Horseradish peroxidase (HRP), injected intramuscularly, specifically labeled motoneurons innervating antagonistic jaw muscles in the cottonmouth mocassin, Agkistrodon piscivorus piscivorus. Adductor mandibulae profundus, part 3a, motoneurons were localized in the lateral regions of the ventral and intermediate subnuclei of the trigeminal (V) motor nucleus. These were large cells containing fine, granular reaction product characteristic of α-motoneurons. Small cells, which contained large coarse reaction granules characteristic of γ-motoneurons were localized in a separate cluster in the lateral regions of the dorsal subnucleus of the V motor nucleus. Depressor mandibulae motoneurons were localized in the ventromedial regions of the facial (VII) motor nucleus, primarily in the caudal half. Cell sizes ranged from 30–50 μm in diameter and HRP staining characteristics were variable, indicating a mixed population of motoneuron functional types without the segregation noted in the V motor nucleus.
Boutons which made synaptic contact with labeled somata or processes were classified according to morphological type and their frequency of appearance. Boutons containing spherical vesicles (S-, C-, T-) were distributed similarly on motoneurons of both muscles, but more F-boutons, those with flattened vesicles, synapsed on the adductor motoneurons.
Comparison of snake bouton distribution with that in mammalian spinal cord indicates that synaptology on all these motoneurons are remarkably similar. The more frequent occurrence of C-boutons, those with a subsynaptic cistern, on reptilian motoneurons may indicate a stronger intrasegmental input, as determined from mammalian degeneration studies.