Comparative studies of cerebellar electrophysiology indicated that surface stimulation of the cerebellar cortex evokes a sharp, superficial, positive-negative wave produced by direct stimulation of the parallel fibers, followed by a secondary negativity (cf. ref. 4). In most vertebrates this latter negativity, which is produced by the activation of the parallel fiber-Purkinje cell junction, reverses to a positive wave at 100 gm depth and remains positive throughout the depth of the molecular layer. The above comparative study further determined that in pigeons, as in alligators (2), the superficial negativity does not reverse in depth. This particular distribution of the late negative wave has been found to be correlated with the active current sinks which characterize dendritic electroresponsiveness in the cerebellar cortex (2, 3).The Fig. 1A. Analysis of the laminar field potential confirmed the presence of large negative potentials attributable to inward ionic transmembrane current in the molecular layer between 100 and 300 ,m from the surface. Characteristically, this negative field demonstrated a shift in latency with depth, as indicated by the second broken line in the figure. This delay has been interpreted as the conduction time of dendritic current sinks from their site of origin to the somatic level (2).Following Loc stimulation it is possible to obtain, at 400 ,m depth, intrasomatically recorded spikes from Purkinje cells which are identified by their antidromic invasion (3, 5). These intracellular records are characterized by a resting potential of -60 to -65 mV and by the generation of increasing numbers of action potentials as the Loc stimulus intensity is increased (Fig. 1B) (Fig. 1G). As reported for the alligator, hyperpolarizing current pulses injected through the recording pipette uncovered several distinct all-or-nothing components, indicating that these action potentials had more than one site of spike initiation (Fig. 1). As in other Purkinje cells (3), the dendritic spikes thus appear to be conducted to the soma in a noncontinuous manner and produce, at that level, the prolonged after-depolarization seen in Fig. 1B
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