Analysis of reflex responses in preganglionic parasympathetic fibres innervating submandibular glands of rabbits.

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Abstract
The volume of submandibular salivary secretion and electrical activities in preganglionic parasympathetic fibers innervating the submandibular gland were recorded in rabbits, anesthetized with chloralose-urethane, to evaluate the neural control mechanisms subserving reflex salivary secretion. Single, paired or repetitive electrical shocks were applied to the various parts of the oral region in sympathetically decentralized animals. A copious salivary secretion was induced when the repetitive electrical stimulation was applied to anterior ipsilateral parts of the oral region, especially the upper lip. The optimum frequency of stimulation was 10-20 Hz. About 90% of the preganglionic parasympathetic fibers responded to single electrical shock applied to a respective confined area of the oral region with a mean latency of 10.8 ms. The remainder responded to wider areas of the oral region with a mean latency of 31.5 ms. Preganglionic fibers were classified into 3 types (E-type, N-type and I-type) in accordance with the mode of impulse discharges. Spontaneous discharges of E-type fibers (41%) were increased, while those of N-type fibers (36%) were unchanged and those of I-type fibers (23%) were decreased by repetitive stimulation at over 10 Hz. Most dominant responses in E-type fibers were induced by electrical shocks with a frequency of 10-20 Hz. When paired shocks with varying intershock intervals were applied, excitability of E-type fibers was enhanced for .apprx. 20 ms and then inhibited for .apprx. 100 ms, while that of N-type was inhibited for 150-200 ms and I-type was inhibited for 500-700 ms after onset of the first shock. The volume of reflex submandibular salivation evoked by varying frequencies of electrical shocks applied to the oral region correlated statistically significantly with the magnitude of responses in E-type fibers. E-type fibers are apparently secretory fibers. The functional significance of N-type and I-type fibers is discussed.