The synaptic basis of a bilateral lingual-hypoglossal reflex in cats.

1. Reflex discharge of some hypoglossal neurones innervating the intrinsic musculature of the tongue occurred following single stimuli delivered to low threshold afferents in the ipsilateral or contralateral lingual nerves of decerebrate cats.2. The discharge produced by contralateral lingual nerve stimuli was smaller and had a longer latency than that produced by ipsilateral lingual nerve stimuli. The timing of the ipsilateral response suggested that two synapses were included in the reflex arc.3. Intracellular recordings in hypoglossal motoneurones revealed complex depolarizing and later hyperpolarizing synaptic respones following single nerve stimuli. The direction of the early synaptic action in any cell was the same whether low threshold afferents in the ipsilateral or the contralateral lingual nerve were stimulated. The timing of these synaptic responses and the occurrence of ripples on the rising phase of many of the depolarizing potentials was consistent with the presence of an internuncial neurone in the pathway from the afferent fibres to the motoneurone.4. Recordings were made of the discharges of neurones in or near the sensory nucleus of the spinal trigeminal tract caused by lingual nerve stimuli. The latencies of these discharges and the characters of the bursts of responses which followed single lingual nerve stimuli indicated that these neurones could be the internuncials on the reflex pathway.5. The relationship of these findings to the observations of Miller & Sherrington (1915) on the ;licking' reflex is discussed.