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Hypothalamic-Pituitary Axis

Pearson
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Recall that the pituitary gland, also called the hypophysis, lies beneath the hypothalamus. It is attached to the hypothalamus by a stalk called the infundibulum. The pituitary gland in humans consists of two lobes: The anterior lobe of the pituitary gland is endocrine tissue and a posterior lobe that is made up of neural tissue. The anterior lobe and a small portion of the infundibulum make up the adenohypophysis, and posterior lobe and most of the infundibulum make up the neurohypophysis. We have reviewed six hormones secreted by the anterior lobe of the pituitary. They are: Thyroid stimulating hormone, or thyrotropin, Follicle stimulating hormone and luteinizing hormone, the gonadotropins, Adreno corticotropic hormone, or corticotrophin, Growth hormone, and prolactin. These are considered the classic anterior pituitary hormones because their structures and physiological functions in humans are well understood. Four of these hormones, TSH, FSH, LH, and ACTH, are called tropic hormones because they cause the release of hormones from other endocrine glands. TSH stimulates the thyroid gland to release thyroid hormones FSH and LH stimulate estrogen and progesterone production in females and testosterone production in males. ACTH stimulates the adrenal cortex to release cortisol and other glucocorticoids. In addition to functions of each hormone that we have already reviewed, the anterior pituitary hormones exert important growth effects on target tissues keeping them healthy and functional. FSH and LH are produced by the same type of cells. Each of the other anterior pituitary hormones is secreted by a different type of cells. This follows the pattern we have observed for other endocrine glands and tissues that produce more than one hormone. Understanding the pituitary gland requires careful study of the vascular supply. We have noted that the six classic anterior pituitary hormones are released directly into the systemic circulation. The ventral hypothalamic hormones regulate the function of the anterior pituitary cells. They are produced in neurons and travel down the axons to terminals where they are stored. The axon terminals do not extend into the pituitary gland; they end at the base of the hypothalamus. Stimuli that induce action potentials in the ventral hypothalamic neurons cause release of hormones into a capillary bed that drains into a second capillary bed before entering the systemic circulation. These two capillary beds are connected by the hypophyseal portal veins, special vessels that connect capillary beds. This design allows small amounts of ventral hypothalamic hormones to be present in high concentrations at the anterior pituitary cells. Recall that most of the ventral hypothalamic hormones cause the secretion of anterior pituitary hormones. We learned that the posterior pituitary hormones, vasopressin also called antidiuretic hormone and oxytocin are synthesized in individual hypothalamic neurons of the supraoptic and paraventricular nuclei. Hormones travel down the axons of these neurons through the infundibulum to axon terminals in the posterior lobe where they are stored. Stimuli that induce action potentials in the hypothalamic neurons cause release of the hormones into the systemic circulation. Note that the release of posterior pituitary and ventral hypothalamic hormones is identical to the release of neurotransmitters. In addition, molecules that function as hormones in the hypothalamic-pituitary axis are often found in other parts of the brain or the body where they act as neurotransmitters, neuromodulators, or local chemical messengers called paracrines. The ventral hypothalamic hormones that influence the release of tropic hormones and growth hormone are the first hormones in a series that ultimately regulate the secretion of hormones from target glands. The target glands include the thyroid gland, gonads, adrenal cortex, and liver (it secretes insulin like growth factor). A ventral hypothalamic hormone controls secretion of an anterior pituitary hormone that stimulates secretion of a hormone from a target gland. The final hormone in the chain acts on target tissues of the body. This kind of complex control system provides many input sites where other hormones and neurons can act to achieve fine control over the endocrine system. Let’s look first at the influence of hormones: For each hormone series, negative feedback loops control circulating levels of the target gland hormone. Negative feedback exerted by the target hormone can be directed at the anterior pituitary, the ventral hypothalamus, or both. Target hormones from one series can also influence the secretion of hypothalamic or anterior pituitary hormones from another hormone series. Let’s review negative feedback control for thyroid hormone and cortisol. Thyrotropin releasing hormone acts on cells of the anterior pituitary to increase secretion of thyrotropin, TSH, which in turns acts on the thyroid gland to increase secretion of T3 and T4. The thyroid hormones feedback almost exclusively to the anterior pituitary to inhibit secretion of thyrotropin. Thyroid hormone is also essential for anterior pituitary cells to secrete growth hormone. Corticotropin releasing hormone acts on cells of the anterior pituitary to increase secretion of corticotrophin, ACTH. Corticotropin acts on the adrenal cortex to increase secretion of cortisol. Cortisol feeds back to both the anterior pituitary, to inhibit secretion of corticotrophin, and to the ventral hypothalamus, to inhibit secretion of corticotropin releasing hormone. The cortisol system is subject to far more dynamic changes than the thyroid system, and all hormones of the cortisol series exhibit a circadian rhythm. Let’s look now at the release of prolactin. Prolactin