REGULATION OF ORGANISM‘S FUNCTION BY HYPOTHALAMO-HYPOPHYSEAL SYSTEM AND ADRENAL GLANDS

Regulation of BODY functionS by hypothalamo-hypophysIAL system and adrenal glands.

In general, the endocrine system is in charge of body processes that happen slowly, such as cell growth. Faster processes like breathing and body movement are monitored by the nervous system. But even though the nervous system and endocrine system are separate systems, they often work together to help the body function properly.

Endocrine glands and the hormones secreted

· Hypothalamus produces

o Thyrotropin-releasing hormone (TRH)

o Gonadotropin-releasing hormone (GnRH)

o Growth hormone-releasing hormone (GHRH)

o Corticotropin-releasing hormone (CRH)

o Somatostatin (SS; also GHIH, growth factor-inhibiting hormone)

o Dopamine (DA)

· Pineal Gland produces

o Dimethyltryptamine

o Melatonin

· Pituitary gland (hypophysis) produces

o Anterior pituitary lobe (adenohypophysis)

§ Growth hormone (GH)

§ Prolactin (PRL)

§ Adrenocorticotropic hormone (ACTH, corticotropin)

§ Thyroid-stimulating hormone (TSH, thyrotropin)

§ Follicle-stimulating hormone (FSH, a gonadotropin)

§ Luteinizing hormone (LH, a gonadotropin)

o Posterior pituitary lobe (neurohypophysis)

§ Oxytocin (ocytocin)

§ Arginine vasopressin (AVP; also ADH, antidiuretic hormone)

§ Lipotropin

· Adrenal glands produce

o Adrenal cortex

§ Glucocorticoids (chiefly cortisol)

§ Mineralocorticoids (chiefly aldosterone)

§ Androgens (including DHEA and testosterone)

o Adrenal medulla

§ Adrenaline (epinephrine)

§ Noradrenaline (norepinephrine)

o Testosterone

Anterior Pituitary Hormones.

(1) Growth hormone: causes growth of almost all cells and tissues of the body.

(2) Adrenocorticotropin: causes the adrenal cortex to secrete adrenocortical hormones.

(3) Thyroid-stimulating hormone: causes the thyroid gland to secrete thyroxine and triiodothyronine.

(4) Follicle-stimulating hormone: causes growth of follicles in the ovaries prior to ovulation, promotes the formation of sperm in the testes.

(5) Luteinizing hormone: plays an important role in causing ovulation; also causes secretion of female sex hormones by the ovaries and testosterone by the testes.

(6) Prolactin: promotes development of the breasts and secretion of milk. Posterior Pituitary Hormones.

(1) Antidiuretic hormone (also called vasopressin): causes the kidneys to retain water, thus increasing the water content of the body; also, in high concentrations, causes constriction of the blood vessels throughout the body and elevates the blood pressure.

(2) Oxytocin: contracts the uterus during the birthing process, thus perhaps helping expel the baby; also contracts myoepithelial cells in the breasts, thereby expressing milk from the breasts when the baby suckles.

Adrenal Cortex.

(1) Cortisol: have multiple metabolic functions for control of the metabolism of proteins, carbohydrates, and fats.

(2) Aldosterone: reduces sodium excretion by the kidneys and increases potassium excretion, thus increasing sodium in the body while decreasing the amount of potassium.

The foundations of the endocrine system are the hormones and glands. As the body's chemical messengers, hormones (pronounced: hor-moanz) transfer information and instructions from one set of cells to another. Many different hormones move through the bloodstream, but each type of hormone is designed to affect only certain cells.

A gland is a group of cells that produces and secretes, or gives off, chemicals. A gland selects and removes materials from the blood, processes them, and secretes the finished chemical product for use somewhere in the body. Some types of glands release their secretions in specific areas. For instance, exocrine (pronounced: ek-suh-krin) glands, such as the sweat and salivary glands, release secretions in the skin or inside the mouth. Endocrine glands, on the other hand, release more than 20 major hormones directly into the bloodstream where they can be transported to cells in other parts of the body.

The major glands that make up the human endocrine system include the:

· hypothalamus

· pituitary gland

· thyroid

· parathyroids

· adrenal glands

· pineal body

· reproductive glands (which include the ovaries and testes)

The Hypothalamus (pronounced: hi-po-tha-luh-mus), a collection of specialized cells that is located in the lower central part of the brain, is the main link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.

Pituitary (pronounced: puh-too-uh-ter-ee) gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It's often called the "master gland" because it makes hormones that control several other endocrine glands. The production and secretion of pituitary hormones can be influenced by factors such as emotions and changes in the seasons. To accomplish this, the hypothalamus provides information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.

