Samuel W. Cadden

The digastric reflex evoked by tooth-pulp stimulation in the cat and its modulation by stimuli applied to the limbs

The digastric reflex evoked by electrical stimulation of tooth pulp in anaesthetized cats was studied together with the effects on this reflex of stimulating other parts of the body. The threshold for the digastric reflex generally lay in the range of stimulus intensities which would excite a large proportion of the pulpal afferent fibres which suggested that a large amount of central summation was required to evoke the reflex. During the course of 25/27 experiments, the threshold for the reflex increased. It was also found that repeated application of suprathreshold stimuli produced first an increase and then a decrease in the reflex response. The application of noxious but not of non-noxious mechanical conditioning stimuli to the limbs produced strong, long-lasting depressions of the digastric reflex. Electrical conditioning stimuli applied to the limbs also depressed the reflex; this depression had a latency of onset of 20-50 ms and lasted for up to 500 ms. When conditioning stimuli were applied to the saphenous nerve, the depression of the reflex occurred only when the stimuli were of an intensity sufficient to excite fibres conducting at less than 40 m X s-1; it may be assumed that some of these fibres would have been high threshold mechanoreceptors or nociceptors. These results show that noxious stimulation of anatomically remote structures can depress the activity of a population of trigeminal brainstem neurones. The opiate antagonist, naloxone, had no detectable effect on either the digastric reflex or the depression of the reflex produced by stimulating other parts of the body. The serotonin antagonists, methysergide and cinanserin, strongly depressed the digastric reflex but it was not clear whether these drugs also affected the depression of the reflex by the conditioning stimuli.

Suppression of an inhibitory jaw reflex by the anticipation of pain in man

&NA; Electromyographic recordings (EMGs) were made from the active masseter muscle, of the inhibitory reflex evoked by application of electrical stimuli to the skin of the upper lip in 15 human subjects. In control sequences, the reflex had a mean latency and duration (± S.E.M.) of 45.4 ± 1.3 msec and 47.9 ± 2.8 msec, respectively. Significant decreases in the reflex as well as increases in heart rate and anxiety levels assessed by a visual analogue scale, occurred when the subjects were stressed by the anticipation of receiving painful electrical stimuli above the ankle (P < 0.00005; Students t‐tests). During such sequences, the magnitude of the reflex measured by integration of the EMG, was reduced by 47.7 ± 5.6%. This effect involved a reduction in both the duration and depth of the inhibitory wave. It occurred regardless of whether the painful stimuli were applied during or after the recording of the reflex and of whether the baseline activity in the muscle was inadvertently raised or lowered during the stressful sequences. It is concluded that stress induced by the anticipation of pain, can markedly reduce an inhibitory jaw reflex in man by exerting an influence on the reflex pathway prior to the motoneurones.

A comparison of reflex depressions of activity in jaw-closing muscles evoked by intra- and peri-oral stimuli in man.

The effects on activity in the masseter muscle of applying electrical stimuli to discrete areas within the mouth or on peri-oral skin were studied electromyographically in 8 subjects. In all subjects, the intra-oral stimuli produced two phases of depressed masseteric activity with mean latencies of 14 ms and 47 ms. By contrast, this shorter latency response was generally not obtained with peri-oral stimuli: responses evoked from vermilion-border skin had an intermediate latency (mean 25 ms) and consisted of either one or two periods of depressed activity, while the responses to stimulation of the hairy skin of the lip generally consisted of a single phase of depressed activity, the time course of which was similar to the later phase produced by intra-oral stimuli (mean latency, 48 ms). These findings were consistently obtained with both single and short trains of electrical stimuli and contrast with reports that intra- and peri-oral stimuli produce similar reflexes. The finding that the short-latency reflex depression of masseteric activity could be evoked only by stimulating nerves supplying intra-oral tissues is consistent with the notion that this reflex may play a role in functions such as mastication, and indeed may indicate that the reflex exists specifically for such a purpose.

