Responses of human soleus motor units to low-threshold stimulation of the tibial nerve

Abstract

The peristimulus frequencygram (PSF) has recently been shown to illustrate postsynaptic potentials of motoneurones much more reliably than the peristimulus time histogram (PSTH). The aim of this investigation was to examine the profile of the postsynaptic potential (PSP) in soleus motoneurones in response to an H-reflex with and without accompanying M waves of different magnitude by using PSTH and PSF profiles of single motor units. Nine men and five women healthy subjects participated in this study. Electrical stimuli were delivered to the tibial nerve in the popliteal fossa. The reflex response of the soleus muscle was recorded using both surface electromyogram and single motor unit potentials. The PSTH analysis demonstrated that there were four different synaptic events following low-intensity stimulation of the tibial nerve: primary enhancement in firing probability (H-reflex or E1), primary reduction in firing probability (primary silent period or SP1), secondary reduction in firing probability (secondary silent period or SP2), and secondary enhancement in firing probability (E2). On the other hand, the PSF analysis indicated only two reflex responses, long-lasting enhancement in discharge rate including the H-reflex (LLE) and long-lasting decrease in discharge rate (LLD). The results of the two analyses methods are compared and contrasted. While the PSTH demonstrated that there was a silent period (SP1) immediately following the H-reflex, the PSF indicated an increase in discharge rate during the same period. The PSF also indicated that, during SP2 and E2, the discharge rate actually decreased (LLD). It was therefore suggested that LLD involved activation of several inhibitory pathways including the autogenic inhibition of units via the Golgi tendon organs. It was concluded that the PSF could indicate the details of the postsynaptic potentials and is very useful for bringing out previously unknown effects of electrical stimulation of muscle nerves.

Analysis of motoneuron responses to composite synaptic volleys (computer simulation study)

“…The response in Fig. 7a of the present paper and in other experimental PSFs presented in (Binboğa et al 2011; Prasartwuth et al 2008) is limited to the duration of PSTH peak and often does not exceed pre-stimulus rates; thus, the decaying phase of the EPSP may be responsible for the extra discharges only during its short initial fragment (see PSP profile in Fig. 7b).…”

“…This shortening, corresponding to firing rate increase, is repeatedly interpreted as the influence of long-lasting excitation (e.g. Binboğa et al 2011; Prasartwuth et al 2008). To resolve this discrepancy, we performed simulations aimed toward the reproduction of the ‘H-reflex only’ results obtained from the tibialis anterior muscle by the stimulation of the peroneal nerve (Prasartwuth et al 2008, their Fig.…”

“…Similar time characteristics of Ia EPSPs (5–20 and 2–5 ms, respectively) were estimated in human MNs (Ashby and Zilm 1982a). Yet in later studies, the duration of human Ia EPSPs was claimed to be longer, up to 70 ms (Binboğa et al 2011). …”

“…The analysis revealed that these changes consist of a single primary and several secondary effects; one of the latter is still interpreted in certain experimental studies as “long-lasting excitation” (e.g. Binboğa et al 2011; Prasartwuth et al 2008). That is why we decided to discuss this problem in this paper in more detail.…”

Reflex responses of human masseter motor units to mechanical stimulation of the teeth

“…As has been previously shown in regularly discharging motoneurons in brain slices, the PSTH and SEMG techniques have count and synchronization errors that give wrong impressions regarding the neuronal networks (Türker and Powers 1999. A number of recent studies have compared these methods of spike analysis to illustrate the differences between them (Binboǧa et al 2011;Deriu et al 2005;Kahya et al 2010;Norton et al 2008;Prasartwuth et al 2008;Rogasch et al 2012;Todd et al 2012). In general, the following findings are noted from these recent studies.…”