In this note, we are going to discuss the anatomy of the brachial plexus (plexus brachialis), which is responsible for motor and sensory innervation of the upper limb.

The brachial plexus originates from the interconnection of the anterior rami (rami anteriores) of spinal nerves C5-T1 in a complex arrangement,

forming the following structures:

• 5 roots
• 3 trunks
• 6 divisions
• 3 cords
• 5 terminal branches

Let’s discuss them in greater detail.

Roots

5 roots of the brachial plexus course between the anterior and middle scalene muscles, giving off several branches.

• The long thoracic nerve (n. thoracicus longus) — receives contributions from roots C5, C6 and C7.

This nerve innervates the serratus anterior muscle, which moves the scapula anteriorly.

• The dorsal scapular nerve (n. dorsalis scapulae), which arises from C5.

This nerve provides innervation to the rhomboid major, rhomboid minor and levator scapulae muscles, which elevate and retract the scapula.

• Also, the C5 root gives contributions to the phrenic nerve (nervus phrenicus) of the cervical plexus, which provides motor and sensory innervation of the diaphragm.

Trunks

Having originated, 5 nerve roots unite to form three so-called trunks:

• The superior trunk (truncus superior), which is formed by the roots of C5 and C6

• The middle trunk (truncus medius), which is a direct continuation of the root C7

• The inferior trunk (truncus inferior), which is formed by the roots of C8 and T1

These trunks course between the anterior and middle scalene muscles and behind the subclavian artery.

The superior trunk gives off two branches:

• The suprascapular nerve (n. suprascapularis), which supplies the supraspinatus and infraspinatus muscles, and also provides sensory innervation of the glenohumeral and acromioclavicular joints.

• And the subclavian nerve (n. subclavius), which innervates the subclavius muscle.

Divisions

Then each of the 3 trunks divides into an anterior and posterior division.

We have 3 anterior (divisiones anteriores) and 3 posterior divisions (divisiones posteriores) in total. They pass beneath the clavicle and enter the axillary region.

All the structures of the brachial plexus mentioned before belong to its supraclavicular part (pars supraclavicularis), and the branches that they give off are also called supraclavicular.

The following structures, which we are going to discuss, form the infraclavicular part of the plexus (pars infraclavicularis) and give off, respectively, infraclavicular branches.

Cords

Then the divisions merge in a specific way, forming three cords, which are named after their relationship with the axillary artery.

• The lateral cord (fasciculus lateralis), which is formed by the anterior divisions of the superior and middle trunks

• The posterior cord (fasciculus posterior), which is formed by the posterior divisions of all three trunks

• The medial cord (fasciculus medialis), which is formed by the anterior division of the inferior trunk

Along their course, each of the cords gives off so-called preterminal branches.

The lateral cord gives rise to the lateral pectoral nerve (n. pectoralis lateralis).

It supplies the pectoralis major muscle and provides minor contributions to the pectoralis minor muscle (via the small communicating branch).

The posterior cord gives rise to the subscapular nerves (nn. subscapulares):

• The upper subscapular nerve (n. subscapularis superior), which innervates the subscapularis muscle

• The thoracodorsal nerve (n. thoracodorsalis). It is sometimes called the middle subscapular nerve and innervates the latissimus dorsi

• The lower subscapular nerve (n. subscapularis inferior), which innervates the subscapularis and the teres major muscle.

The medial cord gives rise to the following branches:

• The medial pectoral nerve (n. pectoralis medialis), which supplies the pectoralis major muscle and pectoralis minor muscle

• The medial cutaneous nerve of the arm (n. cutaneus brachii medialis), which provides sensory innervation of the skin area of the arm

• The medial cutaneous nerve of the forearm (n. cutaneus antebrachii medialis), which provides sensory innervation of the skin area of the forearm

Terminal branches

On the level of the inferior margin of the pectoralis minor muscle, the cords of the brachial plexus split into its 5 terminal branches:

• The median nerve (n. medianus), which emerges from the lateral and medial cords, receives contributions from C5, C6, C7, C8, and T1

• The musculocutaneous nerve (n. musculocutaneus), which emerges from the lateral cord, and is made up of contributions from C5, C6, and C7.

• The axillary nerve (n. axillaris), which emerges from the posterior cord and receives contributions from roots С5 and С6.

• The radial nerve (n. radialis), which also emerges from the posterior cord, and is made up of contributions from С5, С6, С7, С8 and Т1

• And the ulnar nerve (n. ulnaris), which emerges from the medial cord, receives contributions from roots С8, Т1, and occasionally C7.

Let’s discuss each of the terminal branches of the brachial plexus in greater detail.

Median nerve

After arising from the brachial plexus, the median nerve descends down the medial compartment of the arm. At the elbow level, moving to the forearm, it shifts to a more anterior position.

Then it passes through the carpal tunnel and terminates by dividing into two terminal branches.

Along its course, the median nerve gives off several muscular branches:

• In the forearm region — to all flexor muscles, except for the flexor carpi ulnaris and the medial half of the flexor digitorum profundus.
• In the hand region — to all thenar muscles (except for the adductor pollicis) and to two lateral lumbrical muscles.

