Morphology of single vestibulospinal collaterals in the upper cervical spinal cord of the cat. II. Collaterals originating from axons outside the ventral funiculi

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Abstract
Recent studies have shown that vestibulospinal axons reach the upper cervical spinal cord of the cat via several different funicular routes. The purpose of this study was to describe the projections of those axons travelling outside the well‐recognized pathways in the ventral funiculi. These axons are located in the dorsal columns, dorsolateral funiculi, and lateral funiculi. Collaterals of these axons were stained following extracellular injections of Phaseolus vulgaris leucoagglutinin in the medial and descending vestibular nuclei. The trajectories of individual collaterals were reconstructed from serial histological sections.Collaterals arising from axons in the same funiculus usually had the same characteristic appearance. Axons in the lateral funiculi, ipsilateral or contralateral to their cells of origin, gave rise to collaterals that had a simple structure and usually followed a horizontal trajectory across laminae VII and VIII. The boutons of these collaterals were distributed throughout the mediolateral extent of laminae VI and VII and the dorsal half of lamina VIII. In contrast, axons in the dorsolateral funiculi, ipsilateral or contralateral to their cells of origin, terminated primarily in laminae IV and V. Many collaterals of these axons projected either rostrally or caudally and had a narrow mediolateral distribution. The combined distribution of boutons from collaterals originating from axons in the dorsal columns included the dorsal horn and intermediate zone. Although these collaterals were less common and formed a heterogeneous group, they were easily distinguished from collaterals originating from axons travelling in other funiculi.These results indicate that vestibulospinal axons travelling outside the ventral funiculi comprise several distinct systems. Each system travels by a different funicular route and is distinguished by differences in collateral morphology and termination zones. Compared to vestibulospinal axons travelling in the ventromedial funiculus, vestibulospinal axons in lateral and dorsal funiculi have a more dorsal termination zone. Vestibulospinal projections to the upper cervical spinal cord appear to consist of several functional subdivisions that correspond to different funicular pathways.

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