Impulse patterns of Ia afferents and Ia-activated DSCT neurons during sinusoidal muscle stretch in cats.

The cat gastrocnemius muscles of one hind leg were stretched sinusoidally with amplitudes between 10 micrometer and 2.5 mm and frequencies between 1 and 30 Hz. The stretch response of deefferented muscle spindle afferents and of Ia-activated cells within Clarke's column were investigated by means of extra-cellular recordings of action potentials. The responses to 20-50 cycles were displayed in impulse patterns (raster diagrams) of the responding action potentials. The impulse patterns of Ia afferents exhibited a high degree of phase-locking (regularity) on the stretch cycle of amplitudes of about 50 micrometers at 3 Hz and all higher amplitudes or frequencies. At comparable stretch parameters the regularity in Ia afferents was 4-6 times larger than in Ia-activated DSCT neurons. The regularity in the DSCT patterns increased with an increase in stretch frequency. The impulse patterns of DSCT cells exhibited a high negative correlation between successive interspike intervals (-0.4 to -0.6) at low stretch frequencies (less than 3 Hz), which decreased with an increase in stretch frequency.