Excitability and firing behavior of single slow motor axons transmitting natural repetitive firing of human motoneurons.

Excitability of motor axons is critically important for realizing their main function, i.e., transmitting motoneuron firing to muscle fibers. The present study was designed to explore excitability recovery and firing behavior in single slow axons transmitting human motoneuron firing during voluntary muscle contractions. The abductor digiti minimi, flexor carpi ulnaris, and tibialis anterior were investigated during threshold stimulation of corresponding motor nerves. Motor unit (MU) firing index in response to testing volleys evoking M-responses was used as a physiological measure of axonal excitability and its changes throughout a target interspike interval (ISI) were explored. It was shown that axons displayed an early irresponsive period (within the first ~2-5 ms of a target ISI) that was followed by a responsive period (for the next 5-17 ms of the ISI), in which MUs fired axonal doublets, and a later irresponsive period. At the beginning of the responsive period, M-responses showed small latency delays. However, since at that ISI moment, MUs displayed excitability recovery with high firing index, slight latency changes may be considered as a functionally insignificant phenomenon. The duration of axonal doublet ISIs did not depend on motoneuron firing frequencies (range 4.3-14.6 imp/s). The question of whether or not traditionally described axonal recovery excitability cycle is realistic in natural motor control is discussed. In conclusion, the present approach, exploring, for the first time, excitability recovery in single slow axons during motoneuron natural activation, can provide further insight into axonal firing behavior in normal states and diseases.