Differentiating condition-induced facilitation, inhibition and disinhibition in a complex series of reflexes in an...

In man, the principal exteroceptive reflexes evoked by intra-oral stimulation involve the jaw-closing muscles and include inhibitory and excitatory responses [H.W. van der Glas, A. De Laat, D. van Steenberghe, Oral pressure receptors mediate a series of inhibitory and excitatory periods in the masseteric post-stimulus EMG complex following tapping of a tooth in man, Brain Res. 337 (1985) 117-125.]. These reflexes can be observed in electromyograms (EMGs) recorded with bipolar surface electrodes. The likelihood that these reflexes play important roles in the integrative actions of the jaw has led to interest in the physiological control mechanisms by which they may be modulated. It has been reported recently that the complex series of jaw reflexes evoked by non-painful tapping on human teeth can be modulated by the application of noxious stimulation to the hand [S.W. Cadden, H.W. van der Glas, F. Lobbezoo, A. van der Bilt, Effects of remote noxious stimulation on exteroceptive reflexes in human jaw closing muscles, Brain Res. 726 (1996) 189-197.] or by exercises which produce a change in mental state [S.W. Cadden, H.W. van der Glas, F. Lobbezoo, A. van der Bilt, The influence of attentional factors on short- and long-latency jaw reflexes in man, Arch. Oral Biol. 41 (1996) 995-998.]. The effects of remote noxious stimuli and mental exercises usually involved transient increases in electromyographic (EMG) activity around the interfaces between the successive inhibitory and excitatory reflexes. As the mechanisms underlying the tap-induced inhibitory and excitatory reflexes may show some temporal overlap [H.W. van der Glas, A. De Laat, D. van Steenberghe, Oral pressure receptors mediate a series of inhibitory and excitatory periods in the masseteric post-stimulus EMG complex following tapping of a tooth in man, Brain Res. 337 (1985) 117-125.], these condition-induced increases in EMG activity could, in the simplest hypothesis, have been due to either (i) a condition-induced inhibition of the tap-induced inhibitory influences on the motoneurones (i.e., disinhibition) and/or (ii) a condition-induced facilitation of the tap-induced excitatory influences underlying the subsequent excitatory reflexes. In the present protocol, we describe how it is possible to differentiate between these different underlying mechanisms. 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. The analysis is applied on reflex data pooled from various subjects. Although this method of data analysis is illustrated with trigeminal reflexes, it is potentially of use for other complex extracellular recordings including those in other fields of motor control (e.g., EMGs from muscles other than jaw ones).