The action of antidromic impulses on the cerebellar Purkinje cells.

1. Antidromic impulses have been set up in the axons of Purkinje cells of the cerebellar vermis by stimulation in the juxta-fastigial (J.F.) region. Most experiments were performed on the normal cat cerebellum, but in nine the cerebellum was chronically deafferented by bilateral pedunculotomy 9-23 days previously.2. Intra- and extracellular recording from Purkinje cells both showed a characteristic inflexion on the rising phase of the spike potential (the characteristic IS-SD inflexion) that presumably signals a delay in invasion between the axon and the large soma-dendritic expansion.3. Laminar field analysis of the antidromic spike potentials showed that the antidromic impulses invaded at least 200 mu of the main dendrites as well as the soma, there being then a steep decrement to the surface. At superficial levels there was even an inverse antidromic spike potential. There appeared to be a synchronous invasion of the soma-dendritic complex, perhaps due to trigger zones of low threshold on the dendrites.4. Antidromic soma-dendritic invasion was modified in the expected manner by a volley in the parallel fibres; there was inhibition of transmission into the soma and up the main dendrites (maximum effect at 200-300 mu depth) due to the inhibitory action of the basket and superficial stellate cells that are excited by the parallel fibres; there was facilitation of transmission in the dendrites at levels superficial to 200 mu due to the direct excitatory action of parallel fibres. Both the inhibitory and excitatory actions had a duration in excess of 100 msec.5. In the chronically deafferented cerebellum a second J.F. stimulation evoked a full size antidromic spike potential at an interval of 3 msec. There was a gradual decline in size down to intervals of about 2 msec, and at briefer intervals, to 1 msec, there was a small residual spike potential that possibly is due to transmission into the Purkinje cell axon collaterals at intervals too brief for soma-dendritic invasion. With repetitive stimulation there was a well maintained soma-dendritic invasion at a frequency as high as 300/sec.6. In the chronically deafferented cerebellum an antidromic volley in the Purkinje cell axons caused a brief inhibitory silence of rhythmically discharging Purkinje cells. It is suggested that this is a direct inhibitory action of the Purkinje axon collaterals, that parallels their direct inhibitory action that has been demonstrated by Ito and collaborators (1964) on the intracerebellar nuclei and Deiters nucleus.7. In the chronically deafferented cerebellum an antidromic volley in the Purkinje axons produced not only the large negative spike potential indicative of antidromic soma-dendritic invasion, but also a later small and slow positive wave that appeared to be closely linked with the negative spike. It is shown how this would arise by current flow into the dendrites that had been depolarized but not excited by the initial antidromic invasion.