Trigeminocervical reflexes elicited by stimulation of the infraorbital nerve: Head retraction reflex

In the current study, the effects of stimulation of the infraorbital nerve (ION) on the trigeminocervical reflexes (TCRs), recorded from the posterior neck muscles, was investigated and the results were compared with the resu lts recorded by stimulation of the supraorbital nerve (SON). TCRs obtained by stimulation of the ION was evaluated as the electrophysiologic counterpa rt of the head retraction reflex, Twenty normal control subjects, 10 men an d 10 women, were enrolled in the study. The SON and the ION were stimulated by using a bipolar surface electrode. Results were recorded by using eithe r concentric needle electrodes inserted into the semispinalis capitis muscl e at the level of the third or fourth cervical vertebra or by surface elect rodes placed at the C3 and C7 vertebrae on the midline. It was found that s timulation of the supraorbital and infraorbital branches of the trigeminal nerve had different reflexive effects on the posterior neck muscles. A stab le positive (or negative-positive) wave, with a very early latency and high amplitude was always recorded after maximal stimulation of the ION, which could never be detected by stimulation of the SON. The C3 response of the T CR, evoked by SON stimulation was always evoked, by stimulation of the ION, at a low threshold. These findings suggest that the head retraction reflex is composed of two phases: inhibitory and excitatory. The early, fixed pos itive wave represents the general inhibition of the cranial and neck muscle s, just before withdrawal of the face and head, from unexpected stimuli, wh ich precedes the dense C3 response, demonstrating activation of the posteri or neck muscles.