Control Systems Involving the Hypothalamus and Pituitary

The pituitary gland, or hypophysis, lies in a pocket (the sella turcica) of the sphenoid bone at the base of the brain (Figure 10-12), just below the brain area called the hypothalamus. The pituitary is connected to the hypothalamus by the infundibulum, a stalk containing nerve fibers and small blood vessels. In adult human beings, the pituitary gland is composed of two adjacent lobes—the anterior pituitary (toward the front of the head) and the posterior pituitary (toward the back of the head)—each of which is a more or less distinct gland. (In many mammalian species an intermediate lobe is found between the anterior and posterior portions of the pituitary, but this is not the case in humans.) The anterior pituitary and its part of the stalk are termed the adenohypophysis, and the posterior pituitary with its part of the stalk is the neurohy-pophysis.

The posterior pituitary is an outgrowth of the hypothalamus and is neural tissue. The axons of two well-defined clusters of hypothalamic neurons (the supraoptic and paraventricular nuclei) pass down the infundibulum and end within the posterior pituitary in close proximity to capillaries (the smallest of blood vessels) (Figure 10-12b).

In contrast to the neural connections between the hypothalamus and posterior pituitary, there are no important neural connections between the hypothalamus and anterior pituitary. There is, however, an unusual blood-vessel connection (Figure 10-12b). The capillaries at the base of the hypothalamus (the median eminence) recombine to form the hypothalamo-pituitary portal vessels—the term "portal" denotes vessels that connect distinct capillary beds. The hypothalamo-pituitary portal vessels pass down the stalk connecting the hypothalamus and pituitary and enter the anterior pituitary where they drain into a second capillary bed, the anterior pituitary capillaries. Thus, the hypothalamo-pituitary portal vessels offer a local route for blood flow directly from the hypothalamus to the anterior pituitary.

Posterior Pituitary Hormones

We stressed above that the posterior pituitary is really a neural extension of the hypothalamus (Figure 10-12). The term "posterior pituitary hormones" is somewhat of a misnomer, therefore, since the hormones are not synthesized in the posterior pituitary itself but in the hypothalamus, specifically in the cell bodies of the hypothalamic neurons whose axons pass down the infundibulum and end in the posterior pituitary. Only one hormone is produced by any particular neuron in the two relevant hypothalamic nuclei. Enclosed in small vesicles, the hormone moves down the neural axons to accumulate at the axon terminals in the posterior pituitary. Stimuli (either hormones or neurotransmitters) act to generate action potentials in the neurons; these action potentials propagate to the axon terminals and trigger the release of the hormone by exocytosis. The hormone then enters the capillaries to be carried away by the blood returning to the heart.

The two posterior pituitary hormones are oxytocin and vasopressin (the latter is also known as antidi-uretic hormone, or ADH). Vasopressin participates in the control of water excretion by the kidneys and of blood pressure (Chapters 16 and 14). Oxytocin acts on smooth muscle cells in the breasts and uterus (Chapter 19); its functions, if any, in the male are uncertain.

Vasopressin and oxytocin are also produced in other areas of the brain and serve in those sites as neu-rotransmitters or neuromodulators.

The Hypothalamus and Anterior Pituitary

Hypothalamic neurons different from those that produce the hormones released from the posterior pituitary, secrete hormones that control, in large part, the secretion of all the anterior pituitary hormones. These hypo-thalamic hormones are collectively termed hypophys-iotropic hormones (recall that another name for the pituitary is hypophysis); they are also commonly called hypothalamic releasing hormones. One more word about terminology is appropriate: "Hypophysiotropic hormones" denotes only those hormones from the hypothalamus that influence the anterior pituitary. We shall see later that nonhypothalamic hormones can also influence the anterior pituitary, but they are not categorized as hypophysiotropic hormones.

