Vestibulospinal effects on neurons in different regions of the gray matter of the cat upper cervical cord
Authors: Isu N. , Thomson D. B. , Wilson V. J. 
Published: December 2017

1. Previous studies of vestibular effects on the upper cervical cord have concentrated on the lateral and medial vestibulospinal tracts and on the actions that they exert on neck motoneurons and other neurons in the ventral horn. It is known, however, that both the rostral and the caudal areas of the vestibular nuclei (VN) give rise to axons that are located in the dorsal and dorsolateral funiculi and that terminate in the dorsal horn. A primary goal of our experiments was to investigate the effect of VN stimulation on neurons dorsal to lamina VII. 2. In decerebrate cats with the caudal cerebellar vermis removed, we stimulated different areas of the VN with an array of electrode. The area of stimulation extended from the caudal tip of the descending nucleus to Deiters' nucleus, and was divided into rostral and caudal halves with the use of the descending nucleus as a reference. For control purposes some stimulating points were placed in the external cuneate nucleus and restiform body. 3. We tested the effects of VN stimulation on spontaneously firing neurons in the ipsilateral C2 and C3 segments. For purposes of classification the gray matter was divided into four zones corresponding approximately to laminae 1-IV, V-VI, VII, and VIII of Rexed. Overall, the activity of 39 of 84 neurons was influenced from one or more stimulating sites. For six cells there was some possibility of current spread to the external cuneate nucleus or to the underlying reticular formation. 4. VN-evoked effects could consist of facilitation, or, less often, inhibition. In the majority of facilitated neurons conditioning stimuli evoked a synchronized, short-latency, increase in firing probability. When evoked by single stimuli this facilitation was considered monosynaptic. Facilitation that was diffuse, or that was only evoked by two or more stimuli, presumably involved more complex pathways. The latency of inhibition could not be measured, but was short. 5. Stimulation of either the rostral or caudal VN had no effect on neurons in laminae I-IV. Electrodes placed rostrally had little effect on neurons in laminae V-VI, but influenced more than half the neurons in laminae VII-VIII. Conversely, electrodes placed caudally were most effective on cells in laminae V-VII, although they also influenced some neurons in lamina VIII. 6. Stimulation of the dorsal rami influenced most neurons in laminae V-VI, and about a quarter of the neurons in laminae VII-VIII. When tested, there was often convergence between vestibulospinal and peripheral inputs. 7. Our results provide physiological evidence that vestibulospinal fibers influence neurons not only in laminae VII and VIII, but also as far dorsally as lamina V. Fibers that influence neurons in laminae V and VI originate primarily in the caudal areas of the VN. As suggested previously on anatomic grounds, the projection to the dorsal laminae, which is predominantly facilitatory, often converges with afferent input and can therefore modulate its influence on spinal neurons.