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

1. Micro-electorde recordings were made from motoneurones in the lumbo-sacral region of the cat spinal cord whose axonal conduction velocities were 10--55 m/sec. Most of these may be presumed to be fusimotor in function. 2. Intracellular records from twelve gamma-motoneurones revealed six with short (2--4 msec) and six with long (30--100 msec) duration after-hyperpolarizations following an antidromically conducted action potential. 3. Using extracellular recording, the excitability of eighty-nine other gamma-motoneurones following an antidromic impulse was tested with a second antidromic action potential. In eighty-four of these neurones, the minimum antidromic response interval was short, 1.5--3.5 msec, implying that in most gamma-motoneurones, after-hyperpolarization was of limited effectiveness and of short duration. In the remaining five neurones, the minimum response interval was longer, 20--80 msec. 4. There was a lack of monosynaptic excitation from group 1 afferent axons in the dorsal roots in eleven of the twelve motoneurones from which intracellular records were obtained. Polysynaptic excitation was commonly observed. 5. In these anaesthetized preparations, there was a lack of recurrent i.p.s.p.s even though such evidence of Renshaw inhibition could be found in the neighbouring alpha-motoneurones. 6. The mean input resistance of gamma-motoneurones was shown to be 1.55 Momega and the principal time constant 8.5 msec by passing hyperpolarizing current through the recording micro-electrode in a bridge circuit. These values are open to error because of the small numbers of neurones investigated, and of the use of the single micro-electrode method. 7. 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. 8. These properties of gamma-motoneurones arediscussed in relation to their function.