13.5 Male reproductive physiology

The organs and structures described on the previous page need to work together under hormonal signaling to release semen with mature and motile sperm out of the penis. As described earlier, in both males and females hormonal signaling begins in the brain.

When LH is released by the pituitary gland, it travels through the body in the bloodstream, and targets the testes. LH stimulates the testes to produce testosterone and several other hormones. Testosterone then circulates through the bloodstream and has effects all over the body and the brain (See table 1 for some effects of testosterone).

Table 2: Some of the effects circulating testosterone has on the body

Location Effect
Face and body Hair growth
Vocal chords Thickening (resulting in lower voice)
Brain Increases libido (sex drive), can increase aggression and competitive drive
Seminiferous tubules Sperm production
Testes and penis Triggers maturation during puberty
Muscles Growth

The production of testosterone by the testes is regulated by a negative feedback loop (Figure 4). In short, negative feedback is when the effect of an action decreases the subsequent action. In this case the testosterone signals the hypothalamus to decrease the amount of GRH produced, therefore reducing the secretion of LH and FSH, and, in turn, decreasing testosterone production. If testosterone levels are low, the hypothalamus releases more GRH which signals the pituitary to secrete LH and FSH, again stimulating testosterone production from the testes. In this way the level of testosterone in the male’s body is kept within a relatively narrow range.

 

Figure 4. Negative feedback: When testosterone (T) levels are low, Gonadotropin Releasing Hormone (GRH) triggers the secretion of Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH) from the pituitary. LH stimulates the testes to produce T. High T inhibits GRH production by the hypothalamus, reducing LH and FSH signaling from the pituitary.

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The Evolution and Biology of Sex by Sehoya Cotner and Deena Wassenberg is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.