrenal medulla is composed of modified neurons that secrete epinephrine and norepinephrine (adrenaline and noradrenaline) under conditions of stress. These hormones are released in response to a variety of stresses and stimulate the fight- or- flight response of the sympathetic nervous system. It results in a faster heart rate, faster blood flow, and dilated airways to facilitate oxygen flow to the lungs. In addition, the level of glucose in the blood is increased to make energy more available. Their secretion is controlled by brain centers (including hypothalamus) via sympathetic nerves, not by pituitary hormones. Gonads LH and FSH from the anterior pituitary stimulate the gonads (ovaries and testes). LH stimulates the testes to produce several kinds of steroid hormones called androgens. One of these androgens is testosterone, the main sex hormone in males. LH stimulates the ovaries produce estrogen and progesterone, the female sex hormones. Sex hormones are responsible for the development of secondary sex characteristics, which develop at puberty. Some examples of secondary sex characteristics in males are deepening of the voice (due to a large larynx), growth of facial hair, and muscle development. Some secondary sex characteristics in females are development of the breasts and broadening of the pelvis. Both sexes show increased activity of sweat glands and sebaceous glands (oil glands in the skin), and growth of pubic and axillary (armpit) hair. FSH controls gamete (egg or sperm) production. Pancreas The pancreas is a digestive gland that secretes digestive enzymes into the duodenum through the pancreatic duct. The islets of Langerhans are groups of cells within the pancreas that secrete insulin and glucagon. The islets are endocrine glands because they are ductless; the circulatory system carries their hormones to target cells. Insulin Insulin promotes the removal of glucose from the blood for storage as glycogen (muscle, liver), fats (fat cells), and protein. It promotes the buildup of fats and proteins and inhibits their use as an energy source. Glucagon Glucagon is produced in the islets of Langerhans but by different cells than those that produce insulin. The effects of glucagon are opposite those of insulin. It raises the level of glucose in the blood. It is normally secreted between meals to maintain the concentration of glucose in the blood. Diabetes Mellitus Diabetes mellitus is a disease in which glucose is not sufficiently metabolized. This results in high glucose levels in blood and glucose in the urine. Cells can starve because glucose is not being metabolized. Type I Type I diabetes is also called “juvenile-onset diabetes” or “insulin-dependent diabetes” because the symptoms usually appear during childhood and insulin injections are necessary to treat it. It usually occurs after a viral infection triggers an immune response that results in the body destroying its own insulin-producing cells. Because the disease is caused by a lack of insulin, it can be treated with insulin injections. Type II Type II diabetes is more common than type I. Type II diabetes is caused by a deficiency in insulin production or by changes in insulin receptors on the target cells. In either case, blood glucose level may be high because cells do not receive the message to metabolize glucose. This form of diabetes usually becomes noticeable in middle age. It is treated with a low fat, low sugar diet, regular exercise, weight control. Another treatment is oral medications that make the cells more sensitive to the effects of insulin or that stimulate more insulin production. Thymus Gland The thymus grows during childhood but gradually decreases in size after puberty. Lymphocytes that have passed through the thymus are transformed into T cells. Lymphocytes are white blood cells that function to fight infection. There are two kinds of lymphocytes: B cells and T cells. T cells participate in identifying and destroying body cells that are infected. Thymus hormones called thymosins stimulate the development and differentiation of T lymphocytes. They play a role in regulating the immune system by stimulating other kinds of immune cells as well.