[Discharge frequency and excitability of human firing motoneurone].

The excitability of firing motoneurones activated by voluntary contraction of the flexor carpi ulnaris or tibialis anterior was tested by single excitatory Ia afferent volleys. In order to estimate the stimulation effects, the peri-stimulus time histograms of single motoneurones were plotted, and the firing indices were calculated. It was shown that the firing-frequency effect was absent within the range of 4-14 imp/s during testing by low-intensity excitatory volley. At higher intensity of afferent volley, the excitability increased at a low firing rate. It is suggested that the characteristics of the interspike-interval excitability trajectories underlie these relations. These findings made it possible to explicate some conflicting literature data which were usually reported without taking the afferent volley intensity effect into account. The mechanisms controlling the firing motoneurone excitability and possible trajectories of interspike-interval membrane potential in human motoneurones are discussed.