The brachial plexus is a network of nerve fibres that supplies the skin and musculature of the upper limb. It begins in the root of the neck, passes through the axilla, and runs through the entire upper extremity.

The plexus is formed by the anterior rami (divisions) of cervical spinal nerves C5, C6, C7 and C8, and the first thoracic spinal nerve, T1.

 The brachial plexus is divided into five parts; roots, trunks, divisions, cords and branches. There are no functional differences between these divisions – they are simply used to aid explanation of the brachial plexus.


The ‘roots’ refer the anterior rami of the spinal nerves that comprise the brachial plexus. These are the anterior rami of spinal nerves C5, C6, C7, C8, and T1.

At each vertebral level, paired spinal nerves arise. They leave the spinal cord via the intervertebral foramina of the vertebral column.

Each spinal nerve then divides into an anterior and a posterior ramus. The roots of the brachial plexus are formed by the anterior rami of spinal nerves C5-T1 (the posterior divisions innervate the skin and musculature of the intrinsic back muscles).

After their formation, these nerves pass between the anterior and medial scalene muscles to enter the base of the neck.


At the base of the neck, the roots of the brachial plexus converge to form three trunks. These structures are named by their relative anatomical location:

  • Superior trunk – a combination of C5 and C6 roots.
  • Middle trunk – continuation of C7.
  • Inferior trunk – combination of C8 and T1 roots.

The trunks traverse laterally, crossing the posterior triangle of the neck.


Each trunk divides into two branches within the posterior triangle of the neck. One division moves anteriorly (toward the front of the body) and the other posteriorly (towards the back of the body).     Thus, they are known as the anterior and posterior divisions.

We now have three anterior and three posterior nerve fibres. These divisions leave the posterior              triangle  and pass into the axilla. They recombine into the cords of the brachial plexus.


Once the anterior and posterior divisions have entered the axilla, they combine together to form three cords, named by their position relative to the axillary artery.

The lateral cord is formed by:

  • The anterior division of the superior trunk
  • The anterior division of the middle trunk

The posterior cord is formed by:

  • The posterior division of the superior trunk
  • The posterior division of the middle trunk
  • The posterior division of the inferior trunk

The medial cord is formed by:

  • The anterior division of the inferior trunk.
  • The cords give rise to the major branches of the brachial plexus.
Brachial plexus network
Brachial plexus network



C5 and C6.


The axillary nerve is formed within the axilla area of the upper limb. It is a direct continuation of the posterior cord from the brachial plexus – and therefore contains fibres from the C5 and C6 nerve roots.

In the axilla, the axillary nerve is located posterior to the axillary artery and anterior to the subscapularis muscle. It exits the axilla at the inferior border of subscapularis via the quadrangular space, often accompanied by the posterior circumflex humeral artery and vein.

The axillary nerve then passes medially to the surgical neck of the humerus, where it divides into three terminal branches:

Posterior terminal branch – provides motor innervation to the posterior aspect of the deltoid muscle and teres minor. It also innervates the skin over the inferior part of the deltoid as the upper lateral cutaneous nerve of the arm.

Anterior terminal branch – winds around the surgical neck of the humerus and provides motor innervation to the anterior aspect of the deltoid muscle. It terminates with cutaneous branches to the anterior and anterolateral shoulder.

Articular branch – supplies the glenohumeral joint.


The axillary nerve innervates teres minor and deltoid muscles.

  • Teres minor – part of the rotator cuff muscles which act to stabilise the glenohumeral joint. It acts to externally rotate the shoulder joint and is innervated by the posterior terminal branch of the axillary nerve.
  • Deltoid – situated at the superior aspect of the shoulder. It performs abduction of the upper limb at the glenohumeral joint and is innervated by the anterior terminal branch of the axillary nerve.

NB: There is some evidence from research on cadavers that the axillary nerve can also innervate the lateral head of triceps brachii muscle.


The sensory component of the axillary nerve is delivered via its posterior terminal branch.

After the posterior terminal branch of the axillary nerve has innervated the teres minor, it continues as the upper lateral cutaneous nerve of the arm.

 It innervates the skin over the inferior portion of the deltoid (the ‘regimental badge area’).

In a patient with axillary nerve damage, sensation at the regimental badge area may be impaired or absent. The patient may also report paraesthesia (pins and needles) in the distribution of the axillary nerve.

Sensory distribution of the axillary nerve.
 Sensory distribution of the axillary nerve.



C5, C6, C7.


The musculocutaneous nerve is the terminal branch of the lateral cord of the brachial plexus (C5, C6 and C7) and emerges at the inferior border of pectoralis minor muscle.

