Head extensor reflex evoked by trigeminal stimulation in humans

Objective: Excitatory and inhibitory responses have been recognized in human cervical muscles following trigeminal stimulation. However, no evidence has so far been published of a crossed, short-latency, excitatory response resembling the early head extensor reflex seen in the cat. We seek its existence in humans. Methods: The study was carried out in 14 voluntary healthy subjects. Percutaneous and surface electrical stimulation of the supraorbital and infraorbital nerves was performed with single, double and repetitive stimuli. Signals were recorded from the relaxed splenius and sternomastoid muscles bilaterally. 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 facilitated by double or repetitive stimuli. Single stimuli also gave rise to two larger responses in all 4 muscles in the latency ranges 50-70 ms (HR2) and 100-160 ms (HR3). Surface stimulation of one nerve alone could not elicit any early activity. Single surface stimuli delivered simultaneously to the supraorbital and infraorbital nerves evoked HR1 in only 5 subjects. Conclusions: We detected a crossed early reflex of the head extensor muscles 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 oligosynaptic circuit and that it needs a strong spatial summation at central synapses. © 2001 Elsevier Science Ireland Ltd. All rights reserved.