TWITCH INTERPOLATION IN THE ASSESSMENT OF THE MAXIMUM FORCE-GENERATING CAPACITY OF THE JAW-CLOSING MUSCLES IN MAN

The method of twitch interpolation was employed to study the maximum potential bite forces of humans. Transcutaneous electrical stimuli were applied to parts of one or both masseter muscles in eight volunteers while they bit with a variable but controlled isometric force on a unidirectional force transducer held between the anterior teeth. In all participants the twitch force produced by a single 1-ms pulse, of 25 50 mA intensity, was inversely and linearly related to the voluntary bite force. For each participant the slope of the regression between twitch force and bite force depended on the stimulus intensity and not on whether the stimulus was applied to one or both masseters. Extrapolation of the regression lines to zero twitch force showed that they converged towards a bite-force value that, for any given participant varied only a small amount between different stimulus intensities. For most participants this bite force lay above the maximum that they produced voluntarily: voluntary maximum bites ranged from 153 to 593 N, while the extrapolations predicted a narrower and higher potential range of 282-629 N. It was concluded that, for the masseters at least, there is often spare force-generating capacity which individuals are either unable or not prepared to utilize. This method is non-invasive and may help to define better the maximum bite-force potential of humans.

EFFECTS OF REMOTE DEEP SOMATIC NOXIOUS STIMULI ON A JAW REFLEX IN MAN

The effects on a inhibitory jaw reflex of activating deep somatic afferent nerves in a remote part of the body (the arm) were studied in 13 humans. Electromyographic recordings were made from the active masseter of the long-latency (mean 42.0 +/- 1.1 ms) inhibitory reflex evoked by electrical stimulation of the upper lip. Immediately after a 1-min conditioning period during which the participants compressed a hand-held spring once a second while ischaemia was produced in the arm with an inflated pneumatic cuff, the magnitude of the inhibitory reflex decreased significantly (by 43%). The reflex recovered within 5 min to a magnitude that was not significantly different from its pre-conditioning value. The arm exercise or the ischaemia alone produced no significant changes in the reflex. Furthermore, neither of these last two conditions was reported to be painful, whereas the ischaemic exercise produced pain in all but one participant. It is concluded that activation of remote nociceptive, but not non-nociceptive, deep somatic nerves can modulate jaw reflexes in man.

Analysis of differences between conditioned and control reflex series in EMG recordings

Two methods are presented for the data analysis of signals derived by subtracting conditioned from control reflex data in full-wave rectified and averaged electromyographic (EMG) signals. The first method uses the ratios of the mean amplitudes and standard errors of the mean (S.E.M.s) (i.e. Students t values) of a series of data points in such a difference signal, and deals with the detection of latencies of reflex components which are susceptible to a conditioning stimulus or experimental situation. The second method applies a modified cumulative sum (CUSUM) technique to full-wave rectified difference signals. This modified CUSUM technique determines the magnitude of the effects of the conditioning situation above that expected due to chance fluctuations, taking into account the effect of reflex modulations on such chance fluctuations in a post-stimulus period. The modified CUSUM technique proved particularly useful when various subtle but consistent, opposing changes occurred sequentially in the conditioned series thus yielding a complicated pattern of effects in a difference signal, with a low signal-to-noise ratio.

Mechanisms underlying the effects of remote noxious stimulation and mental activities on exteroceptive jaw reflexes in man.

Abstract Successive inhibitory, excitatory, inhibitory and excitatory reflexes (the Q, R, S and T waves of the post‐stimulus electromyographic complex (PSEC)), evoked by applying non‐painful taps to an incisor tooth, were recorded from the jaw‐closing muscles of 15 subjects. The effects on these reflexes of the subjects undertaking mental exercises (MEx) in the form of arithmetic calculations were compared with those of remote noxious stimulation (RNS; application of 3°C to a hand). This was done to investigate whether the previously established effects of RNS were likely to be related to a change in the subjects mental state and/or to direct nociceptive mechanisms. Both MEx and RNS caused increases in EMG activity around the Q–R and S–T transitions of the PSEC, which resulted principally from shortenings of the inhibitory Q and S waves. Reducing the intensity of the tap stimuli, which mimicked condition‐induced disinhibition, caused shortenings of the inhibitory waves at latencies similar to the shortenings induced by MEx or RNS. The magnitude of the RNS‐induced effect on the ST segment of the PSEC was greater (P<0.01) than that on the QR segment. By contrast, MEx induced similar effects on both segments. Regression analyses were performed for the relationship between condition‐induced changes in amplitude of the excitatory waves and their control amplitudes. These analyses were performed to reveal any condition‐induced inhibition or facilitation of the tap‐induced influences on the motoneurons. Overall, the evidence suggested that: (1) mental exercise induced a similar degree of inhibition of the two tap‐induced inhibitory jaw reflexes and a facilitation of the excitatory ones, and (2) remote noxious stimulation induced an inhibition of the second tap‐induced inhibitory reflex which was greater than that of the first one, and an inhibition of the first excitatory reflex. Thus, although factors related to altered mental activity could play a role in the modulation of jaw reflexes by RNS, the differences between the effects of MEx and RNS suggest that alternative or complementary mechanisms are also likely to be involved.

