The adrenal medulla develops from the neuroectoderm of the embryo.
Medulla consists of chromaffin cells or phaeochromocytes. These cells are connected with the preganglionic motor fibres of the sympathetic nervous system.
Hormones : The medulla of the adrenal glands secretes two hormones: norepinephrine (noradrenaline) and epinephrine
(adrenaline). Norepinephrine and epinephrine are derived from the aminoacid tyrosine.
(i) Norepinephrine (Noradrenaline). It regulates the blood pressure under normal condition. It causes constriction of essentially all the blood vessels of the body. It causes increased activity of the heart, inhibition of gastrointestinal tract, dilation of the pupils of the eyes and so furth.
(ii) Epinephrine (Adrenaline). It is secreted at the time of emergency. Hence it is also called emergency hormone. It causes almost the same effects as those caused by norepinephrine, but the effects differ in the following respects.
- First, epinephrine has a greater effect on cardiac activity than norepinephrine.
- Second, epinephrine causes only weak constriction of the blood vessels of the muscles in comparison with a much stronger constriction that results from norepinephrine.
- A third difference between the action of epinephrine and norepinephrine relates to their effects on tissue metabolism.
- Epinephrine probably has several times as great a metabolic effect as norepinephrine.
Target Cells. Both adrenaline and noradrenaline acts on the cells of skeletal, cardiac and smooth muscles and blood vessels and fat cells.
Because of the role of their hormones, the adrenal glands are also called 'glands of emergency'.