Physiological identification of afferent fibers and postsynaptic sensory neurons in the spinal cord of the intact, awake cat

A method was developed to record from spinal cord cells in the awake, intact, partially restrained cat. Units were classified as afferent fibers or postsynaptic cells based on their ability to follow 100-Hz peripheral stimulation, the duration and configuration of the action potential waveform, and the number of spikes evoked by a single electrical pulse. These criteria are supported by independent observations of the location of the recording site, size of the receptive field, and adequate stimulus. Of 84 cutaneously activated units, 29 were classified as afferent fibers, 28 as postsynaptic cells, and 27 were not classified. No cutaneously activated unit was spontaneously active. In contract, all 28 units (7 postsynaptic and 21 not classified) responding to joint position or movement were spontaneously active (5 to 40 Hz). No unit responded to both cutaneous and proprioceptive inputs. Evidence for convergence of cutaneous input from different types of receptors was limited to five postsynaptic neurons that responded to hair movement and to stimuli applied to the skin; two of these cells responded differentially to noxious pinch. Two of 24 postsynaptic cutaneous units ceased responding to electrical cutaneous stimuli when the cat was eating. The responses of 29 primary afferent fibers were not altered by the behavior of the cat. These results suggest that, in the awake cat (i) criteria based on neuronal responsiveness and action potential waveform can be used to distinguish adequately between afferent fibers and postsynaptic cells; and (ii) there is a tonic inhibitory control, greater than in the anesthetized or spinally transected cat, that varies with behavioral state and is directed primarily at spinal neurons receiving cutaneous input.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27419/1/0000457.pd