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Reflex (unloading) and (defensive capitulation) responses in human neck muscle
S Corna1, Y Ito, M von Brevern
1Salvatore Maugeri Foundation, Division of Physical Therapy and Rehabilitation, Instituto di Ricovero E Cura A Carattere Scientifico, Novara, Italy.
The Journal of Physiology
|October 15, 1996
Summary
This study investigated human neck muscle responses to sudden changes in head-supported weight. Results show distinct muscle inhibition and excitation patterns, revealing the role of the vestibular system in maintaining head stability.
Area of Science:
- Neuroscience
- Biomechanics
- Human Physiology
Background:
- Neck muscles play a crucial role in head stabilization and responding to external perturbations.
- The vestibular system, particularly the labyrinth, is known to influence postural control and muscle activity.
- Understanding reflex responses in neck muscles is essential for diagnosing and treating conditions affecting head and neck stability.
Purpose of the Study:
- To investigate the unloading and stretch responses of human neck muscles during controlled head movements against a load.
- To determine the latencies of electromyographic (EMG) responses in neck muscles following unloading and stretch.
- To explore the influence of the vestibular system on these muscle responses.
Main Methods:
- Subjects performed head tracking in pitch and yaw while a 2-3 kg weight was electromagnetically applied or released.
- EMG activity was recorded from neck muscles, including sternocleidomastoid and splenius capitis (SpC).
- Responses were analyzed in both normal subjects and those with bilateral labyrinthine deficits.
Main Results:
- Unloading the weight during pitch tracking induced EMG inhibition in sternocleidomastoid at ~25 ms (normal) and ~41 ms (labyrinthine-defective), with antagonist excitation.
- Unloading during yaw tracking caused SpC inhibition at ~20 ms (normal) and ~25 ms (labyrinthine-defective), with antagonist excitation.
- Resisting a sideways tug elicited a stretch response in SpC at ~54 ms, while relaxation showed shortening excitation at ~76 ms, potentially a defensive reflex.
Conclusions:
- Neck muscle responses to unloading are modulated by the vestibular system, with altered latencies observed in labyrinthine-defective subjects.
- Distinct short-latency inhibitory and excitatory reflexes exist in neck muscles, contributing to head stabilization.
- A longer-latency excitation during muscle shortening may represent a protective or "defensive reflex" assisting imposed motion.