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

1. The volume of submandibular salivary secretion and electrical activities in preganglionic parasympathetic fibres innervating the submandibular gland were recorded in rabbits, anaesthetized with chloralose-urethane, to evaluate the neural control mechanisms subserving reflex salivary secretion. 2. 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. 3. About 90% of the preganglionic parasympathetic fibres responded to single electrical shock applied to a respective confined area of the oral region with a mean latency of 10.8 msec. The remainder responded to wider areas of the oral region with a mean latency of 31.5 msec. 4. Preganglionic fibres were classified into three types (E-type, N-type and I-type) in accordance with the mode of impulse discharges. Spontaneous discharges of E-type fibres (41%) were increased, while those of N-type fibres (36%) were unchanged and those of I-type fibres (23%) were decreased by repetitive stimulation at over 10 Hz. Most dominant responses in E-type fibres were induced by electrical shocks with a frequency of 10--20 Hz. 5. When paired shocks with varying intershock intervals were applied, excitability of E-type fibres was enhanced for about 20 msec, and then inhibited for about 100 msec, while that of N-type was inhibited for 150--200 msec, and I-type was inhibited for 500--700 msec after onset of the first shock. 6. 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 fibres. This result strongly suggests that E-type fibres are secretory fibres. The functional significance of N-type and I-type fibres is discussed.