THE FUNCTIONAL PROPERTIES OF VENTROBASAL THALAMIC NEURONS STUDIED IN UNANESTHETIZED MONKEYS

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Abstract
The method of single unit analysis has been used to study the functional properties of the third order neurons of the lemniscal component of the somatic afferent system. These cells compose the ventrobasal nuclear complex, the thalamic relay nucleus of the system. The experiments were made in unanesthetized monkeys, whose heads were, several weeks before the experiment, de-afferented by an intracranial retrogasserian neurectomy, and by transection of the ascending branches of the cervical plexus. The properties of the mode of the adequate exciting peripheral stimulus and the locus of the peripheral receptive field have been determined for more than 1000 thalamic neurons, each of which was later shown by the reconstruction of serial sections to be located within the cytoarchitectural confines of the ventrobasal nuclear complex. The specificity of the lemniscal system, previously observed in anesthetized animals, has been confirmed in the unanesthetized state. Each ventrobasal cell was activated by a mechanical stimulus: by touch-pressure of the skin, by stimulation of deep fascia or periosteum, or by the gentle rotation of a joint. Each cell was excited by one of these modes of stimulation delivered to a contralateral receptive field. The fields for the entire ensemble of ventrobasal cells compose the patterns of representation of the body form at the thalamic level, one determined more by peripheral innervation density than by bodily geometry. Within this pattern there is a differential distribution of cells by modality, a distribution determined, it is thought, by the heavy innervation of the skin of apical regions of the body, and the equally distorted distribution of deep receptors, which are most plentiful in proximal parts. The results show that such measures of neuronal activity as the recovery cycles and the capacity for frequency following are so markedly depressed by an anesthetic agent that the more dynamic, time-dependent aspects of central neural actions must be studied in unanesthetized animals. A newly designed system for the acquisition of data in experiments on the CNS is described, as well as a second system for data reduction. The latter yields automatically the total information available concerning the temporal aspects of the record of impulse discharge by a single nerve cell, and in a form readily processible for computer input.