The Adrenal Medulla

The adrenal medulla was discussed as part of the sympathetic nervous system in chapter 15. It arises from the neural crest and is not fully formed until the age of three. It is actually a sympathetic ganglion consisting of modified neu-

Figure 17.9 The Parathyroid Glands.

rons, called chromaffin cells, that lack dendrites and axons. These cells are richly innervated by sympathetic pregan-glionic fibers and respond to stimulation by secreting cate-cholamines, especially epinephrine and norepinephrine. About three-quarters of the output is epinephrine.

These hormones supplement the effects of the sympathetic nervous system, but their effects last much longer (about 30 min.) because the hormones circulate in the blood. They prepare the body for physical activity in several ways. They raise the blood pressure and heart rate, increase circulation to the skeletal muscles, increase pulmonary airflow, and inhibit such temporarily inessential functions as digestion and urine formation. They stimulate glycogenolysis (hydrolysis of glycogen to glucose) and gluconeogenesis (the synthesis of glucose from amino acids and other substrates), thus raising the blood glucose level. In order to further ensure an adequate supply of glucose to the brain, epinephrine inhibits insulin secretion and thus, the uptake and use of glucose by the muscles and other insulin-dependent organs. Thus, epinephrine has a glucose-sparing effect, sparing it from needless consumption by organs that can use alternative fuels to ensure that the nervous system has an adequate supply.

The medulla and cortex are not as functionally independent as once thought. The boundary between them is indistinct and some cells of the medulla extend into the cortex. When stress activates the sympathetic nervous system, these medullary cells secrete catecholamines that stimulate the cortex to secrete corticosterone.

Saladin: Anatomy & 17. The Endocrine System Text © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 17 The Endocrine System 649

Chapter 17 The Endocrine System 649

Adrenal Histology
Figure 17.10 The Adrenal Gland. (a) Gross anatomy; (b) histology.

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