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a ready fuel during stress. Other physiological changes attributable to the adrenal catecholamines include changes in blood pressure and heart function, which also occur through adrenergic receptors.

The overall system is not essential to life in the same sense as the requirement for the adrenal cortex. The adrenal medulla can be removed (sympathectomy), and, clinically, this is done sometimes without endangering survival in cases of oversecretion of catecholamines. However, when the adrenal cortex is removed, life can be sustained only in a totally nonstressful environment, with salt provided to the experimental animal unless glucocorticoid therapy is given. It is clear that when the adrenal is not secreting Cortisol (in the human), the hormone must be supplied orally for continued survival.

II. ANATOMICAL, MORPHOLOGICAL, AND PHYSIOLOGICAL RELATIONSHIPS

A. Introduction

The adrenal glands are embedded and enclosed by coverings located (Figure 10-1) in fat above the kidneys. The blood circulation of the adrenal is described in Figure 10-2. The important detail to remember is that blood drains from the adrenal cortex, carrying its secretions (e.g., Cortisol, aldosterone) into the medulla, and then joins larger vessels that ultimately merge with the bloodstream. Thus, the intimacy of the cortical steroids with the medulla is apparent. Since the glucocorticoids induce the enzyme converting norepinephrine to epinephrine in the chromaffin cell, and since glucocorticoids as well as epinephrine are increased by stress, the capacity for the response of the specific secretion of the adrenal medulla is guaranteed by the events occurring in the cortex.

B. Development of the Adrenal Medulla

The adrenal medulla arises from the neural crest and is ectodermal in origin. As described in Chapter 10, the development of the primary cortex is signaled by the appearance of a suprarenal groove above the dorsal mesentary on each side (Figure 10-3). The cells lining the groove proliferate into the primary cortex. Neural crest cells, which give rise to sympathetic neuroblasts, move down toward the cortex and invade it to form the precursor of the medulla. The cells of the medulla, postnatally, display a segmental arrangement, which may relate to the structures of the nerve fibers from the sixth thoracic to the first lumbar segments, probably representing the sources of the migrat ing nerve cells forming the medulla. Ultimately, the medulla becomes invested with cells that secrete norepinephrine, in addition to those secreting epinephrine. The persistence of the former suggests their relationship with the postganglionic sympathetic neurons.

C. Autonomic Nervous System and the Adrenal Medulla

It is becoming clear that the same structures are involved, at the central nervous system level, for triggering the adrenal cortex and the medulla. Thus, the limbic system is a candidate for early stimulation followed by the hypothalamus. It has been shown experimentally that electrical stimulation of the dorsomedial nuclei and posterior hypothalamic areas leads to increased secretion of epinephrine and norepinephrine from the adrenal medulla. One possibility is that separate hypothalamic centers exist that control the secretion of norepinephrine and epinephrine, since there are situations when one type of medullary catecholamine is secreted predominantly. Signals are transmitted from the hypothalamus to the sympathetic nervous system, specifically to a neuron located in the lateral horn gray matter of the thoracolumbar spinal cord (Figure 11-1). This neuron sends out a long axon that travels in splanchnic nerves, passing through the celiac ganglion and continuing to the adrenal medulla. This first neuron releases acetylcholine from its nerve ending. The acetylcholine-containing secretor vesicles are about 40-65 nm in diameter. The acetylcholine released binds to a receptor in the synaptic junction to the cell body of a second neuron in this chain, which contains the acetylcholine receptor. In this case, the second neuron of the sympathetic autonomic nervous system is the specialized chromaffin cell.

The chromaffin cell itself may be considered to be a modified postganglionic sympathetic neuron. It is analogous to the second neuron of the sympathetic nervous system and releases catecholamines. Whereas the catecholamines of the usual postganglionic second neuron synapse with an effector organ or tissue, chromaffin cells release catecholamines directly into the blood circulation. The nature of the precise signals at the beginning of the overall network is unclear. It would not be surprising to learn that the limbic system stimulates the hypothalamus electrically by alteration in the firing rate of the electrical signal. Presumably, further transmission from the hypothalamus to the sympathetic system is neuronal.

D. Histology of the Adrenal Medulla

The cells of the medulla are polyhedral, epithelioid, and arranged in connecting cords. Many capillaries and venules are present (Figure 11-1). As previously

(EPINEPHRINE, ENKEPHALINS.

NOREPINEPHRINE]

FIGURE 11-1 Autonomic nervous system and the innervation of chromaffin cells of the adrenal medulla. (A) Connections between the central nervous system and the adrenal medulla. Signals emanate from the hypothalamus down the spinal cord by way of descending autonomic pathways to the appropriate level in the spinal cord (segment 7 of the thoracic spinal cord). The signal is transported further from segment 7 over the long preganglionic axon, which passes through the celiac ganglion and innervates the adrenal medulla by releasing acetylcholine at the level of the chromaffin cell. (B) Details of the pathway from the spinal column described in A. Redrawn from notes compiled by Dr. Laurie Paavola, Department of Anatomy, Temple University School of Medicine, with permission.

(EPINEPHRINE, ENKEPHALINS.

NOREPINEPHRINE]

FIGURE 11-1 Autonomic nervous system and the innervation of chromaffin cells of the adrenal medulla. (A) Connections between the central nervous system and the adrenal medulla. Signals emanate from the hypothalamus down the spinal cord by way of descending autonomic pathways to the appropriate level in the spinal cord (segment 7 of the thoracic spinal cord). The signal is transported further from segment 7 over the long preganglionic axon, which passes through the celiac ganglion and innervates the adrenal medulla by releasing acetylcholine at the level of the chromaffin cell. (B) Details of the pathway from the spinal column described in A. Redrawn from notes compiled by Dr. Laurie Paavola, Department of Anatomy, Temple University School of Medicine, with permission.

described, the neuron of the autonomic sympathetic nervous system delivers acetylcholine to the chromaffin cell and makes terminal contact with it. In most mammals, the epinephrine-containing chromaffin cell is more abundant than the norepinephrine chromaffin cell.

Chromaffin cells are of two types and they share many common features. The cell is polyhedral, with a large round nucleus containing one or more nucleoli. A large number of catecholamine-containing granules 1000-3000 A in diameter are obvious in the cytoplasm. Chromaffin cells are also found in the kidney, ovary, testis, liver, heart, and gastrointestinal tract. Similar cell types are found in the carotid and aortic bodies, which function as chemoreceptors (see Chapter 4).

A target tissue of interest with respect to epinephrine is the hepatocyte. A discussion of the biology of this cell and the liver was presented in Chapter 10. Important target cells for the action of epinephrine are pericytes and vascular smooth muscle cells. The membranes of these cells contain a-receptors that, when occupied by ligand, lead to movements of ions in and out of the cell and activation of contractile ele ments of the cell. The pericyte is found outside the endothelial lining of arterioles, capillaries, and sinuses. Although the function of these cells is not clearly understood, they have been suggested to regulate the size of the vessel lumen in that they possess contractile elements. There are tight junctions between pericytes and endothelial cells suggestive of ion transfer. Smooth muscle cells are elongated with a contractile apparatus probably similar to the myosin system. Actin is undoubtedly present in these cells, but the scarcity of thick filaments characteristic of myosin makes it difficult to explain contractile behavior on a basis similar to that occurring in striated muscle.

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