Does the inactivity of the esophageal body at rest indicate that there is no tonic activity of excitatory nerves to this region ?
J. Christensen (Iowa City)
Most of the esophageal body in man is made up of smooth muscle, as it is in the opossum. In the animal that is not swallowing, this part of the esophagus exhibits no spontaneous acivity. In this inactivity at rest, the esophagus is unique in the gastrointestinal tract. Nearly all other parts of the gut show constant contractions, although the frequency and magnitude of these contractions vary a good deal from time to time. The smooth muscle part of the esophageal body never seems to exhibit spontaneous resting contractions in normal people.
There could be several reasons for such resting inactivity: first, it could be that the excitatory motor nerves to this muscle are simply inactive, whereas such excitatory motor nerves are tonically active elsewhere in the gut. It could be alternatively that inhibitory motor nerves are tonically active and so, suppress any contractions that might be excited by tonic excitatory nerves or might be due to some underlying tendency of the muscle itself to show spontaneous contractions.
These possibilities have been investigated in isolated strips of esophageal muscle, removed from the animal (the opossum) and put into an organ bath. Such isolated muscle strips show no tendency at all to undergo spontaneous contractions. Like the esophageal muscle in situ, the esophageal muscle in vitro is completely quiet.
In vitro, the muscle can be treated in ways that are not possible in the whole animal. When the muscle is exposed in vitro to a general neurotoxin like tetrodotoxin, an agent that abolishes all nerve activity, the muscle exhibits no change in its activity. It remains completely flaccid. This shows that the muscle itself has no intrinsic tendency for spontaneous contractions. When the muscle is exposed to atropine, which will selectively oppose any possible tonic activity of the major excitatory nerves of the gut, those that act by releasing acetylcholine, the muscle remains inactive. Similarly, when the muscle is exposed to agents that oppose the actions of other neurohormonal transmitters, like catecholamines, no spontaneous
activity appears. This is the same with all antagonists to both excitatory and inhibitory neurohormonal transmitters.
The conclusion that must be reached is that there is no tonic activity, both in the excitatory and in the inhibitory nerves to this muscle, and that this muscle has no tendency to undergo spontaneous contractions in itself.
1. Christensen J (1987) The motor functions of the pharynx and esophagus, in: Physiology of the Gastrointestinal Tract, Johnson, Christensen, Jackson, Walsh, and Jacobson (eds.), Raven Press, New York Second Edition pp. 595-612.