[Functional specialization of stretch receptors in the triceps surae muscle of cats].

The behavior of 300 receptors in response to stepwise changes in muscle length was studied in passive m. triceps surae of cats anesthetized with nembutal. Distinct differences in receptor behavior were demonstrated, and receptors were divided into three functional groups according to their sensitivity to slow stretch (0.6 mm/sec in the limit range of 10 mm). The 1st group (elementary dynamic receptors) showed obvious sensitivity (over 70/sec) to dynamic components of stimulus and did not respond to static stretch in the physiological range. It was impossible to study the silent period during muscle contraction and stretch. The units of the 2nd group (elementary static receptors) reached maximal discharge frequency of 20--25/sec in the same range of muscle stationary stretch. The dynamic response of these receptors slightly exceeds the static response. The 3rd group, comprising 80% of units, served as universal detector of both dynamic and static phases of a stimulus. Some units showed significant dynamic response (over 70/sec) and moderate static response (about 30--40/sec), others exhibited moderate dynamic and static responses. The behavior of the receptors of universal type is similar to that of the well--known primary and secondary endings of the muscle spindles. Asto the endings of two elementary types they should be further indentified.