Modulation of an inhibitory reflex in single motor units in human masseter at different joint angles

doi:10.1016/0304-3940(89)90677-0

Neuroscience Letters

Volume 100, Issues 1–3, 22 May 1989, Pages 157-163

https://doi.org/10.1016/0304-3940(89)90677-0 Get rights and content

Abstract

The inhibitory reflex evoked in single motor units in human masseter by lip stimulation was recorded at several joint angles. The pre-stimulus firing frequency of the unit was kept constant. Some modulation of the inhibitory response at different vertical jaw positions (i.e., temporomandibular joint angle) was observed in most masseter units, with the majority of the modulated units being inhibited less when the teeth were closer together.

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Citation Excerpt :
The synaptic mechanism underlying the JM can be studied further with the help of a computer program to trigger the stimulator only when the unit is firing in a preset frequency range (Türker et al., 1989).
For many decades, the Jendrassik maneuver (JM) has been used as a reinforcement for stretch reflexes, although the underlying mechanism of this reinforcement is still not fully understood. Moreover, the term JM has been used for many different muscle contraction strategies as there is no fixed movement for the maneuver in the literature. In this study, we aimed to investigate the effects of clenched hand pull, teeth clenching, and their combined effects to reach standardization.
Achilles tendon tap reflex responses in the soleus were recorded during rest (R), hand pull (HP), teeth clench (TC), and HP + TC combined, hereafter referred to as the JM.
Reflex response amplitudes significantly increased during JM, HP, and TC in the soleus. HP and JM significantly changed the background activity in the soleus, but TC alone did not.
These results suggest that dominantly presynaptic disinhibitory mechanisms may be responsible for the increase in the tendon tap reflex during HP, TC, and JM.
Because the findings indicate that HP increases the background activity of the soleus, we suggest that researchers should use only TC during the Jendrassik maneuver to avoid any confounding background activity change.
Black box revisited: A technique for estimating postsynaptic potentials in neurons
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Our understanding of the operation of the brain depends on knowledge of its wiring. Currently, the wiring of the human brain is estimated by counting the number of neuron discharges that occur at specific times following a stimulus. There is now strong evidence that this approach generates significant errors. Recently, the accuracy of this ‘count’ method has been compared directly with an alternative ‘rate’ method in rat brain slices. The results confirmed that the count method generates significant errors that are minimized by the rate method, because the rate of discharge of a neuron accurately displays its excitability at the time of discharge. Therefore, it is now crucial that the rate method be used to reassess previous estimates of the characteristics of wiring in the brain.
Motor-unit firing frequency can be used for the estimation of synaptic potentials in human motoneurones
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This paper describes a new method that uses the frequency of firing of motor units to estimate the stimulus-induced net post-synaptic potential (PSP) and the synaptic noise in the membrane of voluntarily active human motoneurones. Unlike the peri-stimulus time histogram (PSTH) which is the most commonly used method for assessing stimulus-induced synaptic potentials in human motoneurones, this new approach overcomes contamination of the results caused by the synchronizing effect of the stimulus on the firing pattern of the motor units. However, even after overcoming the contamination by synchronized firing, the new method does not directly represent the true net synaptic potential in the motoneurone membrane. Therefore, a new term estimated net synaptic potential (ENSP) has been introduced. This term highlights the fact that the stimulus-induced net synaptic potential has been determined indirectly and that the size and the shape of this synaptic potential may depend on the level of activity of the recording medium (i.e., pre-stimulus firing frequency of the motor unit). This paper also puts forward a normalization procedure that allows the value of the ENSP and the amplitude of the synaptic noise to be read from the ENSP graph. The normalization procedure, therefore, allows comparisons of those values within and between subjects.
Effects of jaw clenching, jaw movement and static jaw position on facial skin sensitivity to non-painful electrical stimulation in man
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The effects of isometric jaw clenching, static jaw position and jaw movement on electrically evoked perception thresholds of the facial skin of the mental region were studied in healthy human subjects. Exercise consisted of brief (1 s) isometric contractions of jaw-closing muscles against a static load (30% of the maximum force), or continuous jaw movements at two different frequencies (1 and 3 Hz). A visual cue was used to indicate the start and end of the isometric exercise (duration 1 s.). Isometric jaw clenching induced a significant elevation of perception thresholds in the skin of the lower jaw just before and during the early electromyographic response of the jaw-closing muscles. This elevation was attenuated before the end of the exercise. Corresponding thresholds evoked by electrical stimulation applied to the dorsum of the hand were not changed by isometric jaw clenching. Changes in static jaw position did not have any effect on detection thresholds. Continuous ‘masticatory-like’ jaw movements produced a velocity-dependent reduction of sensitivity in the facial skin. The suppression was significantly stronger than that produced by isometric jaw exercise. An imagined isometric biting exercise, which presumably activated the supplementary motor cortex, did not cause any threshold elevations. The results indicate that isometric jaw clenching as well as cyclical jaw movements produce segmentally a phasic, rapidly attenuating masking of facial skin sensitivity. The observed effect could be due to corollary efferent signals from the primary motor cortex or from subcortical masticatory rhythm-generating circuits to relay neurones within the main sensory nucleus of the spinal trigeminal tract. Afferent inhibitory mechanisms might also contribute to the threshold changes observed during continuous jaw movement.
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Modulation of an inhibitory reflex in single motor units in human masseter at different joint angles

Abstract

Brain Res.

Electroencephalogr. Clin. Neurophysiol.

Neuroscience

Exp. Neurol.

Prog. Neurobiol.

Arch. Oral Biol.

Integration in spinal neuronal systems