Physiological Relationships

A. Hypothalamus-Pituitary-Testicular Axis

1. General Comments

Puberty, or the integral series of anatomical, physiological, and endocrinological changes occurring to produce a male competent for sexual reproduction, occurs between 10 and 17 years of age in the human. The onset of puberty is believed to occur as a consequence of a change in the steady state of the prepubescent pituitary-gonadal system, resulting in a dramatic rise in the plasma testosterone level (see Figure 12-7). It has been hypothesized that there is a decrease in the feedback sensitivity of the central nervous system-pituitary axis over years 6-10, resulting in an increase in the secretion of GnRH, which in turn initiates the increased secretion of LH and FSH above the prepubescent low basal levels. Thus, LH secretion rises and reaches adult levels by age 15, while FSH secretion rises more slowly, achieving adult levels only by age 17. In addition, in both males and females during the interval of puberty, there are intermittent bursts of both FSH and LH secretion that occur during sleep; their etiology is not known.

2. Integrated functioning of the Hypothalamus-Pituitary-Testicular Axis

The hypothalamus-pituitary-testicular axis is diagrammed schematically in Figure 12-8. The production and secretion of LH is governed by the medial-basal

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