ELECTRICALLY-INDUCED BLINK REFLEX IN HORSES

The electrically induced blink reflex was studied electromyographicall y in 21 healthy adult, detomidine-sedated horses. Using surface electr odes, the supraorbital nerve was electrically stimulated at the suprao rbital foramen, The responses were recorded from the ipsilateral and c ontralateral orbicularis oculi muscles with concentric needle electrod es inserted in the lateral aspect of the ventral eyelids, Ipsilateral and contralateral recordings were made on successive stimulations of t he same side of the face, maintaining a constant stimulus intensity, T he electromyographically recorded responses consisted of an early R(1) response in the orbicularis oculi muscle ipsilateral to the side of s timulation, a bilateral late response (ipsilateral R(2) and contralate ral R(c)) and a third, R(3) response, in the ipsilateral orbicularis o culi muscle. All the responses were polyphasic muscle potentials of va riable duration and peak to peak amplitudes. The reflex latency of the R(1) response was, as in man, fairly stable, The R(2) response showed greater variability both within and between individual horses. The R( c) response was recorded in only 13 of the 21 horses and showed a slig htly longer latency than the corresponding R(2). The R(3) response, wh ich is significantly related to pain sensation in man, appeared in 19 horses and showed the greatest variability in latency.