Musculocutaneous nerve
Encyclopedia
The musculocutaneous nerve arises from the lateral cord
of the brachial plexus
, opposite the lower border of the Pectoralis major, its fibers being derived from C5, C6 and C7.
In its course through the arm it innervates the Coracobrachialis, Biceps brachii, and the greater part of the Brachialis.
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, and it supplies the dorsal surface of the thumb when the superficial branch of the radial nerve is absent.
Isolated injury, causes weakness of elbow flexion and supination of the forearm.
A discrete sensory disturbance is present on the radial side of the forearm. Biceps reflex is also affected.
The nerve is usually involved in an upper brachial plexus palsy
Injury can occur before entering the coracobrachialis due to dislocation or apparently due to stretch due to throwing injury
Heavy backpacks can cause damage to the upper trunk of the brachial plexus – dysfunction can be severe and prolonged with similar injury as occurs with Erb's palsy
from breech deliveries. Early detection is important – the combination of time, avoidance of wearing a backpack
, and strengthening of the shoulder muscles will probably be effective.
Distal to the coracobrachialis, the MC cause appears to be weight lifting
– either through compression due to hypertrophy
or entrapment between the biceps and brachialis, the nerve may lead to a painless loss of muscle strength in flexion and supination of the forearm. Initial treatment should include avoidance of biceps curls or other biceps exercises.
Lateral cord
The Lateral cord is a division of the brachial plexus.The lateral cord gives rise to the following nerves:*The lateral pectoral nerve, C5, C6 and C7 to the pectoralis major muscle*The musculocutaneous nerve which innervates the biceps muscle...
of the brachial plexus
Brachial plexus
The brachial plexus is a network of nerve fibers, running from the spine, formed by the ventral rami of the lower four cervical and first thoracic nerve roots...
, opposite the lower border of the Pectoralis major, its fibers being derived from C5, C6 and C7.
Path
It penetrates the Coracobrachialis muscle and passes obliquely between the Biceps brachii and the Brachialis, to the lateral side of the arm; a little above the elbow it pierces the deep fascia lateral to the tendon of the Biceps brachii and is continued into the forearm as the lateral cutaneous nerve of the forearm.In its course through the arm it innervates the Coracobrachialis, Biceps brachii, and the greater part of the Brachialis.
- The branch to the Coracobrachialis is given off from the nerve close to its origin, and in some instances as a separate filament from the lateral cordLateral cordThe Lateral cord is a division of the brachial plexus.The lateral cord gives rise to the following nerves:*The lateral pectoral nerve, C5, C6 and C7 to the pectoralis major muscle*The musculocutaneous nerve which innervates the biceps muscle...
of the plexus; it is derived from the seventh, cervical nerve.
- The branches to the Biceps brachii and Brachialis are given off after the musculocutaneous has pierced the Coracobrachialis; that supplying the Brachialis gives a filament to the elbow-joint.
- The nerve also sends a small branch to the bone, which enters the nutrient foramen with the accompanying artery.
Irregularities
The musculocutaneous nerve presents frequent irregularities.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, and it supplies the dorsal surface of the thumb when the superficial branch of the radial nerve is absent.
Damage
Although rare, the musculocutaneous n. can be affected through compression due to hypertrophy or entrapment between the biceps aponeurosis and brachialis fascia or it may be injured through stretch as occurs in dislocations and sometimes in surgery.Isolated injury, causes weakness of elbow flexion and supination of the forearm.
A discrete sensory disturbance is present on the radial side of the forearm. Biceps reflex is also affected.
The nerve is usually involved in an upper brachial plexus palsy
Injury can occur before entering the coracobrachialis due to dislocation or apparently due to stretch due to throwing injury
Heavy backpacks can cause damage to the upper trunk of the brachial plexus – dysfunction can be severe and prolonged with similar injury as occurs with Erb's palsy
Erb's palsy
Erb's palsy is a paralysis of the arm caused by injury to the upper group of the arm's main nerves, specifically the upper trunk C5-C6 is severed. These form part of the brachial plexus, comprising the ventral rami of spinal nerves C5-C8, and T1. These injuries arise most commonly, but not...
from breech deliveries. Early detection is important – the combination of time, avoidance of wearing a backpack
Backpack
A backpack is, in its simplest form, a cloth sack carried on one's back and secured with two straps that go over the shoulders, but there can be exceptions...
, and strengthening of the shoulder muscles will probably be effective.
Distal to the coracobrachialis, the MC cause appears to be weight lifting
Weight training
Weight training is a common type of strength training for developing the strength and size of skeletal muscles. It uses the weight force of gravity to oppose the force generated by muscle through concentric or eccentric contraction...
– either through compression due to hypertrophy
Hypertrophy
Hypertrophy is the increase in the volume of an organ or tissue due to the enlargement of its component cells. It should be distinguished from hyperplasia, in which the cells remain approximately the same size but increase in number...
or entrapment between the biceps and brachialis, the nerve may lead to a painless loss of muscle strength in flexion and supination of the forearm. Initial treatment should include avoidance of biceps curls or other biceps exercises.