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Transcript
Hormones in Animals (2) Endocrinology D R Davies School of Biological Sciences Purves Life: the Science of Biology Chapters 41 (Animal Hormones) and 15 (Cell Signalling) Lecture 14 Learning Outcomes At the end of this lecture you should be able to: • Describe in general terms how endocrine control mechanisms regulate homeostasis • Explain the endocrine functions of the hypothalamus and pituitary gland • Discuss at least one complex endocrine control mechanism involving the interplay of several different hormones Feedback Control Systems • Thus a feedback control system will consist of an effector (gland) which produces and maintains a variable signal (hormone concentration). • This is measured by a sensor (receptor) which monitors the changes in the signal and sends a further signal to the pituitary gland (comparator) which compares the feedback signal to a reference (called the set point) • produces an error signal in the form of a trophic hormone which regulates the effector and restores the variable to its proper value, Control Systems • Feedback loop • Effector (gland) • Variable Hormone concn • Sensor (hormone receptor) • Comparator (pituitary) • Error signal The hypothalamus and pituitary • The hypothalamus is a region of the brain controlling many aspects of physiological activities such as hunger, thirst, sleep and body temperature and consisting of a distinctive cells which produce hormones regulating these activities. Pituitary Gland 12/30/03 Richard H. Pointer 34 The hypothalamus can regulate physiological activity via three routes: •Via the autonomic nervous system e.g. via the release of adrenaline by the adrenal medulla •Neurones in the hypothalamus produce releasing hormones into the blood portal system causing the release of hormones by specific cells in the anterior pituitary gland (adenohypophysis) . These tropic hormones are then released into the general circulation where they affect a whole range of physiological activities in various target cells •Via the posterior pituitary gland (neurohypohysis) where the release of two closely related peptide hormones (oxytocin and vasopressin) at nerve endings. These peptides have totally different functions Hormonal Regulatory Mechanisms • Positive Feedback e.g. increase in oestradiol triggers ovulation • Negative Feedback e.g circulating thyroid hormone inhibits release of TRH and TSH Negative Feedback Loops Much of endocrine physiology is concerned with the maintenance of homeostasis. Control systems are of different degrees of complexity but usually consist of a feedback loop with a signal from the output used to regulate the input. Here the feedback signal produces an opposite response to a change in the variable. Hormonal Regulatory Mechanisms • Pulsatile Secretion Hypothalamic hormones are released in bursts typically 60 - 180 min apart • Permissive Effects e.g hormones may regulate the action of other hormones, oestrogens increase the number of progesterone receptors • Synergistic Effects Hormonal Regulatory Mechanisms • Up-Regulation- hormones increase the number of their own receptors thus increasing the sensitivity of the target cell to the hormone • Down-Regulation- hormones decrease the number of their own receptors thus reducing the sensitivity of the target cell to the hormone • The components of the signal transduction mechanism may also become sensitized or desensitized to the signal Regulation of reproductive function • Androgens, such as testosterone, trigger the development of male characteristics – In male elephant seals, androgens account for bodies weighing 2 tons or more, a thick hide, and aggressive behavior Figure 26.12 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Hormonal Regulation of Ovulation Hormonal Changes in Pregnancy Hormonal regulation of cyclic ovulation • Pregnancy is triggered by an increase in oestradiol (oestrogen) arising from the developing ovarian follicle • This is monitored by the hypothalamus and pituitary and results in an increased frequency of GnRH secretion followed by a surge in LH and FSH • LH surge causes the rupture of the ovarian follicle and the release of the oocyte • Oestradiol causes development of the endometrium lining the uterus and oestrogen and progesterone together maintain the uterus as a preparation for pregnancy Hormonal Regulation of Pregnancy • If pregnancy is maintained the coprpus luteum is maintained by the production of chorionic gonadotrophin (hCG) • the levels of oestradiol and progesterone increase • These steroid hormones maintain the development of the endometrium in a suitable state for implantation. • Eventually the placenta takes over the production of the two steroids • The levels of oestrogens and progesterone continue to increase through pregnancy until just before full term Role of the adrenal gland in the response to stress • How the adrenal glands control our responses to stress Adrenal medulla Adrenal gland Adrenal cortex ST RESS Nerve signals Hypothalamus Releasing hormone Kidney Nerve cell Sp inal co rd (cross section) Blood vessel Nerve cell Adrenal medulla Ep inephrine and norep inephrine SHORT-TERM STRESS RESPONSE Figure 26.10 Anterior pituitary 1. Glycog en broken down to glu cose; increased blood gluco se 2. Increased blood pressu re 3. Increased breathing rate 4. Increased metabolic rate 5. Change in blood-flow patterns, leading to increased alertness and decreased digestive and kidney activity Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings ACTH ACTH Min eralocortico ids Adrenal cortex Glucocorticoids LONG-TERM STRESS RESPONSE 1. Reten tion of sodium ion s and water by kidneys 2. Increased blood volume and blood pressure 1. Proteins and fats broken down and converted to glucose, leading to increased blood glucose 2. Immune system may be suppressed