The Adrenal Medulla

Anatomy

The adrenal medulla is embedded in the centre of the adrenal cortex. It is quite small, making up only about 10 percent of the total adrenal weight.

Functions

The adrenal medulla helps a person in coping with physical and emotional stress. The adrenal medulla secretes the following hormones:

• epinephrine (also called adrenaline) - this hormone increases the heart rate and force of heart contractions, facilitates blood flow to the muscles and brain, raises blood glucose levels; increases blood pressure, heart rate, sweating, respiratory rate, causes relaxation of smooth muscles, helps with conversion of glycogen to glucose in the liver, and other activities.
• norepinephrine (also called noradrenaline) - this hormone has little effect on smooth muscle, metabolic processes, and cardiac output, but has strong vasoconstrictive effects, thus increasing blood pressure.

Indeed, the adrenal medulla forms an integral part of the sympathetic nervous system sympathetic nervous system, a major subdivision of the autonomic nervous system (see the autonomic nervous system), and the combined activities have been referred to as the sympathoadrenal system.

Included among the medullary hormones, the catecholamines, are dopamine, norepinephrine, and epinephrine, all of which are synthesized in the brain and sympathetic nerve endings. The adrenal medulla differs from most other endocrine glands in that the major stimulus for the release of the catecholamines is by stimulating sympathetic nerve endings to release acetylcholine (ACh), an important neurotransmitter of the peripheral nervous system (nerves and ganglia located outside the central nervous system, or the brain and spinal cord). When stimulated, the medullary cell ejects the chromaffin granules from the cytoplasm into the bloodstream, a process known as exocytosis. Thus, the adrenal medulla is a neurohemal organ.

Catecholamines

The catecholamines are synthesized from the amino acid l-tyrosine. Serial changes in chemical structure are catalyzed by enzymes, leading to the following synthetic sequence: l-tyrosine ® l-dopa (dihydroxyphenylalanine) ® dopamine ® l-norepinephrine (noradrenaline) ® l-epinephrine (adrenaline).

L-dopa is well known for its role in the treatment of parkinsonism, but its biological importance lies in the fact that it is a precursor of dopamine, a neurotransmitter widely distributed in the central nervous system, including the basal ganglia of the brain (groups of nuclei within the cerebral hemispheres that collectively control muscle tone, inhibit movement, and control tremor). It is a deficiency of dopamine in these ganglia that leads to parkinsonism, a deficiency that is at least partially repaired by the administration of l-dopa. Under ordinary circumstances, far more epinephrine than norepinephrine is released from the adrenal medulla; in the catecholamine neurotransmitting function throughout the body, norepinephrine is far more widespread. It is likely that the full complement of hormones secreted by the adrenal medulla is not yet completely known. There is strong evidence to indicate, for example, that enkephalins (neurotransmitters with opiate-like effects) are contained within chromaffin granules and are secreted into the general circulation.

In physiological terms, a major action of the hormones of the adrenal medulla conjoined with the sympathetic nervous system is to initiate a rapid, generalized bodily response described by Walter Cannon as “fight or flight.” This response may be triggered by a fall in blood pressure, pain (including burns), or abrupt emotional upheavals. An injection of epinephrine, in fact, closely mimics the symptoms of an anxiety attack (sweating, tremor, greatly increased heart rate). Metabolic changes also stimulate catecholamine secretions as evidenced by the rapid rise in plasma epinephrine levels when an individual becomes hypoglycemic (has a greatly decreased glucose level). Thus, much of what is called a hypoglycemic reaction is the result of a large epinephrine discharge.

Release of adrenaline and noradrenaline is triggered by nervous stimulation in response to physical or mental stress. The hormones bind to adrenergic receptors - transmembrane proteins in the plasma membrane of many cell types.

Some of the effects are:

• increase in the rate and strength of the heartbeat resulting in increased blood pressure
• blood shunted from the skin and viscera to the skeletal muscles, coronary arteries, liver, and brain
• rise in blood sugar
• increased metabolic rate
• bronchi dilate
• pupils dilate
• hair stands on end ("gooseflesh" in humans)
• clotting time of the blood is reduced
• increased ACTH secretion from the anterior lobe of the pituitary

The catecholamines increase hormonal secretion and affect the thyroid and parathyroid, the gonads ( ovary and testis), and the placenta.

Adrenomedullary dysfunction

Isolated loss of the medulla of both adrenals does not occur; such destruction is always accompanied by impairment of the function of the cortex of both adrenals. Any effects that can be attributed to the loss of the medulla are overshadowed by the predominating signs of Addison's disease.

Tumors of the adrenomedullary chromaffin cells, called pheochromocytomas, do occur and may produce striking, largely predictable signs and symptoms that are exaggerations of the physiological actions of the catecholamines. Pheochromocytomas are tumors of the chromaffin cell, usually benign but occasionally malignant.

High blood pressure is an invariable finding in adrenomedullary hyperfunction. It may be constant, and it may be difficult to distinguish from the common forms of hypertension. In some instances, however, there is a sudden increase in norepinephrine secretion, provoking the sudden explosive onset of a severe headache, excessive sweating, palpitation of the heart, ashen pallor, tremor, and anxiety. These attacks may end abruptly and the patient may appear to be normal following the attack. They may last from minutes to hours and may occur at intervals ranging from, for example, once a month to several per day. In persons in whom tumors secrete an appreciable amount of epinephrine, anxiety may be more marked and the patient may lose weight, be feverish, and show evidence of diabetes mellitus.

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