An autoradiographic and HRP study of vestibulocollic pathways in the pigeon

This study examines the connections underlying the vestibulocollic system in the adult pigeon by using retrogradely transported horseradish peroxidase (HRP) to identify neck muscle motoneurons in one set of animals, and transneural anterograde transport of tritiated proline-fucose to delineate the descending medial (MVST) and lateral (LVST) vestibulospinal tracts in a second set of animals. Correlations of location and distribution of HRP-labeled motoneurons and autoradiographically labeled fiber tracts and terminal fields were performed between the two sets of experiments. The right biventer cervicis and complexus neck muscles were subdivided into rostral and caudal halves in ten animals and HRP injected into only half of one of the two muscles in each experiment. Following a 16–48-hour survival, the brain was fixed by intracarotid catheterization and perfusion and the HRP in the brain sections reacted with the tetramethylbenzidine (TMB) blue reaction process. Three groups of HRP-labeled motoneurons were identified in the ipilateral ventral horn of the upper cervical spinal cord: a ventromedial and ventrolateral group within lamina VIII innervating the biventer cervicis and the more rostral part of the complexus muscle, and a dorsolateral group of motoneurons within lamina VII innervating the caudal part of the complexus muscle. The dorsolateral motoneurons with their HRP-labeled axons leaving the cord through the dorsal root are homologous to the spinal accessory nucleus of mammals. Labeled motoneurons were also noted in the ipsilateral medulla adjacent to the medial longitudinal fasiculus (ELM) in a location previously identified as the hy-poglossal nucleus. Additional experiments were performed in which HRP was injected directly into the base of the tongue. The resultant HRP-labeled hypoglossal motoneurons were separate and dorsolateral to the collic motoneurons. Descending vestibulospinal projections from one vestibular labyrinth were identified autoradiographicalry (ARG) by transneural anterograde transport of 3H-proline-fucose injected into the left labyrinthine endolytine endolymph in five animals. Heavily labeled MVST fibers were observed crossing the midline of the brain to enter and descend in the contralateral ELM. Labeled MVST fibers were noted to leave the contralateral FLM and surround the previously identified collie motoneurons in the medulla with intense terminal fields suggestive of synaptic contact. Labeled MVST fibers in the contralateral ventral funiculus of the cord were also noted to innervate the HRP-identified ventromedial and ventrolateral cervical motoneurons, but not the dorsolateral motoneurons in lamina VII. Ipsilateral (left) descending MVST and LVST fibers were less heavily labeled at all levels in the medulla and upper cervical cord. Labeled ipsilateral (left) vestibulospinal fibers were also observed to leave the lateralmost aspect of the left ventrolateral funiculus in the upper cervical cord to terminate among left ventrolateral motoneurons. Our findings are compared and contrasted with previous studies of vestibulocollic pathways.