Acta Neurol. Scand., zyx 1985:602-605 zy Key zyxwvuts words: Habituation;tactile-pressure afferents; trigemino facial reflex. The role of the tactile-pressure afferents in the habituation phenomenon of trigemino-facial reflex B. Rossi, C. Giannini, G. Siciliano and F. Sartucci lstituto di Clinica Neurologica, Pisa, Italy ABSTRACT - In ten healthy subjects the trigemino-facial reflex was elicited by double shock (delta t = 0.5-1 zyxwvutsr s). The first stimulus was subliminal for pain and the second supramaximal for it. The percentage integral values, with respect to single shock, of the direct and consensual conditioned responses obtained in all subjects were averaged and compared with those conditioned by supramax.imal stimuli for pain fibres. Significant differences between subliminal and supramaximal con- ditioning stimuli for pain fibres is revealed for only consensual R3 with delta t = 1 s: greater inhibition with painful stimuli. There were no significant differences in any of the other values. Our results show that a subliminal conditioning stimulus for pain threshold can inhibit the late responses to the test stimulus, both for R2 and especially for R3, suggesting that this mechanism can contribute to the trigemino- facial reflex habituation phenomenon. zyxwvut Accepted zyxwvutsrq for publication zyxwvutsrqp June 25, I985 The phenomenon of habituation of the late trig- emino facial reflexes (TFR), i.e. the reduced amplitude to repetitive stimulation, is not yet fully understood, but some neurophysiological data suggest the prevalent involvement of cortical-sub- cortical mechanisms (1, 2, 3, 4, 5, 6, 7, 8, 9, 10). The fact that polysynaptic, nociceptive re- sponses evoked in the flexor muscles of the lower limbs (femoral biceps muscle) by means of pain- ful stimuli to the sural nerve are smaller if pre- ceded by touch stimuli, may be of some interest in understanding the phenomenon. This inhibi- tion lasts a few hundred ms and is at its maxi- mum at stimulus intervals of 100-300 ms. (11). The subjective pain sensation is less if the test stimulus is preceded by a non-painful con- ditioning stimulus (11, 12, 13). On the other hand, with intensely nociceptive stimuli se- quences the corresponding polysynaptic respon- ses have a greater amplitude, and the sensation of pain is also greater (11, 14). The inhibition process provoked in pain afferents by the touch afferents occurs by means of both presynaptic (1 1, 12, 15) and postsynaptic (16) mechanisms. In the trigeminal area too, as Kimura (18) demonstrated by means of a TFR recovery cycle, a conditioning stimulus clearly inhibits the late polysynaptic components of the test stimulus. The maximum inhibition interval lies between 80-200 ms, and lasts up to 800 ms, though with a less marked effect. However, in these experiments the stimuli used were of equal suprathreshold intensity for the pain fibres, making it very difficult to distinguish the contributions of the different touch or pain afferents to the inhibition of the late TFR com- ponents. It is possible, however, given the above neurophysiological data on spinal reflexes, that in the trigeminal zone too the input correspond- ing to tactile-pressure sensitivity contributes to provoking inhibition of the late TFR responses.