Intramural Innervation of the Opossum Sphincter of Oddi
The predominate motor activity of the opossum SO, in vitro as in vivo, is spontaneous peristaltic contractions of myogenic origin that propagate antegrade toward the duodenum. We studied the response of the opossum SO in vitro to intramural nerve stimulation using SO segments from 15 animals. The SO segment, about 3 cm. long, was suspended in a muscle bath. Force transducers recorded circular muscle tension at four sites along the SO. To stimulate intramural nerves, 10–30 s trains of current pulses (16–40 ma, 0.5 ms duration, 5 hz) were delivered to one of three bipolar electrodes implanted along the SO. Antagonists were added to the bath to test their effects on SO responses to nerve stimulation. Results: Nerve stimulation in the proximal, mid or distal SO elicited repetitive peristaltic contractions that invariably originated in the proximal SO and propagated antegrade. These contractions were of similar wave form and duration (2–3 s) to spontaneous contractions. The repetitive contractions usually continued for the duration of the stimulus and occurred at a rate of 12–20/min. This rate was greater than the frequency of spontaneous contractions, 4–8/min. Atropine 10−6M or tetrodotoxin 10−6M completely blocked the excitatory response to nerve stimulation.