Brachial plexus

Brachial plexusAnatomy and function of the brachial plexus. Median, musculocutaneous, axillary, radial and ulnar nerves
~ 9 min

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.

Brachial plexus (plexus brachialis)
Brachial plexus (plexus brachialis)

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

Anterior ramus (ramus anterior)
Anterior ramus (ramus anterior)
Brachial plexus (plexus brachialis)
Brachial plexus (plexus brachialis)

forming the following structures:

  • 5 roots
  • 3 trunks
  • 6 divisions
  • 3 cords
  • 5 terminal branches
Brachial plexus (plexus brachialis)
Brachial plexus (plexus brachialis)

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.

Roots of the brachial plexus
Roots of the brachial plexus
  • The long thoracic nerve (n. thoracicus longus) — receives contributions from roots C5, C6 and C7.
Long thoracic nerve (n. thoracicus longus)
Long thoracic nerve (n. thoracicus longus)
Long thoracic nerve (n. thoracicus longus)
Long thoracic nerve (n. thoracicus longus)

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

Serratus anterior muscle (m. serratus anterior)
Serratus anterior muscle (m. serratus anterior)
  • The dorsal scapular nerve (n. dorsalis scapulae), which arises from C5.
Dorsal scapular nerve (n. dorsalis scapulae)
Dorsal scapular nerve (n. dorsalis scapulae)

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

Rhomboid major muscle (m. rhomboideus major)
Rhomboid major muscle (m. rhomboideus major)
Rhomboid minor muscle (m. rhomboideus minor)
Rhomboid minor muscle (m. rhomboideus minor)
Levator scapulae muscle (m. levator scapulae)
Levator scapulae muscle (m. levator scapulae)
  • 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.
Phrenic nerve (n. phrenicus)
Phrenic nerve (n. phrenicus)
Diaphragm (diaphragma)
Diaphragm (diaphragma)

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
Superior trunk (truncus superior)
Superior trunk (truncus superior)
  • The middle trunk (truncus medius), which is a direct continuation of the root C7
Middle trunk (truncus medius)
Middle trunk (truncus medius)
  • The inferior trunk (truncus inferior), which is formed by the roots of C8 and T1
Inferior trunk (truncus medius)
Inferior trunk (truncus medius)

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

Subclavian artery (a. subclavia)
Subclavian artery (a. subclavia)

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.
Suprascapular nerve (n. suprascapularis)
Suprascapular nerve (n. suprascapularis)
Supraspinatus muscle (m. supraspinatis)
Supraspinatus muscle (m. supraspinatis)
Infraspinatus muscle (m. supraspinatis)
Infraspinatus muscle (m. supraspinatis)
  • And the subclavian nerve (n. subclavius), which innervates the subclavius muscle.
Subclavian nerve (n. subclavius)
Subclavian nerve (n. subclavius)
Subclavian muscle (m. subclavius)
Subclavian muscle (m. subclavius)

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
Lateral cord (fasciculus lateralis)
Lateral cord (fasciculus lateralis)
  • The posterior cord (fasciculus posterior), which is formed by the posterior divisions of all three trunks
Posterior cord (fasciculus posterior)
Posterior cord (fasciculus posterior)
  • The medial cord (fasciculus medialis), which is formed by the anterior division of the inferior trunk
Medial cord (fasciculus medialis)
Medial cord (fasciculus medialis)

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).

Lateral pectoral nerve (n. pectoralis lateralis)
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).

Pectoralis major muscle (m. pectoralis major)
Pectoralis major muscle (m. pectoralis major)
Pectoralis minor muscle (m. pectoralis minor)
Pectoralis minor muscle (m. pectoralis minor)

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

Subscapular nerves (nn. subscapulares)
Subscapular nerves (nn. subscapulares)
  • The upper subscapular nerve (n. subscapularis superior), which innervates the subscapularis muscle
Subscapularis muscle (m. subscapularis)
Subscapularis muscle (m. subscapularis)
  • The thoracodorsal nerve (n. thoracodorsalis). It is sometimes called the middle subscapular nerve and innervates the latissimus dorsi
Thoracodorsal nerve (n. thoracodorsalis)
Thoracodorsal nerve (n. thoracodorsalis)
Latissimus dorsi muscule (m. latissimus dorsi)
Latissimus dorsi muscule (m. latissimus dorsi)
  • The lower subscapular nerve (n. subscapularis inferior), which innervates the subscapularis and the teres major muscle.
Teres major muscle (m. teres major)
Teres major muscle (m. teres major)

