Contralateral projections of trigeminal mandibular primary afferents in the guinea pig as seen by transganglionic transport of horseradish peroxidase

Abstract

Transganglionic transport of horseradish peroxidase (HRP) was used to investigate contralateral projections of trigeminal mandibular fibers in the guinea pig. After application of HRP to the buccal, lingual, auriculotemporal, mylohyoid, mental and inferior alveolar nerves, crossing fibers and contralateral endings were found in the caudal region of the nucleus of the solitary tract (most of these belonging to the buccal and lingual nerves), the dorsomedial region of the subnucleus caudalis of the trigeminal sensory nuclear complex (TSNC), and the dorsal horns of the first 5 cervical spinal cord segments (C 1–C 5). The greatest numbers of crossing fibers in the medullary and cervical dorsal horn segments belonged to the mental and mylohyoid nerves, though these nerves did not project contralaterally to C 4–C 5. Contralateral buccal and lingual endings were scattered sparsely from the subnucleus caudalis to C 5, and only very few contralateral auriculotemporal terminals were observed. Though laminae I–V of the dorsomedial region of the medullary and cervical dorsal horns all exhibited contralateral endings of the mental and mylohyoid nerves, most such endings were found in laminae II i–III, followed by lamina IV, which suggests their involvement in the reception of mechanical stimuli and in the sensory motor reflexes of the orofacial region. The contralateral buccal and lingual terminals were distributed somatotopically in the first 5 cervical cord segments, with the lingual endings rostral to the buccal terminals within each segment. In C 4 and C 5 lingual endings appeared exclusively in laminae I and II o, suggesting that like the ipsilateral lingual projections at this level, which also terminate in these laminae, they may be involved in pain and temperature sensation.