SOMATOSENSORY-EVOKED BLINK RESPONSE - INVESTIGATION OF THE PHYSIOLOGICAL MECHANISM

The somatosensory-evoked blink response (SBR) is a newly identified bl ink reflex elicited by electrical stimulation of peripheral nerves. Th e present study was performed to investigate the physiological mechani sm underlying the SBR elicited by median nerve stimulation in normal s ubjects. The peripheral afferents responsible for the SBR included low -threshold cutaneous fibres. In the SBR-positive subjects, the late (R 2) component of the blink reflex elicited by supraorbital nerve stimul ation and the SBR facilitated each other when both responses were indu ced at the same time, but they each caused long-lasting inhibition in the other when one stimulus was given as a conditioning stimulus. The extent of inhibition was correlated with the size of the preceding SBR . In the SBR-negative subjects, simultaneous inhibition of R2 was obse rved when median nerve stimulation was applied as a conditioning stimu lus. Brainstem excitability, as evaluated by blink-reflex recovery stu dies, did not differ between SBR-positive and SBR-negative subjects. T herefore, based on anatomical and physiological findings, it appears t hat the reflex pathways of the SBR and R2 converge within the brainste m and compete with each other presumably by presynaptic inhibition at the premotor level, before entering the common blink-reflex pathway. T he influence of median nerve stimulation upon tonic contraction of the orbicularis oculi muscle was studied to detect the latent SBR. There was not only a facilitatory period corresponding to the SBR but also a n active inhibitory period (exteroceptive suppression), suggesting tha t the mechanism generating the SBR is not only influenced by blink-ref lex volleys but also by active exteroceptive suppression. Thus, the SB R may appear as a result of integration of facilitatory and inhibitory mechanisms within the brainstem.