The tiny pituitary is divided into two parts: the anterior lobe and the posterior lobe. The anterior lobe regulates the activity of the thyroid, adrenals, and reproductive glands. The anterior lobe produces hormones such as:

· growth hormone, which stimulates the growth of bone and other body tissues and plays a role in the body's handling of nutrients and minerals

· prolactin (pronounced: pro-lak-tin), which activates milk production in women who are breastfeeding

· thyrotropin (pronounced: thy-ruh-tro-pin), which stimulates the thyroid gland to produce thyroid hormones

· corticotropin (pronounced: kor-tih-ko-tro-pin), which stimulates the adrenal gland to produce certain hormones

The pituitary also secretes endorphins (pronounced: en-dor-fin), chemicals that act on the nervous system and reduce feelings of pain. In addition, the pituitary secretes hormones that signal the reproductive organs to make sex hormones. The pituitary gland also controls ovulation and the menstrual cycle in women.

The posterior lobe of the pituitary releases antidiuretic (pronounced: an-ty-dy-uh-reh-tik) hormone, which helps control the balance of water in the body. Antidiuretic hormone also affects the production of oxytocin (pronounced: ahk-see-toe-sin), which triggers the contractions of the uterus in a woman having a baby.

The hypothalamic releasing hormones.

The stimulating hormone first binds with a specific "receptor" for that hormone on the membrane surface of the target cell. The specificity of the receptor determines which hormone will affect the target cell. After binding with the membrane receptor, the combination of hormone and receptor activates the protein enzyme adenyl cyclase. This enzyme is also located in the membrane and is either bound directly with the receptor protein or closely associated with it. However, a large portion of the adenyl cyclase enzyme protrudes through the inner surface of the membrane into the cytoplasm and, when activated, causes immediate conversion of much of the cytoplasmic ATP into cyclic AMP.

Once cyclic AMP is formed inside the cell it activates still other enzymes. In fact, it usually activates a cascade of enzymes. That is, a first enzyme is activated and this activates another enzyme, which activates still a third, and so forth. The importance of this mechanism is that only a few molecules of activated adenyl cyclase in the cell membrane can cause many more molecules of the next enzyme to be activated, which can cause still many times that many molecules of the third enzyme to be activated, and so forth. In this way, even the slightest amount of hormone acting on the cell surface can initiate a very powerful cascading activating force for the entire cell.

The specific action that occurs in response to cyclic AMP in each type of target cell depends upon the nature of the intracellular machinery, some cells having one set of enzymes and other cells having other enzymes. Therefore, different functions are elicited in different target cells— such functions as

(1) initiating synthesis of specific intracellular chemicals,

(2) causing muscle contraction or relaxation,

(3) initiating secretion by the cells,

(4) altering the cell permeability,

(5) and many other possible effects.

ACTION OF STEROID HORMONES ON THE GENES TO CAUSE PROTEIN SYNTHESIS

A second major means by which hormones— specifically the steroid hormones secreted by the adrenal cortex, the ovaries, and the testes—act is to cause synthesis of proteins in the target cells; these proteins then function as enzymes or transport proteins that in turn activate other functions of the cells.

The sequence of events in steroid function is the following:

1. The steroid hormone enters the cytoplasm of the cell, where it binds with a specific receptor protein,

2. The combined receptor protein/hormone then diffuses into or is transported into the nucleus.

3. The combination now activates the transcription process of specific genes to form messenger RNA.

4. The messenger RNA diffuses into the cytoplasm where it promotes the translation process at the ribosomes to form new proteins.

To give an example, aldosterone, one of the hormones secreted by the adrenal cortex, enters the cytoplasm of renal tubular cells, which contain its specific receptor protein. Therefore, in these cells the above sequence of events ensues. After about 45 minutes, proteins begin to appear in the renal tubular cells that promote sodium reabsorption from the tubules and potassium secretion into the tubules. Thus, there is a characteristic delay in the beginning action of the steroid hormone of 45 minutes and up to several hours or even days for full action, which is in marked contrast to the almost instantaneous action of some of the peptide and amino acid-derived hormones, such as vasopressin and norepinephrine.

CONTROL OF PITUITARY SECRETION BY THE HYPOTHALAMUS

Almost all secretion by the pituitary is controlled by either hormonal or nervous signals from the hypothalamus. Indeed, when the pituitary gland is removed from its normal position beneath the hypothalamus and transplanted to some other part of the body, its rates of secretion of the different hormones (except for prolactin) fall to low levels – in the case of some of the hormones, almost to zero.

Secretion from the posterior pituitary is controlled by nerve fibers originating in the hypothalamus and terminating in the posterior pituitary. In contrast, secretion by the anterior pituitary is controlled by hormones called hypothalamic releasing and inhibitory hormones (or factors) secreted within the hypothalamus itself and then conducted to the anterior pituitary through minute blood vessels called hypothalamic-hypophysial portal vessels. In the anterior pituitary these releasing and inhibitory hormones act on the glandular cells to control their secretion.