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Physiology: The Systems Control Systems Companies, 2001 Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Pituitary

Pituitary

Hypothalamus Blood Supply

Hypothalamus

Arterial blood supply Infundibulum

Arterial blood supply

Arterial blood supply Infundibulum

Hypothalamo-pituitary portal vessels

Anterior pituitary

Endocrine cells

To venous circulation

Hypothalamus And Pituitary Gland

Median eminence

Arterial blood supply

To venous circulation

FIGURE 10-12

(a) Relation of the pituitary gland to the brain and hypothalamus. (b) Neural and vascular connections between the hypothalamus and pituitary. Some hypothalamic neurons run down the infundibulum to end in the posterior pituitary, whereas others end in the median eminence. Almost the entire blood supply to the anterior pituitary comes via the hypothalamo-pituitary portal vessels, which originate in the median eminence. (The short portal vessels, which originate in the posterior pituitary, carry only a small fraction of the blood leaving the posterior pituitary and supply only a small fraction of the blood received by the anterior pituitary.) %

Hypothalamo-pituitary portal vessels

Anterior pituitary

Endocrine cells

To venous circulation

Median eminence

Arterial blood supply

To venous circulation

FIGURE 10-12

(a) Relation of the pituitary gland to the brain and hypothalamus. (b) Neural and vascular connections between the hypothalamus and pituitary. Some hypothalamic neurons run down the infundibulum to end in the posterior pituitary, whereas others end in the median eminence. Almost the entire blood supply to the anterior pituitary comes via the hypothalamo-pituitary portal vessels, which originate in the median eminence. (The short portal vessels, which originate in the posterior pituitary, carry only a small fraction of the blood leaving the posterior pituitary and supply only a small fraction of the blood received by the anterior pituitary.) %

With one exception, each of the hypophysiotropic hormones is the first in a three-hormone sequence: (1) A hypophysiotropic hormone controls the secretion of (2) an anterior pituitary hormone, which controls the secretion of (3) a hormone from some other endocrine gland (Figure 10-13). This last hormone then acts on its target cells. As we shall see later, the adaptive value of such chains is that they permit a variety of important hormonal feedbacks. We begin our description of these sequences in the middle—that is, with the anterior pituitary hormones—because the names and actions of the hypophysiotropic hormones are all based on the names of the anterior pituitary hormones.

Anterior Pituitary Hormones As shown in Table 10-1, the anterior pituitary secretes at least eight hormones, but only six have well-established functions. All peptides, these six "classical" hormones are follicle-stimulating hormone (FSH), luteinizing hormone (LH), growth hormone (GH), thyroid-stimulating hormone (TSH, thyrotropin), prolactin, and adrenocorticotropic hormone (ACTH, corticotropin). Each of the last four is probably secreted by a distinct cell type in the anterior pituitary, whereas FSH and LH, collectively termed gonadotropic hormones (or go-nadotropins) because they stimulate the gonads, are both secreted by the same cells.

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Principles of Hormonal Control Systems CHAPTER TEN

Principles of Hormonal Control Systems CHAPTER TEN

Pituitary Gland Portal Tunnel Opening

FIGURE 10-13

Typical sequential pattern by which a hypophysiotropic hormone (hormone 1 from the hypothalamus) controls the secretion of an anterior pituitary hormone, which in turn controls the secretion of a hormone by a third endocrine gland. The hypothalamo-pituitary portal vessels are illustrated in Figure 10-12. %

What about the other two peptides—beta-lipotropin and beta-endorphin—secreted by the anterior pituitary? Their functions are not presently known, but some possibilities are described in Chapters 9 and 13.

The target organs and major functions of the six classical anterior pituitary hormones are summarized in Figure 10-14. Note that the only major function of two of the six is to stimulate secretion of other hormones by their target cells (and to maintain the growth and function of these cells): thyroid-stimulating hormone induces secretion of thyroxine and triiodothyronine from the thyroid; adrenocorticotropic hormone, meaning "hormone that stimulates the adrenal cortex," stimulates the secretion of cortisol by that gland.

