Anatomy & Physiology

Anatomy & Physiology Study Guide poorly mineralized, weak, and flexible. For example, in rickets, a condition typically caused by inadequate calcitriol production as a result of vitamin D deficiency in growing children, the lower limb bones are so weak that they bend under the body’s weight. Reproductive Hormones The activity of osteoblasts in key locations and the growth of specific cell populations are affected by the presence or absence of reproductive hormones (androgens in males, estrogens in females). These sex hormones stimulate cell growth and differentiation in their target tissues. The targets differ for androgens and estrogens, and the differential growth induced by each account for gender-related differences in skeletal proportions and secondary sex characteristics. 17.11 The Alarm Phase During the alarm phase, an immediate response to stress occurs. This response is orchestrated by the sympathetic division of the autonomic nervous system. In the alarm phase, energy reserves are mobilized, (mainly in the form of glucose) and the body prepares to deal with the stress-causing factor by “fight or flight” responses. Epinephrine is the primary hormone of the alarm phase, and its secretion accompanies a generalized sympathetic activation. The characteristics of the alarm phase include increased mental alertness, increased energy consumption by skeletal muscles and many other tissues, the mobilization of energy reserves (glycogen and lipids), changes in circulation, a drastic reduction in digestion and urine production, increased sweat gland secretion, and increases in blood pressure, heart rate, and respiratory rate. 17.12 The Effects of Hormones on Behavior Many endocrine functions are regulated by the hypothalamus, and hypothalamic neurons monitor the levels of many circulating hormones. Other portions of the CNS are also quite sensitive to hormonal stimulation. The clearest demonstrations of the behavioral effects of specific hormones involve individuals whose endocrine glands are oversecreting or undersecreting. Even normal changes in circulating hormone levels can cause behavioral changes. In precocious (premature) puberty, sex hormones are produced at an inappropriate time, perhaps as early as age 5 or 6. An affected child not only begins to develop adult secondary sex characteristics but also undergoes significant behavioral changes. The “nice little kid” disappears, and the child becomes aggressive and assertive due to the effects of sex hormones on CNS function. In adults, changes in the combination of hormones reaching the CNS can have significant effects on intellectual capabilities, memory, learning, and emotional states. 17.13 Aging and Hormone Production The endocrine system undergoes relatively few functional changes with age. The most dramatic exception is the decline in the concentrations of reproductive hormones. Blood and tissue Achieve Page 216 of 368 ©2018