The morphology of single lateral vestibulospinal tract axons in the lower cervical spinal cord of the cat

The intraspinal morphology of single lateral vestibulospinal tract (LVST) axons was investigated with the method of intra‐axonal staining with horseradish peroxidase (HRP) and three‐dimensional reconstruction of the axonal trajectory. Axons penetrated in the ventral funiculus at C5‐C8 were identified as LVST axons by their monosynaptic responses to stimulation of the ipsilateral vestibular nerve and by their direct responses to stimulation of the ipsilateral Deiters' nucleus and LVST. Reconstructions were made from 34 well‐stained LVST axons. Of these, 23 terminated in the brachial segments (C5‐Thl) and the other 11 projected below Th2. These axons were traced over distances of 2.9‐16.3 mm rostrocaudally. Within these lengths, one to seven axon collaterals (mean ± S. D., 3.2 ± 2.0, N = 19) were given off at right angles from the stem axons of LVST axons terminating in the brachial segments. The mean diameters of stem axons and primary collaterals were 4.5 μm and 1.6 μm, respectively. In the gray matter, collaterals ramified successively, pursued a delta‐like path, and terminated mainly in laminae VII and VIII or lamina IX. The rostrocaudal extension of a single collateral was very restricted (mean ± S. D., 760 ± 220 μm, N = 16), in contrast to the extensive dorsoventral and mediolateral extent of the terminal arborization. There were usually gaps between adjacent collateral arborizations from the same stem axons, since the intercollateral distances ranged from 400 to 4,300 μm (mean = 1,490 μm). LVST axons terminating in brachial segments were divided into two groups‐a medial group and a lateral group‐on the basis of their projection sites in the transverse plane of the gray matter. The axons of the medial type had their main projection to laminae VII and VIII of Rexed, while those of the lateral type terminated in lamina IX. The terminal arborizations of the‐medial type LVST axons were mainly distributed over lamina VIII, where synaptic boutons appeared to make contact with proximal dendrites or somata of medium‐sized and large neurons in the ventromedial nucleus and also in the medial portion of lamina VII adjacent to the central canal and dorsal to lamina VIII. Five out of 15 medial type axons had a bilateral projection. One or two collaterals of each of these axons crossed the midline through the anterior commissure and terminated in lamina VII or VIII. It was concluded that the contralateral projection was sparse. The terminal arborizations of the lateral type LVST axons were distributed mainly in lamina IX, where apparent axosomatic and axodendritic contacts were observed on large neurons, and in regions of lamina VII adjacent to lamina IX. Terminal branches carried boutons of both the en passant type and the terminal type. The number of boutons per axon collateral was 38‐262 (mean ± S. D., 161 ± 76, N = 5) for the medial type axons and 98‐271 (182 ± 69, N = 5) for the lateral type axons. In 11 LVST axons projecting below Th2, only 4 axons had axon collaterals to the lower cervical cord.