Parasympathetic nervous control of reflex salivary secretion

The volumes of submandibular salivary secretion and efferent discharges of the pregangl-ionic parasympathetic fibers innervating the submandibular gland were recorded in anesthetized rabbits to evaluate the neural mechanisms subserving the reflex salivary secretion.Copious salivary secretion was induced when repetitive electrical stimulation was applied to the anterior ipsilateral parts of the oral region in sympathetically decentralized animals. The optimum frequency of stimulation was 10-20 Hz. Ninety percent of the recorded preganglionic parasympathetic fibers responded to a single electrical shock applied to the respective confined areas of the oral region with a mean latency of 10.8 msec (upper lip, 20%; anterior palate, 25%; anterior tongue, 27%; lower lip, 18%). The remainder responded to wider areas of the oral region with a mean latency of 31.5 msec. These fibers were classified into 3 types (E-type, N-type and I-type) in accordance with the mode of impulse discharges. Reflex discharges of the E-type fibers (41%) increased, while those of the N-type fibers (36%) were unchanged and those of the I -type fibers (23%) decreased by repetitive stimulation of over 10 Hz. When paired shocks with varying intershock intervals were applied, excitability of the E-type fibers was enhanced for about 20 msec, then inhibited for about 100 msec, while that of the N-type was inhibited for 150-200 msec, and the I-type for 500-700 msec after onset of the first shock. The volume of reflex submandibular salivation evoked by varying frequ encies of electrical shocks applied to the oral regions correlated statistically significantly with the magnitude of response in the E-type fibers, and this fact suggests that E-type fibers are secretory fibers.