Chapter 2 Vestibular projections to the spinal cord: the morphology of single vestibulospinal axons

Abstract

The three-dimensional distribution of LVST and MVST axons was examined in the cat cervical spinal cord using an intra-axonal staining method. LVST and MVST axons were electrophysiologically identified by their responses to stimulation of the vestibular nucleus, bilateral vestibular primary afferents, the LVST and the MVST were stained with injection of HRP. The axonal trajectory was reconstructed from serial histological sections. LVST axons were found to have multiple axon collaterals in the cervical cord. The maximum number of the identified collaterals for one neuron was 7. These collaterals were observed in either LVST axons terminating at the cervical cord or those projecting below Th2. The rostro-caudal extension of terminals for each collateral was very restricted (mean = 760 μm) and much narrower than intercollateral intervals (mean = 1470 μm). In the gray matter, collaterals ramified successively, pursued a delta-like path, and terminated mainly in lamina VIII and in the medial part of lamina VII and many boutons made apparent contact with the cell bodies and the proximal dendrites of motoneurons in the ventromedial nucleus. Some terminals were also distributed to the ventrolateral part of lamina VII adjacent to lamina IX. One group of LVST axons projected to lamina IX in the lateral ventral horn and terminated on large neurons there, probably motoneurons of forelimb muscles. MVST axons had one to seven axon collaterals at C1–C3 within the range of the stained axon. Stem axons ran in the ventromedial funiculus and primary collaterals arose from them at right angles. Each collateral had a very nar-row rostrocaudal spread as in LVST axons. Terminals were distributed in laminae VIII and IX, including the ventromedial nucleus, the spinal accessory nucleus, and the commissural nucleus. Many terminals seemed to make contact with retrogradely labelled motoneurons of neck muscles. Both crossed and uncrossed MVST axons had these characteristics.