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
Medial pectoral nerve (n. pectoralis medialis)
Medial pectoral nerve (n. pectoralis medialis)
  • The medial cutaneous nerve of the arm (n. cutaneus brachii medialis), which provides sensory innervation of the skin area of the arm
Medial cutaneous nerve of the arm (n. cutaneus brachii medialis)
Medial cutaneous nerve of the arm (n. cutaneus brachii medialis)
Medial cutaneous nerve of the arm - area of innervation (n. pectoralis medialis)
Medial cutaneous nerve of the arm - area of  innervation (n. pectoralis medialis)
  • The medial cutaneous nerve of the forearm (n. cutaneus antebrachii medialis), which provides sensory innervation of the skin area of the forearm
Medial cutaneous nerve of the forearm (n. cutaneus antebrachii medialis)
Medial cutaneous nerve of the forearm (n. cutaneus antebrachii medialis)
Medial cutaneous nerve of the forearm -
area of innervation
Medial cutaneous nerve of the forearm -<br />
area of  innervation

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
Median nerve (n. medianus)
Median nerve (n. medianus)
  • The musculocutaneous nerve (n. musculocutaneus), which emerges from the lateral cord, and is made up of contributions from C5, C6, and C7.
Musculocutaneous nerve (n. musculocutaneus)
 Musculocutaneous nerve (n. musculocutaneus)
  • The axillary nerve (n. axillaris), which emerges from the posterior cord and receives contributions from roots С5 and С6.
Axillary nerve (n. axillaris)
Axillary nerve (n. axillaris)
  • 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
Radial nerve (n. radialis)
Radial nerve (n. radialis)
  • And the ulnar nerve (n. ulnaris), which emerges from the medial cord, receives contributions from roots С8, Т1, and occasionally C7.
Ulnar nerve (n. ulnaris)
Ulnar nerve (n. ulnaris)

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.

Median nerve (n. medianus)
Median nerve (n. medianus)
Median nerve (n. medianus)
Median nerve (n. medianus)

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

Median nerve (n. medianus)
Median nerve (n. medianus)

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
Median nerve - area of innervation
Median nerve - area of  innervation
  • It also provides innervation to the distal radioulnar, wrist and intercarpal joints
Median nerve - area of innervation
Median nerve - area of  innervation

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.

Мышечно-кожный нерв (n. musculocutaneus)
Мышечно-кожный нерв (n. musculocutaneus)

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.

Lateral cutaneous nerve of the forearm (n. cutaneus antebrachii lateralis)
Lateral cutaneous nerve of the forearm (n. cutaneus antebrachii lateralis)
Lateral cutaneous nerve of the forearm (n. cutaneus antebrachii lateralis)
Lateral cutaneous nerve of the forearm (n. cutaneus antebrachii lateralis)
Lateral cutaneous nerve of the forearm - area of innervation
Lateral cutaneous nerve of the forearm - area of  innervation
Lateral cutaneous nerve of the forearm - area of innervation
Lateral cutaneous nerve of the forearm - area of  innervation

Axillary nerve

The axillary nerve runs along the posterior circumflex humeral artery.

Axillary nerve (n. axillaris)
Axillary nerve (n. axillaris)
Posterior circumflex humeral artery (a. circumflexa humeri posterior)
Posterior circumflex humeral artery (a. circumflexa humeri posterior)

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

Quadrangular space or foramen (foramen quadrilaterum)
Quadrangular space or foramen (foramen quadrilaterum)
Nerve position within the quadrangular space
Nerve position within the quadrangular space

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.

Axillary nerve - area of innervation
Axillary nerve - area of  innervation
Axillary nerve - area of innervation
Axillary nerve - area of  innervation

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.

Radial nerve (n. radialis)
Radial nerve (n. radialis)

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).

Radial nerve (n. radialis)
Radial nerve (n. radialis)
Radial groove (sulcus nervi radialis)
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.

Superficial (sensory) branch (r. superficialis)
Superficial (sensory) branch (r. superficialis)
Superficial (sensory) branch (r. superficialis)
Superficial (sensory) branch (r. superficialis)
Deep (motor) branch (r. profundus)
Deep (motor) branch (r. profundus)
Deep (motor) branch (r. profundus)
Deep (motor) branch (r. profundus)

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.
Radial nerve - area of the innervation
Radial nerve - area of the innervation
Radial nerve - area of the innervation
Radial nerve - area of the innervation

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.

Ulnar nerve (n. ulnaris)
Ulnar nerve (n. ulnaris)

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.

Ulnar nerve (n. ulnaris)
Ulnar nerve (n. ulnaris)
Groove for the ulnar nerve (sulcus nervi ulnaris)
Groove for the ulnar nerve (sulcus nervi ulnaris)

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.

Ulnar nerve (n. ulnaris)
Ulnar nerve (n. ulnaris)

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.

Ulnar nerve (n. ulnaris)
Ulnar nerve (n. ulnaris)
Ulnar nerve (n. ulnaris)
Ulnar nerve (n. ulnaris)
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
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