Tendon tap induces a single long-lasting excitatory reflex in the motoneurons of human soleus muscle.

The reflex responses of the soleus motor units to Achilles tendon taps were investigated. Two different techniques were used to analyse the motor unit data. The first approach was the classical technique which involved building peristimulus time histograms (PSTH) from the firing times of single motor units. The second approach was a relatively unused technique that involved plotting the instantaneous discharge frequency of the single motor unit against time (peristimulus frequencygram or PSF). Using PSTH as the indicator, we found that the tap to the tendon induced three separate reflex responses: the first response was a very short-lasting excitatory response or the jerk reflex, the second was a period of relative silence (silent period or the "trough"), and the third was a broad peak 170 ms after the stimulus. Using the same motor unit data, the PSF technique indicated that the tap to the tendon induced a single long-lasting excitatory reflex. The PSF displayed an increase starting from the latency of the jerk reflex and continuing for about 65 ms. There was no significant change in the discharge frequency at the end of the first excitatory response. Since the discharge frequency of a motoneuron has a strong positive linear relationship with the effective synaptic current it receives, it is suggested that throughout the 65-ms period the net (effective) synaptic drive to the soleus motoneurons was excitatory. It is therefore concluded that tendon tap induces a single long-lasting excitatory reflex in the motoneurons of the soleus muscle.