SPIKE DISCHARGES OF SKELETOMOTOR NEURONS DURING RANDOM NOISE MODULATED TRANSMEMBRANE CURRENT STIMULATION AND MUSCLE STRETCH

Spike discharges of skeletomotor neurons innervating triceps surae mus cles elicited by white noise modulated transmembrane current stimulati on and muscle stretch were studied in decerebrated cats. The white noi se modulated current intensity ranged from 4.3 to 63.2 nA peak-to-peak , while muscle stretches ranged from 100 mu m to 4.26 mm peak-to-peak. The neuronal responses were studied by averaging the muscle length re cords centered at the skeletomotor action potentials (peri-spike avera ge, PSA) and by Wiener analysis. Skeletomotor spikes appeared after a sharp peak in PSA of the injected current, preceded by a longer-lastin g smaller wavelet of either depolarizing or hyperpolarizing direction. The PSA amplitude was not related to the injected current amplitude n or showed any differences related to the motor unit type. The PSA ampl itudes were virtually independent of the stretching amplitude sigma, a fter an initial increase with stretching amplitudes in the range of 15 -40 mu m (S.D.), or 100-270 mu m peak-to-peak. Analyses of cross-spect ra indicated a small or absent increase in gain with frequency in resp onse to injected current, but about 20 dB/decade in the range 10-100 H z in response to muscle stretch. The peaks of both Wiener kernels in r esponse to current injection appear to decrease with the amplitude of injected current, but this decrease was not statistically significant. The narrow first-order kernels suggest that the transfer function bet ween the current input and spike discharge is lowpass with a wide pass band, i.e. there is very little change in dynamics. The values of the second-order kernels appear to be nonzero only along the main diagonal . This is characteristic of a simple Hammerstein type cascade, i.e. a zero memory nonlinearity followed by a linear system. Small values of second-order kernels away from the origin and narrow first-order kerne ls suggest that the linear cascade contributes very little to the over all dynamic response. In contrast to Wiener kernels found in response to current injection, the Wiener kernels in response to stretch showed a decreasing trend with stretch amplitude. The size of the second-ord er kernels decreased to a somewhat larger extent with input amplitude than that of the first-order kernels, indicating an amplitude-dependen t nonlinearity. Overall, the transformation between length and spike o utput was described as an LNNL cascade with second-order nonlinearitie s.