Brainstem Excitability is Increased in Subjects with Palmomental Reflex

Abstract

The palmomental reflex (PMR) is a brief contraction of the mentalis muscles caused by a scratch over the thenar eminence, i.e. a brainstem reflex to afferents of upper limb. Using electrophysio-logic methods, we studied the characteristics of brainstem excitability in PMR subjects. Methods: Ten healthy PMR subjects were included in the study. Brainstem excitability was assessed with electrical stimulation at the trigeminal nerve, median nerve, ulnar nerve, and sural nerve with recordings at the mentalis muscles. A comparison was made by the probability between the mechanical scratch and the electrical stimulation to evoke the visible muscle contraction of mentalis. Results: An electrical stimulus was able to elicit mentalis muscle responses (MMRelectrical) in all the subjects if the stimulus was of sufficient strength. Using electrical stimulation, the median nerve at the wrist was the best site to evoke MMRelectrical. However, in PMR subjects, the probability of MMRelectrical to median nerve stimulation was less than that of MMRscratch, i.e. the clinical findings of PMR. Significantly lower thresholds and higher amplitudes were noted in PMR subjects only when the median nerve was stimulated. The onset latency did not show any difference between the two groups despite the stimulation sites. Conclusion: The facial motor neurons to median nerve stimulation are more sensitive in PMR subjects. In healthy PMR subjects, this indicates that the excitability increases only in the specific neuronal circuits between the lower cervical spinal cord and the facial motor nucleus in the rostral medulla. MMRelectrical is a physiologic phenomenon, and PMR is a sign of increased brainstem excitability.

Keywords

• brainstem excitability
• facial motor neuron
• median nerve
• palmomental reflex