Feedback Control of the Anterior Pituitary

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In view of its secretion of releasing and inhibiting hormones, the hypothalamus might be considered the "master gland." The chain of command, however, is not linear; the hypothalamus and anterior pituitary are controlled by the effects of their own actions. In the endocrine system, to use an analogy, the general takes orders from the private. The hypothalamus and anterior pituitary are not master glands because their secretions are controlled by the target glands they regulate.

Anterior pituitary secretion of ACTH, TSH, and the gonadotropins (FSH and LH) is controlled by negative feedback inhibition from the target gland hormones. Secretion of ACTH is inhibited by a rise in corticosteroid secretion, for example, and TSH is inhibited by a rise in the secretion of thyroxine from the thyroid. These negative feedback relationships are easily demonstrated by removal of the target glands. Castration (surgical removal of the gonads), for example, produces a rise in the secretion of FSH and LH. In a similar manner, removal of the adrenals or the thyroid results in an abnormal increase in ACTH or TSH secretion from the anterior pituitary.

Clinical Investigation Clue

Remember that Rosemary has a blood ACTH level that was only about one-fiftieth of normal.

What might account for the low ACTH secretion from the anterior pituitary?

The effects of removal of the target glands demonstrate that, under normal conditions, these glands exert an inhibitory effect on the anterior pituitary. This inhibitory effect can occur at two levels: (1) the target gland hormones can act on the hypothalamus and inhibit the secretion of releasing hormones, and (2) the target gland hormones can act on the anterior pituitary and inhibit its response to the releasing hormones. Thyroxine,

304 Chapter Eleven

Hypothalamus

Thyrotropin-releasing hormone (TRH)

Thyrotropin-releasing hormone (TRH)

Anterior pituitary

Thyroid-stimulating hormone (TSH)

Thyroid

■ Figure 11.16 The hypothalamus-pituitary-thyroid axis (control system). The secretion of thyroxine from the thyroid is stimulated by thyroid-stimulating hormone (TSH) from the anterior pituitary. The secretion of TSH is stimulated by thyrotropin-releasing hormone (TRH) secreted from the hypothalamus. This stimulation is balanced by negative feedback inhibition (blue arrow) from thyroxine, which decreases the responsiveness of the anterior pituitary to stimulation by TRH.

Inhibits responsiveness to TRH l I I I I I I I

Thyroxine

Growth of thyroid

■ Figure 11.16 The hypothalamus-pituitary-thyroid axis (control system). The secretion of thyroxine from the thyroid is stimulated by thyroid-stimulating hormone (TSH) from the anterior pituitary. The secretion of TSH is stimulated by thyrotropin-releasing hormone (TRH) secreted from the hypothalamus. This stimulation is balanced by negative feedback inhibition (blue arrow) from thyroxine, which decreases the responsiveness of the anterior pituitary to stimulation by TRH.

Negative feedback

Inhibits secretion of GnRH

Hypothalamus

Gonadotropin-releasing hormone (GnRH)

Anterior pituitary

Gonadotropins (FSH and LH)

Negative feedback

Gonadotropins (FSH and LH)

Gonads

Inhibits responsiveness to GnRH

hormones (estrogens and androgens)

■ Figure 11.17 The hypothalamus-pituitary-gonad axis (control system). The hypothalamus secretes GnRH, which stimulates the anterior pituitary to secrete the gonadotropins (FSH and LH). These, in turn, stimulate the gonads to secrete the sex steroids. The secretions of the hypothalamus and anterior pituitary are themselves regulated by negative feedback inhibition (blue arrows) from the sex steroids.

for example, appears to inhibit the response of the anterior pituitary to TRH and thus acts to reduce TSH secretion (fig. 11.16). Sex steroids, by contrast, reduce the secretion of gonadotropins by inhibiting both GnRH secretion and the ability of the anterior pituitary to respond to stimulation by GnRH (fig. 11.17).

Evidence suggests that there may be retrograde transport of blood from the anterior pituitary to the hypothalamus. This may permit a short feedback loop in which a particular trophic hormone inhibits the secretion of its releasing hormone from the hypothalamus. A high secretion of TSH, for example, may inhibit further secretion of TRH by this means.

In addition to negative feedback control of the anterior pituitary, there is one instance of a hormone from a target organ that actually stimulates the secretion of an anterior pituitary hormone. Toward the middle of the menstrual cycle, the rising secretion of estradiol from the ovaries stimulates the anterior pituitary to secrete a "surge" of LH, which results in ovulation. This is commonly described as a positive feedback effect to distinguish it from the more usual negative feedback inhibition of target gland hormones on anterior pituitary secretion. Interestingly, higher levels of estradiol at a later stage of the menstrual cycle exert the opposite effect—negative feedback inhibition— on LH secretion. The control of gonadotropin secretion is discussed in more detail in chapter 20.

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Essentials of Human Physiology

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Responses

  • antje
    Are thyrotropin and thyroxine an example of negative feedback?
    8 years ago
  • Mikolaj
    What does thyrotropinreleasing hormone trh target and response?
    8 years ago
  • myles
    Does tsh feedback on teh pituitary to regulate its own secretion?
    8 years ago
  • Iago Sackville-Baggins
    How does the anterior pituitary gland respond to negative feedback?
    7 years ago
  • SAVANNA
    Is the anterior pituitary gland positive feedback or negative feedback?
    6 years ago
  • Mackenzie
    Which Anterior Pituitary Hormones are positive feedback?
    2 years ago
  • almaz luwam
    Does tsh negatively feed back to tje pituitary?
    1 year ago
  • saimi
    Does pituitary glan have a postiove or negtive on trh?
    8 months ago
  • sara
    How does negative feedback effect the anterior pituitary gland?
    1 month ago
  • Olli-Pekka Rautavaara
    Which hormones from anterior pituitary use negative feedback?
    3 days ago

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