Head extensor reflex evoked by trigeminal stimulation in humans

Objective: Excitatory and inhibitory responses have been recognized in huma n cervical muscles following trigeminal stimulation. However, no evidence h as so far been published of a crossed, short-latency, excitatory response r esembling the early head extensor reflex seen in the cat. We seek its exist ence in humans. Methods: The study was carried out in 14 voluntary healthy subjects. Percut aneous and surface electrical stimulation of the supraorbital and infraorbi tal nerves was performed with single, double and repetitive stimuli. Signal s were recorded from the relaxed splenius and sternomastoid muscles bilater ally. Results: Percutaneous stimulation of infraorbital nerve with single stimuli evoked an early response in the contralateral splenius muscle, with onset latency ranging from 11 to 14 ms (HR1). This response was greatly facilitat ed by double or repetitive stimuli. Single stimuli also gave rise to two la rger responses in all 4 muscles in the latency ranges 50-70 ms (HR2) and 10 0-160 ms (HR3). Surface stimulation of one nerve alone could not elicit any early activity. Single surface stimuli delivered simultaneously to the sup raorbital and infraorbital nerves evoked HR1 in only 5 subjects. Conclusions: We detected a crossed early reflex of the head extensor muscle s to trigeminal stimuli. Its timing is similar to the 8-ms response seen in cats. The evidence provided suggests that the reflex is mediated by an oli gosynaptic circuit and that it needs a strong spatial summation at central synapses. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.