Adrenal glands are paired organs located above the kidneys. Each adrenal consists of two separate glands, an outer adrenal cortex and an inner adrenal medulla. Though the two glands have different embryological origin, structure, and hormonal secretions, at least with respect to responses to stress, their functions are synergistic and aimed at a common goal
CHROMAFFIN CELLS AND CATECHOLAMINES.
The adrenal medulla is essentially part of the sympathetic division of the autonomic nervous system, being in fact a modified sympathetic ganglion. The secretory cells of the adrenal medulla, called the chromaffin cells, are equivalent to postganglionic sympathetic neurons that have lost their axons. The chromaffin cells contain vesicles filled with epinephrine and norepinephrine. These biogenic amines, collectively called catecholamines, are produced from the amino acid pheynlalanine via several enzymatic chemical reactions in the chromaffin cells as follows: Phenylananine–p-tyrosine–Dopa–Dopamine–Norepinephrine–Epinephrine. Recently, the opiate peptide endorphin has also been found in the chromaffin cells. This analgesic (anti-pain) peptide is co-secreted with the catecholamines.
Upon stimulation of the adrenal medulla by the sympathetic nervous system, the vesicles release their content of epinephrine and norepinephrine into the blood to act as the hormones of the adrenal medulla. There are probably two cell types in the medulla, one secreting epinephrine and the other, norephinephrine. In humans the secretion of epinephrine comprises 80% of the total catacholamine output, the remainder being that of norepinephrine.
Each time the sympathetic nervous system is strongly stimulated, the activity of the adrenal medulla, increases. Thus, during fear and excitement or stressful muscular exercise (running, physical exertion, and struggle), stimuli from various parts of the nervous system impinge on the hypothalamus, which, among other things, acts as the highest center for the regulation of sympathetic responses.
Excitatory fibers from hypothalamic neurons descend in the spinal cord, stimulating the preganglionic sympathetic neurons. The preganglionic fibers enters into the two chains of sympathetic ganglia, one on each side of the vertebral column, releasing acetylcholine and synaptically stimulating the postganglionic neurons. The latter send their fibers to the visceral organs and skin. Norepinephrine is the neurotransmitter at these nerve endings. The adrenal medulla receives a long preganglionic sympathetic fiber (via a splanchic nerve).
CATECHOLAMONE ALPHA AND BETA RECEPTORS
Catecholamine hormones reach their target organs and bind with specific adrenegic receptors present on the cell membranes of their target organs. The adrenegic receptors are divided into the alpha and beta types. Norepinephrine binds mostly with the alpha receptors; epinephrine can bind with both types. The particular responses of the target organs depend on the kind and number of receptors present in the cells of the organ. Also, because sympathetic nerve fibers release only norepinephrine they tend mainly to activate the alpha receptors. The adrenal medullary secretion, being a mixture of both catecholamines, tends to activate both types of responses.
CATECHOLAMINE IN FIGHT -FLIGHT RESPONSES
The functions of the adrenal medulla and the effects of its hormones are best understood in terms of the preparation of the body for the unexpected stressful situations such as fight or flight, or exercise. In all these responses, the intense muscular activity demands increased blood flow, nutrients, and oxygen supply.
Consider a person who is running fast. The need for increased oxygen and fuel for muscles demands increased delivery of blood by the heart. Thus, cardiac output (heart rate and cardiac contractibility) must be increased. At the same time, the blood vessels to the heart and muscles must be dilated while those to the skin and visceral organs must be constricted, shunting the blood to where it is most needed (muscles and heart). The respiratory activity must be increased and the bronchioles dilated to supply more oxygen and remove more carbon dioxide. All these responses are brought about by various effects of epinephrine and norepinephrine acting on the target organs.
Epinephrine acts mainly on the heart, causing increased rate and contractility; norepinephrine acts on the visceral blood vessels (arterioles) to cause vasoconstriction, increasing blood pressure and shunting blood to muscles. This differential response occurs because the heart contains mainly beta receptors which bind preferentially with epinephrine, and visceral arterioles have the alpha type, which bind with norepinephrine. The smooth muscles of bronchioles and those of arterioles of the heart and muscle contain beta receptors. These receptors, when activated by epinephrine, relax the smooth muscles, causing vasodilation and bronchiolar dilation.
Metabolically, the body demands increased nutrient supply. Epinephrine increases glycogen breakdown in the liver and fat in the adipose tissue to mobilize ample fuel substances (glucose and fatty acids). Lastly, catecholamines act on the brain to increase arousal, alertness, and excitability. They also act on the iris of the eye to dilate the pupil, thus permitting more light into the eyes and enhancing peripheral vision.
In short, the functions of the adrenal medulla should be construed as complementary and synergistic with the functions of the sympathetic nervous system.
Daryl Conant, M.Ed