The Brachial Plexus

Lateral pectoral

Spinal segments: C5 to C7 Motor: pectoralis major, pectoralis minor

Brachial Plexus Layout-Trunks

The C5-T1 rami will merge to form three trunks. The superior trunk is formed from the C5 and C6 rami. The middle trunk is just the continuation of the C7 ramus. The inferior trunk is formed by the C8 and T1 rami.

The carpal tunnel

The median nerve passes through a fibro-osseous space formed by the carpal bones and flexor retinaculum, accompanied by tendons of the flexor digitorum superficialis, flexor digitorum profundus and flexor pollicis longus Here it is subject to compression due to inflammation resulting from repetitive use injury.

Thoracodorsal

Spinal segments: C6 to C8 motor: latissimus dorsi

Brachial Plexus Layout-Divisions

Each of the trunks splits into an anterior division and a posterior division. Each division will contain neurons from the spinal levels that made up the parent trunk. The divisions result directly from the splitting of the limb hypomeres into anterior and posterior muscle masses. The posterior divisions will go on to innervate the dorsal or extensor muscles while the anterior divisions will innervate the anterior or flexor muscles.

Median

Origin: Medial and lateral cords Spinal segments: C5 to C7 via lateral cord; C8,T1 via medial cord Motor: all muscles in the anterior compartment of the forearm (except flexor carpi ulnaris and medial half of flexor digitorum profundus), three thenar muscles of the thumb and two lateral lumbrical muscles cutaneous: palmar surface of the lateral three and one-half digits and over the lateral side of the palm and middle of the wrist

Nerve to subclavius

Origin: Superior trunk Spinal segments: C5,C6 Motor: subclavius

Suprascapular

Origin: Superior trunk Spinal segments: C5,C6 Motor: supraspinatus, infraspinatus

The 'M' of the brachial plexus

The 'M' of the brachial plexus is formed from the typical branching pattern of the musculocutaneous nerve, medial root of the median, lateral root of the median and ulnar nerve. This is a very helpful landmark when trying to find your way around the plexus.

Ulnar nerve

Branches from the medial cord, carrying C8 and T1 fibers No innervation in the arm, then courses posterior to the medial epicondyle to then serve two muscles in the flexor compartment of the forearm: Flexor carpi ulnaris Flexor digitorum profundus Funny bone! - the position of the ulnar nerve along the humerus at the elbow and being very superficial makes it vulnerable to impact. It then continues on to serve some of the intrinsic muscles of the hand: Flexor digiti minimi, Abductor digiti minimi, Opponens digiti minimi, Adductor pollicis, Lumbricals 3 and 4, all Interossei Palmaris brevis Its cutaneous field is on the medial hand.

Dorsal scapular

Origin: C5 ventral ramus Spinal segment: C5 Motor: rhomboid major, rhomboid minor, levator scapulae

Musculocutaneous

Spinal segments: C5 to C7 motor: all muscles in the anterior compartment of the arm cutaneous: lateral side of forearm

Upper (superior) subscapular

Spinal segments: C5,C6 motor: subscapularis

Embryologic Origin of Brachial Plexus

The brachial plexus is the site of reorganization (merging and splitting) of spinal nerves to provide innervation to the pectoral girdle and upper limb.

Lateral pectoral nerve and Medial pectoral nerve

The lateral pectoral nerve branches from the lateral cord, then courses medial to pectoralis minor to enter pectoralis major in the area of the deltopectoral triangle. After branching from the medial cord, the medial pectoral nerve innervates and pierces the pectoralis minor to then serve the pectoralis major.

Brachial Plexus Layout-Cords

Three cords are formed by combining divisions: The lateral cord forms from the anterior divisions of the superior and middle trunks. The medial cord is just the continuation of the anterior division of the inferior trunk. The posterior cord forms from the posterior division of all three trunks. We can see then that the lateral cord will contain neurons from C5, C6 and C7. The medial cord will contain neurons from C8 and T1. The posterior cord will contain neurons from C5, C6, C7, C8 and T1.

Musculocutaneous Nerve

Branches from the lateral cord, carrying C5, C6 and C7 fibers Characteristically pierces the coracobrachialis muscle Serves the flexor compartment of the arm : biceps brachii, brachialis, and coracobrachialis Ends as the lateral cutaneous nerve of the forearm (a.k.a. lateral antebrachial cutaneous nerve)

Brachial Plexus Layout- Branches

Each cord will end in its terminal branches. Lateral cord Musculocutaneous nerve [C5, C6 and C7] Lateral root of the median nerve [C5, C6 and C7] Medial cord Ulnar nerve [C8 and T1] Medial root of the median nerve [C8 and T1] Posterior cord Radial nerve [C5, C6, C7, C8 and T1] Axillary nerve [C5 and C6]

Upper plexus injury - Erb's palsy

Occurs with a forcibly increased angle between neck and shoulder: Common injury during delivery of babies, Stretches upper trunk and/or C5/C6 spinal nerves, Symptoms: adducted and medially rotated arm; extended elbow; flexed wrist; sensory loss on lateral shoulder, arm, forearm and hand This is called the "Waiter's tip posture" This results from loss of most of the shoulder muscles, except pectoralis major and minor (so the arm is adducted and medially rotated), the forearm flexors and supinators (all of these are primarily C5,C6).

Long thoracic

Origin: Origin: C5 to C7 ventral rami Spinal segments: C5 to C7 Motor: serratus anterior

Median nerve

Originates from both the medial and lateral cord, carrying C5, C6, C7, C8 and T1 fibers The median nerve does nothing in the arm. It serves muscles of the flexor compartment of the forearm: Pronator teres, Flexor carpi radialis, Palmaris longus, Flexor digitorum superficialis, Flexor digitorum profundus, Flexor pollicis longus, Pronator quadratus It then continues to innervate some of the intrinsic hand muscles: Flexor pollicis brevis, Abductor pollicis brevis, Opponens pollicis, Lumbricals 1 and 2 Its cutaneous field is on the lateral hand only.

Posterior Cord Branches

Radial nerve- This is the largest branch of the entire brachial plexus. Axillary nerve- Passes through the quadrangular space to gain access to the posterior shoulder. It innervates the deltoid and teres minor muscles and has a cutaneous field on the shoulder. Upper subscapular nerve- This is a small nerve that just innervates the medial part of subscapularis, lying on its anterior surface. Lower subscapular nerve- Another small nerve innervating the lateral part of subscapularis as well as teres major. Thoracodorsal nerve- Branches from the posterior cord between the two subscapular nerves. It innervates latissimus dorsi.

Ulnar

Spinal segments: (C7),C8,T1 motor: all intrinsic muscles of the hand (except three thenar muscles and two lateral lumbricals); also flexor carpi ulnaris and the medial half of flexor digitorum profundus in the forearm cutaneous : palmar surface of the medial one and one-half digits and associated palm and wrist, and dorsal surface of the medial one and one-half digits

Radial

Spinal segments: C5 to T1 motor: all muscles in the posterior compartments of arm and forearm cutaneous: posterior aspects of the arm and forearm, the lower lateral surface of the arm, and the dorsal lateral surface of the hand

Axillary

Spinal segments: C5,C6 motor: deltoid, teres minor cutaneous: upper lateral part of arm

Lower (inferior) subscapular

Spinal segments: C5,C6 motor: subscapularis, teres major

Medial cutaneous of forearm

Spinal segments: C8,T1 Cutaneous: medial side of forearm

Medial pectoral

Spinal segments: C8,T1 motor: pectoralis major, pectoralis minor

Medial cutaneous of arm

Spinal segments: T1 Cutaneous: medial side of distal one-third of arm

Nerves of the posterior shoulder

Subscapular nerve - passes under the suprascapular (transverse scapular) ligament, while the artery passes over it. Both nerve and artery then course through the glenoid notch to reach the infraspinous fossa. Axillary nerve - passes along with the posterior humeral circumflex artery through the quadrangular space. (Quadrangular space is formed by the borders of teres minor, teres major, long head of the triceps and surgical neck of the humerus.) Radial nerve - passes, along with the profunda brachii (a.k.a. deep brachial) artery, through the triangular interval Triangular interval is the space formed by the long head of the triceps, teres major and shaft of the humerus. This should not be confused with the triangular space which is between teres minor, teres major and the long head of the triceps, through which the circumflex scapular artery passes.

Embryologic Origin of Brachial Plexus Continued- WHY?

The limb bud develops adjacent to the C5-T1 spinal levels so those nerves enter the limb along with the muscle mass from those somite levels These five somitic muscle masses split and recombine in a predictable pattern. Nerves remain with their original muscle mass, so correspondingly split and merge. Every muscle in the upper extremity is innervated by neurons from more than one spinal nerve and every spinal nerve innervates more than one muscle. The Brachial plexus is the site of this reorganization of neurons. Limb muscles form from the myotome (somitic mesoderm) The myotome splits into the epimere, which forms the deep back muscles, and the hypomere, which forms the ventral trunk muscles. The hypomeres of C5-T1 also invade the limb bud. The hypomeres split into dorsal and ventral muscle masses within the limb bud. Each muscle mass carries part of the spinal nerve with it.