It leaves the axilla and pierces the coracobrachialis muscle near its point of insertion on the humerus. It gives a branch to this muscle. The musculocutaneous nerve then passes down the flexor compartment of the upper arm, superficial to brachialis but deep to the biceps brachii muscle. It innervates both these muscles and gives articular branches to the humerus and the elbow.

The nerve then pierces the deep fascia lateral to biceps brachii to emerge lateral to the biceps tendon and brachioradialis. It continues into the forearm as the lateral cutaneous nerve and provides sensory innervation to the lateral aspect of the forearm.

Note: The musculocutaneous nerve is well recognised to have a varied anatomical course. It can interact with the median nerve, adhering to the nerve and exchanging fibres. Alternatively, it may pass under the coracobrachialis instead of through it, and/or through the biceps brachii.

musculocutaneous nerve
musculocutaneous nerve


The musculocutaneous nerve innervates the muscles in the anterior compartment of the arm:

  • Biceps brachii
  • Brachialis
  • Coracobrachialis

These muscles flex the upper arm at the shoulder and the elbow. In addition, the biceps brachii also supinates the forearm.


The musculocutaneous nerve gives rise to the lateral cutaneous nerve of forearm.

This nerve initially enters the deep forearm, but then pierces the deep fascia to become subcutaneous. In this region, it can be found close to the cephalic vein.

The lateral cutaneous nerve of forearm innervates the skin of the anterolateral aspect of the forearm.

 Sensory distribution of the axillary nerve.
 Sensory distribution of the axillary nerve.



C6 – T1 (also contains fibres from C5 in some individuals).


The median nerve is derived from the medial and lateral cords of the brachial plexus,and It contains fibres from roots C6-T1 and can contain fibres from C5 in some individuals.

After originating from the brachial plexus in the axilla, the median nerve descends down the arm, initially lateral to the brachial artery. Halfway down the arm, the nerve crosses over the brachial artery, and becomes situated medially. The median nerve enters the anterior compartment of the forearm via the cubital fossa.

In the forearm, the nerve travels between the flexor digitorum profundus and flexor digitorum superficialis muscles. The median nerve gives off two major branches in the forearm:

  • Anterior interosseous nerve – supplies the deep muscles in the anterior forearm.
  • Palmar cutaneous nerve – innervates the skin of the lateral palm.

 After giving off the anterior interosseous and palmar cutaneous branches, the median nerve enters the hand via the carpal tunnel – where it terminates by dividing into two branches:

  • Recurrent branch – innervates the thenar muscles.
  • Palmar digital branch – innervates the palmar surface and fingertips of the lateral three and half digits. Also innervates the lateral two lumbrical muscles.
Median nerve
Median nerve


The median nerve innervates the majority of the muscles in the anterior forearm, and some intrinsic hand muscles.

Anterior Forearm

In the forearm, the median nerve directly innervates muscles in the superficial and intermediate layers:

  • Superficial layer: Pronator teres, flexor carpi radialis and palmaris longus.
  • Intermediate layer: Flexor digitorum superficialis.

The median nerve also gives rise to the anterior interosseous nerve, which supplies the deep flexors:

  • Deep layer: Flexor pollicis longus, pronator quadratus, and the lateral half of the flexor digitorum profundus (the medial half of the muscle is innervated by the ulnar nerve).

In general, these muscles perform pronation of the forearm, flexion of the wrist and flexion of the digits of the hand.


The median nerve innervates some of the muscles in the hand via two branches.  The recurrent branch of the median nerve innervates the thenar muscles – muscles associated with movements of the thumb. The palmar digital branch innervates the lateral two lumbricals – these muscles perform flexion at the metacarpophalangeal joints and extension at the interphalangeal joints of the index and middle fingers

(The remaining muscles in the anterior forearm and hand are innervated by the ulnar nerve).


  • Palmar cutaneous branch – arises in the forearm and travels into the hand. It innervates the lateral aspect of the palm. This nerve does not pass through the carpal tunnel, and is spared in carpal tunnel syndrome.
  • Palmar digital cutaneous branch – arises in the hand. Innervates the palmar surface and fingertips of the lateral three and half digits.
Sensory distribution of the median nerve.
Sensory distribution of the median nerve.





The radial nerve is the terminal continuation of the posterior cord of the brachial plexus. It therefore contains fibres from nerve roots C5 – T1.

The nerve arises in the axilla region, where it is situated posteriorly to the axillary artery. It exits the axilla inferiorly (via the triangular interval), and supplies branches to the long and lateral heads of the triceps brachii.

The radial nerve then descends down the arm, travelling in a shallow depression within the surface of the humerus, known as the radial groove.

