Control by the Endocrine and Nervous Systems

Two types of control of skeletal muscle are evident in this scene. First are the quick, moment-by-moment moves of the game. To return the ball with a backhand shot, this player runs to a good position to execute his return, plants his feet, pulls back the racket, focuses his eyes on the ball, and swings—all in a few seconds. His nervous system controls the muscle contractions responsible for these quick moves. In addition, this player’s muscles are far different than they were before he entered puberty. His muscles have become more highly developed, larger, and stronger since puberty began—a process requiring years. His endocrine system has controlled these long-term muscle changes.

Castration revealed long ago that the testes—although they are just one part of the body—play a critical role throughout the body in male maturation. No one knows how the practice of castration began, because it started in prehistorical times, before written historical records were kept. Presumably, farmers discovered that removal of the testes rendered male farm animals more docile, and then the practice was extended to people such as harem servants. Boys castrated prior to puberty do not undergo normal adolescent muscular and skeletal development, and their voices do not deepen because their vocal cords do not increase in length. By 1700, several thousand prepubescent boys were castrated each year in Italy to create adult male singers with high voices. Some of these castrati, as they were called, are legendary for being among the greatest opera singers the world has ever known.

The effects of castration pointed to the involvement of the testes in male maturation for thousands of years, but not until 1849 did someone carry out scientific experiments to understand why the testes matter. The German physiologist Arnold Berthold knew from farmers that when male chickens are castrated prior to sexual maturation, they do not develop adult male characteristics. The fleshy crest (comb) on the top of a castrated male’s head remains immature, for example. After Berthold castrated young male chickens, he transplanted surgically isolated testes into their bodies at abnormal locations. These chickens then matured normally, complete with adult combs.

These experiments started the modern study of endocrinology. Berthold concluded that the testes secrete a substance into the blood that circulates with the blood to all parts of the body and controls the development of other tissues. Substances of this type were named hormones about 50 years later, and in 1935, the key substance secreted by the testes was chemically identified and named testosterone.