Synaptic organization of tectal-facial pathways in cat. II. Synaptic potentials following midbrain tegmentum stimulation.

1. The organization of the synaptic pathways underlying midbrain tegmentum influence over the facial musculature was studied with the use of an acute electrophysiological approach in the cat. Under pentobarbital sodium anesthesia, synaptic potentials were recorded intracellularly in antidromically identified facial motoneurons following electrical stimulation of the paralemniscal zone. The cells of origin and the pathways responsible for the potentials evoked from the paralemniscal zone were defined with the use of retrograde transport of horseradish peroxidase (HRP). The putative role of the paralemniscal zone with regard to the production of disynaptic, tectally evoked potentials in facial motoneurons was investigated both by inactivating this nucleus with injections of lidocaine and by making acute brain stem lesions to sever the paralemniscal-facial and other afferent pathways. 2. Following paralemniscal stimulation, monosynaptic, excitatory postsynaptic potentials (EPSPs) with latencies ranging from 0.6 to 0.9 ms, steep rising phases, and amplitudes in excess of 4.0 mV were recorded in motoneurons of the temporal and auriculoposterior subdivisions, which supply the pinna muscles. Smaller amplitude EPSPs (less than 1.0 mV) with monosynaptic latencies were observed in the zygomatic subdivision. Polysynaptic EPSPs with latencies ranging from 1.0 to 1.8 ms were also observed in all three of these subdivisions. However, only long-latency EPSPs, arriving at 2.0 ms or later, were present in ventral subdivision motoneurons. 3. Inhibitory postsynaptic potentials (IPSPs) were also frequently recorded in facial motoneurons after paralemniscal stimulation. Monosynaptic IPSPs with latencies ranging from 0.8 to 1.2 ms and amplitudes in excess of 4.0 mV were recorded in facial motoneurons of the temporozygomatic and auriculoposterior subdivisions, as were polysynaptic IPSPs with latencies ranging from 1.2 to 1.8 ms. IPSPs were sometimes observed in combination with a smaller, shorter latency EPSPs. Only long-latency IPSPs of greater than 2.0 ms were recorded in ventral subdivision motoneurons. In all cases, both the EPSPs and the IPSPs were graded in character and could be augmented by multiple stimuli. 4. The contralateral paralemniscal zone and the supraoculomotor area, bilaterally, represented the two most prominent afferent sources labeled after HRP injection of the facial nucleus. The superior colliculus and numerous reticular formation regions were also identified as facial nucleus afferents by the presence of retrogradely labeled cells. The retrogradely labeled cells in the paralemniscal zone exhibited heterogeneous soma size. HRP-labeled axons of the paralemniscal-facial pathway were observed to cross the midline by traveling ventral to the brachium conjunctivum in the caudal mesencephalon.(ABSTRACT TRUNCATED AT 400 WORDS)