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 musc le 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 superimpos ed a sinusoidal stretch (10 Hz) of five different amplitudes (50, 100, 500, 1000 and 2000 mu m peak to peak). The response of the Ia afferents to the sinusoidally overlaid ramp-and-hold stretch was subjected to computer analy sis, by means of which the response to the superimposed sinusoids and the r esponse to the underlying ramp-and-hold stretch were each obtained separate ly. Four basic discharge frequencies were determined from the response to t he underlying ramp-and-hold stretch (obtained after elimination of the resp onse to the concomitant sinusoidal stretch). The evaluation yielded the res ult that the concomitant sinusoidal stretch affected the response to the un derlying ramp-and-hold stretch: the level of discharge of the Ia afferent w as significantly increased and the dynamic and static indices were signific antly diminished. These two effects intensified with increasing amplitude o f 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 frequ encies were read. Evaluation showed that the response to the underlying ram p-and-hold stretch affected the modulation depth of the oscillatory respons es, 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 ra mp-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 curre nts occurring at the Ia sensory terminals. It is assumed that the probabili ty of the stretch activated (SA) channels opening is increased by a stretch ing of the sensory terminals. A depolarizing Na+ and Ca++ inward current du e to the activation of the SA channels in its turn activates a Ca++-activat ed 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 o utward currents that are sinusoid-related and those that are ramp-and-hold- stretch related interact. (C) 1999 Published by Elsevier Science B.V. All r ights reserved.