Figure 2.
Spontaneous discharge originating in the axon rather than in the cell soma. A, The experimental setup. CAP, Suction electrode recording compound action potential; R&SIC, intracellular recording micropipette; SCN, sciatic nerve; SN, stimulating electrode on nerve. B, This neuron fired spontaneous spikes at Vr (-58 mV) but did not have subthreshold oscillations. Depolarization induced the appearance of a DAP after each spike, which sometimes triggered subsequent spikes, resulting in burst firing (traces at -37 and -40 mV). Hyperpolarization to -70 mV caused failure of soma spikes, leaving only residual (axonal) m-spikes. Thus, the soma spikes (singlets and bursts) at more depolarized potentials were triggered by action potentials of axonal origin. These axonal spikes also propagated into the L5 DR as demonstrated by the spike-triggered average of the signal recorded on the DR suction electrode (inset adjacent to the trace at -58 mV). C, Consistent with this interpretation, the frequency of the singlet spikes at -58 mV matched that of m-spikes at -70 mV (and spike bursts at -37 mV). The plot shows the cumulative distribution of singlet spike and m-spike ISI (d = 0.16; p > 0.2; K-S test; http://www.physics.csbsju.edu/stats/KS-test.html). D, Electrical stimulus pulses delivered to the spinal nerve (1 Hz; asterisks) triggered soma spikes. These summed with the spontaneously occurring spikes. Transection of the spinal nerve just distal to the stimulating electrode (arrow) eliminated the spontaneous spikes but not the stimulation-evoked spikes.