Changes in primary muscle spindle ending excitability induced by a ramp-and-hold stretch

36 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 5 or 8.5 mm) of the muscle, on which a sinusoidal stretch (50 Hz) of four different amplitudes (25, 50, 250 and 500 microns) was superimposed. In 54 discharge patterns a Ia afferent subjected to a ramp-and-hold stretch with a sinusoidal stretch superimposed responded only to the superimposed sinusoidal stretch. In 25 of the cases the Ia afferent responded with an one-to-one driven action potential (AP) and in 29 of the cases with two APs per sinusoidal stretch. For these 54 discharge patterns the phase of the sinusoidal cycle was determined at which each AP occurred. Where the Ia afferent responded with one AP per cycle an accelerating phase advance was observed during the ramp stage of the underlying ramp-and-hold stretch and a decelerating phase advance during the plateau. This phase shift means that the excitability of the site generating the AP increased during the ramp stage and decreased during the plateau. If the Ia afferent responded with two APs per superimposed cycle, the second AP per cycle evinced a decelerating phase advance during the ramp and an accelerating phase advance during the plateau. The phase of the second AP per cycle showed a second, contrary change in excitability at the AP generating site. The excitability decreased during the ramp and increased during the plateau. The first kind of excitability change is interpreted as a consequence of an inward current at the AP generating site. The second, contrary type of excitability points to an interplay between an inward and an outward current. An increasing outward current lowers the excitability for the second AP per cycle during the ramp. A decreasing outward current raises the excitability during the plateau.