DIGESTWE DISEASES

Third International Symposium

on Gastrointestinal Motility

Stockholm, September 1971

Summary Comments on Session on Electrophysiology

The electrical activity recorded from the stomach and small bowel consists of two easily distinguishable components. The intial slow component is triggered at high (50 to 60 mV inside negative) levels of membrane potential, and it does not normally give rise to contrac- tions. This component corresponds with the slowest wave of the electromyogram, and is known variously as the slow wave, the paceset- ter potential, the basic electrical rhytkm, and control activity of the stomach and intestine. The electrical excitability of slow waves, and the basis of their coordination, was the theme of the first papers delivered during this session.

Contractions of the stomach and small bowel are associated with an additional component of electrical activity which often consists of spikes. These are initiated at levels of membrane po- tential varying from 30 to 40 mV (inside nega- tive). Their amplitude may be much larger than that of the slow waves recorded in the absence of spikes (up to 60 mV compared to up to 35 mV). In extracellular records, this activity is

associated with rapid negative-going spikes. Slow waves are generated in the longitudinal

layer of the stomach and small intestine and have not been observed in isolated circular muscle. Spikes originate from both layers; those from circular muscle are often larger in the electromyogram. It is well-established that the intrinsic frequency (that in isolated segments) for generating slow waves decreases distally either in the stomach or in the small bowel. In the stomach, the intrinsic frequency gradient decreases from 6 to 1 cycle/sec and in the small bowel, from 18 to 13 cycles/sec. These obser- vations about intrinsic frequency (observed in isolated segments) demand an explanation as to why: a) the stomach has but one frequency of activity in vivo; b) the frequency of duodenal slow waves is constant (and higher than the in- trinsic frequency of any isolated segment) and e) the frequency of lower parts of the small bowel diminishes irregularly from the lower .je- junum to the terminal ileum.

A number of attempts have been made to find

Digestive Diseases, Vol. 17, No. 4 (April 1972) 287

COMPARATIVE GASTROENTEROLOGY

an electric model which would mimic this be- havior. Sarna, Daniel and Kingma (Edmonton) described a model consisting of mutually cou- pled relaxation oscillators. Relaxation oscilla- tors, as opposed to sinusoidal oscillators, were essential for this and previous models, since sine wave oscillators interact with each other by addition of wave forms yielding continuous fluctuation in membrane potential, or "beat - ing," unless their frequencies are harmonics of one another. The oscillation frequency of indi- vidual sine-wave oscillators is not changed on coupling. Relaxation oscillators, on the other hand, can interact in a way that enables fre- quency entra inment (all oscillators beating at the same rate) or frequency pull ing (the higher- frequency oscillator increasing the frequency of the lower-frequency oscillators connected to it). The cardiac action potential behaves as a re- laxation oscillator in the sense that it has a finite threshold for firing and a refractory period.

Evidence was presented in this session that slow waves of the stomach and small bowel can be entrained bv electrical stimulations. Bortoff and Specht were able to drive the slow waves generated in longitudinal strips (containing circular muscle) from rabbit small bowel at frequencies up to twice that of the control. The entrained oscillations were initiated by cathodal currents, as in the stomach, where Kelly and LaForce were able to entrain the slow waves of conscious dogs by electrical st imulations with electrodes encircling the stomach with pulses of 0.8 second duration. In addition to their clinical i m p o r t a n c e , these f ind ings p r o v i d e f u r t h e r support for the kind of model outlined above.

The pa thway for current flow when the os- cillators are being driven electrically or by each other remains controversial. Daniel, Duchon and Henderson carried out electron microscopic studies on the longitudinal and circular layers of the dog intestine, and found many gapjunc- tior~s--the probable morphologic correlate of a low-resistance pa thway between neighboring cells in the circular layer. But these authors

found no such junctions in the longitudinal layer where slow waves are generated and pre- sumably couple with each other!

Electrical activity similar to that in animals (slow waves and faster spike activity) was dem- onstrated in the human stomach bv Monges and Salducci, and from the gastroduodenal area by Duthie, Brown, Ng, Kwong, Whi t taker and Frank. Both groups used suction electrodes on the mucosa. Duthie et al found that gastric electrical activity does spread, with appreciable phase lag, into the duodenum. This suggests that coupling between the activities of these two areas may be of physiologic and pathologic importance. As in the dog, Duthie et al found frequency entrainnqent of slow waves, in that a frequency plateau was present; but in the hu- man duodenum, there were time-dependent variations in phase relations between oscilla- tions at various electrodes.

There were two papers dealing with the in- teraction of longitudinal and circular layers of the small intestine. Bortoff questioned the need to postulate reciprocal interaction of the two layers, contending that the circular and longi- tudinal layers always contracted simultaneously in his preparat ions. On the other hand, Gonella presented evidence suggesting that contraction of the circular muscle causes inhibition of the spikes and contraction associated with the sub- sequent slow wave of the longitudinal layer, apparent ly by a long-lasting inhibitory reflex.

The session concluded with an account bv Drewes of a new approach to the problems in- volved in at tempting to correlate pressure changes and the E M G with the t ransport of intestinal contents.

E. E. DANIEL, PHD Department of Pharmacology The ~:%i~ersity of Alberta Edmonton, Canada

MOLLm E. HOLMAN, PHD Department of Physiology Monask University Clayton, Victoria, Australia

288 Digestive Diseases, Vol. 17, No. 4 (April 1972)