As it descends, the radial nerve wraps around the humerus laterally, and supplies a branch to the medial head of the triceps brachii. During much of its course within the arm, it is accompanied by the deep branch of the brachial artery.

To enter the forearm, the radial nerve travels anterior to the lateral epicondyle of the humerus, through the cubital fossa. The nerve then terminates by dividing into two branches:

  • Deep branch (motor) – innervates the muscles in the posterior compartment of the forearm.
  • Superficial branch (sensory) – contributes to the cutaneous innervation of the dorsal hand and fingers.


The radial nerve innervates the muscles located in the posterior arm and posterior forearm.

In the arm, it innervates the three heads of the triceps brachii, which acts to extend the arm at the elbow. The radial nerve also gives rise to branches that supply the brachioradialis and extensor carpi radialis longus (muscles of the posterior forearm).

A terminal branch of the radial nerve, the deep branch, innervates the remaining muscles of the posterior forearm. As a generalisation, these muscles act to extend at the wrist and finger joints, and supinate the forearm.

Note: When the deep branch of the radial nerve penetrates the supinator muscle of the forearm, it is termed the posterior interosseous nerve for the remainder of its course.

A   deep branch of the radial nerve and
posterior interosseous nerve
 A   deep branch of the radial nerve and posterior interosseous nerve


There are four branches of the radial nerve that provide cutaneous innervation to the skin of the upper limb. Three of these branches arise in the upper arm:

  • Lower lateral cutaneous nerve of arm – Innervates the lateral aspect of the arm, inferior to the insertion of the deltoid muscle.
  • Posterior cutaneous nerve of arm – Innervates the posterior surface of the arm.
  • Posterior cutaneous nerve of forearm – Innervates a strip of skin down the middle of the posterior forearm.

The fourth branch – the superficial branch – is a terminal division of the radial nerve. It innervates the dorsal surface of the lateral three and half digits and the associated area on the dorsum of the hand.

The Four branches of the radial nerve
The Four branches of the radial nerve





The ulnar nerve arises from the brachial plexus within the axilla region. It is a continuation of the medial cord and contains fibres from spinal roots C8 and T1.

After arising from the brachial plexus, the ulnar nerve descends in a plane between the axillary artery (lateral) and the axillary vein (medial). It proceeds down the medial aspect of the arm with the brachial artery located lateral.

At the mid-point of the arm, the ulnar nerve penetrates the medial fascial septum to enter the posterior compartment of the arm. It passes posterior to the elbow through the ulnar tunnel (small space between the medial epicondyle and olecranon). Here, it also gives arise to an articular branch which supplies the elbow joint.

In the forearm, the ulnar nerve pierces the two heads of the flexor carpi ulnaris, and travels deep to the muscle, alongside the ulna. Three main branches arise in the forearm:

  • Muscular branch – innervates two muscles in the anterior compartment of the forearm.
  • Palmar cutaneous branch – innervates the medial half of the palm.
  • Dorsal cutaneous branch – innervates the dorsal surface of the medial one and a half fingers, and the associated dorsal hand area.

At the wrist, the ulnar nerve travels superficially to the flexor retinaculum, and is medial to the ulnar artery. It enters the hand via the ulnar canal (Guyon’s canal). In the hand, the nerve terminates by giving rise to superficial and deep branches.


The ulnar nerve innervates muscles in the anterior compartment of the forearm, and in the hand.


In the anterior forearm, the muscular branch of the ulnar nerve supplies two muscles:

  • Flexor carpi ulnaris – flexes and adducts the hand at the wrist.
  • Flexor digitorum profundus (medial half) – flexes the ring and little fingers at the distal interphalangeal joint

The remaining muscles in the anterior forearm are innervated by the median nerve.


The majority of the intrinsic hand muscles are innervated by the deep branch of the ulnar nerve:

  • Hypothenar muscles (a group of muscles associated with the little finger)
  • Medial two lumbricals
  • Adductor pollicis
  • Palmar and dorsal interossei of the hand
  • Palmaris brevis

The other muscles in the hand (such as the thenar eminence) are innervated by the median nerve.


There are three branches of the ulnar nerve that are responsible for its sensory innervation.

  • Palmar cutaneous branch – innervates the medial half of the palm.
  •  Dorsal cutaneous branch – innervates the dorsal surface of the medial one and a half fingers, and the associated dorsal hand area.

Superficial branch – innervates the palmar surface of the medial one and a half fingers.

The ulnar nerve Three main branches sensory distribution
The ulnar nerve Three main branches sensory distribution


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18. The Ulnar Nerve. (2021). Teach Me Anatomy. https://teachmeanatomy.info/upper- limb/nerves/ulnar-nerve/

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