The brachial plexus has a most extensive distribution, and the order in which the nerves come off is of value in determining the site of any lesion. This is of particular importance in traumatic lesions, where the prognosis and treatment are closely related to the level of injury.
branches from nerve roots
The first branches of the plexus to be given off arise from the nerve roots themselves. Two important branches in this category are:
1. The nerve to the rhomboids (dorsal scapular nerve). It arises from the C5 root alone.
2. The nerve to serratus anterior (long thoracic nerve). It has contributions from C5, 6 and 7. Its most proximal part arises in conjunction with the nerve to rhomboids.
C5 also contributes to the phrenic nerve, and C5, 6, 7 and 8 supply the scalenes and longus colli. Although not strictly branches of the brachial plexus, these segmental branches are of some importance; paralysis of the hemidiaphragm, when found after a brachial plexus injury, indicates a proximal lesion.
branches from the trunks
There are two branches only at this level:
1. The suprascapular nerve is important, supplying the supraspinatus and infraspinatus.
2. The nerve to subclavius: this is of little clinical significance.
Both these nerves arise from the upper trunk. All the branches from the nerve roots and trunks arise above the clavicle (the supraclavicular branches).
1. In Erb's (upper obstetrical) palsy (E) the C5-6 roots are affected but the nerve to rhomboids and the long thoracic nerve are spared.
2. In Klumpke's (lower obstetrical) palsy (K) the C8-T1 roots are involved. The sympathetic nerve supply to the eye (arising from Tl) is often also affected, leading to a Horner's syndrome. It was said that 80% of birth injuries to the plexus make a full recovery by 13 months, and persisting severe sensory or motor deficits in the hand are rare; recent work suggests that this view is somewhat optimistic. Note that a number of obstetrical injuries to the plexus are accompanied by facial nerve palsy and posterior dislocation of the shoulder.
3. In traumatic plexus lesions in adults the commonest patterns of injury are (a) C5-6 (Erb type); (b) C5, 6, 7; (c) C5-T1 inclusive.
the brachial plexus: axillary part___
The cords for the most part lie in the axilla, and are closely related to the axillary artery. (The axillary artery commences at the outer border of the first rib and ends at the lower border of teres major. The second part of the axillary artery lies behind the pectoralis minor, with the first and third parts of the artery lying above and below it. The three cords enter the axilla above the first part, embrace the second part in the position indicated by their names, and give off their branches around the third part.)
branches from the cords
The lateral cord (C5, 6, 7) This gives off the following branches:
1. The lateral pectoral (which supplies pectoralis major)
2. The musculocutaneous (which supplies coracobrachial!s and biceps)
3. The lateral root of the median nerve.
The medial cord (C8, Tl) This gives off:
1. The medial pectoral nerve (which supplies pectoralis major)
2. The medial cutaneous nerve of the arm (which supplies the skin over the front and the medial side of the arm)
3. The medial cutaneous nerve of the forearm (which supplies the skin over the lower part of the arm and the medial side of the forearm)
4. The medial root of the median nerve
5. The ulnar nerve (in 90% of cases the ulnar nerve receives a branch (C6, 7) from the lateral cord).
The posterior cord (C5, 6, 7, 8, Tl) This gives off:
1. The upper subscapular nerve (C5, 6), which partly supplies subscapularis
2. The lower subscapular nerve (C5, 6), which supplies subscapularis and teres major
3. The thoracodorsal nerve (C6, 7, 8), which supplies latissimus dorsi
Details of the most important branches (median, ulnar, radial, axillary) are given later.
2.2. Segmental distribution: Where you suspect involvement of spinal nerves rather than peripheral nerves (e.g. injuries to the spine or brachial plexus, cervical spondylosis etc.) you must examine myotomes and dermatomes. These are the muscle masses and areas of skin supplied by single spinal nerves (no matter how the nerve fibres within these spinal nerves are finally distributed via the limb plexuses and peripheral nerves).
2.3. Myotomes (1): Normally two roots produce movement of a joint in one direction, and two in another. This is true at the elbow, where weakness of elbow flexion and an absent biceps tendon jerk indicate C5.6 involvement; and where weakness of extension and an absent triceps jerk suggest a C7.8 lesion. This general rule is followed throughout the lower limb, but is modified in the highly specialized upper limb.
2.4. Myotomes (2): In a distal or proximal joint the four spinal segments involved differ by plus or minus one, so that theoretically the shoulder should be controlled by C4,5,6,7. However, C4 has been suppressed, with the result that abduction is mediated through C5 alone (deltoid, supraspinatus etc). Adduction (involving principally pectoralis major) is controlled by C6.7).
2.5. Myotomes (3): At the wri*. where C6,7 would have been expected to control palmar flexion only, it is the case that these two segments control dorsiflexion as well.
2.6. Myotomes (4): Both flexion and extension of the fingers are controlled by C7,8.
2.7. Myotomes (5): In the case of pronation and supination a single spinal segment is involved, namely C6.
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