Effects of calcium on the discharge pattern of primary and secondary endings of isolated cat muscle spindles recorded under a ramp-and-hold stretch

The impulse activity of muscle spindles isolated from the cat tenuissimus m uscle was investigated under varying concentrations of external calcium (Ca 2+). The outer capsule of the muscle spindle represents an effective diffus ion barrier for Ca2+ ions since activity changes were strong and rapid only if the capsule was partly removed from the sensory region of the receptor. The impulse activity of both primary and secondary muscle spindle endings was lowered by an increase in the external Ca2+ concentration from 1.8 mM ( normal Ringer's solution) to 2.7 mM and raised by a decrease in the Ca2+ co ncentration from 1.8 to 0.9 mM. Primary endings were generally more strongl y affected than secondary endings. With primary endings the firing rate cha nged by 23-52% when the external Ca2+ concentration was altered by 0.9 mM. With secondary endings the discharge frequency changed by 15-24%. The affer ent discharge patterns were obtained under repetitive ramp-and-hold stretch es and were analyzed with regard to influences of external Ca2+ ions on the static and dynamic components of the endings' responses. The stretch sensi tivity and the adaptive response of both types of ending increased in the l ow Ca2+ solution and decreased in the high Ca2+ solution, but a specific ef fect on a single component of the responses to stretch was not observed. Th ese findings indicate an overall change in excitability when the external C a2+ concentration was varied. The mechanical properties of the receptor wer e probably not affected since changes in the Ca2+ concentration did not eli cit a contraction or relaxation of the intrafusal muscle fibers. On the one hand, the observed effects can be explained according to the surface poten tial theory by an indirect influence of extracellular Ca2+ ions on ion chan nels of the sensory nerve terminals, with Ca2+ ions binding to negative cha rged sires at the endings' outer membrane. On the other hand, the results a re consistent with the supposition that Ca2+ ions act directly on ion chann els of the sensory membrane of muscle spindle endings. (C) 2000 Elsevier Sc ience B.V. All rights reserved.