Commissural interneurons for masticatory motoneurons: A light and electron microscope study using the horseradish peroxidase tracer technique

Abstract

Commissural interneurons for masticatory motoneurons of the cat were investigated using the horseradish peroxidase tracing technique. After injection of horseradish peroxidase into the motor trigeminal nucleus, neurons labeled with the enzyme were seen contralaterally around the motor trigeminal nucleus, mainly in the supra- and intertrigeminal regions and the lateral tegmental regions close to the medial border of the motor trigeminal nucleus, and additionally within the confines of the motor and main sensory trigeminal nuclei and the parabrachial nuclei. After placing lesions in the supra- and intertrigeminal regions, degenerated axon terminals were found electron microscopically in the motor trigeminal nucleus contralateral to the lesion. In accordance with these findings, axon terminals in the motor trigeminal nucleus were labeled orthodromically with horseradish peroxidase injected contralaterally into the supra- and intertrigeminal regions; both terminals filled with round synaptic vesicles and those containing flattened vesicles were labeled with the enzyme. The existence of many commissural neurons around the motor trigeminal nucleus would be consistent with the intricacy of bilateral mechanisms of jaw movements.