[Correlation of the discharge frequency of a firing motor neuron and the efficiency of an excitatory volley of Ia-afferents in man].

Single motor unit potentials of the flexor carpi ulnaris muscle were recorded during voluntary contraction and H-reflex was evoked. The afferent volley effectiveness was estimated by the firing index of a motoneuron. The background firing rate of motor units ranged from 3.3 to 16.0 imp/s. Afferent volley effectiveness depended on the background firing rate of fast muscle units under study like slow soleus muscle units investigated earlier. The effectiveness of afferent volley decreased with the motor units firing rate growth. At all range of firing rate the efficiency of afferent volley was higher for motor units of fast muscle as compared to slow muscle. It is suggested that the motoneuron excitability changes within the interspike interval underlay the relationship between the motoneuron firing rate and effectiveness of the excitatory afferent volley.