The hypothalamus receives signals from almost all possible sources in the nervous system. Thus, when a person is exposed to pain, a portion of the pain signal is transmitted into the hypothalamus. Likewise, when a person experiences some powerful depressing or exciting thought, a portion of the signal is transmitted into the hypothalamus. Olfactory stimuli denoting pleasant or unpleasant smells transmit strong signal components directly and through the amygdaloid nuclei into the hypothalamus. Even the concentrations of nutrients, electrolytes, water, and various hormones in the blood excite or inhibit various portions of the hypothalamus. Thus, the hypothalamus is a collecting center for information concerned with the internal well-being of the body, and in turn much of this information is used to control secretions of the many globally important pituitary hormones.

THE HYPOTHALAMIC-HYPOPHYSIAL PORTAL SYSTEM

The anterior pituitary is a highly vascular gland with extensive capillary sinuses among the glandular cells. Almost all the blood that enters these sinuses passes first through a capillary bed in the tissue of the lower tip of the hypothalamus and then through sma:ll hypothalamic-hypophysial portal vessels into the anterior pituitary sinuses. Small blood vessels project into the substance of the median eminence and then return to its surface, coalescing to form the hypothalamic-hypophysial portal vessels. These in turn pass downward along the pituitary stalk to supply blood to the anterior pituitary sinuses.

Secretion of Hypothalamic Releasing and Inhibitory Hormones into the Median Eminence. Special neurons in the hypothalamus synthesize and secrete hormones called hypothalamic releasing and inhibitory hormones (or releasing and inhibitory factors) that control the secretion of the anterior pituitary hormones. These neurons originate in various parts of the hypothalamus and send their nerve fibers into the median eminence and the tuber cinereum, the hypothalamic tissue that extends into the pituitary stalk. The endings of these fibers are different from most endings in the central nervous system in that their function is not to transmit signals from one neuron to another but merely to secrete the hypothalamic releasing and inhibitory hormones (factors) into the tissue fluids. These hormones are immediately absorbed into the capillaries of the hypothalamic-hypophysial portal system and carried directly to the sinuses of the anterior pituitary gland.

(To avoid confusion, the student needs to know the difference between a "factor" and a "hormone." A substance that has the actions of a hormone but that has not been purified and identified as a distinct chemical compound is called a factor. Once it has been so identified it is thereafter known as a hormone instead of simply a factor.)

Function of the Releasing and Inhibitory Hormones. The function of the releasing and inhibitory hormones is to control the secretion of the anterior pituitary hormones. For each type of anterior pituitary hormone there is usually a corresponding hypothalamic releasing hormone; for some of the anterior pituitary hormones there is also a corresponding hypothalamic inhibitory factor. For most of the anterior pituitary hormones it is the releasing hormone that is important; but, for prolactin, an inhibitory hormone probably exerts most control. The hypothalamic releasing and inhibitory hormones (or factors) that are of major importance are:

1. Thyroid-stimulating hormone releasing hormone (TRH), which causes release of thyroid-stimulating hormone

2. Corticotropin -releasing (CRF), which causes release of adrenocorticotropin

3. Growth hormone releasing hormone (GHRH), which causes release of growth hormone, and growth hormone inhibitory hormone (GHIH), which is the same as the hormone somatostatin and which inhibits the release of growth hormone

4. Luteinizing hormone releasing hormone (LRH), which causes release of both luteinizing hormone and follicle-stimulating hormone – this hormone is also called gonadotropin-releasing hormone (GnRH)

5. Prolactin inhibitory factor (PIF), which causes inhibition of prolactin secretion

In addition to these more important hypothalamic hormones, still another excites the secretion of prolactin, and several hypothalamic inhibitory hormones inhibit some of the other anterior pituitary hormones. Each of the more important hypothalamic hormones will be discussed in detail at the time that the specific hormonal system controlled by them is presented in this and subsequent chapters.

Other Hypothalamic Substances That May Have Hormonal Effects. Multiple other substances, especially many small peptides, are found in the neurons of the hypothalamus. However, functions for these as hormones are only speculative. Yet, because they are of research interest they are listed here: (1) substance P, (2) neurotensin, (3) angiotensin II, (4) enkephalins, (5) endorphins, (6) uasoactive inhibitory polypeptide, and (7) cholecystokinin-8. Many of these same substances are also found in neurons elsewhere in the brain, suggesting that they may function as neurotransmitters both in the hypothalamus and elsewhere. In addition, some of them are in the neurons of the enteric nervous system of the gastrointestinal tract, functioning there also as neurotransmitters possibly as hormones released into the circulating blood from the nerve endings.

