Short-latency trigemino-cervical reflexes in man.

We describe a reflex evoked in neck muscles by stimulation of afferent fibres in the trigeminal nerve. The clearest responses were seen in averaged, unrectified, monopolar surface electromyographic (EMG) recordings from active sternocleidomastoid muscles after stimulation of the infraorbital nerve. They consisted of a bilateral positive/negative (p19, n31) wave with a mean onset latency of 12.9 ms which corresponded to a period of inhibition in the underlying motor unit activity. Responses also could be seen in splenius and trapezius, but not in arm muscles. Stimuli to other branches of the trigeminal nerve (supraorbital or mental) did not produce such clear effects. The threshold for the reflex was relatively low (2-4 times perceptual threshold) and its size scaled with the level of background EMG in an approximately linear fashion. Responses to infraorbital stimulation did not interact with other short-latency inhibitory responses in the sternocleidomastoid muscle evoked by loud acoustic clicks or stimulation of the median nerve at the wrist. We suggest that the infraorbital response is part of a head withdrawal reflex involving an oligosynaptic trigemino-cervical system similar to that described in the cat.