Chapter 12 Comparison of the branching patterns of lateral and medial vestibulospinal tract axons in the cervical spinal cord

Abstract

The morphology of single physiologically-identified lateral and medial vestibulospinal tract (LVST and MVST) axons was analysed, using intracellular staining with horseradish peroxidase (HRP) and three-dimensional reconstruction of axonal trajectories in the cat. Axons were penetrated in the cervical cord at C1-C8 with a microelectrode filled with 7% HRP. These axons were identified as vestibulospinal axons by their monosynaptic responses to stimulation of the vestibular nerve and further classified as either LVST or MVST axons by their responses to stimulation of the LVST and MVST. The stained axons could be traced over distances of 3-16 mm rostrocaudally. Within these lengths, both LVST and MVST axons were found to have multiple axon collaterals at different segments in the cervical cord. Up to seven collaterals were given off from the stems of MVST axons and LVST axons. The LVST axons included both neurones terminating at the cervical cord and those projecting further caudally to the thoracic or lumbar cord. Each collateral of these LVST axons, after entering into the gray matter, ramified successively in a delta-like fashion and terminated mainly in lamina VIII and in the medial part of lamina VII. Many boutons of both terminal and en passant types seemed to make contact with the cell bodies and proximal dendrites of neurones in the ventromedial nucleus (VM). Each collateral had a narrow rostrocaudal extension (0.2-1.6 mm, average 0.8 mm) in the gray matter in contrast to a much wider intercollateral interval (average 1.5 mm), so that there were gaps free from terminal boutons between adjacent collateral arborizations. The morphology of axon collaterals of MVST axons was very similar to that of LVST axons. The rostrocaudal extent of single axon collaterals was very restricted (0.3-2.1 mm) in contrast to the wide spread in a mediolateral or a dorsoventral direction. MVST axons had intensive projections to the upper cervical cord with multiple axon collaterals. One to seven collaterals of single MVST axons were found at C1-C3. Terminals of MVST axons were distributed in laminae VII, VIII and IX, including the VM, the nucleus spinalis n. accessorii (SA), and the commissural nucleus. Many terminals seemed to make contact with retrogradely-labelled motoneurones of neck muscles. Both axosomatic and axodendritic contacts were observed on motoneurones in various sizes. Some collaterals gave rise to terminal arborizations in both the VM and the SA. These results suggest that single LVST and MVST axons may control excitability of multiple dorsal axial muscles concurrently with their multiple axon collaterals at multisegmental levels.