The adrenal medulla is a very special example of an gland innervated by the autonomic nervous system. You will hear more about the adrenal gland when you cover the endocrine system. It is a small endocrine gland that sits on top of each kidney and consists of a shell region (cortex) that secretes a large number of steroid hormones, surrounding a core region (medulla) that secretes adrenaline (epinephrine to our American friends) and noradrenaline (norepinephrine).
In many respects the cells of the adrenal medulla can be considered to be sympathetic postganglionic neurones in that they are innervated by sympathetic preganglionic neurones and secrete substances which act on adrenergic receptors.
The major difference between the adrenal medulla and sympathetic postganglionic neurones is that the chemicals secreted by the adrenal medulla are hormones that enter the blood stream and consequently can exert influence over cells throughout the body.
Sympathetic postganglionic neurones on the otherhand release noradrenaline into the narrow cleft of the neuroeffector junction and have a very specific and localised effect on a small number of cells.
The relationship between the sympathetic division of the autonomic nervous system and the adrenal medulla is summarised in the figure opposite.
The endocrine secretions of the adrenal medulla consist of approximately 80% adrenaline and 20% noradrenaline. The secretion of these hormones is enhanced by an increase in sympathetic tone so the local and highly specific effects of activation of the sympathetic division of the autonomic nervous system are reinforced further by the more widespread actions of the hormones.