RECEPTIVE-FIELD PROPERTIES OF TRIGEMINOTHALAMIC NEURONS IN THE ROSTRAL TRIGEMINAL SENSORY NUCLEI OF CATS

This study described topographic and receptive field representation in the region of the rostral trigeminal nuclei, and evaluated whether thalamic neurons from the principal sensory nucleus relay muscle afferent information to the thalamus. Extracellular single-unit activity was recorded from anesthetized cats. Units were tested for responses to natural stimuli (i.e., air bursts, brushing, light pressure, and pinch) applied to the face and oral cavity, electrical stimulation of the masseter nerve, and ramp-and-hold movements of the jaw. The receptive fields and physiological properties for 110 units were studied; we were able to verify the recording site for 96 of these units. Most of the units had discrete receptive fields in the oral cavity, skin, hair, and masseter muscle. Only 2 units received convergent inputs. Stimulation of the ipsilateral and contralateral ventroposteromedial nucleus of the thalamus was performed to identify antidromically activated units. The results showed that the dorsal principal sensory nucleus received its input primarily from the oral cavity. Most of the units (85%) that were activated by antidromic stimulation from the ipsilateral thalamus were located in this nucleus. In contrast, 82% of the units that projected to the contralateral thalamus were located in the ventral principal sensory nucleus. A complete somatotopic representation of the ipsilateral face and oral cavity was observed in the ventral principal sensory nucleus. Although 24 units had muscle receptive fields, none were activated by stimulation of the ipsilateral thalamus, and only 1 responded to stimulation of the contralateral thalamus. Most of the units that were not antidromically driven were recorded outside of the cytoarchitectual boundaries of the principal sensory nucleus. Retrograde labeling of the rostral trigeminal nuclei indicated that most of the neurons in the dorsal principal sensory nucleus projected to the ipsilateral thalamus, whereas those in the ventral principal sensory nucleus projected to the contralateral thalamus. Taken together, these observations do not support the presence of a substantial relay for muscle afferent input from the dorsal principal sensory nucleus to the ventrobasal thalamus in cats.