Dr Asfar AzimeeJ.N Medical

collegeAligarh

The brachial plexus is formed by the union of the anterior primary divisions (ventral rami) of the fifth through eighth cervical nerves ( C5,C6,C7,C8 ) and first thoracic (T1) nerve, with variable contributions from C4 and T2.

It consists of roots, trunks, divisions, cords and terminal branches.

The roots of the plexus (anterior primary rami of C5-T1 nerves) are between the scalene muscles, the trunks in the posterior triangle, the divisions behind the clavicle, and the cords arranged around the second part of axillary artery.

About 10% of plexuses are prefixed (from C4-C8) and 10% postfixed (from C6-T2).

COURSEThe nerves roots leave the intervertebral

foramina and course anterolaterally and inferiorly to lie between the anterior and middle scalene muscles.

Between these muscles, these nerve roots unite to form 3 vertically arranged trunks- upper(C5,C6), middle(C7), lower(C8,T1).

The prevertebral fascia invests the anterior and middle scalene muscles and fuses laterally to enclose the brachial plexus in a fascial sheath.

The trunks emerge from the interscalene space to lie cephaloposterior to the subclavian artery as it courses along the upper surface of the first rib.

As the trunks pass over the lateral border of the first rib and under the clavicle, each divides into anterior and posterior divisions.

The divisions pass posterior to the middle of the clavicle to enter the axilla, surrounded by the fascial sheath descended from the prevertebral fascia.

Here they combine again to form three cords which are named according to their relationship to the second part of axillary artery.

Lateral cord- formed by the union of anterior divisions of superior and middle trunks.

Medial cord- formed by the anterior division of inferior trunk.

Posterior cord- formed by the posterior divisions of all three trunks.

At the lateral border of pectoralis minor

each cord divides into the peripheral nerves of the upper extremity.

Lateral cord- gives off lateral branch of Median nerve; terminates as Musculocutaneous nerve.

Medial cord- gives off medial branch of Median nerve and median cutaneous nerves of arm and forearm; terminates as Ulnar nerve.

Posterior cord- gives off Axillary nerve; terminates as Radial nerve.

Anterior scalene muscle: arises from the anterior tubercles of transverse processes of the third , fourth, fifth and sixth cervical vertebrae and is inserted into the scalene tubercle on the inner border of the first rib.

The muscle lies anterior to the brachial plexus, being seperated from it below by the subclavian artery.

Its lateral border, if palpable, is a guide to the position of the plexus.

Medial scalene muscle: arises from the posterior tubercles of the six lowest cervical vertebrae, and is inserted into the upper surface of the first rib behind the groove made by the brachial plexus and the subclavian artery. The plexus thus lies in front of the muscle.

First rib: the first rib lies in an almost horizontal plane, being inclined slightly downwards and forwards.

It passes below the clavicle at about the junction of its inner and middle thirds.

Head: the head has a single articular facet, which articulates with the body of the first thoracic vertebra.

Tubercle: the tubercle articulates with the transverse process of the same vertebra.

Upper surface: it has two articular grooves- an anterior one for the subclavian vein and a posterior one for the subclavian artery and the lowest trunk of the brachial plexus. On the inner border, between the grooves, is the scalene tubercle.

The subclavius muscle arises in front of the anterior groove and the scalenus medius is inserted behind the posterior groove.

Lower surfaceLower surface:: the lower surface of the first rib has no costal groove- the inner border embraces the dome of the pleura and the outer border gives origin to the first slip of the serratus anterior.

Subclavian artery: the subclavian artery extends from its origin to the outer border of the first rib. The right subclavian artery originates from the innominate artery and the left from the aortic arch.

A major portion of the brachial plexus is located parallel and lateral to the third part of the subclavian and first part of the axillary artery, just above and below the clavicle, respectively.

The third part of the subclavian artery extends from the outer border of the anterior scalene muscle laterally and caudally to the outer border of the first rib, where it becomes the first part of the axillary artery.

