Gastrointestinal Smooth Muscle

Overview
The smooth muscle of the gastrointestinal (GI) system regulates the digestion and peristalsis of ingested food. Smooth muscle cells exhibit two types of electrical activity. An oscillatory slow wave known as either the basic electrical rhythm (BER) or the electrical control activity (ECA) is always present and mediates a higher frequency plateau and spiking activity that is associated with muscle contraction known as the electrical response activity (ERA). Two separate muscular layers, one oriented circumferentially and one oriented longitudinally, mix and propel gastric and intestinal contents. The slow wave activity exhibits a frequency characteristic of a particular location in the gastrointestinal tract. In the stomach, the (BER) oscillates at about 3 cycles per minutes (cpm). In the intestine, the (BER)frequency is about 12 cpm in the duodenum and decreases in a stepwise fashion to about 8 cpm in the terminal ileum.

Our studies have shown that the (BER) frequency correlates with intestinal viability and disease states; during mesenteric ischemia, the BER frequency decreases. If the ischemic insult is removed before permanent injury has occurred and bowel reperfusion restored, the BER frequency may return to normal levels.

Invasive serosal or mucosal electrodes provide an accurate measure of gastrointestinal electrical activity, but there is currently no reliable noninvasive method of detection. Electric potentials from sources of GI activity are attenuated and smoothed by low-conductivity abdominal layers so that cutaneous electrodes are unable to detect GI activity reliably. Our studies have shown that the magnetic fields associated with electrical activity in the GI tract are not affected by the conductivity profile of the abdomen, and thus offer the first possibility for reliable noninvasive measurement of GI activity.

We have used Superconducting QUantum Interference Device (SQUID) magnetometers to noninvasively measure and characterize the BER in normal human volunteers. Our studies have identified the characteristic BER frequency gradient of the small intestine magnetically. We have also performed the first measurements of gastric propagation direction and propagation velocity using SQUID magnetometers. In the future, we intend to investigate several parameters to characterize the BER and provide surgeons with a quantitative measure of intestinal viability. Additionally, using optical detection techniques with voltage sensitive dyes (developed for studies of cardiac electrophysiology) in addition to electrodes and SQUID magnetometers, we will investigate the mechanisms of pacemaking and propagation in gastrointestinal smooth muscle.

Alan Bradshaw bradshla@ctrvax.vanderbilt.edu