Location of, and peripheral convergence on, the interneuron in the disynaptic path from the coronal gyrus of the cerebral cortex to the trigeminal motoneurons in the cat.

Primary afferent and descending cortico-bulbar convergence on 186 interneurons located in the intertrigeminal area was investigated. The experiments were performed on cats anaesthetized with chloralose. Nerves from the three trigeminal dermatomes were stimulated electrically at intensities below and above twice the threshold level. Nerves from oral, perioral and periorbital structures, and afferents from the masseteric and digastric muscles were included. The surface of the cerebral cortex was stimulated electrically in systematically selected, maximally receptive points within the trigeminal primary projection fields. The intertrigeminal neurons generally responded to stimulation of low-threshold afferents from periodontal, lingual or perioral cutaneous receptors with a polysynaptic latency. Inputs from 3-5 nerves were common but one afferent input was usually most effective. The neurons were generally discharged from two or more cortical points, as a rule those of the oral and perioral projection fields in areas 3a and 3b of the coronal gyrus. The fastest path from the cerebral cortex to the intertrigeminal area was monosynaptic. However, the median latency was 4-5 ms which indicates an oligosynaptic path. The path went through the pyramid at the pontine level. The discharge pattern of the intertrigeminal neurons was 1-4 spikes in 54% of the neurons and a high frequency train of spikes in 46%. Cortical excitation followed by inhibition of the neurons was observed. The neurons were not discharged by electrical stimulation in the defence-attack area of the hypothalamus. Transsynaptic responses evoked from the mesencephalon were seen in 1/3 of the tested neurons.