Brachial plexus layout- Ventral rami

The plexus originates from the ventral rami of spinal nerves C5 through T1. These are also commonly called the "roots of the plexus", but should not be confused with the roots of the spinal nerves. In addition to contributing to the brachial plexus, these ventral rami will also have small branches to the deep ventral neck muscles. C5 also contributes to the phrenic nerve and a branch of T1 serves muscles in the first intercostal space. Remember that the dorsal rami do NOT participate in the plexus, but rather serve the back. (C4 and T2 may try to sneak in; this is not consistent, so don't worry about them)

Radial nerve

The radial nerve serves the posterior (extensor) compartment of the arm and forearm and has an extensive cutaneous field in the posterior arm, forearm and hand. It runs along radial groove of the humerus (where it is vulnerable in a humeral break) giving off muscular branches to the triceps and anconeus and cutaneous branches to the posterior arm. It then splits into superficial and deep branches in the cubital fossa. The superficial branch is cutaneous only to the posterior forearm and dorsum of the hand. The deep branch (a.k.a., posterior interosseous nerve) is muscular only, serving the entire extensor forearm compartment: Brachioradialis, Extensor carpi radialis longus, Extensor carpi radialis brevis, Extensor carpi ulnaris, Extensor digitorum, Extensor digiti minimi, Extensor indicis, Abductor pollicis longus, Extensor pollicis longus, Extensor pollicis brevis, Supinator

Supraclavicular Branches

We will however see several branches which emerge from the upper part of the plexus, what are termed the supraclavicular branches (i.e., originating from the plexus superior to the clavicle). Dorsal scapular nerve - this will be seen coursing medial to the scapula, deep to the rhomboid . It innervates the rhomboids as well as the levator scapulae. Suprascapular nerve - this nerve passes over the suprascapular notch to enter the supraspinous fossa. It then wraps around the glenoid notch to enter the infraspinous fossa. The supraspinatus and infraspinatus are innervated by this nerve. Long thoracic nerve - this will be seen coursing along the surface of the serratus anterior, which is its sole innervation. Nerve to subclavius - this small nerve is rarely seen and only innervates the small subclavius muscle.

Upper limb compartmentalization- Ventral muscle mass

anterior (flexor) compartment of arm anterior (flexor) compartment of forearm intrinsic muscles of the hand pectoralis major and minor

Upper limb compartmentalization- Dorsal muscle mass

posterior (extensor) compartment of the arm posterior (extensor) compartment of the forearm all other shoulder muscles (except trapezius): deltoid, latissimus dorsi, serratus anterior, teres major and minor, infraspinatus, supraspinatus, subscapularis, rhomboid major and minor, levator scapula Trapezius: CN XI innervation; from head somitomeric mesenchyme (occipital and upper cervical somites)

Lower plexus injury - Klumpke's palsy

This occurs when the arm forcefully abducted and pulled Another common injury during delivery Stretches lower trunk and/or C8/T1 spinal nerves Symptoms: paralysis of intrinsic hand muscles (complete claw hand); sensory loss over the medial arm, forearm and hand

Medial cutaneous nerve of the arm and medial cutaneous nerve of the forearm

a.k.a. medial brachial cutaneous nerve and medial antebrachial cutaneous nerve These may be separate nerves or a single entity. In either case they provide cutaneous innervation to the medial side of the arm and forearm, but innervate no muscles.

Nerve Regeneration

Regeneration is possible in the peripheral nervous system but does not naturally occur in the CNS (some experimental treatments are promising) Regenerating nerve fibers follow existing connective tissue paths (endoneurium), So, if the endoneurium is intact, regeneration of axons and reconnection to their appropriate target is likely and nearly complete return of function is possible. If a nerve is transected and the cut ends are not reunited, useful nerve regeneration is unlikely. If cut ends are surgically reunited, partial return of function is possible. Realignment is critical since nerves must go through the proper endoneurium to reach their target. Nerve axon regeneration is slow (maybe 0.5-3 mm/day) and is even slower the farther away from the cell body it gets. Proximal process of sensory neurons (DRG to the spinal cord) cannot make useful contacts in the cord, so this type of injury is not good.

Brachial Plexus and Peripheral Nerve Injuries

Other, more specific injuries can occur with trauma (punctures. lacerations, gunshot) or pathology (tumors, cervical rib) that affect single cords or branches. The symptoms of a peripheral nerve lesion will vary depending on exactly where along its length the lesion is situated Also, in the real world, injury to a part of the plexus or nerve branch is often not complete. If a nerve is not completely transected, some activity will remain. Muscles may be weakened but not paralyzed and sensory loss may be only partial. Muscle weakness and partial sensory loss are detected by comparing the injured side to the well side.