Slow waves and unitary activity evoked by cutaneous stimulation from the rat cuneate nucleus.

Depth profiles of averaged evoked potentials (AEPs) and simultaneously generated unitary activity have been recorded from the cuneate nucleus of the rat in response to controlled tactile stimulation of the ipsilateral forepaw. Four separate components of the AEPs were isolated, N1, N2, P, and N3. N1 corresponds to the classical N wave previously described by other workers; four fractions of N1 are described. The classical P wave which follows N1 reverses at 150--350 micrometers depth to become a negative wave of identical time course, the N2 wave, at deeper locations. N2 peaks deeper than N1 within the non-relay portion of the cuneate nucleus, or below in the subnuclear reticular formation where it is the only significant evoked component. Its strong susceptibility to high Mg++ C.S.F. superperfusion suggests a polysynaptic origin. It is argued that the depth distribution and time course of N2 does not support its function relating to depolarisation of primary afferents (PAD) in the vicinity of synaptically driven cuneate cells. Alternative possibilities for its origin are discussed. An additional sustained component of the AEP, the N3 component, is described and evaluated. N3 is co-extensive with N1, has a long time course and simple exponential decay, and is the component most resistant to high Mg++ C.S.F. superperfusion. A similar component to N3 has been described by previous workers in the spinal cord, where it has been shown to arise from glia depolarised by K+ effluxing from discharging afferents and cells. A similar origin for N3 is suggested, and its possible involvement with PAD discussed.