Direct and indirect activation

Intrinsic nervous system

Gastrointestinal motility is enhanced by stimulation of cholinergic receptors located in the smooth and long muscle layers. Stimulation can occur both directly (extrinsic stimulation of the cholingergic receptors) or indirectly (intrinsic stimulation of the myenteric plexus and then the cholinergic receptors). The myenteric plexus can be stimulated by neuronal factors (i.e. serotonin, dopamine), neurocrines (i.e.

substance P, motilin, cholecystokinin) and GI peptides (i.e. gastrin, VIP).

The effects of the various nervous systems are as follows: Extrinsic parasympathetic neurons (from vagus and pelvic nerves) are predominantly excitatory, but may also be inhibitory. Most vagal fibers project to the enteric nervous system neurons. Extrinsic sympathetic postganglionic nerve fibers will cause an inhibition of motility.

1

Which of the following is a compound for inhibitory neurons in ENS: