Brachial Plexus <br />The nerves entering the upper limb provide the following <br />important functions: sensory innervation to the skin and <br />deep structures, such as the joints; motor innervation to <br />the muscles; influence over the diameters of the blood vessels <br />by the sympathetic vasomotor nerves; and sympathetic <br />secretomotor supply to the sweat glands. <br />At the root of the neck, the nerves form a complicated <br />plexus called the brachial plexus. This allows the nerve fibers <br />derived from different segments of the spinal cord to be <br />arranged and distributed efficiently in different nerve trunks <br />to the various parts of the upper limb. The brachial plexus <br />is formed in the posterior triangle of the neck by the union <br />of the anterior rami of the 5th, 6th, 7th, and 8th cervical and <br />the 1st thoracic spinal nerves (Figs. 9.18 and 9.19). <br />The plexus can be divided into roots, trunks, divisions, <br />and cords (Fig. 9.18). The roots of C5 and 6 unite to form the <br />upper trunk, the root of C7 continues as the middle trunk, <br />and the roots of C8 and T1 unite to form the lower trunk. <br />Each trunk then divides into anterior and posterior divisions. <br />The anterior divisions of the upper and middle trunks <br />unite to form the lateral cord, the anterior division of the <br />lower trunk continues as the medial cord, and the posterior <br />divisions of all three trunks join to form the posterior cord. <br />The roots, trunks, and divisions of the brachial plexus <br />reside in the lower part of the posterior triangle of the neck <br />and are fully described on page XXX. The cords become <br />arranged around the axillary artery in the axilla (Fig. 9.15). <br />Here, the brachial plexus and the axillary artery and vein <br />are enclosed in the axillary sheath. <br />Cords of the Brachial Plexus All three cords of the <br />brachial plexus lie above and lateral to the first part of <br />the axillary artery (Figs. 9.15 and 9.20). The medial cord <br />crosses behind the artery to reach the medial side of the <br />second part of the artery (Fig. 9.20). The posterior cord <br />lies behind the second part of the artery, and the lateral <br />cord lies on the lateral side of the second part of the artery <br />(Fig. 9.20). Thus, the cords of the plexus have the relationship <br />to the second part of the axillary artery that is indicated <br />by their names. <br />Most branches of the cords that form the main nerve <br />trunks of the upper limb continue this relationship to the <br />artery in its third part (Fig. 9.20). <br />The branches of the different parts of the brachial <br />plexus (Figs. 9.19 and 9.21) are as follows: <br />■■ Roots <br />Dorsal scapular nerve (C5) <br />Long thoracic nerve (C5, 6, and 7) <br />■■ Upper trunk <br />Nerve to subclavius (C5 and 6) <br />Suprascapular nerve (supplies the supraspinatus and <br />infraspinatus muscles) <br />etc snell
