Stretch reflex in human tibialis anterior motor units

The reflex responses of the tibialis anterior motor units to stretch stimulus were investigated to pinpoint synaptic connection of muscle spindle afferents to tibialis anterior motoneurones in human subjects. Three types of stretch stimuli were applied and results were analysed using two different methods. The stimuli were simple tap-like stretch; ramp-and-hold stretch; and half-sine stretch. Stimulus induced responses in single motor units were analysed using the classical technique which involved building averaged surface electromyogram (SEMG) and peristimulus time histograms (PSTH) from the firing times of motor units; and, peristimulus frequencygram (PSF) from the instantaneous discharge rates of the single motor units. Using SEMG and PSTH as the indicator, we found that the tap-like stretch stimulus on average induced five separate reflex responses, the first response was a short lasting excitatory response (M1) or the stretch reflex, the second was a silent period (SP1), the third was a medium latency excitation or M2, the fourth was a long latency silent period (SP2, latency around 85 ms), and the fifth was a broad peak (M3). Using the same single motor unit data, the PSF technique indicated that the tap to the tendon induced only two excitatory reflex responses, M1 and M2. The PSF displayed that the first two of the excitatory reflexes in the PSTH did in fact induce significant increases in the discharge rate of the underlying unit. The third excitatory response in the PSTH, on the other hand, did not indicate any increase in the instantaneous discharge rate. Furthermore, the silent periods in the PSTH were in fact continuations of the net excitatory effects as the discharge rate of the low number of occurrence during these ‘silent’ periods were in fact were higher than the prestimulus discharge rates. Stretch stimuli in the shape of ramp-and-hold induced several peaks and troughs in the SEMG and PSTH. PSF analyses displayed genuine increases in discharge rates underlying most of the peaks and troughs. Half-sine stretch stimulus induced a very long lasting excitation followed by a long lasting inhibition in SEMG and PSTH. The increase in the discharge rate however lasted for the entire duration of the stimulus and continued even during the ‘inhibitory’ phase as indicated in the SEMG and PSTH. The fact that the discharge rate of a motoneuron has a strong positive linear relationship with the effective synaptic current it receives and hence represents changes in the membrane potential more accurately. However, it is suggested that both probability and discharge rate based methods should be used as most of our current knowledge have been obtained using SEMG and PSTH methods.