Innervation to the Upper Limbs
Brachial Plexus
The innervation of the upper limbs can be summarized as:
Brachial plexus
This is a somatic plexus of nerves formed from the anterior rami of C5-C8 and T1 spinal nerves
Its spinal nerve roots divide into superior (C5-C6), middle (C7), and inferior (C8-T1) trunks
These trunks then divide into anterior and posterior divisions to later form cords
The anterior divisions of the superior and middle trunks form the lateral cord while the inferior trunk continues as the medial cord
The posterior divisions of the superior, middle, and inferior trunks unite to form the posterior cord
The lateral and medial cords and its division into the musculocutaneous, median, and ulnar nerves, is seen as the appearance of an ‘M’ shape
Smaller branches of the brachial plexus are:
Dorsal scapula - innervates the rhomboids
Long thoracic - innervates the serratus anterior muscle
Suprascapular - innervates the supra and infraspinatus muscles
Nerve to subclavius - innervates the subclavius muscle
Superior and inferior subscapular - innervate the subscapularis and teresmajor
Thoracodorsal - innervates the latissimus dorsi muscle
Lateral and medial pectoral - innervate the pectoralis major and minor muscles
Median cutaneous nerves of the the arm and forearm - innervate the skin of the medial arm and forearm
The main branches of the brachial plexus can be summarized as:
Axillary nerve
This nerve is the continuation of the posterior cord of the brachial plexus
It provides motor innervation to the deltoid and teres major muscles and sensory innervation to the skin over the inferior region of the deltoid muscle and shoulder joint
Musculocutaneous nerve
This nerve is the continuation of the lateral cord of the brachial plexus
It provides motor innervation to anterior arm flexors and sensory innervation to the skin over the region of the lateral forearm and the elbow joint
Radial nerve
This nerve is the continuation of the posterior cord of the brachial plexus
It has deep (motor) and superficial (cutaneous) branches
It provides motor innervation to the muscles in the posterior compartment of the arm and forearm and sensory innervation to the skin in the region of the posterior and inferolateral arm, posterior arm, and skin of the dorsum of the hand
Ulnar nerve
This nerve is the continuation of the medial cord of the brachial plexus
It provides motor innervation to the intrinsic muscles of the hand except thenar muscles and the two lateral lumbricals, and sensory innervation to the medial one and half digits of the hand
Median nerve
The medial root of this nerve is the continuation of the medial cord of the brachial plexus while the lateral root of this nerve is from the lateral cord of the brachial plexus
It provides motor innervation to all the muscles of the anterior forearm (except flexor carpi ulnaris and the medial half of flexor digitorum profundus muscles) as well as the thenar muscles and first two lumbricals in the hand, and sensory innervation to the lateral three and a half digits of the hand
Dermatomes and Myotomes
Dermatomes receive sensory innervation by the posterior root of a spinal nerves C5 to T2 and can be summarized as:
C5 - Over lateral region of the arm.
C6 - Over radial region of the forearm and region of the thumb.
C7 - Over the middle aspect of the posterior forearm and the middle finger.
C8 - Over the ulnar region of the forearm and hand and the 5th digit (pinky finger).
T1 - Over the medial region of the forearm and distal arm.
T2 - Over the medial and proximal regions of the arm and over the region of the axilla.
Myotomes are a group of muscles that are innervated by one spinal nerve. They allow for clinical evaluation of a patient in order to determine if neurological deficits exists, and can be summarized as:
C5 - Abduction, lateral rotation, and flexion of the arm at the shoulder joint.
C6 - Supination at the shoulder joint.
C6 and C7 - Extension of the forearm at the elbow joint, and, flexion and extension of the hand at the wrist joint.
C6, C7, and C8 - Medial rotation, adduction, and extension of the arm at the shoulder joint.
C7 and C8 - Pronation at the shoulder joint, and, flexion and extension of the digits of the hand at the metacarpophalangeal joints and the interphalangeal joints.
C8 - Extension of the thumb at the metacarpophalangeal joint, and ulnar flexion at the wrist joint.
T1 - Intrinsic muscles of the hand.
Clinical Notes:
On the brachial plexus:
Erb’s palsy occurs when there is injury to the superior parts of the brachial plexus (between the neck and shoulder). As a result of Erb’s palsy, the adducted arm on the affected side hangs limply by the side in medial rotation and extension of the elbow joint.
On the axillary nerve:
May be injured during intramuscular injections into the deltoid muscle
On the radial nerve:
Wrist drop occurs when there is an injury to the region of the humeral shaft (i.e. fracture) or continued pressure in the axillar (i.e. ill fitting crutches). As a result of wrist drop, there is loss of extension at the elbow joint and loss of sensations to the lateral 2/3s of the dorsum of the hand
On the ulnar nerve:
Clawed hands occur when there is injury to the subcutaneous regions behind the medial epicondyle and at the wrist. An ulnar claw leads to hyperextension of the 4th and 5th digits at the metacarpophalangeal joints and hyperflexion at the interphalangeal joints. An ulnar paradox is when the higher the lesion is, the less apparent the claw deformity and in wrist lesions, the flexor digitorum profundus muscle is not damaged leading to marked flexion of the terminal phalanges whereas in higher lesions, the flexor digitorum profundus may be paralyzed therefore decreasing flexion at the terminal phalanges.
On the median nerve:
Benediction sign occurs when there is loss of flexion at the interphalangeal joints of the second and third digits, loss of flexion at the wrist joint, and due to pronator muscle paralysis there is forearm is maintained in a supine position. This can occur as a result of supracondylar fracture or other damage to the median nerve at the region of the elbow joint.
Carpal tunnel syndrome occurs when there is compression of the median nerve in the carpal tunnel. This can lead to motor function deficits like paralysis or weakness of the thenar muscles and loss of opposition or sensory function deficits like paresthesia, hypoesthesia, or anesthesia of the lateral three and a half digits.