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REFLEX CONTROL OF HUMAN JAW MUSCLES

Department of Physiology, University of Adelaide, SA 5005, Australia; kemal.turker{at}adelaide.edu.au

View larger version (35K): [in a new window]   Figure 1. Methodology for a reproducible tooth, mucosa, and skin stimulation under static conditions. The subject sits comfortably with the upper teeth held in fixed relation to a probe mounted on the moving coil of a vibrator and bites into an impression of the upper and lower teeth on a metal bite bar. The impression material on the top part of the bite bar is cut away from around the area of stimulation (in this Fig., it is the upper central incisor tooth) so that the probe can 'move' the tooth. The shape, amplitude, and frequency of the stimulus wave are determined by means of a wave-generating computer program. The computer program is set to initiate random stimuli, with the interstimulus interval varying between 1 and 5 sec. Electrodes are placed to record the surface EMG from the ipsi- and contralateral masseter and digastric muscles. Force applied to the tooth is measured with strain gauges mounted in series with the stimulating probe. For feedback, the ipsilateral EMG signal is rectified and low-pass-filtered at 0.1 Hz, and displayed on an oscilloscope screen. The subject is asked to bite in such a way as to keep the level of activity of the ipsilateral masseter muscle at a pre-determined level (10-20% of the maximal voluntary contraction). In all experiments, the sound of the mechanical stimulus is masked by white noise played into earphones at 80-90 dB.

View larger version (26K): [in a new window]   Figure 2. Cumulative sum (CUSUM; Ellaway, 1978) of the ipsilateral masseter surface electromyography (SEMG) and bite force responses to mechanical stimulation of an upper incisor tooth. Calibration was in 'k' (average pre-stimulus background activity) for the CUSUM and in Newtons (the calibration bar for the force data indicates 1 N) for the bite force. Thick lines are the pre-local-anesthetic records, and thin lines are the records taken during the local anesthetic block. Stimulus delivery was at 0 ms. The response to unloading stimulus (2 N load taken off within 5 ms; 400 N/s) is shown at the top; the response to the tap stimulus (2 N load is delivered within 5 ms; 400 N/s) is shown at the bottom. The vertical dotted lines indicate the minimum reaction time for the subject. The relationship between the change in the load and the reflex response is illustrated in an expanded time scale in Fig. 3. Note that the inhibitory reflex disappears but the excitatory reflex stays the same or increases in size during the local anesthetic block of the stimulated tooth. Therefore, both unload and tap stimuli activated not only the PMRs but the spindles as well. Spindle response became obvious once the PMRs were blocked by the local anesthesia. See text for details.

View larger version (19K): [in a new window]   Figure 3. The relationship between the 'stretch' stimulus and the first excitatory response. All records are from trials during the local anesthetic block of the stimulated tooth. Thick lines represent the CUSUM of the ipsilateral masseter SEMG; thin lines represent the bite force. Note that the unload stimulus induces a reduction in the force record, and the tap stimulus induces an increase, followed by a decrease in the force record. Since there was no change in the electrical activity of the jaw muscles preceding these changes in the bite force, they are labeled as mechanical artifacts of the stimuli. This subtle yet consistent mechanical artifact is used for eliciting and studying the muscle spindle stretch reflex responses. Note also that the latency of the early excitatory reflex response is shorter in the trials with the unload stimuli, since the 'stretch' arrives earlier with the unload stimulus.

View larger version (27K): [in a new window]   Figure 4. Wiring diagram of human masticatory system. Current knowledge on the wiring diagram of the human masticatory muscles estimated under static conditions only. Note that the positions of the cell bodies of the neurons and the motoneurons are all deduced from reduced animal experiments.

View larger version (25K): [in a new window]   Figure 5. Peristimulus time histogram (PSTH) vs. peristimulus frequencygram (PSF) in representing the underlying synaptic potential. The PSTH (upper trace) and PSF (lower trace) produced by an EPSP. The dashed line in both traces is the estimated EPSP produced by the current transient injected into a rat hypoglossal motoneuron in a slice preparation. The PSTH exhibited a peak during the rising phase of the EPSP, followed by a trough. Note that following the increase in firing probability during the rising phase of the EPSP, two more peaks appeared in the PSTH, denoted by two pairs of vertical lines to the right of time zero. Looking at the PSTH alone, one would have to say that there was a series of excitations followed by inhibitions. On the other hand, the increase in discharge frequency in the PSF followed the time course of the EPSP correctly, indicating that there is only one long-lasting excitatory event occurring in the motoneuron. The three horizontal lines in the PSF record represent the mean background discharge rate calculated over negative time lags (solid line) ± 2 SD (dotted lines). There were 198 stimuli. Adapted from Türker and Powers (1999) with permission from the American Physiological Society.

View larger version (28K): [in a new window]   Figure 6. Reflex response of a single human soleus motor unit to the Achilles tendon tap. This Fig. shows two different approaches to estimating the synaptic potential in humans. The reflex response of a soleus motor unit to 838 tap stimuli is expressed by PSTH and PSF. PSTH records confirmed the classic belief that a tap to a tendon generates three different reflexes: tendon jerk reflex, inhibitory (refractory, silent) period, and long-latency excitation. The tendon jerk reflex is signified by a large number of counts in the PSTH at a latency of about 44 ms. This reflex response lasted for 5 ms and was followed by a silent period of about 60 ms. The silent period is signified by a small number of counts in the PSTH. There was also a 'long-latency' reflex starting around 120 ms after the stimulus. Alternatively, the PSF record showed only one reflex response, a single long-lasting excitation. There was a rapid increase in the discharge frequency coincident with the tendon jerk reflex latency. This increase lasted for about 65 ms, after which the discharge rate returned to the pre-stimulus level. Time zero (arrows) indicates the timing of the tendon tap. Adapted from Türker et al. (1997b) with permission from Springer-Verlag.

View larger version (31K): [in a new window]   Figure 7. Effects of an IPSP on motoneuron discharge probability and rate. The PSTH (upper trace) and PSF (lower trace) produced by an IPSP (dashed trace). Note that following the decrease in firing probability during the falling phase of the IPSP, three peaks appeared in the PSTH, denoted by three pairs of vertical lines to the right of time zero. The PSF produced by the same IPSP showed that the deepest part of the IPSP induced a gap in the PSF which was followed by a lower-frequency discharge during the rising phase of the IPSP. Looking at the PSTH alone, one would have to say that there was a series of inhibitions followed by excitations. The discharge rate of the PSF, however, did not change much after the initial reduction, indicating that there was only one inhibitory event. There were 198 stimuli. Adapted from Türker and Powers (1999) with permission from the American Physiological Society.

View larger version (55K): [in a new window]   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.