Muscle spindle afferent studies in the baboon.

1. The muscle spindle afferent conduction velocity and response to muscle twitch and stretch in young baboons has been recorded in order to find a conduction velocity that can be used to separate primary and secondary afferents.2. A number of the features of the response of the spindle afferents to stretch were examined. It was found that none could distinguish between primary and secondary afferents with greater certainty than the conduction velocity.3. Spindle afferents with conduction velocities below 50 m/sec can be classified as secondary and those with conduction velocities above 68 m/sec can be classified as primary with a reasonable degree of certainty.4. The spindle afferents with conduction velocities between 51 and 67 m/sec are most likely not a separate intermediate group but represent a region of overlap between the two groups.5. Comparisons of the difference between the response of primary and secondary afferents to overstretch suggested a mechanism to explain the saturation of the primary afferent response at frequencies far below those at which it is capable of firing.