Firing behaviour of dorsal spinocerebellar tract neurones.

1. The repetitive discharge evoked by constant current injection from an intracellular micropipette has been studied in dorsal spinocerebellar tract cells of the cat. 2. The discharge frequency decreased with time, the decrease being more pronounced at high current intensities. Most of the frequency change occurred during the first ten intervals but the decrease continued slowly for several seconds. In some cells the frequency rose initially, the first interspike interval being larger than immediately succeeding ones. 3. The frequency-current (f/I) curves for the first interspike intervals were S-shaped, as found in spinal motoneurones. 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 msec after current onset. 4. The experimental f/I curves were compared with the f/I curves obtained with a simple neurone model based on the properties of the postspike afterhyperpolarization. For the first interspike interval there was a good agreement between the experimental and calculated f/I curves of individual neurones 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 neurones, 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 neurone model. 5. It is concluded that the conductance process underlying the postspike afterhyperpolarization is a major factor in the regulation of repetitive firing in dorsal spinocerebellar tract neurones.