The scalene reflex: relationship between increased median or ulnar nerve pressure and scalene muscle activity

Neck pain, headaches, upper thoracic pain, and dystonic scalene muscles are common findings in patients who have severe entrapment neuropathies of the upper extremities. This problem was taken to the laboratory in an attempt to discover the correlation between distal entrapment neuropathies, brachial plexus entrapments, and prominent scalenus muscles. When increased pressure (over 40 mmHg) was applied to the median and ulnar nerves in the forelimbs of eight goats, increased electromyographic activity was noted in the ipsilateral scalenus muscle. Pressures ranging from 100 to 150 mmHg caused increased electromyographic activity on the contralateral scalene muscle, and the authors postulate that it is mediated by the gamma afferent and efferent system. This relationship may explain the commonly found neck pain and muscle spasm in patients with peripheral neuropathies, and it represents a link between the somatic efferent nerves and the gamma motor neuron system. At present, the same phenomenon has been documented in 30 humans with the diagnosis of brachial plexus entrapment.