Biology Stack Exchange is a question and answer site for biology researchers, academics, and students. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I learned that the adrenal medulla releases epinephrine, which is a vasodilator of the vessels that lead to the skeletal muscle.

I also learned that somatic nervous system only uses Achetycholine as its hormone (not epinephrine).

I also learned that the somatic nervous system controls skeletal muscle.

So, why is epinephrine not considered a part of the somatic nervous system hormones, if it is controlling the skeletal muscle? What is wrong here?

share|improve this question
up vote 2 down vote accepted

You are confusing different functional systems. Epinephrine released by the adrenal medulla circulates in the blood and indeed dilates the blood vessels in skeletal muscle. This ensures that enough oxygen and nutrients are available for the muscles to perform in a fight/flight response.

The somatic nervous system utilises acetylcholine not as a hormone but as a neurotransmitter. It is not released into the blood but only into the tiny (about 10nm wide) gap (synapse) between the nerve cell's axon terminal and the next nerve cell or muscle cell respectively.

This release by the somatic nerve cells causes an electrical potential in the muscle which makes it contract.

In summary:

The somatic nervous system controls the muscles because it stimulates the contraction which makes e.g. your arm move. Epinephrine released into the blood by the adrenal medulla serves to ensure the muscles have the blood supply they need in a stress response (e.g. a dangerous situation).

Along with muscular blood vessel dilation, epinephrine in the blood for example also stimulates the heart to beat faster and stronger, and dampens down mechanisms involved in digestion - essentially it does everything necessary to allow a quick escape or a strong fight.

share|improve this answer
Ah, yes, thank you. Confusion between hormones and neurotransmitters. Thanks! – CodeGuy Jun 3 '13 at 23:21

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.