Frog muscle spindles with unbranched myelinated afferent axons: the response to stretch and the length of the first myelinated segment.

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Abstract
Muscle spindles (25) innervated by unbranched myelinated axons in the capsule were isolated from sartorius muscles in young frogs [Rana nigromaculata] (2.2-9.5 g in weight, 28-47 mm in rostro-caudal length). The lengths and the diameters of the 1st myelinated segments varied from 30-170 .mu.m and from 9-20 .mu.m, respectively. There was no relationship between the lengths and the diameters. Dynamic and static components were analyzed from discharge rates of the muscle spindles during ramp-and-hold stretches of 0.8 mm from different initial lengths. The values of the dynamic components to a certain stretch stimulation increased with shortening in the length of the 1st myelinated segment. The values of the static components were independent in length. The amplitudes of action potentials recorded from the spindle terminal decreased during the dynamic phase of the stretch. The ratio of amplitude decrease at the end of the dynamic phase vs. the initial length depended upon the length of the 1st myelinated segment. The discharges during stretch may arise at the 1st node, though the spontaneous discharges may be generated at the terminal.