Higher Brain Function and Pituitary Secretion

The relationship between the anterior pituitary and a particular target gland is described as an axis; the pituitary-gonad axis, for example, refers to the action of gonadotropic hormones on the testes and ovaries. This axis is stimulated by GnRH from the hypothalamus, as previously described. Since the hypothalamus receives neural input from "higher brain centers," however, it is not surprising that the pituitary-gonad axis can be affected by emotions. Indeed, the ability of intense emotions to alter the timing of ovulation or menstruation is well known. Psychological stress, as another example, also stimulates another axis—the pituitary-adrenal axis (described in the next section).

Stressors, as described later in this chapter, produce an increase in CRH secretion from the hypothalamus, which in turn results in elevated ACTH and corticosteroid secretion. In addition, the influence of higher brain centers produces circadian ("about a day") rhythms in the secretion of many anterior pituitary hormones. The secretion of growth hormone, for example, is highest during sleep and decreases during wakefulness, although its secretion is also stimulated by the absorption of particular amino acids following a meal.

Fox: Human Physiology, I 11. Endocrine Glands: I Text I I © The McGraw-Hill

Eighth Edition Secretion and Action of Companies, 2003


Endocrine Glands 305

The influence of higher brain centers on the pituitary-gonad axis helps to explain the "dormitory effect"— that is, the tendency for the menstrual cycles of female roommates to synchronize. This synchronization will not occur in a new roommate if her nasal cavity is plugged with cotton, suggesting that the dormitory effect is due to the action of chemicals called pheromones. These chemicals are excreted to the outside of the body and act through the olfactory sense to modify the physiology or behavior of another member of the same species. Pheromones are important regulatory molecules in the urine, vaginal fluid, and other secretions of most mammals, and help to regulate their reproductive cycles and behavior. The role of pheromones in humans is difficult to assess. Recently, however, scientists discovered that pheromones produced in the axillae (underarms) of women may contribute to the dormitory effect.

Test Yourself Before You Continue

1. Describe the embryonic origins of the adenohypophysis and neurohypophysis, and list the parts of each. Which of these parts is also called the anterior pituitary? Which is called the posterior pituitary?

2. List the hormones released by the posterior pituitary. Where do these hormones originate and how are their secretions regulated?

3. List the hormones secreted by the anterior pituitary and explain how the hypothalamus controls the secretion of each.

4. Draw a negative feedback loop showing the control of ACTH secretion. Explain how this system would be affected by (a) an injection of ACTH, (b) surgical removal of the pituitary, (c) an injection of corticosteroids, and (d) surgical removal of the adrenal glands.

. _Adrenal medulla

glomerulosa —I

Zona fasciculata—

Zona reticularis-

glomerulosa —I

- Connective tissue capsule

. Adrenal cortex

. Adrenal medulla

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