VESTIBULOSPINAL EFFECTS ON NEURONS IN DIFFERENT REGIONS OF THE GRAY-MATTER OF THE CAT UPPER CERVICAL CORD

1. Previous studies of vestibular effects on the upper cervical cord h ave concentrated on the lateral and medial vestibulospinal tracts and on the actions that they exert on neck motoneurons and other neurons i n the ventral horn. It is known, however, that both the rostral and th e caudal areas of the vestibular nuclei (VN) give rise to axons that a re located in the dorsal and dorsolateral funiculi and that terminate in the dorsal horn. A primary goal of our experiments was to investiga te the effect of VN stimulation on neurons dorsal to lamina VII. 2. In decerebrate cats with the caudal cerebellar vermis re moved, we stimu lated different areas of the VN with an array of electrodes. The area of stimulation extended from the caudal tip of the descending nucleus to Deiters' nucleus, and was divided into rostral and caudal halves wi th the use of the descending nucleus as a reference. For control purpo ses some stimulating points were placed in the external cuneate nucleu s and restiform body. 3. We tested the effects of VN stimulation on sp ontaneously firing neurons in the ipsilateral C-2 and C-3 segments. Fo r purposes of classification the gray matter was divided into four zon es corresponding approximately to laminae I-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 possibilit y of current spread to the external cuneate nucleus or to the underlyi ng reticular formation. 4. VN-evoked effects could consist of facilita tion, or, less often, inhibition. In the majority of facilitated neuro ns conditioning stimuli evoked a synchronized, short-latency, increase in firing probability. When evoked by single stimuli this facilitatio n was considered monosynaptic. Facilitation that was diffuse, or that was only evoked by two or more stimuli, presumably involved more compl ex 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 ef fect on neurons in laminae V-VII but influenced more than half the neu rons in laminae VII-VIII. Conversely, electrodes placed caudally were most effective on cells in laminae V-VII, although they also influence d some neurons in lamina VIII. 6. Stimulation of the dorsal rami influ enced 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 physiol ogical 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 facilitat ory, often converges with afferent input and can therefore modulate it s influence on spinal neurons.