Electrophysiological responses of 519 single and 405 multiple neurons located in the distal forelimb zone of the second somatic sensory cortex (SII) of 11 intact cats were characterized according to their submodality and receptive field properties. In 4 of these animals, 46 single and 134 multiple neuronal responses were studied after transection of the dorsal columns contralateral to the cortical recording sites. Receptive field positions overlapped considerably in SII during orthogonal electrode penetrations, but were shifted during tangential penetrations. Analysis of the receptive field positions for neurons encountered in tangential penetrations indicated that receptive fields rarely overlapped when the neurons were separated by more than 750 microns. Using a variety of hand-held stimuli, neuronal responses were assessed according to several criteria including: velocity, adaptation, following rate, spontaneous activity, and whether the response was elicited by stimulating hairs, skin, claws, or deep tissue. Based on these parameters, it was possible to discern several types of neuronal responses in SII. Among these, over 60% of the neurons in our sample responded best to movement of hairs. A smaller number of neurons responded as though they received inputs from Pacinian receptors or rapidly adapting receptors in the glabrous skin. In about 20% of the single neuron sample, it was not possible to identify a selective adequate stimulus, however, these cells responded to somatic stimuli, such as taps. Approximately 5% of the neurons could not be driven with somatic sensory stimuli. Following dorsal column lesions, some neurons in SII still responded to cutaneous stimulation, primarily hair movement. Most SII neurons were more difficult to drive, the responses were more sluggish and receptive fields were less well-defined. A greater proportion of single neuron responses (> 60%) could not be activated by any type of somatic sensory stimulus. These results indicate that the dorsal columns provide a potent, but not exclusive, source of afferent input to SII.
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