Three other anterior pituitary hormones also stimulate secretion of another hormone but have an additional function as well. Follicle-stimulating hormone and luteinizing hormone stimulate secretion of the sex hormones by the gonads—estradiol and progesterone from the ovaries, or testosterone from the testes—but in addition, they regulate the growth and development of ova and sperm. Growth hormone stimulates the liver and many other body cells to secrete a growth-promoting peptide hormone known as insulin-like growth factor I (IGF-I), and in addition, growth hormone exerts direct effects on the metabolism of protein, carbohydrate, and lipid by various organs and tissues (Chapter 18).

Prolactin is unique among the anterior pituitary hormones in that it does not exert major control over the secretion of a hormone by another endocrine gland. Rather, its most important action is to stimulate development of the mammary glands and milk production by direct effects upon the breasts. In the male, prolactin may facilitate several components of reproductive function.

It should be emphasized that many, perhaps all, of the anterior pituitary hormones are also secreted by cells elsewhere in the body and may, in those other sites, exert functions (as neurotransmitters, neuro-modulators, or paracrine/autocrine agents) quite different from those summarized in Figure 10-14). For example, ACTH is an important neurotransmitter in several brain areas.

Hypophysiotropic Hormones As stated above, secretion of the anterior pituitary hormones is largely regulated by hormones produced by the hypothalamus and collectively called hypophysiotropic hormones. These hormones are secreted by neurons that originate in diverse areas of the hypothalamus and terminate in the median eminence around the capillaries that are the origins of the hypothalamo-pituitary portal vessels. The generation of action potentials in these neurons causes them to release their hormones, which enter the capillaries and are carried by the hypothalamo-pituitary portal vessels to the anterior pituitary (Figure 10-15). There they act upon the various anterior pituitary cells to control their hormone secretions.

Thus, these hypothalamic neurons secrete hormones in a manner identical to that described previously for the hypothalamic neurons whose axons end in the posterior pituitary. In both cases the hormones are made in hypothalamic neurons, pass down axons

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

10. Principles of Hormonal Control Systems

© The McGraw-Hill Companies, 2001

PART TWO Biological Control Systems

Anterior pituitary

Growth hormone

Gonads

Germ cell ' Secrete hormones

development

Female

Male

¡ i

Estrogen,

Testos

proges-

terone

terone

Secrete IGF-I

Many organs and tissues

Protein synthesis carbohydrate and lipid metabolism

Prolactin

ACTH

Thyroid

Secretes thyroxine, triiodothyronine

Breasts

Breast development and milk production (in male may facilitate reproductive function)

Breasts

Breast development and milk production (in male may facilitate reproductive function)

Adrenal

cortex

Secretes

cortisol

/

Targets and major functions of the six classical anterior pituitary hormones.

Hypothalamo-pituitary portal vessels

Hypothalamus

Hypothalamic-

FIGURE 10-14

Targets and major functions of the six classical anterior pituitary hormones.

Hypothalamo-pituitary portal vessels

Hypothalamus

Hypothalamic-

Cortisol Control From The Hypothalamus

Arterial inflow from heart

Capillaries in median eminence pituitary

Capillaries in anterior pituitary

Arterial inflow from heart

Capillaries in median eminence pituitary

Anterior — pituitary gland cells

Venous outflow to heart

Capillaries in anterior pituitary

O Hypophysiotropic hormone A Anterior pituitary hormone

FIGURE 10-15

Hormone secretion by the anterior pituitary is controlled by hypophysiotropic hormones released by hypothalamic neurons and reaching the anterior pituitary by way of the hypothalamo-pituitary portal vessels.

to the neuron terminals, and are released in response to action potentials in the neurons. The crucial differences, however, between the two systems need emphasizing: (1) The axons of the hypothalamic neurons that secrete the posterior pituitary hormones leave the hypothalamus and end in the posterior pituitary, whereas those that secrete the hypophysiotropic hormones remain in the hypothalamus, ending in its median eminence. (2) Most of the posterior pituitary capillaries into which the posterior pituitary hormones are secreted immediately drain into the main bloodstream, which carries the hormones to the heart to be distributed to the entire body. In contrast, the hypophys-iotropic hormones enter capillaries in the median eminence of the hypothalamus that do not directly join the main bloodstream, but empty into the hypothalamo-pituitary portal vessels, which carry them to the anterior pituitary (from which the blood then returns to the heart). Thus, the anterior pituitary is uniquely exposed to plasma concentrations of the hypophys-iotropic hormones much higher than those existing in the general bloodstream. The hypothalamo-pituitary portal system allows these hormones to act preferentially on the anterior pituitary without influencing other organs and tissues that may have receptors for them.

