Contribution to the Lateral Cord
The anterior divisions of the superior and middle trunks unite to form the lateral cord, which branches off to the lateral pectoral nerve.
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The anterior division of the inferior trunk continues distally as the medial cord, whose branches are the medial pectoral, medial brachial cutaneous, and medial antebrachial cutaneous nerves. The posterior divisions of all three trunks unite to form the posterior cord, and its branches are the upper and lower subscapular and thoracodorsal nerves. The three cords are named according to their relation to the axillary artery, which passes through the plexus at this level. The terminal branches of the brachial plexus are the axillary, musculocutaneous, median, ulnar, and radial nerves.
The axillary nerve (C5 and C6) arises from the posterior cord and courses posteriorly around the surgical neck of the humerus,where it is at risk for injury. The posterior circumflex humeral artery accompanies the nerve in this course. The axillary nerve supplies the deltoid and teres minor muscles, is sensory to the skin over the lower portion of the deltoid, and is optimally tested on the "shoulder patch" portion of the upper arm. Axillary nerve injury, such as that due to fracture at the surgical neck of the humerus, results in an inability to abduct the arm at the shoulder to a horizontal position and in sensory loss in the shoulder patch area.
The musculocutaneous nerves (C5 and C6) and axillary nerves (C5 and C6). (Reproduced, with permission, from Waxman SG. Clinical neuroanatomy, 25th ed. New York: McGraw-Hill, 2003:350.)
The musculocutaneous nerve (C5–C7) is the continuation of the lateral cord. It courses distally through the coracobrachialis muscle to innervate it in addition to the biceps brachii and brachialis muscles. The lateral antebrachial cutaneous nerve to the skin of the lateral forearm represents the terminal continuation of this nerve. Damage to the musculocutaneous nerve causes weakness in supination and flexion of the shoulder and elbow.
The ulnar nerve (C8 and T1) is a continuation of the medial cord, which enters the posterior compartment through the medial intermuscular septum and passes distally to enter the forearm by curving posteriorly to the medial epicondyle. Here it is superficial and at risk for injury. It enters the anterior compartment of the forearm, where it innervates the flexor carpi ulnaris and the bellies of the flexor digitorum profundus to the ring and little fingers. The ulnar nerve enters the hand through a canal (Guyon canal) superficial to the flexor retinaculum. The nerve supplies all the intrinsic muscles of the hand except for the three thenar musclesand the lumbricals of the index and middle fingers. It is sensory to the medial border of the hand, the little finger, and the medial aspect of the ring finger. Damage to the ulnar nerve in the upper forearm causes lateral (radial) deviation of the hand, with weakness in flexion and adduction of the hand at the wrist and loss of flexion at the distal interphalangeal joint of the ring and little fingers. Damage to the ulnar nerve in the upper forearm or at the wrist also results in loss of abduction and adduction of the index, middle, ring, and little fingers due to paralysis of the interossei muscles. A "claw hand" deformity results, and with longstanding damage, atrophy of the interosseous muscles occurs.
Injury to the lower brachial plexus, known as Klumpke palsy, occurs by a similar mechanism, that is, an abnormal widening of the angle between the upper extremity and the thorax. This may occur at obstetrical delivery by traction on the fetal head or when an individual reaches out to interrupt a fall. The roots from C8 and T1 and/or the inferior trunk are stretched or torn. Spinal cord segments C8 and T1 form the ulnar nerve and a significant portion of the median nerve. Most of the muscles of the anterior forearm are innervated by the median nerve (see Case 4) and will display weakness. Most of the muscles of the hand are innervated by the ulnar nerve. There will be loss of sensation along the median aspect of the arm, forearm, hypothenar eminence, and little finger (C8 and T1 dermatome).
Compression of the brachial plexus cords may occur with prolonged hyper-abduction during performance of overhead tasks. The hyperabduction syndrome of pain down the arm, paresthesia, hand weakness, and skin redness may result from compression of the cords between the coracoid process and pectoralis minor. An axillary-type crutch that is too long can compress the posterior cord, leading to radial nerve palsy.
The roots of the plexus emerge from between the anterior and middle scalene muscles together with the subclavian artery. Arising from the roots are branches to the longus colli and scalene muscles and the dorsal scapular and long thoracic nerves. The roots unite to form superior, middle, and inferior trunks.
The suprascapular nerve and the nerve to the subclavius muscle arise from the superior trunk.
Each trunk is divided into anterior and posterior divisions, which will innervate musculature of the anterior and posterior compartments.
Roots of the Brachial Plexus
The five roots are the ventral rami originating from spinal nerve levels C5–T1. The roots course between the anterior and middle scalene muscles, along with the subclavian artery. The following two nerves originate from the roots:
- Dorsal scapular nerve (C5). Innervates the rhomboid and levator scapular muscles.
- Long thoracic nerve (C5–C7). Innervates the serratus anterior muscle.