The terminal part of the subclavian artery lies behind the clavicle near its midpoint. At this level the inferior trunk of the brachial plexus is posterior to the third part of the third part of the subclavian artery. The upper two trunks of the brachial plexus lie superior and lateral to the subclavian artery.

Subclavian vein: the subclavian vein is separated from the brachial plexus by the anterior scalene muscle. As it is well protected by the clavicle, it is unlikely to be punctured.

The brachial line: the brachial line runs in a straight line from the transverse process of the sixth cervical vertebra to the axillary artery in the axilla. It runs inferolateral at 45 degrees from the horizontal and slightly forwards at 15 degrees.

Branches of brachial plexus

Branches of the roots:C5- dorsal scapular C5,6- nerve to subclaviusC5,6,7- long thoracic nerve

The dorsal scapular nerve (C5) runs down deep to levator scapulae and the two rhomboids, supplying all three muscles.

Lying on serratus posterior superior, it forms neurovascular bundle with the descending scapular vessels alongside the vertebral border of the scapula

The nerve to subclavius (C5,6) passes down over the trunks of the plexus and in front of the subclavian vein. It frequently contains contains accessory phrenic fibres which join the phrenic nerve in the superior mediastinum.

The long thoracic nerve (C5,6,7) ,forms on the first digitation of the serratus anterior muscle and runs vertically downwards just behind the midaxillary line, deep to the fascia over the muscle.

Branches of the upper trunk:

The supraclavicular nerve (C5,6), prominent beneath the fascial floor of the posterior triangle, passes beneath the transverse scapular ligament and round the lateral border of the scapular spine. The nerve supplies supraspinatus, infraspinatous, and the shoulder and acromioclavicular joints.

Branches of the lateral cord: (LML)

C5,6,7- lateral pectoralC5,6,7- musculocutaneousC5,6,7- lateral root of median

The lateral pectoral nerve (C5,6,7) passes through the clavipectoral fascia and supplies the upper fibres of the pectoralis major.

It also contributes to the supply of pectoralis minor via a communicating branch.

The musculocutaneous nerve (C5,6,7) supplies the flexors of the arm- coracobrachialis, biceps brachii and brachialis.

Below the elbow, it is known as the ‘ lateral cutaneous nerve of the forearm’ and gives the cutaneous supply from elbow to wrist by an

anterior and a posterior branch along the radial border of the forearm.

The lateral root of the median nerve (C5,6,7) is joined by the medial root to form the main nerve.

Branches of the medial cord: (M4U)C8,T1- medial pectoralC8,T1- medial root of medianC8,T1- medial cutaneous nerve of armC8,T1- medial cutaneous nerve of

forearmC8,T1- ulnar

The medial pectoral nerve gives a branch to pectoralis minor and then pierces it to supply the sternocostal fibres of the pectoralis major.

The medial root of the median nerve crosses the axillary artery to join its companion and form the median nerve at the lateral side of the artery.

The median cutaneous nerve of arm is the smallest branch of the plexus. It supplies the skin on the medial aspect of the arm.

The medial cutaneous nerve of the forearm supplies the skin over the lower part of the arm and the medial side of the forearm.

The ulnar nerve is the largest branch of the medial cord and supplies most of the small muscles of the hand.

Branches of the posterior cord:

C5,6- upper subscapularC6,7,8- thoracodorsalC5,6- lower subscapularC5,6- axillaryC5,6,7,8,T1- radial

The upper and lower subscapular nervesupper and lower subscapular nerves supply the respective parts of the subscapularis, with the lower nerve also supplying teres major.

The thoracodorsal nerve supplies the latissimus dorsi. It is thrown into prominence in the position of lateral rotation and abduction of the humerus and is thus in danger in surgeries on the lower axilla.

The axillary nerve divides into anterior and posterior branches after supplying the shoulder joint. The anterior branch supplies the deltoid and a small area of overlying skin. The posterior branch supplies the teres minor and deltoid, and continues as the ‘upper lateral cutaneous nerve of the arm’.

The radial nerve is the continuation of the posterior cord and is the largest branch of the brachial plexus.

In the arm, it supplies the triceps and the skin along the posterior surface (‘posterior cutaneous nerve of the arm).