There are multiple discrete hypophysiotropic hormones, each secreted by a particular group of hypo-thalamic neurons and influencing the release of one or, in at least one case, two of the anterior pituitary

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Principles of Hormonal Control Systems CHAPTER TEN

Principles of Hormonal Control Systems CHAPTER TEN

Hypothalamus

Hypothalamus

Pih Pituitary

FSH and LH

Growth hormone

Prolactin

ACTH

FSH and LH

Growth hormone

Prolactin

ACTH

Major known hypophysiotropic hormones

Major effect on anterior pituitary

Corticotropin releasing hormone (CRH)-

Thyrotropin releasing hormone (TRH)* -

Growth hormone releasing hormone (GHRH)-

Somatostatin (SS, also known as growth hormone -

release inhibiting hormone, GIH)

Gonadotropin releasing hormone (GnRH)-

Dopamine (DA, also known as prolactin-inhibiting-

hormone, PIH)^

-Stimulates secretion of ACTH

->- Stimulates secretion of TSH

->- Stimulates secretion of GH

-Inhibits secretion of GH

-Stimulates secretion of LH and FSH

-Inhibits secretion of prolactin

*TRH can also stimulate the release of prolactin, but whether this occurs physiologically is unclear. ^Dopamine is a catecholamine; all the other hypophysiotropic hormones are peptides.

*TRH can also stimulate the release of prolactin, but whether this occurs physiologically is unclear. ^Dopamine is a catecholamine; all the other hypophysiotropic hormones are peptides.

FIGURE 10-16

The effects of definitely established hypophysiotropic hormones on the anterior pituitary. The hypophysiotropic hormones reach the anterior pituitary via the hypothalamo-pituitary portal vessels.

hormones. For simplicity, Figure 10-16 and the text of this chapter summarize only those hypophysiotropic hormones that are known to play important physiological roles and whose structures have been identified. These hypophysiotropic hormones may also exert effects in addition to those described here, but these effects have not been established with certainty under physiological conditions.

Each hypophysiotropic hormone is named for the anterior pituitary hormone whose secretion it controls. Thus, secretion of ACTH (corticotropin) is stimulated by corticotropin releasing hormone (CRH), secretion of growth hormone is stimulated by growth hormone releasing hormone (GHRH), secretion of thyroid-stimulating hormone (thyrotropin) is stimulated by thyrotropin releasing hormone (TRH), and secretion of both luteinizing hormone and follicle-stimulating hormone (the gonadotropins) is stimulated by go-nadotropin releasing hormone (GnRH).

Note, however, in Figure 10-16, that two of the hypophysiotropic hormones do not stimulate release of an anterior pituitary hormone but rather inhibit its release.

One of them inhibits secretion of growth hormone and is most commonly called somatostatin (SS). The other, dopamine (also termed, in this location, prolactin-inhibiting hormone, PIH), inhibits secretion of prolactin.

As shown in Figure 10-16, growth hormone is controlled by two hypophysiotropic hormones—somato-statin, which inhibits its release, and growth hormone releasing hormone, which stimulates it. The rate of growth hormone secretion depends, therefore, upon the relative amounts of the opposing hormones released by the hypothalamic neurons, as well as upon the relative sensitivities of the anterior pituitary to them. Such dual controls may also exist for prolactin, but the identity of the hypothesized "prolactin releasing hormone" (not shown in Figure 10-16) and its importance in humans remain uncertain; this will be discussed further in Chapter 19.

