Monday, 27 April 2009

Which hormones does the adrenal medulla secrete?

The adrenal gland is anatomically divided into the cortex and medullary regions, and for once this division is actually important.  So different are the two areas that they might as well be separate organs.  

The adrenal medulla secretes only catecholamines - adrenaline (epinephrine*), noradrenaline (norepinephrine) and dopamine.  Of these three hormones, adrenaline makes up the lion's share of the catecholamine output here - about 80%, to be specific.

Unusually for an endocrine organ, the secretion of these hormones is regulated by direct nervous innervation to the gland.  Preganglionic sympathetic nerves secrete acetylcholine, and this causes the adrenal medulla to secrete its catecholamine load.

In many ways, it is best to conceive of the adrenal medulla a giant sympathetic ganglion, capable of rapidly boosting the sympathetic nervous system's response to a "fight or flight" situation.  As you know, one of the autonomic nervous system's characteristics is that it has two effector neurones instead of one.  The preganglionic neurone, originating in the brain stem or spinal cord, synapses with the postganglionic neurone at a ganglion.  The former secretes acetylcholine, and, in the case of the sympathetic nervous system, the latter usually secretes noradrenaline (norepinephrine).  This is a local response: the amount of adrenaline secreted by the postganglionic neurone is so small that it only affects those few cells in the immediate vicinity. 

In the case of the adrenal medulla, the mechanism is the same ('preganglionic' acetylcholine secretion, catecholamine response), but instead of a postganglionic neurone, the very cells of the medulla produce catecholamines directly.  This is why I say that the it can be thought of as a sympathetic ganglion that produces its own catecholamines directly, without needing a second neurone. 

The reason for this modification of the usual sympathetic nervous systemic pattern is so that the medulla can quickly and directly generate huge amounts of catecholamine for the systemic circulation (another neurone in the pathway would only slow things down).  As a consequence of all this, the whole body can rapidly respond to a sympathetic signal without each cell having to be stimulated individually.

*See here for the epinephrine vs adrenaline story.

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