Short-latency trigemino-cervical reflexes in man

Area of Science:

• Neuroscience
• Human Physiology
• Motor Control

Background:

• The trigeminal nerve plays a crucial role in facial sensation and motor control.
• Neck muscle reflexes are vital for maintaining posture and head stability.
• Understanding trigeminal nerve reflexes can elucidate protective head movement mechanisms.

Purpose of the Study:

• To investigate and characterize a reflex evoked in neck muscles by stimulating the trigeminal nerve.
• To determine the specific branches of the trigeminal nerve that elicit this reflex.
• To explore the characteristics and potential function of this newly described reflex.

Main Methods:

• Surface electromyography (EMG) recordings from neck muscles (sternocleidomastoid, splenius, trapezius).
• Electrical stimulation of infraorbital, supraorbital, and mental nerves (branches of the trigeminal nerve).
• Analysis of reflex latency, amplitude, threshold, and interaction with other reflexes.

Main Results:

• A consistent, bilateral reflex response (positive/negative wave, 12.9 ms latency) was observed in sternocleidomastoid muscles upon infraorbital nerve stimulation.
• This reflex involved a period of motor unit inhibition and was also present in splenius and trapezius muscles, but not arm muscles.
• The reflex had a low threshold, scaled linearly with background EMG, and did not interact with acoustic or median nerve-evoked reflexes.

Conclusions:

• The infraorbital nerve stimulation evokes a distinct reflex in neck muscles.
• This reflex is likely part of a head withdrawal mechanism mediated by a rapid, oligosynaptic trigemino-cervical pathway.
• The findings suggest a conserved protective reflex system across species, similar to that seen in cats.