Sensory (or skin) branches of the median nerve provide sensory innervation to the:

• Lateral half of the palm
• Anterior aspect of the lateral 3 and one-half fingers
• Posterior aspect of the distal regions of the lateral 3 and one-half fingers

• It also provides innervation to the distal radioulnar, wrist and intercarpal joints

Musculocutaneous nerve

The musculocutaneous nerve leaves the axillary fossa, pierces and innervates the coracobrachialis muscle.

From here it descends in the anterior compartment of the arm, giving off the branches to the adjacent muscles.

After giving articular branches to the elbow joint and a branch to the humerus, the nerve continues as the lateral cutaneous nerve of the forearm (n. cutaneus antebrachii lateralis).

Cutaneous branches of the nerve innervate the anterolateral skin of the forearm.

Axillary nerve

The axillary nerve runs along the posterior circumflex humeral artery.

It emerges from the axilla by traversing the quadrangular space or foramen (foramen quadrilaterum).

Then the axillary nerve loops around the surgical neck of the humerus and divides into its terminal branches.

It innervates the deltoid and teres minor muscles, as well as the skin over the lower part of the deltoid and over the upper part of the triceps.

The articular branch supplies the glenohumeral joint.

Radial nerve

The radial nerve is the largest branch of the brachial plexus.

Having originated, it runs posteriorly to the axillary artery and leaves the axillary fossa.

It descends through the posterior aspect of the arm through a depression on the posterior surface of the humerus called the radial groove (sulcus nervi radialis).

In the distal part of the arm it courses anterior to the lateral condyle of humerus, giving off muscular branches to the anconeus and brachioradialis muscles.

Then it splits into two terminal branches: the superficial (sensory) branch and the deep (motor) branch.

The deep branch provides motor innervation to the muscles of the posterior compartment of the arm and forearm, as well as the abductor pollicis longus in the hand.

The radial nerve provides sensory innervation to the:

• Posterolateral surface of the arm
• Posterior surface of the forearm
• Lateral surface of the dorsum of the hand
• Small region of the palmar surface of the thenar
• Posterior aspect of the lateral 2 and one-half fingers. In this case, only the proximal divisions are innervated on the second and third fingers.

Ulnar nerve

In the arm region, the ulnar nerve passes medial to the axillary artery. In the mid-portion, it courses within the posterior compartment.

Then it passes between the medial epicondyle and olecranon in the groove for the ulnar nerve (sulcus nervi ulnaris) and enters the anterior compartment of the forearm.

The ulnar nerve gives off articular branches to the elbow joint.

In the distal portion of the forearm, it descends more medially, then along the ulnar artery, superficial to the flexor retinaculum and at the wrist — through Guyon’s canal.

The ulnar nerve provides motor supply to the flexor carpi ulnaris, the medial half of the flexor digitorum profundus, hypothenar muscles, interossei muscles, medial two lumbricals and the adductor pollicis.

Dictionary

Brachial plexus

Brachial plexus

plexus brachialis

Anterior ramus

ramus anterior

Long thoracic nerve

n. thoracicus longus

Serratus anterior muscle

m. serratus anterior

Dorsal scapular nerve

n. dorsalis scapulae

Rhomboid major

m. rhomboideus major

Rhomboid minor

m. rhomboideus minor

Levator scapulae muscle

m. levator scapulae

Phrenic nerve

n. phrenicus

Diaphragm

diaphragma

Superior trunk

truncus superior

Middle trunk

truncus medius

Inferior trunk

truncus inferior

Suprascapular nerv

n. suprascapularis

Supraspinatus muscle

m. supraspinatus

Infraspinatus muscle

m. infraspinatus

Subclavian nerv

n. subclavius

Subclavius muscle

m. subclavius

Lateral cord

fasciculus lateralis

Posterior cord

fasciculus posterior

Medial cord

fasciculus medialis

Lateral pectoral nerve

n. pectoralis lateralis

Pectoralis major muscle

m. pectoralis major

Pectoralis minor muscle

m. pectoralis minor

Subscapular nerves

nn. subscapulares

Subscapularis muscle

m. subscapularis

Thoracodorsal nerve

n. thoracodorsalis

Latissimus dorsi muscle

m. latissimus dorsi

Teres major muscle

m. teres major

Medial pectoral nerve

n. pectoralis medialis

Medial cutaneous nerve of the arm

n. cutaneus brachii medialis

Medial cutaneous nerve of the forearm

n. cutaneus antebrachii medialis

Median nerve

n. medianus

Musculocutaneous nerve

n. musculocutaneus

Axillary nerve

n. axillaris

Radial nerve

n. radialis

Ulnar nerve

n. ulnaris

Lateral cutaneous nerve of the forearm

n. cutaneus antebrachii lateralis

Radial groove

sulcus nervi radialis

Superficial branch

r. superficialis

Deep branch

r. profundus

Groove for the ulnar nerve

sulcus nervi ulnaris