Firing behaviour of dorsal spinocerebellar tract neurones.

Abstract

The repetitive discharge evoked by constant current injection from an intracellular micropipette was studied in dorsal spinocerebellar tract cells of the cat. The discharge frequency decreased with time, the decrease being more pronounced at high current intensities. Most of the frequency change occurred during the first 10 intervals but the decrease continued slowly for several seconds. In some cells the frequency rose initially, the 1st interspike interval being larger than immediately succeeding ones. The frequency-current (f/I) curves for the 1st interspike intervals were S-shaped, as found in spinal motoneurons. With successive intervals the lower leg of the f/I curve extended to higher frequencies, giving a progressive linearization of the f/I curves. In almost all cells this linearization was completed at 200 ms after current onset. The experimental f/I curves were compared with the f/I curves obtained with a simple neuron model based on the properties of the postspike afterhyperpolarization. For the 1st interspike interval there was a good agreement between the experimental and calculated f/I curves of individual neurons up to frequencies of several hundred impulses per second. In the high frequency range, it was necessary to compensate for changes in initial postspike voltage trajectories caused by the injected current. Other aspects of the firing of real neurons, such as the progressive linearization of the f/I curves, the negative adaptation and the changes in the interspike voltage trajectories with increasing current were also reproduced by the neuron model. Apparently the conductance process underlying the postspike afterhyperpolarization is a major factor in the regulation of repetitive firing in dorsal spinocerebellar tract neurons.