The musculocutaneous nerve is a mixed branch of the

lateral cord The lateral cord is the part of the brachial plexus formed by the anterior divisions of the upper (C5-C6) and middle trunks (C7). Its name comes from it being lateral to the axillary artery as it passes through the axilla. The other cords of the b ...

of the

brachial plexus The brachial plexus is a network of nerves (nerve plexus) formed by the anterior rami of the lower four Spinal nerve#Cervical nerves, cervical nerves and first Spinal nerve#Thoracic nerves, thoracic nerve (cervical spinal nerve 5, C5, Cervical spi ...

derived from cervical spinal nerves C5-C7. It arises opposite the lower border of the

pectoralis minor Pectoralis minor muscle () is a thin, triangular muscle, situated at the upper part of the chest, beneath the pectoralis major in the human body. It arises from ribs III-V; it inserts onto the coracoid process of the scapula. It is innervated by ...

. It provides motor innervation to the muscles of the

anterior compartment of the arm Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

: the

coracobrachialis The coracobrachialis muscle muscle in the upper medial part of the arm. It is located within the anterior compartment of the arm. It originates from the coracoid process of the scapula; it inserts onto the middle of the medial aspect of the body ...

biceps brachii The biceps or biceps brachii (, "two-headed muscle of the arm") is a large muscle that lies on the front of the upper arm between the shoulder and the elbow. Both heads of the muscle arise on the scapula and join to form a single muscle bell ...

, and

brachialis The brachialis (brachialis anticus) is a muscle in the upper arm that flexes the elbow. It lies beneath the biceps brachii, and makes up part of the floor of the region known as the cubital fossa (elbow pit). It originates from the anterior aspec ...

. It provides sensory innervation to the lateral forearm (via its terminal branch). It courses through the anterior part of the arm, terminating 2 cm above elbow; after passing the lateral edge of the tendon of

it is becomes known as the

lateral cutaneous nerve of the forearm Lateral is a geometric term of location which may also refer to: Biology and healthcare * Lateral (anatomy), a term of location meaning "towards the side" * Lateral cricoarytenoid muscle, an intrinsic muscle of the larynx * Lateral release (sur ...

Structure

Course

Musculocutaneous nerve arises from the lateral cord of the brachial

plexus In anatomy, a plexus (from the Latin term for 'braid') is a branching network of blood vessels, lymphatic vessels, or nerves. The nerves are typically axons outside the central nervous system. The standard plural form in English is plexuses. Al ...

with root value of C5 to C7 of the spinal cord. It follows the course of the third part of the

axillary artery In human anatomy, the axillary artery is a large blood vessel that conveys oxygenated blood to the lateral aspect of the thorax, the axilla (armpit) and the upper limb. Its origin is at the lateral margin of the first rib, before which it is c ...

(part of the axillary artery distal to the

) laterally and enters the frontal aspect of the arm where it penetrates the

muscle. It then passes downwards and laterally between the

(above) and the

muscles (below), to the lateral side of the arm; at 2 cm above the elbow it pierces the

deep fascia Deep fascia (or investing fascia) is a fascia, a layer of dense connective tissue that can surround individual muscles and groups of muscles to separate into fascial compartments. This fibrous connective tissue interpenetrates and surrounds the m ...

lateral to the tendon of the biceps brachii and is continued into the forearm as the

Distribution

Motor innervation

In its course through the arm it innervates the coracobrachialis, biceps brachii, and the greater part of the brachialis.

Sensory innervation

Its terminal branch, the lateral cutaneous nerve of the forearm, supplies the sensation of the lateral side of the forearm from the elbow to the wrist. The musculocutaneous nerve also gives articular branches to the elbow joint and to the humerus.

Variation

The musculocutaneous nerve presents frequent variations and communications with the

median nerve The median nerve is a nerve in humans and other animals in the upper limb. It is one of the five main nerves originating from the brachial plexus. The median nerve originates from the lateral and medial cords of the brachial plexus, and has cont ...

. #It may adhere for some distance to the median and then pass outward, beneath the biceps brachii, instead of through the coracobrachialis. #Some of the fibers of the median may run for some distance in the musculocutaneous and then leave it to join their proper trunk; less frequently the reverse is the case, and the median sends a branch to join the musculocutaneous. #The nerve may pass under the coracobrachialis or through the biceps brachii. #Occasionally it gives a filament to the

pronator teres The pronator teres is a muscle (located mainly in the forearm) that, along with the pronator quadratus, serves to pronate the forearm (turning it so that the palm faces posteriorly when from the anatomical position). It has two origins, at th ...