Trunks of the Brachial Plexus
Once the roots exit between the anterior and middle scalene muscles, they unite to form three trunks.
- Superior trunk. Formed from the union of the C5 and C6 roots at the lateral border of the middle scalene muscle. The superior trunk gives rise to the following two nerves:
- Nerve to subclavius (C5). Provides innervation to the subclavius muscle.
- Suprascapular nerve (C5–C6). Provides innervation to the supraspinatus and infraspinatus muscles.
- Middle trunk. A continuation of the C7 root.
- Inferior trunk. Located posterior to the anterior scalene and formed from the union of C8 and T1 roots.
Divisions of the Brachial Plexus
The trunks divide into three anterior and three posterior divisions, which are associated with the ventral and dorsal musculature, respectively. The axillary artery separates the anterior and posterior divisions of the brachial plexus deep to the clavicle.
- Anterior divisions. Give rise to the nerves that will eventually innervate the flexors of the arm and forearm in the anterior compartments.
- Posterior divisions. Give rise to the nerves that will eventually innervate the extensors of the arm and forearm in the posterior compartments.
Cords of the Brachial Plexus
The anterior and posterior divisions form three cords, named according to their anatomic position relative to the axillary artery.
- Lateral cord. Gives rise to the lateral pectoral nerve (C5–C7), which innervates the pectoralis minor muscle.
- Medial cord. Gives rise to the following nerves:
- Medial pectoral nerve (C8–T1). Innervates the pectoralis major and pectoralis minor muscles.
- Medial cutaneous nerve of the arm (C8–T1). Provides cutaneous innervation to the medial surface of the arm.
- Medial cutaneous nerve of the forearm (C8–T1). Provides cutaneous innervation to the medial surface of the forearm.
- Posterior cord. Gives rise to the following nerves:
- Upper subscapular nerve (C5–C6). Innervates the subscapularis muscle.
- Thoracodorsal (middle subscapular) nerve (C6–C8). Innervates the latissimus dorsi muscle.
- Lower subscapular nerve (C5–C6). Innervates the subscapularis and teres major muscles.
Terminal Branches of the Brachial Plexus
The brachial plexus terminates in the following branches, discussed briefly below and in greater detail in Chapters 30 to 33.
- Musculocutaneous nerve (C5–C7). Provides most of the motor innervation to the anterior compartment of the arm and the sensory innervation to the lateral forearm.
- Median nerve (C6–T1). Provides most of the innervation to the anterior forearm, excluding one and one half muscles, the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus muscle, which are innervated by the ulnar nerve. The median nerve continues into the hand to innervate the thenar eminence and lumbricals 1 and 2. It provides cutaneous innervation to the medial palmar side of the hand and the palmar surface of digits 1 through 3 and half of digit 4.
- Ulnar nerve (C7–T1). Provides motor innervation to one and one half muscles of the forearm (the flexor carpi ulnaris and the ulnar half of the flexor digitorum profundus muscle) and all of the hand musculature except the thenar eminence and lumbricals 1 and 2. The ulnar nerve provides cutaneous innervation to the medial half of digit 4, to digit 5, and to the medial palmar surface of the hand.
- Radial nerve (C5–T1). Provides motor innervation to the posterior compartment of the arm and forearm. The radial nerve also provides cutaneous innervation to the posterior and inferior lateral portion of the arm, posterior forearm, and lateral dorsum of the hand and the dorsum of digits 1 through 3 and half of digit 4.
- Axial nerve (C5–C6). Provides motor innervation to the deltoid and the teres minor muscles. The axial nerve also provides cutaneous innervation to the superior portion of the lateral arm.
Study Questions
A 12-year-old boy is diagnosed with an upper brachial plexus injury after falling from a tree. He presents with his right upper arm lying limp at his side because of loss of abduction. Which of the following muscles are primarily responsible for abduction of the arm at the shoulder?
The correct answer is B.
B. The deltoid and supraspinatus muscles, which are innervated by the axillary and suprascapular nerves, respectively, are the primary abductors of the arm at the shoulder.
Case Studies
A 32-year-old woman delivered a large (4800-g) baby vaginally after a somewhat difficult labor. Her prenatal course was complicated by diabetes, which developed during pregnancy. At delivery, the infant's head emerged, but the shoulders were stuck behind the maternal symphysis pubis, requiring the obstetrician to execute maneuvers to release the infant's shoulders and complete the delivery. The infant was noted to have a good cry and pink color but was not moving its right arm.
A 32-year-old woman delivered a large (4800-g) baby vaginally after a somewhat difficult labor. Her prenatal course was complicated by diabetes, which developed during pregnancy. At delivery, the infant's head emerged, but the shoulders were stuck behind the maternal symphysis pubis, requiring the obstetrician to execute maneuvers to release the infant's shoulders and complete the delivery. The infant was noted to have a good cry and pink color but was not moving its right arm.
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