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

1. Twenty-five muscle spindles innervated by unbranched myelinated axons in the capsule were isolated from sartorius muscle in young frogs (2-2--9-5 g in weight, 28-47 mm in rostro-caudal length). 2. The lengths and the diameters of the first myelinated segments varied from 30 to 170 mum and from 9 to 20 mum respectively. There was no relationship between the lengths and the diameters. 3. Dynamic and static components were analysed 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 first myelinated segment. The values of the static components were independent in length. 4. 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 versus the initial length depended upon the length of the first myelinated segment. 5. These results suggest that the discharges during stretch may arise at the first node, though the spontaneous discharges may be generated at the terminal.