The Pineal (pronounced: pih-nee-ul) body, also called the pineal gland, is located in the middle of the brain.

It secretes melatonin (pronounced: meh-luh-toe-nin), a hormone that may help regulate when you sleep at night and when you wake in the morning.

Sex hormones

The gonads are the main source of sex hormones. Most people don't realize it, but both guys and girls have gonads. In guys the male gonads, or testes (pronounced: tes-teez), are located in the scrotum. They secrete hormones called androgens (pronounced: an-druh-junz), the most important of which is testosterone (pronounced: teh-stass-tuh-rone). These hormones tell a guy's body when it's time to make the changes associated with puberty, like penis and height growth, deepening voice, and growth in facial and pubic hair. Working with hormones from the pituitary gland, testosterone also tells a guy's body when it's time to produce sperm in the testes.

A girl's gonads, the ovaries (pronounced: oh-vuh-reez), are located in her pelvis. They produce eggs and secrete the female hormones estrogen (pronounced: es-truh-jen) and progesterone (pronounced: pro-jes-tuh-rone). Estrogen is involved when a girl begins to go through puberty.

During puberty, a girl will experience breast growth, will begin to accumulate body fat around the hips and thighs, and will have a growth spurt. Estrogen and progesterone are also involved in the regulation of a girl's menstrual cycle. These hormones also play a role in pregnancy.

Although the endocrine glands are the body's main hormone producers, some other organs not in the endocrine system - such as the brain, heart, lungs, kidneys, liver, and skin - also produce and release hormones. The pancreas (pronounced: pan-kree-us) is also part of the body's hormone-secreting system, even though it is also associated with the digestive system because it produces and secretes digestive enzymes. The pancreas produces (in addition to others) two important hormones, insulin (pronounced: in-suh-lin) and glucagon (pronounced: gloo-kuh-gawn). They work together to maintain a steady level of glucose, or sugar, in the blood and to keep the body supplied with fuel to produce and maintain stores of energy.

What Does the Endocrine System Do?

Once a hormone is secreted, it travels from the endocrine gland that produced it through the bloodstream to the cells designed to receive its message. These cells are called target cells. Along the way to the target cells, special proteins bind to some of the hormones.

These proteins act as carriers that control the amount of hormone that is available for the cells to use. The target cells have receptors that latch onto only specific hormones, and each hormone has its own receptor, so that each hormone will communicate only with specific target cells that have receptors for that hormone. When the hormone reaches its target cell, it locks onto the cell's specific receptors and these hormone-receptor combinations transmit chemical instructions to the inner workings of the cell.

When hormone levels reach a certain normal amount, the endocrine system helps the body to keep that level of hormone in the blood. For example, if the thyroid gland has secreted the right amount of thyroid hormones into the blood, the pituitary gland senses the normal levels of thyroid hormone in the bloodstream. Then the pituitary gland adjusts its release of thyrotropin, the hormone that stimulates the thyroid gland to produce thyroid hormones.

Another example of this process is parathyroid hormone. Parathyroid hormone increases the level of calcium in the blood. When the blood calcium level rises, the parathyroid glands sense the change and reduce their secretion of parathyroid hormone. This turnoff process is called a negative feedback system.

Things That Can Go Wrong With the Endocrine System

Too much or too little of any hormone can be harmful to your body. For example, if the pituitary gland produces too much growth hormone, a teen may grow excessively tall. If it produces too little, a teen may be unusually short. Doctors can often treat problems with the endocrine system by controlling the production of hormones or replacing certain hormones with medication. Some endocrine problems that affect teens include:

Adrenal insufficiency.

This condition occurs when the adrenal glands don't work properly or don't produce enough corticosteroids. The symptoms of adrenal insufficiency may include weakness, fatigue, abdominal pain, nausea, dehydration, and skin changes. Doctors treat adrenal insufficiency with medications to replace corticosteroid hormones.

Growth hormone problems.

Too much growth hormone in kids and teens who are still growing will make their bones and other body parts grow excessively. This rare condition (sometimes called gigantism) is usually caused by a pituitary tumor and can be treated by removing the tumor. The opposite can happen when a kid or teen has a pituitary glad that doesn't produce enough growth hormone. Doctors may treat these growth problems with medication.

References:

1. Review of Medical Physiology // W.F. Ganong. – Twentieth edition, 2001. – P. 233-242, 307-368, 383-438.

2. Textbook of Medical Physiology // A.C. Guyton, J.E. Hall. – Tenth edition, 2002. – P. 684, 706, 836-844, 846-856, 858-865, 869-880, 884-894, 899-910, 916-926, 929-939, 948-950, 958-959, 965-966.