ANS Pharmacology Problems/Answers

The following few exercises should help you to understand the effects of neurotransmitters and of the peripheral sympathetic and parasympathetic nerves on various effector organs.

1. Effect of ACh on intestinal muscle:

ACh is released from parasympathetic nerves onto the muscle cells. Thinking of the "restive state" example from lecture, the intestine would be active or excited. Exogenous ACh would act on muscarinic receptors (most peripheral!). Stimulation of the parasympathetic ganglia by ACh would give the same response.

2. Sympathetic nerve action on the heart:

NE released would act on beta 1 receptors (found only on the heart). Thinking of the "Fight or Flight" example, NE would increase heart rate and the strength of contraction.

3. NE actions on blood flow to intestine:

During "Fight or Flight" (remember, NE from sympathetic nerves which give us this state), we know that blood flow to this organ would be reduced as the circular muscle of the blood vessel contracts. Thus, NE must excite these muscle cells, hence most of the adrenergic receptors in these vessels must be alpha 1.

4. ACh on Blood flow:

A very tricky question. The only place one finds muscarinic receptors on blood vessels is in some of them supplying skeletal muscles. In this case, ACh is released from sympathetic nerves. Following our "Fight or Flight" scenario, we can predict that ACh will dilate these vessels. However, if we give an i.v. dose of ACh, heart rate will go down (parasympathetic action) and this may decrease blood flow in the total system.

5. Some points to remember:

- ACh is released from sympathetic nerves to sweat glands and some BVs to skeletal muscle. It will stimulate the sweat glands and dilate (relax or inhibit) the BVs

- The diameter of pupil of the eye is regulated by two opposing sets of muscles. Activation of the sphincter muscle of the iris by ACh (muscarinic) constricts the pupil while stimulation of the radial muscle by NE (alpha 1) opens it.

- Likewise the pacemaker of the heart (the SA node) is dually innervated. ACh (parasympathetic) and NE or Epi (sympathetic) have predictable actions.

- The adrenal medulla releases Epi and some NE into the blood during sympathetic stimulation. The gland is really a modified post-ganglionic sympathetic nerve, excited by the release of ACh from the pre-ganglionic nerve onto nicotinic receptors. This results in the excitation of the gland (i.e., post-ganglionic "nerve") and the release of Epi and NE from this adrenergic structure.