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.   TOS is a disorder of nerves. Various other tissues, such as muscles, blood vessels, and the gastrointestinal tract, can be affected, but this is due to the primary disturbance in the nerves. The nerves affected course from the neck and chest (C4-5 to T1-2, the brachial plexus) through the space above the collarbone, which is bound by small muscles, scalenes, first rib, and the collarbone (clavicle).They go on to innervate primarily the arm, coursing below the prominent shoulder bone where they can also be compromised. Offshoots from these nerves turn backward to the spine, frequently involving the neck and head; forward to the front of the neck, involving the voice box and occasionally the esophagus; backward to the shoulder blade; and downward to the front of the chest and breast. The accompanying drawings illustrate some of these relationships. The nerves emanating from the spine crisscross in the space above the clavicle in close proximity to a large artery and vein, which can contribute to the irritation that occurs in TOS. Fibrous bands can also squeeze these complicated nerve bundles, although they often occur without giving rise to any symptoms.   .