With one exception, all the known hypophys-iotropic hormones are peptides that also occur in locations other than the hypothalamus, particularly in other areas of the central nervous system, where they function as neurotransmitters or neuromodulators,

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems

Hypothalamus

GnRH

GHRH

FSH

and

LH

Q Anterior pituitary Q

Growth Hormone

Prolactin

Gonads

Germ cell

Secrete hormones

development

Female

Male

;

i

Estrogen,

Testos-

proges-

terone

terone

other cells

Liver and

Secrete IGF-I

ACTH

Many organs and tissues

Protein synthesis, carbohydrate and lipid metabolism

Thyroid

Secretes thyroxine, triiodothyronine

\Z

\Z

/

/

/

Breasts

Adrenal

Breast

cortex

development and milk production (in male may facilitate

Secretes cortisol

reproductive function)

/

A combination of Figures 10-14 and 10-16 summarizes the hypothalamic-anterior-pituitary system. %

and in the gastrointestinal tract. The one hypophys-iotropic hormone that is not a peptide is dopamine. As described earlier, this substance is an amine and a member of the catecholamine family.

Figure 10-17 summarizes the information presented in Figures 10-14 and 10-16 to demonstrate the full sequence of hypothalamic control of endocrine function.

Given that the hypophysiotropic hormones control anterior pituitary function, we must now ask: What controls secretion of the hypophysiotropic hormones? Some of the neurons that secrete hypophysiotropic hormones may possess spontaneous activity, but the firing of most of them requires neural and hormonal input. First, let us deal with the neural input.

Neural Control of Hypophysiotropic Hormones

Neurons of the hypothalamus receive synaptic input, both stimulatory and inhibitory, from virtually all areas of the central nervous system, and specific neural pathways influence secretion of the individual hypophys-iotropic hormones. A large number of neurotransmit-

ters (for example, the catecholamines and serotonin) are released at the synapses on the hormone-secreting hypothalamic neurons, and this explains why the secretion of the hypophysiotropic hormones can be altered by drugs that influence these neurotransmitters.

Figure 10-18 illustrates one example of the role of neural input to the hypothalamus. Corticotropin releasing hormone (CRH) from the hypothalamus stimulates the anterior pituitary to secrete ACTH, which in turn stimulates the adrenal cortex to secrete cortisol. A wide variety of stresses, both physical and emotional, act via neural pathways to the hypothalamus to increase CRH secretion, and, hence, ACTH and cortisol secretion, markedly above basal values. Thus, stress is the common denominator of reflexes leading to increased cortisol secretion. Cortisol then functions to facilitate an individual's response to stress. Even in an unstressed person, however, the secretion of cortisol varies in a highly stereotyped manner during a 24-h period because neural rhythms within the central nervous system also impinge upon the hypothalamic neurons that secrete CRH.

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Principles of Hormonal Control Systems CHAPTER TEN

Principles of Hormonal Control Systems CHAPTER TEN

Crh Acth Cortisol Sequence

FIGURE 10-18

CRH-ACTH-cortisol sequence. The "stress" input to the hypothalamus is via neural pathways. Cortisol exerts a negative-feedback control over the system by acting on (1) the hypothalamus to inhibit CRH secretion and (2) the anterior pituitary to reduce responsiveness to CRH.

FIGURE 10-18

CRH-ACTH-cortisol sequence. The "stress" input to the hypothalamus is via neural pathways. Cortisol exerts a negative-feedback control over the system by acting on (1) the hypothalamus to inhibit CRH secretion and (2) the anterior pituitary to reduce responsiveness to CRH.

