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The main function of the axillary nerve is to provide the motor supply to the deltoid. The axillary nerve originates from the spinal cord at the C5 and C6 levels, with occasional contribution from the C4 level. The nerve travels below the coracoid process, then obliquely along the anterior surface of the subscapularis.

Approximately 3 to 5 mm medial to the musculotendinous junction of the subscapularis, the axillary nerve receives a sensory branch from the anterior articular capsule of the shoulder and dives to the inferolateral border of subscapularis. The nerve then takes a posterior course, adjacent to inferomedial capsule, and passes through the quadrilateral space.

The borders of the quadrilateral space include the long head of the triceps medially, the humeral shaft laterally, the teres minor muscle superiorly, the teres major and latissimus dorsi muscles inferiorly, and the subscapularis muscle anterior. The axillary nerve enters the quadrilateral space with the posterior circumflex humeral artery, then courses around the posterolateral humeral neck (surgical neck).

Finally, the nerve branches into anterior and posterior branches supplying the deltoid muscle. Forty-five percent of the strength of external rotation is from the teres minor, whereas the deltoid generally is identified as the prime mover of the shoulder.

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