Secretomotor

Secretomotor refers to the capacity of a structure (often a nerve) to induce a gland to secrete a substance (usually mucus or serous fluid). More specifically, Secretomotor refers to the excitable motor neurons of enteric nervous system in mucosal ganglia innervating the mucosa of the small and large intestines.

Physiology
Whether secretomotor is excitatory or inhibitory depends on the mediation of sympathetic nervous system's postganglionic neurons, enteric nervous system, and paracrine signalings from nonneural cell types in the mucosa and submucosa such as enterochromaffin cells and inflammatory cells.

Whenever secretomotor neurons get excited, they release vasoactive intestinal peptide (VIP) and acetylcholine as its neurotransmitters to communicate while the role of acetylcholine that binds to cholinergic secretomotor neurons is secondary to VIP during this procedure.

It is the release of acetylcholine triggering the release of nitric oxide from the endothelium that dilates the blood vessel and, thereafter, increases blood flow activating secretions of serous fluid made out of water, NaCl, bicarbonate, and mucus into gut lumen from the intestinal gland.

Norepinephrine released from sympathetic postganglionic neurons' terminals acts at alpha-2A adrenergic receptors which hyperpolarizes the membrane potential of secretomotor neurons, decreasing the firing and thus the release of acetylcholine liable for stimulating intestinal gland's secretion. This explains the reason for why whenever sympathetic nervous system is in excitatory state, gastrointestinal becomes less watery. This is because the sympathetic nervous system tends to divert blood from the splanchnic to the systemic circulation.

Neurotransmitters, endocrine, or exocrine that excites secretomotor receptor

 * Acetylcholine (acting at nicotinic receptors, and the muscarinic M1 subtype), ATP (acting at the P2Y1 receptor subtype), vasoactive intestinal peptide, substance P (acting at NK1 and/or NK3 receptor subtypes), serotonin (acting at 5-HT3 and perhaps 5-HT2 receptor subtypes), and prostaglandin.
 * Pro-inflammatory factors that trigger histamine release.

Neurotransmitters, endocrine, or exocrine that hyperpolarizes secretomotor receptor

 * Somatostatin, norepinephrine.

Pathophysiology
The inherent nature of secretomotor neurons making it directly connected with neurogenic secretory diarrhea and neurogenic constipation.

Generally speaking, excitatory secretomotor is liable for neurogenic secretory diarrhea; hyperpolarized secretomotor is liable for neurogenic constipation.

Antidiarrhoeal such as opiates, clonidine, and somatostatin analogs are associated with drier and thus harder stools.

Acetylcholine, vasoactive intestinal peptide (VIP), serotonin, and histamine are associated with more watery stools.

Norepinephrine release inhibition results in the removal of the sympathetic braking action on the neurons leading to overstimulation of secretomotor neurons.

Significant inhibitions of enzymatic degradation, serotonin's movement into the blood and binding to blood platelets, or serotonin reuptake transporter (SERT) leads to accumulation of 5-HT that pooled around secretomotor neurons.

5-HT3 antagonist's proven efficacy in the treatment of diarrhea in the women with irritable bowel syndrome suggests that overstimulation of secretomotor neurons by serotonin from the enterochromaffin could be a significant cause.