Hormonal Feedback Control of the Hypothalamus and Anterior Pituitary A prominent feature of each of the hormonal sequences initiated by a hypophys-iotropic hormone is negative feedback exerted upon the hypothalamo-pituitary system by one or more of the hormones in its sequence. For example, in the CRH-ACTH-cortisol sequence (Figure 10-18), the final hormone, cortisol, acts upon the hypothalamus to reduce secretion of CRH by causing a decrease in the frequency of action potentials in the neurons secreting CRH. In addition, cortisol acts directly on the anterior pituitary to reduce the response of the ACTH-secreting cells to CRH. Thus, by a double-barreled action, cortisol exerts a negative-feedback control over its own secretion.

Such a system is effective in dampening hormonal responses—that is, in limiting the extremes of hormone secretory rates. For example, when a painful stimulus elicits increased secretion, in turn, of CRH, ACTH, and cortisol, the resulting elevation in plasma cortisol concentration feeds back to inhibit the hypothalamus and anterior pituitary. Therefore, cortisol secretion does not rise as much as it would without these negative feedbacks.

Another adaptive function of these negativefeedback mechanisms is that they maintain the plasma concentration of the final hormone in a sequence relatively constant whenever a disease-induced primary change occurs in the secretion or metabolism of that hormone. An example of this is shown in Figure 10-19 for cortisol.

Another example is provided by the TRH-TSH-TH system. The thyroid hormones (TH) exert a feedback inhibition on the hypothalamo-pituitary system (mainly by decreasing the response of anterior pituitary TSH-secreting cells to the stimulatory effects of TRH). Since iodine is essential for the synthesis of TH, individuals with iodine deficiencies tend to have a deficient production of TH. The resulting decrease in plasma TH concentration relieves some of the feedback inhibition TH exerts on the pituitary. Therefore, more TSH is secreted in response to the TRH coming from the hypothalamus, and the increased plasma TSH stimulates the thyroid gland to enlarge and to utilize more efficiently whatever iodine is available (the enlarged gland is known as iodine-deficient goiter). In this manner, plasma TH concentration can be kept quite close to normal.

The situations described above for cortisol and the thyroid hormones, in which the hormone secreted by the third endocrine gland in a sequence exerts a negative-feedback effect over the anterior pituitary and/or hypothalamus, is known as a long-loop negative feedback (Figure 10-20). This type of feedback exists for each of the five three-hormone sequences initiated by a hypophysiotropic hormone.

Long-loop feedback does not exist for prolactin since this is one anterior pituitary hormone that does not have major control over another endocrine gland— that is, it does not participate in a three-hormone sequence. Nonetheless, there is negative feedback in

PART TWO Biological Control Systems

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PART TWO Biological Control Systems

Negative Feedback Mechanism Ovary

FIGURE 10-19

How negative feedback in a hormonal sequence helps maintain the plasma concentration of the final hormone when disease-induced changes in hormone secretion occur. The same analysis would apply if the original reduction in plasma cortisol were due to excessive metabolism of cortisol rather than deficient secretion.

the prolactin system, for this hormone itself acts upon the hypothalamus to stimulate the secretion of dopamine, which then, you will recall, inhibits the secretion of prolactin. The influence of an anterior pituitary hormone on the hypothalamus is known as a short-loop negative feedback (Figure 10-20). Like prolactin, several other anterior pituitary hormones, including growth hormone, also exert such feedback on the hypothalamus.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Principles of Hormonal Control Systems CHAPTER TEN

Principles of Hormonal Control Systems CHAPTER TEN

Hypothalamic Pituitary Portal System

FIGURE 10-20

Short-loop and long-loop feedbacks. Long-loop feedback is exerted on the hypothalamus and/or anterior pituitary by the third hormone in the sequence. Short-loop feedback is exerted by the anterior pituitary hormone on the hypothalamus.

FIGURE 10-20

Short-loop and long-loop feedbacks. Long-loop feedback is exerted on the hypothalamus and/or anterior pituitary by the third hormone in the sequence. Short-loop feedback is exerted by the anterior pituitary hormone on the hypothalamus.

