Centrifugal gastric vagal afferent unit activities: another source of gastric "efferent" control

Our previous studies indicated that in rats about 10% of ventral gastric vagal efferent discharges do not originate from supracervical neural elements. To determine the origin of these efferent activities, an in vitro subdiaphragmatic vagus nerve-esophagus preparation was used. Action potentials with the same amplitude and waveform, and behaving 'all 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 gastric vagal strands, they are ostensibly considered to be efferent activities. However, about 50% of unit action potential samples (21 out of 40) behave like unit activities recorded from mechanoreceptive afferent fibers. They have spot-like or diffuse mechanoreceptive fields on the subdiaphragmatic esophagus. When these receptive fields were stimulated the sensory nerve terminals in the fields generate afferent unit action potentials. These afferent potentials not only propagate orthodromically to the central nerve system, but also can be transmitted centrifugally to the gastric branches of the same vagal afferent neuron. Together with the efferent discharges of gastric vagal motor neurons, these centrifugal sensory potentials can be intercepted from the proximal cut end of gastric vagal nerve strands at gastroesophageal junction. Three types of mechanoresponsive centrifugal afferent unit activities were observed: rapidly adapting (n = 8), with or without after-discharge; slowly adapting (n = 8), with or without after-discharge, and initial high frequency followed by a plateau, with long-lasting after-discharge (n = 5). Of the tested units (n = 24), 25% were either activated or inhibited by esophageal inflation and 23% (n = 22) by esophageal deflation. It is evident that not all centrifugal unit action potentials recorded from the proximal cut end of gastric vagal nerve strands are generated from the vagal motor neurons, the recorded centrifugal unit 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 neurons possess collateral branches innervating the esophagus, activation of esophageal terminals may exert an effect on the gastric terminals via collateral reflex, analogous to the 'axon reflex' mechanism.