The response of muscle spindle primary afferents to simultaneously presented sinusoidal and ramp-and-hold stretches.

Fifteen primary (Ia) muscle spindle afferents from the tibial anterior muscle of the cat were subjected to a ramp-and-hold stretch (stretch rate 10 mm/s, stretch amplitude 8.5 or 7 mm) of the muscle, upon which was superimposed a sinusoidal stretch (10 Hz) of five different amplitudes (50, 100, 500, 1000 and 2000 microm peak to peak). The response of the Ia afferents to the sinusoidally overlaid ramp-and-hold stretch was subjected to computer analysis, by means of which the response to the superimposed sinusoids and the response to the underlying ramp-and-hold stretch were each obtained separately. Four basic discharge frequencies were determined from the response to the underlying ramp-and-hold stretch (obtained after elimination of the response to the concomitant sinusoidal stretch). The evaluation yielded the result that the concomitant sinusoidal stretch affected the response to the underlying ramp-and-hold stretch: the level of discharge of the Ia afferent was significantly increased and the dynamic and static indices were significantly diminished. These two effects intensified with increasing amplitude of the concomitant sinusoids. From among the oscillatory responses of the Ia afferent to the concomitant sinusoidal stretch (obtained after elimination of the response to the underlying ramp-and-hold stretch) the amplitudes of four selected responses were determined. These four oscillatory responses occurred during four time spans during which the four basic discharge frequencies were read. Evaluation showed that the response to the underlying ramp-and-hold stretch affected the modulation depth of the oscillatory responses, which decreased during the ramp and increased again during the plateau. The absolute amount of the change was independent of the amplitude of the sinusoids. The effect of the sinusoids on the response to the underlying ramp-and-hold stretch and that of the underlying ramp-and-hold stretch on the response to the sinusoidal stretches are interpreted in terms of ion currents occurring at the Ia sensory terminals. It is assumed that the probability of the stretch activated (SA) channels opening is increased by a stretching of the sensory terminals. A depolarizing Na+ and Ca++ inward current due to the activation of the SA channels in its turn activates a Ca++-activated and/or a potential-dependent K+ outward current which has a repolarizing effect. Ideas are put forward in the discussion as to how the inward and outward currents that are sinusoid-related and those that are ramp-and-hold-stretch related interact.