Figure 8. Reflex response of one masseter motor unit to near-painful electrical stimulation of the lip. This Fig. shows two different approaches to estimation of the synaptic potential in humans. The reflex response of one human masseter motor unit to 500 stimuli is expressed in PSTH and PSF. This stimulus classically produces two inhibitory responses followed by an excitatory reflex response. The number of counts in the PSTH illustrates this classic belief very precisely. According to the PSTH analysis, the inhibitory period finished at around 70 ms after the stimulus, when the excitatory reflex response started. The PSF record also showed that the inhibitory responses (gaps in the frequency record) do occur. However, according to the PSF, the inhibitory period lasts much longer and terminates at around 120 ms post-stimulus. Note that the reduction in the discharge frequency between 70 and 120 ms post-stimulus signifies the rising phase of the IPSP (from Fig. 7). Therefore, the large peak in the PSTH signifies not an excitatory reflex but rather the occurrence of delayed spikes during the rising phase of the IPSP (hence the continuation of the inhibitory reflex). The discharge frequency then increases, reaching its peak around 140 ms. Therefore, the PSF analysis shows that there were two periods of inhibitory responses that lasted to about 120 ms which were then followed by an excitatory response. The vertical line indicates the timing of the stimulus (time zero). Adapted from Türker and Cheng (1994) with permission from Elsevier Science.