The Journal of Neurophysiology Vol. 79 No. 1 January 1998, pp. 285-303
Copyright ©1998 The American Physiological Society

Relation Between Axon Morphology in C₁ Spinal Cord and Spatial Properties of Medial Vestibulospinal Tract Neurons in the Cat

S. I. Perlmutter, Y. Iwamoto, L. F. Barke, J. F. Baker, and B. W. Peterson

Department of Physiology, Northwestern University School of Medicine, Chicago, Illinois 60611

Perlmutter, S. I., Y. Iwamoto, L. F. Barke, J. F. Baker, and B. W. Peterson. Relation between axon morphology in C₁ spinal cord and spatial properties of medial vestibulospinal tract neurons in the cat. J. Neurophysiol. 79: 285-303, 1998. Twenty-one secondary medial vestibulospinal tract neurons were recorded intraaxonally in the ventromedial funiculi of the C₁ spinal cord in decerebrate, paralyzed cats. Antidromic stimulation in C₆ and the oculomotor nucleus identified the projection pattern of each neuron. Responses to sinusoidal, whole-body rotations in many planes in three-dimensional space were characterized before injection of horseradish peroxidase or Neurobiotin. The spatial response properties of 19 neurons were described by a maximum activation direction vector (MAD), which defines the axis and direction of rotation that maximally 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 fields in the C₁ gray matter. Axons gave off multiple collaterals that terminated ipsilaterally to the stem axon. Collaterals of individual axons rarely overlapped longitudinally but projected to similar regions in the ventral horn when viewed in transverse sections. The number of primary collaterals in C₁ was different for vestibulo-collic, vestibulo-oculo-collic, and C₆-projecting neurons: on average one every 1.34, 1.72, and 4.25 mm, respectively. The heaviest arborization and most terminal boutons were seen in the ventral horn, in laminae VIII and IX. Varicosities on terminal branches in lamina IX were observed adjacent to large cell bodiesputative neck motoneuronsin counterstained tissue. Some collaterals had branches that extended dorsally to lamina VII. Neurons with different spatial properties had terminal fields in different regions of the ventral horn. Axons with type I responses and MADs near those of a semicircular canal pair had widely distributed collateral branches and numerous terminations in the dorsomedial, ventromedial, and spinal accessory nuclei and in lamina VIII. Axons with type I responses that suggested convergent canal pair input, with type II responses, and with spatio-temporal convergent behavior had smaller terminal fields. Some neurons with these more complex spatial properties projected to the dorsomedial and spinal accessory but not to the ventromedial nuclei. Others had focused projections to dorsolateral regions of the ventral horn with few branches in the motor nuclei.

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