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The nervous and endocrine systems are the main control systems of the body. The nervous system controls processes requiring fast responses such as muscle movement. The endocrine system exerts its influence on processes generally requiring long term control such as growth and reproduction.
The endocrine system produces chemical messengers called hormones in special glandular tissue. The hormones are released and travel in the blood to target cells and tissues where they exert their effects.
The location of the body's principal endocrine glands is illustrated below in Figure 01. Each gland is capable of releasing one or more hormones into the blood. There are other endocrine tissues in the body, not shown in the diagram, such as the skin and parts of the digestive system.
Figure 01 - Location of principal endocrine glands - Place cursor over glands to view hormones
Hormones regulate and coordinate many processes in the body including reproduction, growth and development, immune responses and much homoeostatic regulation, including water, electrolyte, nutrient and energy balance.
Endocrine glands respond to specific signals that stimulate both the production and release of hormones into the blood. Some endocrine glands are under control of the nervous system. Some endocrine glands respond to changes in other hormone levels that either stimulate or inhibit the release of the gland's own hormones.
How hormones exert their effects on cells and tissues
Why is it that only certain cells and tissues respond to the stimulation of specific hormones? For example, why does anti-diuretic hormone (ADH) cause the tubules of the nephron to reabsorb water but appears to have no influence on the digestive system?
Hormones communicate with cells by binding to receptors on the cell membrane or inside the cell. Certain cells express specific receptors. For example, the tubules of the nephron express receptors to ADH which is why ADH has an effect on those particular cells. Cells of the digestive system do not express ADH receptors and so are unaffected by the hormone.
Hormones work using a ‘Lock and Key’ mechanism. This is illustrated in Figure 02. Only a specific hormone 'key' will fit a specific receptor 'lock' - and cause the intended effect.
Figure 02. Hormone interact with cells by a 'lock and key' mechanism
Classes of Hormone
Different hormones have different chemical structures and this influences how and where they bind to receptors. There are various ways of classifying hormones but the most simple method groups them according to their solubility:
• Water-soluble (Peptides, glycoproteins, amines)
• Lipid-soluble (Steroids, thyroid hormones)Water-soluble hormones
The majority of hormones are hydrophilic (they like water) and so are soluble in water. Consequently, they are lipophobic (insoluble in lipids as water and lipids do not mix).
Cell membranes are predominantly lipid in structure so these hormones are unable to pass through the membrane and bind to receptors on the cell surface - Figure 03.
Figure 03. Action of water-soluble hormones on cells
Lipid-soluble hormones
Most of the lipid soluble hormones are derived from cholesterol and include steroid hormones such as oestrogen, testosterone and aldosterone. These hormones are lipophilic (they like lipids) and so are hydrophobic and insoluble in water. As lipids, they are able to pass straight through the predominantly lipid cell membrane and bind to receptors inside the cell - Figure 04.
Figure 04. Action of lipid-soluble hormones on cellsWhat happens when a hormone binds to a receptor?
Cells respond to hormones binding to their receptors in a variety of ways, depending on the hormone, the cell and even the type of receptor. Generally biochemical processes are initiated which may result in the activation of an enzyme or the production of a protein.
An example of the result of hormone / receptor interaction occurs when blood glucose levels are elevated. Insulin is released by the pancreas and binds to insulin receptors on cell membranes. This results in more glucose transporter proteins appearing in the cell membrane. Glucose can then enter the cell through the transporters and blood glucose levels fall - Figure 04.
AFigure 05. Action of insulin on cellsAnother example is adrenaline (epinephrine) that binds to cell membrane receptors and activates enzymes that release glucose from intracellular stores.
An example of a hormone binding a receptor inside the cell is the predominantly male steroid hormone testosterone. This is released by the testes and binds to intracellular receptors. One of its effects is an increase in muscle protein synthesis. Occasionally, female athletes have been found to have taken performance enhancing steroids such as testosterone to increase their muscle mass.
Needless to say, the mechanisms involved in all of these processes are quite complex.
The Hypothalamic-Pituitary System
This is one of the most important parts of the endocrine system, producing hormones that affect a number of diverse body functions such as growth, water regulation and lactation. It also controls other endocrine glands such as the thyroid, adrenal and reproductive glands. The pituitary is under control of the hypothalamus and as such is an important area of interface between the nervous and endocrine systems.
Situated at the base of the brain, the pituitary gland is sometimes described as a pea on a stalk. It is divided into two parts – the posterior and anterior - Figure 05. These are actually two very different structures made of different classes of tissues and although both release hormones, the mechanism of release is very different.
Figure 05. Relationship between pituitary and hypothalamusThe Posterior Pituitary
Hormones are synthesised in the hypothalamus, travel down neurons to the posterior pituitary where they are released into the blood stream. Two of these neurohormones (hormones secreted by neurons) are secreted by the posterior pituitary, these are anti-diuretic hormone (ADH) and oxytocin.
The Anterior Pituitary
The release of hormones from the anterior pituitary is under the control of the hypothalamus and many of these hormones are released with a diurnal rhythm as environmental stimuli (such as day/night or sleep patterns) influence the hypothalamus.
The hypothalamus controls the hormonal output of the anterior pituitary via releasing and inhibiting hormones that are secreted into local blood vessels that connect the hypothalamus to the anterior pituitary. These hormones act on the anterior pituitary controlling the release of its hormones into the blood stream.
Several hypothalamic-pituitary hormones are termed trophic, meaning that they exert their effects on other endocrine glands, influencing hormone release elsewhere in the body. An example of this is thyroid-stimulating hormone (TSH) released by the anterior pituitary that causes the release of thyroid hormone by the thyroid gland.
Table 01 below, illustrates the wide range homoeostatic processes that the hypothalamic-pituitary axis helps to regulate.
Table 01. Hormones released by anterior and posterior pituitaries and their action on various tissues
Posterior pituitary
Oxytocin
Breast and uterine tissue - stimulates milk ejection and uterine contractions
Posterior pituitary
Anti-diuretic hormone (ADH) (also called vasopressin)
Kidneys and arterioles - promotes the retention of water and vasoconstriction
Anterior pituitary
Growth hormone (GH)
All tissues - promotes growth and regulates protein metabolism
Anterior pituitary
Adrenocorticotropic hormone (ACTH)
Adrenal cortex - stimulates release of cortisol
Anterior pituitary
Thyroid stimulating hormone (TSH)
Thyroid gland - stimulates release of thyroid hormones
Anterior pituitary
Prolactin
Mammary glands - promotes development of breasts and production of milk
Anterior pituitary
Follicle stimulating hormone (FSH)
Ovaries (females) Testes (males) - promotes development of ovarian follicles (female) and sperm production in males
Anterior pituitary
Luteinising hormone (LH)
Ovaries (females) and testes (males) - causes ovulation in females and testosterone secretion in males
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