Some properties of spinal gamma‐motoneurones in the cat, determined by micro‐electrode recording.

Abstract

Micro-electrode recordings were made from motoneurons in the lumbo-sacral region of the cat spinal cord whose axonal conduction velocities were 10-55 m/s. Most may be fusimotor in function. Intracellular records from 12 .gamma.-motoneurons revealed 6 with short (2-4 ms) and 6 with long (30-100 ms) duration after-hyperpolarizations following an antidromically conducted action potential. Using extracellular recording, the excitability of 89 other .gamma.-motoneurons following an antidromic impulse was tested with a 2nd antidromic action potential. In 84 of these neurons, the minimum antidromic response interval was short, 1.5-3.5 ms, implying that in most .gamma.-motoneurons, after-hyperpolarization was of limited effectiveness and of short duration. In the remaining 5 neurons, the minimum response interval was longer, 20-80 ms. There was a lack of monosynaptic excitation from group 1 afferent axons in the dorsal roots in 11 of the 12 motoneurons from which intracellular records were obtained. Polysynaptic excitation was commonly observed. In these anesthetized preparations, there was a lack of recurrent IPSP (inhibitory post-synaptic potential) even though such evidence of Renshaw inhibition could be found in the neighboring .alpha.-motoneurons. The mean input resistance of .gamma.-motoneurons was 1.55 M.OMEGA. and the principal time constant 8.5 ms by passing hyperpolarizing current through the recording micro-electrode in a bridge circuit. Depolarizing current passed through the recording micro-electrode caused a maintained discharge of action potentials at a high rate. After-hyperpolarization had little effect on discharge rate. The threshold for injected current to cause discharge was very low, and the discharge rate increased rapidly with the magnitude of the current. These properties of .gamma.-motoneurons are discussed in relation to their function.