RELATION BETWEEN AXON MORPHOLOGY IN C-1 SPINAL-CORD AND SPATIAL PROPERTIES OF MEDIAL VESTIBULOSPINAL TRACT NEURONS IN THE CAT

Relation between axon morphology in Ci spinal cord and spatial propert ies of medial vestibulospinal tract neurons in the cat. J. Neurophysio l. 79: 285-303, 1998. Twenty-one secondary medial vestibulospinal trac t neurons were recorded intraaxonally in the ventromedial funiculi of the C-1 spinal cord in decerebrate, paralyzed cats. Antidromic stimula tion in C-6 and the oculomotor nucleus identified the projection patte rn of each neuron. Responses to sinusoidal, whole-body rotations in ma ny planes in three-dimensional space were characterized before injecti on oi horseradish peroxidase or Neurobiotin. The spatial response prop erties of 19 neurons were described by a maximum activation direction vector (MAD), which defines the axis and direction of rotation that ma ximally excites the neuron. The other two neurons had spatio-temporal convergent behavior and no MAD was calculated. Collateral morphologies were reconstructed from serial frontal sections to reveal terminal fi elds in the C-1 gray matter. Axons gave off multiple collaterals that terminated ipsilaterally to the stem axon. Collaterals of individual a xons rarely overlapped longitudinally but projected to similar regions in the ventral horn when viewed in transverse sections. The number of primary collaterals in C-1 was different for vestibulo-collic, vestib ulo-oculo-collic, and C-6-projecting neurons: on average one every 1.3 4, 1.72, and 4.25 mm, respectively. The heaviest arborization and most terminal boutons were seen in the ventral horn, in laminae VIII and I X. Varicosities on terminal branches in lamina IX were observed adjace nt to large cell bodies-putative neck motoneurons-in counterstained ti ssue. Some collaterals had branches that extended dorsally to lamina V II. Neurons with different spatial properties had terminal fields in d ifferent regions of the ventral horn. Axons with type I responses and MADs near those of a semicircular canal pair had widely distributed co llateral branches and numerous terminations in the dorsomedial, ventro medial, and spinal accessory nuclei and in lamina VIII. Axons with typ e I responses that suggested convergent canal pair input, with type II responses, and with spatio-temporal convergent behavior had smaller t erminal fields. Some neurons with these more complex spatial propertie s projected to the dorsomedial and spinal accessory but not to the ven tromedial nuclei. Others had focused projections to dorsolateral regio ns of the ventral horn with few branches in the motor nuclei.