Observations on thalamic and cortical responses to peripheral nerve volley

Abstract

The compound conducted action potential of the superficial radial nerve was investigated with respect to activation of contralateral thalamus and somatic receiving area of the cortex. Other observations relate to the sciatic nerve and its branches. Fifty-three cats, usually carried under light Nembutal anesthesia were used. The contributions of delta and C peripheral axons to the evoked thalamic potential were especially sought. Other matters studied were the phase relationship of the after-discharges of thalamus and cortex, relative latencies of primary responses at those sites, and comparative merits of unipolar and bipolar recording at and about the thalamic locus. 1. 1. Activation of large myelinated A axons of peripheral nerve evoked the positive deflection of shortest latency in the primary response to a nerve volley. Raising stimulus strength markedly over A maximum did not add to the height of this most rapid deflection provided that shock artifact was well balanced. 2. 2. After-discharge also appears with the A nerve spike but may be accentuated by the addition of delta and C components. 3. 3. With increase in stimulus strength over A maximum, a second positive elevation develops in the primary thalamic response which relates to the appearance of the second nerve spike. It is likely that this second nerve spike is due to delta axons. 4. 4. A C effect observed in a minority of reliable experiments was a train of fast spikes which appeared at a latency of 65–80 msec. and migh endure for 300 msec. It was not recorded invariably and we do not have sufficient data at this time to establish it as the significant effect of C activation of the forebrain. Other possible sequalae of C activation are also considered. 5. 5. When initial positive phases of thalamic and cortical primary responses are recorded on the identical sweeps of a dual beam oscilloscope, there is a shifted overlapping between the two indicating that the major synaptic transfer of the thalamic relay is over near the beginning of the second or negative phase of the thalamic response. Allowing for conduction between thalamus and cortex it seems likely that synaptic transfer commences soon after the start of the initial positive phase in the thalamus. 6. 6. We did not discern any invariable phase relationships between thalamic and cortical after-discharges. 7. 7. Bipolar recording from closely spaced electrode tips passed together through the thalamic locus closely parallel the results of recording vs. a distant electrode. They give further evidence that the potential field about the locus has a steep gradient.