Chapter 17 Anatomy & Physiology Seeley/Stephens/Tate Fifth Edition (c) The McGraw-Hill Companies, Inc. 1

FUNCTIONAL ORGANIZATION OF THE ENDOCRINE SYSTEM There are two major means of communication in the body: the neuronal system and the endocrine system. The neuronal system – has clear pathways which are connected with neurons. Thus it is reasonably clear for each neuron where it starts and where it ends. (reflex --- cerebral cortex ---- targets) At the end knob, neurotransmitters are released. Its complexity makes it possible to stimulate more than one tissue and organ simultaneously. The effect is relatively short lived.

The endocrine system – the active substances, hormones, are made in specific organs. The glands may be stimulated with neurons. Hormones have target tissue and organs. Long distance transport of its active substance is done by the circulatory system. Local effect is also possible. Again, one hormone may effect more than one target organs. The effects are usually slow and relatively long lasting.

An overview The word endocrine is in contrast with exocrine. Thus we are concerned with internal secretion. Endocrine glands secrete specific hormones. The strength of a hormone is determined by its concentration, while the strength for neurons is often the frequency of the action potential. Also recall the neurohormones may be released by neurons into the circulatory system. Functional classification of chemical signals is shown in Fig. 17.1

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Review Table 17.1 Autocrine chemical signals – released by cells and have a local effect of the same cell type. Example are prostaglandins and platelets. Paracrine chemical signals – released by cells and effect local other cell types. Somatostatin from pancreas. Pheromones – hormones secreted into environment.

Structure of hormone – there are type major types. Amino acid derivates: epinephrine, norepinephrine, the thyroid hormones, pineal hormone (melatonin) Peptide/protein hormone: antidiuretic hormone, glucagon, oxytocin, growth hormone, prolactin, insulin. Lipid derivatives: Steroid hormones – estrogens, testosterone Prostaglandins – derived from arachidonic acid.

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Control of secretion rate In essence the control is in a form of negative feedback. Three major patterns: By other substance; such as sugar controls the regulation of insulin release. Increased blood sugar Stimulates insulin release from the pancreas Insulin stimulates glucose uptake by tissues Decrease in blood sugar

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Neuronal control of endocrine glands Stress or exercise stimulates sympathetic division of ANS. Release of epinephrine or smaller amounts of norepinephrine from the adrenal medulla. (Fig.17.5) Regulation of hormone secretion by another hormone Thyroid-releasing hormone (TRH) is released from neurons in the hypothalamus. TRH stimulates the release of thyroid stimulating hormone (TSH) from the anterior pituitary gland. TSH stimulates the secretion of thyroid hormones from the thyroid gland. Thyroid hormone stimulates tissues That includes the hypothalamus and the anterior pituitary (negative-feedback) to inhibit both TRH and TSH.(fig.17.6) Regulation is usually complex and more than one factor could affect organs. For example, the level of insulin may be regulated by blood glucose and ANS., Ex: of positive feedback. (fig 17.7)

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The level of hormone in the blood steam may: Maybe relatively short May respond in a matter of hours May respond in a matter of days

Transport Hormones are transported through the circulatory system dissolved in plasma. At the target sites they diffuse out from the capillaries. Some hormones such as, steroids and thyroid hormones, are bound to specific plasma proteins reversibly. hormone + plasma protein = = hormone-protein While this equilibrium is held in plasma, there are free hormones and proteins bound hormones. Free hormones may diffuse out through the walls of capillary pores at the target sites, while proteins bound hormones remain within the capillaries. But eventually they will establish a new equilibrium (carry more hormone) and make more free hormones available (sustained release).

Metabolism and Excretion Once hormones are poured into the blood steam, it is equally important to excrete them from the circulation to avoid excessively long effects of hormones. The lengths of time it takes for elimination by half is called “half-life” time. The half-life time of water soluble hormones, such as proteins, glycoproteins, epinephrine and norepinephrine, is relatively short. The half-life time of lipid soluble hormones, such as steroid and thyroid hormones, is longer.

As shown in Table 17.3 hormones are eliminated from the circulatory system in four ways: Excretion Kidneys-urine; the liver-bile Metabolism Enzymatic digestion Active transport Recycling (neurotransmitters) Conjugation Sulfate and glucuronic acid groups bind to hormones and they are released into urine or bile.

Interaction of hormones with their target tissues At their designated sites hormones interact with the target cells and active their intrinsic function, usually stimulating specific enzymes. Hormone receptors: protein or glycoprotein. Each target cell has a specific binding site for the specific hormone. However, a hormone may bind to the/ other target receptor which are similar. After prolonged exposure to a hormone, the tagged cells may reduce the number of hormone receptors called, down regulation. On the other hand, exposure to stimulation may increase the number of hormone receptors called, up-regulation. Example is LH receptors are up regulated by FSH.

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