differs from the other anterior pituitary hormones in these important ways. Its primary ventral hypothalamic drive is inhibitory. It does not stimulate release of another hormone from a target gland. Prolactin is secreted in small amounts in both males and females. The amount in adult females is slightly higher than the amounts in males and children because estrogen modulates its secretion. Estrogen increases prolactin secretion, but blocks the effects of prolactin on milk production. For example, the high levels of estrogen during pregnancy cause large increases in circulating prolactin. Prolactin, estrogen, and other hormones, promote growth of the breasts, but the high levels of estrogen inhibit milk production. Milk production begins after childbirth when estrogen levels drop due to the removal of the placenta. In summary, endocrine control of the hypothalamic pituitary axis can be exerted by negative feedback from a target hormone to a hormone of its own series and modulation by a target hormone of one series to hypothalamic pituitary hormones of another series. The hypothalamus is the highest level integrating center for the autonomic nervous system, and the coordinating center for neuroendocrine interactions. Fibers from many parts of the brain influence the endocrine system via hypothalamic neurons. The cerebral cortex is widely connected, both directly and indirectly, to the hypothalamus. Parts of the limbic system, thalamus, basal nuclei, reticular formation of the brainstem, and the retina project to the hypothalamus. Strong emotions like fear, body housekeeping signals like ingestion of food, harmful stimuli that produce pain, trauma, or infection, environmental changes like extreme cold, and light signals from the retina can influence the endocrine system through these circuits. Neuroendocrine reflexes like the milk ejection reflex, or milk letdown, are mediated in the hypothalamus. Suckling induces release of oxytocin that causes the myoepithelial cells of the mammary glands to contract and release milk. The sound of a baby crying can cause a similar led down of milk, and anxiety and worry can inhibit the milk ejection reflex. Hypothalamic neurons, including the neurosecretory cells that produce vasopressin, behave like osmoreceptors. They detect osmotic changes in body fluids. For example, an increase in the osmolarity of body fluids due to fluid loss, excessive sweating or diarrhea excites osmoreceptors and stimulates both synthesis and release of vasopressin. Vasopressin acts on the kidneys to promote water reabsorption in an effort to restore normal osmolarity. Neurons of the hypothalamus are pacemakers that generate circadian patterns of hormonal release by the hypothalamic-pituitary axis. Circadian rhythms are approximately 24 hours long. The timing of the rhythm is synchronized to the 24-hour earth day by external signals. We have seen in topic 2 that cortisol and the hypothalamic pituitary hormones in its series exhibit a circadian rhythm that is synchronized by light..
Recall that the pituitary gland, also called the hypophysis, lies beneath the hypothalamus. It is attached to the hypothalamus by a stalk called the infundibulum. The pituitary gland in humans consists of two lobes: The anterior lobe of the pituitary gland is endocrine tissue and a posterior lobe that is made up of neural tissue. The anterior lobe and a small portion of the infundibulum make up the adenohypophysis, and posterior lobe and most of the infundibulum make up the neurohypophysis. We have reviewed six hormones secreted by the anterior lobe of the pituitary. They are: Thyroid stimulating hormone, or thyrotropin, Follicle stimulating hormone and luteinizing hormone, the gonadotropins, Adreno corticotropic hormone, or corticotrophin, Growth hormone, and prolactin. These are considered the classic anterior pituitary hormones because their structures and physiological functions in humans are well understood. Four of these hormones, TSH, FSH, LH, and ACTH, are called tropic hormones because they cause the release of hormones from other endocrine glands. TSH stimulates the thyroid gland to release thyroid hormones FSH and LH stimulate estrogen and progesterone production in females and testosterone production in males. ACTH stimulates the adrenal cortex to release cortisol and other glucocorticoids. In addition to functions of each hormone that we have already reviewed, the anterior pituitary hormones exert important growth effects on target tissues keeping them healthy and functional. FSH and LH are produced by the same type of cells. Each of the other anterior pituitary hormones is secreted by a different type of cells. This follows the pattern we have observed for other endocrine glands and tissues that produce more than one hormone. Understanding the pituitary gland requires careful study of the vascular supply. We have noted that the six classic anterior pituitary hormones are released directly into the systemic circulation. The ventral hypothalamic hormones regulate the function of the anterior pituitary cells. They are produced in neurons and travel down the axons to terminals where they are stored. The axon terminals do not extend into the pituitary gland; they end at the base of the hypothalamus. Stimuli that induce action potentials in the ventral hypothalamic neurons cause release of hormones into a capillary bed that drains into a second capillary bed before entering the systemic circulation. These two capillary beds are connected by the hypophyseal portal veins, special vessels that connect capillary beds. This design allows small amounts of ventral hypothalamic hormones to be present in high concentrations at the anterior pituitary cells. Recall that most of the ventral hypothalamic hormones cause the secretion of anterior pituitary hormones. We learned that the posterior pituitary hormones, vasopressin also called antidiuretic hormone