The musculocutaneous nerve is a mixed nerve whose origin is in the brachial plexus. It also bears the name musculocutaneous nerve.
What is the musculocutaneous nerve?
The musculocutaneous nerve is an external nerve of the brachial plexus (brachial plexus). It is one of the mixed nerves. This means that it has both motor and sensory fibers. These originate from the spinal cord segments C5 to C7, i.e. the 5th to 7th cervical segments. In domestic animals, this is the case at C6 to C8. In hoofed animals, the musculocutaneous nerve on the upper arm connects with the median nerve. One of the most important functions of the musculocutaneous nerve is to supply the flexors of the humerus.
Anatomy and structure
Near the inferior border of the pectoralis minor muscle (small pectoral muscle), the musculocutaneous nerve separates from the lateral fasciculus (lateral fascicle) of the brachial plexus. After a short course, the musculocutaneous nerve passes through the coracobrachialis muscle (hooked arm muscle) and crosses the ventral side of the humerus within the flexor ligament. Between the biceps brachii muscle (arm flexor) and the pectoralis minor muscle, it runs toward the crook of the elbow. There it innervates the outer side of the forearm as a sensitive cutaneous branch. The musculocutaneous nerve comes to the surface within the lateral bicipital sulcus, where it breaks through the brachial fascia at the biceps tendon of the elbow. As the cutaneus antebrachii lateralis nerve, it continues to the hypodermis of the forearm. At this point, the median cubital vein is crossed. The further course leads the musculoskeletal nerve to the radial side of the forearm in the direction of the wrist as well as to the ball of the thumb. Near the crook of the elbow, there is often a division into a dorsal branch and a volar branch. These bear the designations ramus posterior and ramus anterior.
Function and tasks
The function of the musculocutaneous nerve is to provide motor innervation to all flexors in the upper arm. These are the coracobrachialis muscle, the brachialis muscle, and the biceps brachii muscle. The branch toward the coracobrachialis muscle sometimes arises separately as a variant from the lateral fascicle of the brachial plexus. In a sensitive manner, the articular capsule of the elbow (articulatio cubiti) is supplied by the musculocutaneous nerve. The same applies to the volar and dorsal skin areas located on the radial side of the forearm. Flexion of the elbow joint is considered particularly important, as it allows for meaningful use of the upper arm. After the musculocutaneous nerve has delivered its muscle branches, it appears on the surface between the brachialis muscle and the biceps brachii muscle. The resulting skin branch in humans is called the medial cutaneus antebrachii nerve. It performs the task of supplying the skin on the medial forearm. Hoofed animals are equipped with a supply area extending to the fetlock joint, which connects the tubular bone to the pastern bone. This also encompasses the dorsomedial portion of the medial forefoot. Because overlaps and anastomoses (connections between two anatomical structures) exist between the musculocutaneous nerve and the medial cutaneus antebrachii nerve, as well as the superficial ramus of the radial nerve, only minor sensory disturbances are noted even in the event of complete failure of the nerve. The course and strength of the musculocutaneous nerve vary. The anastomoses with the median nerve are considered to be particularly pronounced. This is capable of partially or even completely replacing the sensory and muscular supply of the musculocutaneous nerve.
Diseases
Occasionally, the musculocutaneous nerve can be affected by injury. Isolated impairments of the musculocutaneous nerve rarely present. They may occur as part of a surgical procedure performed to treat habitual shoulder dislocation. Usually, damage to the brachial plexus also affects the musculocutaneous nerve. If the impairment sets in before the nerve crosses the coracobrachialis muscle, it is noticeable by mild weakness when lifting the shoulder, weakness in supination (outward rotation) of the forearm, and weakness in flexion of the elbow. There is also a mild sensory disturbance of the forearm.If the impairment of the musculocutaneous nerve occurs after the passage of the coracobrachialis muscle, the lifting of the shoulder proceeds without problems, while the other complaints are identical. The most common failure symptoms of musculocutaneous nerve damage include problems with bending the elbow joint. Incomplete paralysis of elbow flexion mostly results from a compression syndrome located to the trunk. In this case, the brachioradialis muscle is spared from impairment. In addition, a general reduction of pressure and touch sensitivity of the skin may occur. Pain is not evident in this case. If, on the other hand, the compression takes place at the exit point of the cutaneus antebrachii lateralis nerve near the biceps tendon, pronounced pain develops in the elbow region, which is predominantly noticeable during inward or outward rotations of the damaged arm. In addition, the affected person often suffers from symptoms such as tingling, itching, and a feeling of warmth or cold. Basically, the musculocutaneous nerve has a good ability to regenerate. Thus, pressure paralysis usually heals spontaneously, so that damage to the musculocutaneous nerve can usually be treated conservatively. In some cases, the musculocutaneous nerve is severed in an accident. Neurotization or transplantation of the nerve are then considered the best treatment options. The prognosis for this therapy is generally positive. If there is an isolated injury to the musculocutaneous nerve, its regeneration is more favorable than if there is an injury to the brachial plexus.
