Introduction
In man, the principal exteroceptive reflexes evoked by electrical or mechanical intra-oral stimulation involve the jaw-closing muscles and include inhibitory as well as excitatory responses (e.g. van der Glas et al. 1985). In order to understand how a concerted action of the various reflex mechanisms may occur in the integrated actions of the jaw such as mastication, examining physiological control mechanisms that modulate reflexes, might be helpful. In collaboration with Dr. Sam Cadden (University of Dundee, U.K.), two potent control mechanisms have been investigated, i.e. the influence of noxious stimulation of widespread areas of the body and that of attentional factors.
Reflexes
evoked by applying non-painful taps to an incisor tooth were recorded in the
electromyograms from jaw closing muscles while subjects clenched at a constant level (Fig.
1). A series of inhibitory, excitatory, inhibitory and excitatory waves (the ‘Q, R, S
and T’ waves of the post-stimulus electromyographic complex (PSEC)) occurred in
full-wave rectified and averaged EMGs (Fig. 2). In the first series of experiments (Cadden
et al., 1996a), the reflexes were conditioned by the application of remote noxious
stimulation (RNS). To that end the subject held a hand in 3oC water (Fig. 1).
Conditioning by RNS usually produced increases in EMG activity at the Q-R and S-T
transitions of the PSEC, which yielded a shortening of the inhibitory Q and S waves (Fig.
2). Changes in the amplitudes of the excitatory R and T waves were also found. The
RNS-induced effects were greater when the reflexes were evoked by applying
‘hard’ as opposed to ‘soft’ taps to the tooth. RNS-induced sensations
(scored on Visual Analogue Scales) and increases in systemic arterial blood pressure were
not correlated with the RNS-induced effects. These findings suggest that RNS may affect
particularly those elements of the PSEC evoked by higher threshold afferents and that the
effects are mediated by mechanisms acting directly at the brainstem level and are not
secondary to pain or automatic responses.
The
reflex responses were also affected by an increased level of the subject’s attention
(Cadden et al., 1996b). When subjects undertook mental exercise (MEx) in the form of
arithmetic calculations (Fig. 1), MEx-induced effects occurred which were qualitatively
similar to those induced by remote noxious stimulation. However, dissimilarities were
revealed in a quantitative analysis (van der Glas et al., 2000). The magnitude of the
RNS-induced effect on the ST segment of the PSEC was greater than that on the QR segment.
By contrast, mental exercise induced similar effects on both segments. Furthermore, the
effects of RNS and MEx differed in part in their underlying mechanisms. An RNS or
MEx-induced increase in EMG activity at the transition of an inhibitory and an excitatory
reflex could, in the simplest hypothesis, have been due to two central processes. First,
the increase in EMG activity could have been due to a condition-induced inhibition of the
tap-induced inhibitory influences on the motoneurons, i.e. by disinhibition of an
inhibitory reflex. Second, this increase in EMG activity could have been due to a
condition-induced facilitation of the tap-induced excitatory influences on the motoneurons
underlying an excitatory reflex that follows an inhibitory reflex. A method has been
developed to differentiate between these different underlying mechanisms of
condition-induced effects (van der Glas et al., 1999). The method includes a regression
analysis of the relationship between condition-induced changes in amplitude of a reflex
and the reflex amplitude under control conditions after taking account of the effect of
chance. This analysis revealed that remote noxious stimulation induces an inhibition of
both tap-induced inhibitory reflexes (disinhibition of the Q and the S wave), and an
inhibition of the first tap-induced excitatory reflex (the R wave; van der Glas et al.,
2000). Mental exercise also induces an inhibition of both tap-induced inhibitory reflexes.
However, in contrast to the effect of RNS, mental exercise induces a facilitation of both
tap-induced excitatory reflexes (the R and T waves). Overall, the evidence suggest that
although factors related to altered mental activity could play a role in the modulation of
exteroceptive 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.
The general
inhibitory effect of remote noxious stimulation on exteroceptive reflexes may be part of a
response in which inhibitory controls triggered by noxious stimuli could facilitate the
extraction of nociceptive information from other sensory signals (Le Bars et al., 1986).
If the effects of mental exercise and/or remote noxious stimulation were part of a
response to stress then the suppression of some reflex actions – in both cases the
inhibitory ones and in the case of RNS also the early excitatory one – may be part of
a response whereby the whole body also prepares to counter a threatening situation. The
fewer automatic activities the body is performing, the more it may be prepared for more
voluntary ‘fight and flight’. However, our findings suggest that mental exercise
not only depresses the inhibitory reflexes in jaw-closing muscles but may also facilitate
the excitatory ones. These excitatory responses may have roles in load compensation during
chewing. Thus, one might propose that the MEx-induced increase in gain of the excitatory
responses and loss of the inhibitory ones are compatible with preparing to bite or with
the ability to continue chewing smoothly, while paying attention to the environment. This
could be important to both carnivores and herbivores who could bite forcefully or continue
chewing while hunting or gathering food. For example, it will be important for a cat that
the gain of excitatory jaw reflexes will be enhanced and that of inhibitory reflexes is
diminished for enabling a forcefully bite as soon as a mouse has drawn the cat’s
attention.
Figure 1
The experimental set-up. R, EMG
recordings from jaw-closing muscles. TT, tap stimuli to an upper central incisor tooth.
VF, visual feedback of the EMG to assist in maintaining a stable level of muscle activity.
The reflexes evoked by the tap stimuli were compared under control conditions and when
conditioning procedures were employed. The conditioning procedures were either immersion
of a hand in cold (3oC) water or mental exercise (’17 times table’).
Figure 2
Upper records: Examples of
post-stimulus EMG complexes (PSECs) showing average reflex responses in full-wave
rectified, smoothed and averaged EMG of a jaw-closing muscle, following repeated tap
stimuli (n=36) to the tooth at time 0 ms. Solid line, control response; broken line,
response during conditioning by remote noxious stimulation (RNS; hand immersed in 3oC
water). The horizontal lines indicate the mean pre-stimulus EMG level (100%) and a 95%
confidence interval around this level. The names (Q, R, S, T) of the successive downward-
and upward-going PSEC waves (representing inhibitory and excitatory reflexes respectively)
are indicated. Record in the 2nd row: the differences in EMG level between the
conditioned and the control PSECs shown above. Record in the 3rd row: the
Student’s t-values corresponding to the difference values in the 2nd
record (‘t-signal’). Middle horizontal line: zero level; outer horizontal lines:
95% confidence limits. Shaded areas represent significant changes in the EMG signal during
conditioning. Note that these changes correspond with two phases of an RNS-induced
increase in EMG activity at the Q-R and the S-T transitions in the reflex complex. Bottom
record: integrated values of the absolute (i.e. full-wave rectified) difference signal of
the 2nd row, corrected for contributions of chance fluctuations. The two phases
of RNS-induced EMG increase appear as two successive steps in this signal.
References
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Last modified: July 11, 2002