The Role of "Nonsequence" Hormones on the Hypothalamus and Anterior Pituitary It must be emphasized that there are many stimulatory and inhibitory hormonal influences on the hypothalamus and/or anterior pituitary other than those that fit the feedback patterns just described. In other words, a hormone that is not itself in a particular hormonal sequence may nevertheless exert important influences on the secretion of the hypophysiotropic or anterior pituitary hormones in that sequence. For example, estrogen markedly enhances the secretion of prolactin by the anterior pituitary, even though estrogen secretion is not controlled by prolactin. Thus, one should not view the sequences we have been describing as isolated units.

A Summary Example: Control of Growth Hormone Secretion The three-hormone sequences beginning in the hypothalamus can be extremely complex, incorporating multiple sites of feedback, both long-loop and short-loop, as well as other hormones not in the sequence. Purely for the sake of illustrating this complexity, we describe here the control of growth hormone secretion (Figure 10-21), building upon the information already presented in this chapter.

Recall from Figure 10-16 that the secretion of GH by the anterior pituitary is controlled mainly by two hormones from the hypothalamus: (1) somatostatin, which inhibits GH secretion; and (2) GHRH, which stimulates it. With such a dual control system, the rate of GH secretion at any moment reflects the relative amounts of simultaneous stimulation by GHRH and inhibition by somatostatin. For example, very little growth hormone is secreted during the day in non-stressed persons who are eating normally, but whether this is due to a very low secretion of GHRH or a very high secretion of somatostatin is not yet clear. Similarly, a large number of physiological states (exercise, stress, fasting, a low plasma glucose concentration, and sleep) stimulate growth hormone secretion by decreasing the secretion of somatostatin and/or increasing that of GHRH.

As shown in Figure 10-14 and 10-17, growth hormone stimulates the secretion of another hormone, IGF-I, from the liver and many other target cells of GH. This makes possible a variety of feedbacks by which an increase in plasma GH results in inhibition of GH secretion (Figure 10-21): (1) a short-loop negative feedback exerted by GH on the hypothalamus and (2) a long-loop negative feedback exerted by IGF-I on the hypothalamus. Both of these feedbacks operate by inhibiting secretion of GHRH and/or stimulating secretion of somatostatin, the result in either case being less stimulation of GH secretion. (3) IGF-I also acts directly on the pituitary to inhibit the stimulatory effect of GHRH on GH secretion; again, the result is decreased GH secretion. Finally, the secretion of growth hormone is influenced by several other hormones not in the sequence, for example, by the sex hormones. Some of these hormones may affect GH secretion indirectly by altering the release of somatostatin and/or GHRH from the hypothalamus, whereas others may act directly upon the anterior pituitary.

PART TWO Biological Control Systems

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PART TWO Biological Control Systems

Pituitary Physiology

FIGURE 10-21

Hormonal pathways controlling the secretion of growth hormone (GH) and insulin-like growth factor I (IGF-I). At the hypothalamus, the minus sign Q denotes that the input inhibits the secretion of growth hormone releasing hormone (GHRH) and/or stimulates the release of somatostatin (SS). Not shown in the figure is that several hormones not in the sequence (for example, the thyroid hormones) influence growth hormone secretion via effects on the hypothalamus and/or anterior pituitary.

FIGURE 10-21

Hormonal pathways controlling the secretion of growth hormone (GH) and insulin-like growth factor I (IGF-I). At the hypothalamus, the minus sign Q denotes that the input inhibits the secretion of growth hormone releasing hormone (GHRH) and/or stimulates the release of somatostatin (SS). Not shown in the figure is that several hormones not in the sequence (for example, the thyroid hormones) influence growth hormone secretion via effects on the hypothalamus and/or anterior pituitary.

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Responses

  • LIDYA
    Why is posterior pituitary a misnomer?
    5 years ago
  • dexter
    How the hypothalamus controls the pituitary gland?
    5 years ago
  • rose sackville
    What pituitary is a neural extension of the hypothalamus?
    5 years ago
  • MAURO
    What hormones exert negative control on the hypothalamus and the anterior pituitary in the female?
    5 years ago
  • william
    Is the hypothalamus neural tissue?
    5 years ago
  • ryan mcintyre
    What type of control system is the hypothalamus?
    5 years ago
  • mckenzie
    What are the main functions of those hormones in the posterior pituitary?
    5 years ago
  • Primula
    Is the pituitary attached to the hypothalamus by infundibulum?
    5 years ago
  • Nina
    Does the pituitary neurally connect to the infundibulum?
    5 years ago
  • ELVIA
    What is the feedback control system for hormones of the adenohypophysis?
    5 years ago
  • maria
    What pituitary cells do each hypophysiotropic hormones act upon?
    5 years ago
  • milena
    Is it neural contral that causes hypothalamus to stimulat the anterior pituitary?
    5 years ago
  • Kinfe Kiros
    Why is it critical for the hypothalamus and pituitary to be in close proximity?
    5 years ago
  • marcel
    What other functions are controlled by the hypothalamus?
    5 years ago
  • matthew
    What way is the posterior pituitary an extension of the hypothalamus?
    5 years ago
  • Tobias
    Are hypothalamic neurons in the hypothalamus?
    5 years ago
  • MASON
    What hormones screate by hypothalamus controll anterior pituitary gland?
    5 years ago
  • tomas
    Which body system are controlled by the hypothalamus?
    5 years ago
  • marcel
    What functions are controled by hypothalamus?
    5 years ago
  • mara
    What hormones originate in the hypothalamus?
    5 years ago
  • belinda
    Why it is called hypothalamus is the brain of the brain?
    5 years ago
  • SARADAS NOAKES
    What group of hormones controls the function of the anterior pituitary?
    5 years ago
  • Paul
    HOW DOES THE HYPOTHALAMUS CONTROL THE ENTIRE BODY?
    5 years ago
  • rowan alexander
    How hormones released from pituitary control arterial blood pressure?
    5 years ago
  • Micheal Fazio
    Which artery supplies hypothalamus?
    5 years ago
  • nasih
    What neural connections are there between hypothalamus and pituitary?
    5 years ago
  • Meghan
    What is the primary controling mechanism for the pituitary function?
    5 years ago
  • Marko
    How is the pituitary gland connected to the hypothalamus?
    5 years ago
  • max
    How negative feed back system is shown by hormonal secretion?
    5 years ago
  • Milen Negassi
    What is the sequence of hormone released from the hypothalamus to the pituitary?
    5 years ago
  • amie
    How the hypothalamus controls the rest of the body?
    5 years ago
  • eleanor paterson
    Which body systems control the growth hormone?
    5 years ago
  • clotilde
    What part does infundibulum play in the ADH cotrol system?
    5 years ago
  • JANICE
    What body function is controlled by hormones?
    5 years ago
  • olo
    What body function are controlled by the hypothalamus?
    5 years ago
  • Settimo Conti
    What are primary functions controlled by the pituitary?
    5 years ago
  • gundahar
    What does prolactin feedback on?
    5 years ago
  • Reija
    What is fuction of a hypotthalamus and figure?
    5 years ago
  • robert delagarza
    How the pituitary controls body functions?
    4 years ago
  • jessica
    What two systems are controlled by the hypothalamus?
    4 years ago
  • Scott Runion
    Which part of the pituitary gland has a neural connection with the hypothalamus?
    4 years ago
  • lewis
    Can the pituitary control the devolopement of eggs?
    4 years ago
  • adriana
    What hormones exert negatice control of hypothalamus in female?
    4 years ago
  • Ville
    What 3 hormones are controlled by CRH in the hypolthalamus?
    4 years ago
  • liisa muurinen
    Which hormone does not stimulate another gland?
    4 years ago
  • LEIGH
    How are the posterior and anterior pituitary gland secretions controlled or released?
    4 years ago
  • Barbara Hueber
    How does the pituitary control neurotransmitters?
    4 years ago
  • Mario
    How does pituitary gland control bodyfunction?
    4 years ago
  • mike
    What is hypothalamic control system?
    3 months ago

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