Enhancement of stretch sensitivity of cat primary spindle afferents by low-rate static gamma-action

Abstract

Static and dynamic fusimotor neurones are most easily distinguished by their — sensitivity-reducing vs -increasing — action on the dynamic Ia response to large stretch, during stimulation at high rates (e.g. 100/s). However, occasionally high-rate static γ-action (as recognized by relying on a number of independent criteria; Emonet-Dénand et al., 1977) may also cause a modest increase of the dynamic index. Yet this measure, being a one point estimate of dynamic response, is slightly arbitrary, and rigorous analysis of Ia responses to sinusoidal stretch of widely ranging amplitude has so far revealed a uniform sensitivity-reducing action of static γ-axons. This was true for sine frequencies up to 20 Hz and for high stimulation rates (Goodwin et al., 1975; Hulliger et al., 1977). However, qualitative observations suggested that this reduction of sensitivity might sometimes be restricted to such high stimulation rates, whereas low-rate (20–30/s) γ_s-action could, paradoxically, enhance Ia sensitivity to sinusoids (Emonet-Dénand et al., 1972).