Motoneuron electrophysiological and muscle contractile properties of superior oblique motor units in cat

Abstract

Intracellular techniques were used to study single motor units of the trochlear nucleus and superior oblique muscle in the cat. Motoneuron electrophysiological properties were correlated with muscle-unit contractile characteristics assessed under isometric conditions. Two distinct motor-unit types were identified and designated as twitch and nontwitch. Nontwitch units made up 5% of the total population studied. They responded only to tetanic stimulation with graded force that increased as stimulus frequency was increased up to 300-400 Hz. These units made up a homogeneous population in that they were innervated by slowly conducting axons, produced weak tetanic tensions, and were extremely fatigue resistant. Twitch units made up the majority (95%) of units studied. These units responded to single pulse stimulation with typical twitch contractions. The contraction speed and tension ranges for these units were comparable with those obtained from other extraocular muscle single units. Superior oblique twitch units, mechanically comparable with multiply innervated conducting units, identified in the cat inferior oblique muscle (31) were not observed. The twitch-unit population was heterogeneous in terms of neuromuscular fatigue resistance. Unit fatigability was inversely related to maximal tetanic tension. Motoneuron conduction velocity was related to muscle-unit contractile properties in a way similar to that seen in extremity motor units. The slowest twitch units were weak, fatigue resistant, and innervated by slow conducting axons. The fastest units were, in general, innervated by faster conducting axons, produced greater tetanic tensions, and were more susceptible to fatigue. Correlations among input resistance, rheobase, and conduction velocity were also observed. At present, subdivisions of the twitch-unit population on the basis of any one or combination of unit properties does not seem appropriate.