Pharmacology of the esophageal motor funciton

In the striated muscle of the upper esophageal sphincter, tonic maintenance of closure is probably mediated via tonic central excitation of the extrinsic motor innervation; relaxation represents central inhibition of this mechanism. The motor nerves are probably cholinergic and act through nicotinic receptors like those of somatic striated muscle. In the striated muscle of the esophageal body, swallowing-induced contraction is also probably a cholinergic and nicotinic response. In the smooth muscle of the esophageal body, the control of contractions is cholinergic and muscarinic in part, but there is evidence for a nonadrenergic and noncholinergic component as well. The muscarinic component may arise from the cholinergic innervation of the longitudinal muscle layer. The other component may lie in the cryptic innervation of the circular muscular layer. In the smooth-muscled lower esophageal sphincter, resting closure tension appears to reflect a variety of possible control mechanisms. No single control system predominates. The evidence for muscarinic excitation is equivocal. An excitatory adrenergic alpha mechanism and inhibitory adrenergic beta receptors may contribute. A role for the polypeptide hormones from the gastrointestinal tract seems unlikely. Relaxation of the lower sphincter with swallowing seems not to involve any of these mechanisms, but is apparently accomplished by nonadrenergic noncholinergic inhibitory nerves like those present elsewhere in the gut (87). The possibility that the transmitter of these nerves is an adenine nucleotide has been raised from studies of other parts of the gut, but that hypothesis has not yet been examined critically in the lower esophageal sphincter.