CENTRIFUGAL GASTRIC VAGAL AFFERENT UNIT ACTIVITIES - ANOTHER SOURCE OF GASTRIC EFFERENT CONTROL

Our previous studies indicated that in rats about 10% of ventral gastr ic vagal efferent discharges do not originate from supracervical neura l elements. To determine the origin of these efferent activities, an i n vitro subdiaphragmatic vagus nerve-esophagus preparation was used. A ction potentials with the same amplitude and waveform, and behaving 'a ll or none' characteristic are considered to be recorded from a nerve fiber and defined as an unit activity. Because these centrifugal unit activities were recorded from the proximal cut end of the ventral gast ric vagal strands, they are ostensibly considered to be efferent activ ities. However, about 50% of unit action potential samples (21 out of 40) behave like unit activities recorded from mechanoreceptive afferen t fibers. They have spot-like or diffuse mechanoreceptive fields on th e subdiaphragmatic esophagus. When these receptive fields were stimula ted the sensory nerve terminals in the fields generate afferent unit a ction potentials. These afferent potentials not only propagate orthodr omically to the central nerve system, but also can be transmitted cent rifugally to the gastric branches of the same vagal afferent neuron. T ogether with the efferent discharges of gastric vagal motor neurons, t hese centrifugal sensory potentials can be intercepted from the proxim al cut end of gastric vagal nerve strands at gastroesophageal junction . Three types of mechanoresponsive centrifugal afferent unit activitie s were observed: rapidly adapting (n = 8), with or without after-disch arge; slowly adapting (n = 8), with or without after-discharge, and in itial high frequency followed by a plateau, with long-lasting after-di scharge (n = 5). Of the tested units (n = 24), 25% were either activat ed or inhibited by esophageal inflation and 23% (n = 22) by esophageal deflation. It is evident that not all centrifugal unit action potenti als recorded from the proximal cut end of gastric vagal nerve strands are generated from the vagal motor neurons, the recorded centrifugal u nit activities may contain antidromic unit action potentials generated from the esophageal collateral branches of the gastric vagal afferent nerve fibers. These results suggest that gastric vagal afferent neuro ns possess collateral branches innervating the esophagus, activation o f esophageal terminals may exert an effect on the gastric terminals vi a collateral reflex, analogous to the 'axon reflex' mechanism.