Effect of repetitive activation on the afterhyperpolarization in dorsal spinocerebellar tract neurones.

1. The changes in the afterhyperpolarization (a.h.p.) with repetitive activation have been studied in dorsal spinocerebellar tract cells of the cat using intracellular recording techniques. 2. The a.h.p. following a single spike was conditioned at different interspike intervals by a single preceding spike. In the majority of neurones the a.h.p. following a spike added approximately linearly with that generated by a preceding spike. 3. In other cells the a.h.p. following a spike was instead depressed by a preceding spike. THis depression was approximately constant at interspike intervals less than the a.h.p. duration (50-100 msec). Thereafter the a.h.p. slowly recovered during the next 100-300 msec. There was no associated decrease in the initial brief hyperpolarizing undershoot. 4. With shortlasting repetitive activation at high frequency (greater than 100 impulses/sec) the a.h.p, peak amplitude increased progressively with successive spikes (5-15 spikes). No change in the time constant of decay was observed. A good correspondence was found between the observed increase in peak amplitude of the a.h.p.s and that given by a theoretical linear superposition of the successive a.h.p.s. 5. Changes in the brief hyperpolarizing undershoot with repetitive activation is also described.