Remote noxious stimuli modulate jaw reflexes evoked by activation of periodontal ligament mechanoreceptors in man

The purpose of the study was to investigate whether jaw reflexes evoked by selective stimulation of periodontal ligament mechanoreceptors are susceptible to modulation by remote noxious stimulation. Experiments were performed on 10 volunteer subjects. Skin surface recordings were made from the jaw‐closing masseter muscle. The subjects activated the muscle to approximately 10% of maximum by biting on a rubber impression of their molar teeth while they received visual feedback of the electromyogram (EMG) of the muscle. Reflexes were produced by the application of gentle mechanical stimuli to an upper central incisor tooth. The stimuli were in the form of ‘ramp and hold’ forces with a 5 ms rise‐time and a 1.5 N plateau which lasted 350 ms. The resulting reflexes were recorded both under control conditions and while the subjects received a remote noxious stimulus (immersion of a hand in water at 3 °C). In all 10 subjects, the stimuli produced a single period of inhibition of masseteric activity (latency, 12.8 ± 0.4 ms; duration, 18.1 ± 1.3 ms; means ± S.E.M.), which was usually followed by a period of increased masseteric activity. The period of inhibition constituted a downward wave in full‐wave rectified, averaged signals. The integrals of such waves were significantly smaller (by 17 ± 6.5%; P = 0.027; Students t test) when the reflex was evoked during remote noxious stimulation rather than under control conditions. As such reflexes are believed to play a modulatory role during normal oral function, this finding may be relevant to disorders of mastication associated with pain.

What is a Wide-Dynamic-Range Cell?

Wide-dynamic-range (WDR) neurons are found in the spinal dorsal horn and may be projection neurons and/or interneurons for polysynaptic reflexes. The cutaneous receptive field of a WDR neuron exhibits a gradient of sensitivity with the center responding to any mechanical stimulus while the periphery responds only to noxious stimuli. These neurons also receive signals from viscera, muscles, and joints. Thus WDR neurons continuously capture information from both the interface with the external environment and the internal milieu. This information constitutes a basic somesthetic activity, which may help to build a representation of the whole body. The sizes of the peripheral fields of WDR neurons may change as a result of plasticity in both excitatory and inhibitory mechanisms. The activity of a WDR neuron can be inhibited by noxious stimulation of most of the body outside its excitatory field. Thus a noxious stimulus will both activate a segmental subset of WDR neurons and inhibit the remaining population, thus disrupting the basic somesthetic activity and distorting the body representation in favor of the painful focus.

The role of gingival mechanoreceptors in the reflex control of human jaw-closing muscles

Electromyographic (EMG) experiments were undertaken to investigate the jaw reflexes evoked by activation of gingival receptors in 12 humans. EMG recordings were made from an active masseter muscle whilst ramp-plateau mechanical stimuli were applied to the gingiva. Stimuli with a constant rate of rise (0.2 N/msec) and a variable plateau force (up to 2 N), evoked a complex set of short- and long-latency inhibitory and excitatory responses. These occurred as a sequence of inhibition-excitation-inhibition-excitation, although not all of these elements were seen on every occasion. The median thresholds of these four responses ranged from 0.5 to 1 N but overall there were no significant differences between them (p > 0.05, Friedmans ANOVA). In other experiments, the same reflexes were recorded in response to application to the gingiva of 1 N ramp-plateau stimuli (5 msec rise time) and 1 N tap stimuli applied to the adjacent tooth. The application of a local anaesthetic agent to the stimulated gingiva produced reductions in the mean magnitude of almost all the responses but these were significant (p < 0.05; ANOVA) only for the long-latency inhibitions evoked by ramping the gingiva and the long-latency excitations evoked by either stimulus. It is concluded that mechanoreceptors in the gingiva can mediate long-latency inhibitory and excitatory jaw reflexes, and that these receptors may also contribute to long-latency reflexes evoked by tapping teeth. The scarcity of effects of gingival anaesthesia on the short-latency reflexes may be due to such responses being mediated by receptors deeper in the periodontium.