and oxytocin are synthesized in individual hypothalamic neurons of the supraoptic and paraventricular nuclei. Hormones travel down the axons of these neurons through the infundibulum to axon terminals in the posterior lobe where they are stored. Stimuli that induce action potentials in the hypothalamic neurons cause release of the hormones into the systemic circulation. Note that the release of posterior pituitary and ventral hypothalamic hormones is identical to the release of neurotransmitters. In addition, molecules that function as hormones in the hypothalamic-pituitary axis are often found in other parts of the brain or the body where they act as neurotransmitters, neuromodulators, or local chemical messengers called paracrines. The ventral hypothalamic hormones that influence the release of tropic hormones and growth hormone are the first hormones in a series that ultimately regulate the secretion of hormones from target glands. The target glands include the thyroid gland, gonads, adrenal cortex, and liver (it secretes insulin like growth factor). A ventral hypothalamic hormone controls secretion of an anterior pituitary hormone that stimulates secretion of a hormone from a target gland. The final hormone in the chain acts on target tissues of the body. This kind of complex control system provides many input sites where other hormones and neurons can act to achieve fine control over the endocrine system. Let’s look first at the influence of hormones: For each hormone series, negative feedback loops control circulating levels of the target gland hormone. Negative feedback exerted by the target hormone can be directed at the anterior pituitary, the ventral hypothalamus, or both. Target hormones from one series can also influence the secretion of hypothalamic or anterior pituitary hormones from another hormone series. Let’s review negative feedback control for thyroid hormone and cortisol. Thyrotropin releasing hormone acts on cells of the anterior pituitary to increase secretion of thyrotropin, TSH, which in turns acts on the thyroid gland to increase secretion of T3 and T4. The thyroid hormones feedback almost exclusively to the anterior pituitary to inhibit secretion of thyrotropin. Thyroid hormone is also essential for anterior pituitary cells to secrete growth hormone. Corticotropin releasing hormone acts on cells of the anterior pituitary to increase secretion of corticotrophin, ACTH. Corticotropin acts on the adrenal cortex to increase secretion of cortisol. Cortisol feeds back to both the anterior pituitary, to inhibit secretion of corticotrophin, and to the ventral hypothalamus, to inhibit secretion of corticotropin releasing hormone. The cortisol system is subject to far more dynamic changes than the thyroid system, and all hormones of the cortisol series exhibit a circadian rhythm. Let’s look now at the release of prolactin. Prolactin differs from the other anterior pituitary hormones in these important ways. Its primary ventral hypothalamic drive is inhibitory. It does not stimulate release of another hormone from a target gland. Prolactin is secreted in small amounts in both males and females. The amount in adult females is slightly higher than the amounts in males and children because estrogen modulates its secretion. Estrogen increases prolactin secretion, but blocks the effects of prolactin on milk production. For example, the high levels of estrogen during pregnancy cause large increases in circulating prolactin. Prolactin, estrogen, and other hormones, promote growth of the breasts, but the high levels of estrogen inhibit milk production. Milk production begins after childbirth when estrogen levels drop due to the removal of the placenta. In summary, endocrine control of the hypothalamic pituitary axis can be exerted by negative feedback from a target hormone to a hormone of its own series and modulation by a target hormone of one series to hypothalamic pituitary hormones of another series. The hypothalamus is the highest level integrating center for the autonomic nervous system, and the coordinating center for neuroendocrine interactions. Fibers from many parts of the brain influence the endocrine system via hypothalamic neurons. The cerebral cortex is widely connected, both directly and indirectly, to the hypothalamus. Parts of the limbic system, thalamus, basal nuclei, reticular formation of the brainstem, and the retina project to the hypothalamus. Strong emotions like fear, body housekeeping signals like ingestion of food, harmful stimuli that produce pain, trauma, or infection, environmental changes like extreme cold, and light signals from the retina can influence the endocrine system through these circuits. Neuroendocrine reflexes like the milk ejection reflex, or milk letdown, are mediated in the hypothalamus. Suckling induces release of oxytocin that causes the myoepithelial cells of the mammary glands to contract and release milk. The sound of a baby crying can cause a similar led down of milk, and anxiety and worry can inhibit the milk ejection reflex. Hypothalamic neurons, including the neurosecretory cells that produce vasopressin, behave like osmoreceptors. They detect osmotic changes in body fluids. For example, an increase in the osmolarity of body fluids due to fluid loss, excessive sweating or diarrhea excites osmoreceptors and stimulates both synthesis and release of vasopressin. Vasopressin acts on the kidneys to promote water reabsorption in an effort to restore normal osmolarity. Neurons of the hypothalamus are pacemakers that generate circadian patterns of hormonal release by the hypothalamic-pituitary axis. Circadian rhythms are approximately 24 hours long. The timing of the rhythm is synchronized to the 24-hour earth day by external signals. We have seen in topic 2 that cortisol and the hypothalamic pituitary hormones in its series exhibit a circadian rhythm that is synchronized by light..