Mechanical influence of the extrafusal muscle on the static behaviour of deefferented primary muscle spindle endings in cat

Summary In 16 anesthetized cats 24 afferents of deefferented primary muscle spindle endings from the extensor digitorum longus muscle (EDL) were functionally isolated and their sensitivity to static stretches was tested. Each spindle was carefully located within the EDL using direct focal stimulation. Muscle spindle endings with poor static length sensitivity of stretch were always found in the most peripheral parts of the muscle fibre bundles, near to the aponeurotic and tendineous insertions. Endings having good static sensitivity to stretch were exclusively found in more central portions of the muscle fibre bundles. Using a photographic method, the alterations in the distance between designated points on the muscle surface ware measured during maintained muscle stretch. Only medial portions were markedly extended, whereas little or no increases in length occurred in peripheral parts of the muscle fibre bundles.Quantitatively, a direct relationship was found between the lengthening of those muscle portions, in which the muscle spindles were localized, and the receptors' sensitivity to maintained stretch.Preliminary reports of some of the results were given at the 1. Wissenschaftliche Tagung der Deutschen Gesellschaft für medizinische Physik (Rieboldet al., 1970; Robrechtet al., 1970) and at the Muscle-Spindle Symposium of the Anatomical Society of Great Britain and Ireland, April 4–6, 1974, at the University of Durham (Henatschet al., 1974). The results were presented in more detail in Meyer-Lohmann's Habilitationsschrift (1972).Supported by a grant from the Deutsche Forschungsgemeinschaft (Schwer-punktprogramm Rezeptorphysiologie).