MORPHOLOGY OF SINGLE VESTIBULOSPINAL COLLATERALS IN THE UPPER CERVICAL SPINAL-CORD OF THE CAT .3. COLLATERALS ORIGINATING FROM AXONS IN THEVENTRAL FUNICULUS IPSILATERAL TO THEIR CELLS OF ORIGIN

Some vestibulospinal pathways are composed of a homogeneous collection of axons with similar intraspinal collaterals. Other pathways contain axons whose collaterals vary in terms of shape, distribution, and com plexity. The purpose of the present study was to extend the study of h omogeneity versus heterogeneity of vestibulospinal axons to vestibulos pinal axons that travel in the ventral funiculus ipsilateral to their cells of origin. Collaterals of these axons were stained following ext racellular injections of Phaseolus vulgaris-leucoagglutinin in rostral parts of the medial and descending vestibular nuclei. AU collaterals found in C2 and C3 were reconstructed. Collaterals arising from small diameter (0.5 to 2.9 mu m) axons usually consisted of a single main br anch with short side branches. The termination zones of most of these collaterals formed a narrow path in lamina VIII, but the location of t his pathway was highly variable. Collaterals arising from large-diamet er (3.0-6.1 mu m) axons were usually more complex and consisted of man y branches with en passant and terminal boutons that were located in m otoneuron nuclei as well as laminae VIII and VII. Despite a relationsh ip between termination zone and the position of the parent axon in the ventral funiculus, the variability in collaterals from large-diameter axons precluded a simple classification scheme. These results demonst rate that diversity, instead of homogeneity, is a characteristic featu re of vestibulospinal axons that originate from the medial and descend ing vestibular nuclei and travel in the ipsilateral ventral funiculus. This pathway is therefore composed of multiple anatomical subunits th at, as individuals, may selectively coordinate the activity of specifi c combinations of interneurons and motoneurons. (C) 1996 Wiley-Liss, I nc.