, and it supplies the dorsal surface of the thumb when the superficial branch of the radial nerve is absent.

Nerve injuries

Injury to the musculocutaneous nerve can be caused by three mechanisms: repeated microtrauma, indirect trauma or direct trauma on the nerve. Overuse of coracobrachialis, biceps, and brachialis muscles can cause the stretching or compression of musculocutaneous nerve. Those who have it, can complain of pain, tingling or reduced sensation over the lateral side of the forearm. This symptom can be reproduced by pressing over the region below the

coracoid process The coracoid process (from Greek κόραξ, raven) is a small hook-like structure on the lateral edge of the superior anterior portion of the scapula (hence: coracoid, or "like a raven's beak"). Pointing laterally forward, it, together with the ...

(positive

Tinel's sign Tinel's sign (also Hoffmann-Tinel sign) is a way to detect irritated nerves. It is performed by lightly tapping ( percussing) over the nerve to elicit a sensation of tingling or "pins and needles" in the distribution of the nerve. Percussion is us ...

). Pain can also be reproduced by flexing the arm against resistance. Other differential diagnoses that can mimick the symptoms of musculocutaneous palsy are: C6 radiculopathy (pain can be produced by movement of the neck), long head of biceps tendinopathy (no motor or sensory deficits), pain of the

bicipital groove The bicipital groove (intertubercular groove, sulcus intertubercularis) is a deep groove on the humerus that separates the greater tubercle from the lesser tubercle. It allows for the long tendon of the biceps brachii muscle to pass. Structure ...

(relieved by shoulder joint injection).

Electromyography Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by skeletal muscles. EMG is performed using an instrument called an electromyograph to produce a record called an electromyogram. An electromyo ...

test shows slight neural damage at the biceps and the brachialis muscles with slower motor and sensory conduction over the Erb's point. In indirect trauma, violent abduction and retroposition of the shoulder can stretch the nerve and result tension of the

with musculocutaneous nerve lesion. Those with this type of lesion is presented with pain, reduced sensation, and tingling of the lateral part of forearm (lateral antebrachial cutaneous nerve - terminal sensory only branch of musculocutaneous nerve) with reduced strength of elbow flexion. Tinel's sign can be positive. Differential diagnosis includes C5 and C6 nerve root lesions of the brachial plexus where the abduction, external rotation, and elbow flexion is lost. On the other hand, rupture of the biceps can cause the loss of flexion of the elbow without sensory deficits. Rupture of the SHORT HEAD of the biceps can decrease elbow flexion strength, where the brachialis muscle is intact. Rupture of the LONG HEAD of the biceps results in mild weakening of forearm supination as long as the supinator muscle is intact.

test is negative. In direct trauma, fracture of the humerus, gun shot, glass pieces injuries and more, can cause the musculocutaneous nerve lesion. Iatrogenic nerve injuries (for example during orthopedic surgery involving an internal fixation of the humerus) are relatively common and in a certain percentage of cases probably inevitable, though an adequate knowledge of the surgical anatomy can help to reduce its frequency. To diagnose traumatic nerve injury, operative exploration should be performed without delay. If reconstruction of the motor function of the musculocutaneous nerve (elbow flexion) is needed then there are several options, depending on the injury pattern and timeframes. If the nerve is in-continuity and the fascicles appear healthy under the operating microscope then

Neurolysis Neurolysis is the application of physical or chemical agents to a nerve in order to cause a temporary degeneration of targeted nerve fibers. When the nerve fibers degenerate, an interruption in the transmission of nerve signals occurs. In the medica ...

may be sufficient. When there is a division or segment of non-viable nerve then interpositional autografting is preferred. If reinnervation is likely to take months (e.g. if the injured segment is long and long grafts are needed, or in the case of proximal injuries such as root avulsion or upper trunk injury) then nerve transfer is preferred as this will reinnervate the muscle faster; in the case of upper trunk injuries, the ideal neurotisation appears to be the double Oberlin transfer., although data on this topic are sparse and heterogeneous.

References

External links

* * {{DEFAULTSORT:Musculocutaneous Nerve Nerves of the upper limb