Threshold current for repetitive impulse firing in motoneurones innervating muscle fibres of different fatigue sensitivity in the cat.

In anaesthetized cats, alpha motoneurones of m. gastrocnemius medialis were activated by maintained currents that were injected via an intracellular microelectrode. There was a statistically significant correlation between the threshold current for maintained repetitive firing and axonal conduction velocity. These findings confirmed that slow-axoned motoneurones tend to be more excitable than those with faster axons. Among fast-twitch motoneurones of about the same size as judged by their axonal conduction velocity, the average threshold current was about twice as high for cells innervating fatigue-sensitive muscle fibres (FF units) than for those supplying more fatigue-resistant ones (FR units).