Properties of motor units in a small foot muscle of Xenopus laevis.

1. Extensor digitorum longus IV muscle (EDL IV) of Xenopus laevis was isolated together with its nerve and the 8th and 9th spinal roots. Motor units were functionally isolated and their properties investigated. 2. The motor unit isometric twitch and tetanic tensions, contraction time and axon conduction time and distance were measured. Axon conduction velocity and twitch:tetanus ratio were calculated. 3. The distributions of motor unit properties were continuous and showed no tendency to separate into types. 4. Axon conduction velocity was directly related to unit tetanic tension (expressed as a percentage of whole muscle tetanic tension and to twitch:tetanus ratio and inversely related to contraction time. Unit tetanic tension was directly related to twitch:tetanus ratio and inversely related to contraction time. Contraction time was inversely related to twitch: tetanus ratio. Possible explanations for these relationships are discussed. 5. The units of EDL IV show a greater resistance to fatigue than fast units of other amphibian muscles. There was no separation of units into fatiguable and fatigue resistant types.