Responses of cat dorsal spino-cerebellar tract neurons to sinusoidal stretching of the gastrocnemius muscle.

A defined class of cells within the nucleus dorsalis (Clarke's column) receives excitatory input from Ia afferents of mainly one muscle. Action potentials were recorded from axons of these cells (DSCT neurons) which are excited by Ia afferents of the gastrocnemius muscles. We investigated the response to sinusoidal muscle stretch over a wide range of amplitudes (10 microns-4 mm) and frequencies (0.1-130 Hz) in the deefferented preparation. The dynamic stretch was superimposed on a moderate static muscle stretch to ensure that the muscle was not slack during the phase of release. The response up to 10 Hz was displayed as PST histograms (cycle histograms) and a sinewave of stretch frequency was fitted to the PST histograms to define amplitude and phase of a response sinewave. At a constant frequency of about 3 Hz, the relation between stretch amplitude and response amplitude could well be described by decelerating intensity functions: the hyperbolic or tanh log function and a modified power function (exponent 0.48 +/- 0.12). The phase lead of the response sinewave increased with increasing stretch amplitudes of up to 0.5 mm and then decreased. At constant stretch amplitudes of 0.5-2.0 mm the frequency response was investigated. In relation to stretch frequencies between 0.1 and 1 Hz an increase in the response amplitude of 4.4 dB was observed and an increase for 13 dB/decade between 3 and 10 Hz. At 0.1 Hz the phase of the response sinewave was 48 degrees in advance and increased to a maximum lead of 89 degrees at 6-8 Hz. Above 10 Hz the positions of the responding action potentials with respect to the stretch cycle were used to define a phase, which was in advance up to 60 Hz but decreased and changed to a phase lag at higher frequencies. If in PST histograms no periods of silence occurred during the phase of stretch release, the mean discharge rate was found to be independent of the sinusoidal stretching. If the pauses were present the mean rate increased with increasing stretch frequencies or amplitudes.