THE BRACHIAL PLEXUS
It is formed as a result of the union of the lower four cervical ventral rami and the greater part of the 1st throcacic ventral ramus. It develops as a result of somite migration as they move to form the upper limb. The somites are destined to from the muscles, skin and skeleton. When they migrate they drag along their original segmental nerve supply. As migration continues some of the nerves come into close proximity and then fused in a unique pattern to form the brachial plexus.
FORMATION OF THE BRACHIAL PLEXUS
The brachial plexus is formed as a result of the union of C5, C6, C7, C8 ventral rami and 1st thoracic ventral ramus with contribution sometimes from the 4th cervical ventral ramus and T2 ventral ramus. The participation of T2 & C4, gives the variations in the formation of brachial plexus in that when there is greater contribution from C4 and no contribution from T2 with little or no contribution from T1 such is referred to as prefix brachial plexus. When there is contribution from T2 no contribution from C4 and little or no contribution from C5 this formation is known as the post fixed brachial plexus. The normal configuration of the brachial plexus is C5-T1. It is divided into roots trunks, divisions and cords.
THE ROOT is formed by the ventral rami of C5, C6, C4 C8 and T1. The roots unite to form the trunks in the following manner:
-C5 accepts contribution from C4 and then unites with C6 to form the upper trunk.
- C7 will continue as the middle trunk.
-T1 accepts contribution From T2 and then unites with C8 to form the lower trunk.
The trunks desend laterally above the clavicle and bifurcate into anterior and posterior division. The divisions lie behind the clavicle and they emerge below it to form the cords in the following manner:
· The anterior division of the upper trunk and middle trunk will unite to form the lateral cord.
· The anterior division of the lower trunk continues as the medial cord.
· The posterior divisions of all the trunks will unite to form the posterior cord.
It should be noted that branches that arise from the union of the anterior division will supply the flexor compartment muscles of the upper limb while the branches that arise from the posterior divisions are destined to supply the posterior compartment muscles or extensor muscles.
The cords of the brachial plexus derived their names based on their relationship with the 2nd part of the axillary artery and they all lie in the axilla below the pectoralis minor.
RELATIONS OF THE BRACHIAL PLEXUS
IN THE NECK
It lies at the posterior triangle of the neck between the angle formed by the clavicle and the stenocleidomastoid muscle. It is found to emerge between the scalenus anterior and scalenus medius muscles. It is covered by the skin, deep fascia, and platysma muscle. It is also crossed by suprascapular nerve, external jugular vein, and inferior belle of Omohyoid muscle.
The roots and trunk lie in the posterior triangle but as the brachial plexus passes into the axilla it lies posterior to the clavicle and the subclavius muscle.
IN THE AXILLA
The lateral cord and posterior cord lie laterally to the 1st part of axillary artery while the medial cord lies posterior to it. They lie anterior to the subscapularis muscle.
Below the pectoralis minor the lateral cord lies lateral of the 2nd part of the axillary artery, posterior cord lies posteriorly while the medial cord lies medially to the 2nd part of axillary artery.
Below the p.minor the cords gives their terminal branches which lie in the same relation as their terminal cords.
BRANCHES OF THE BRACHIAL PLEXUS
There are a total of 17 branches arising from the brachial plexus that are destined to supply the upper limb. There are other branches that supplies structures within the neck, they include; nerve to scaleni and a branch that join the phrenic nerve to supply the diaphragm.
Of the seventeen branches of the brachial plexus, three of the branches arise from the root, one from the trunk, three from the lateral cord, five from the medial cord and five from the posterior cord.
BRANCHES FROM THE ROOT
1. Long thoracic nerve of bell (C5,C6,C7).
2. Dorsal scapular nerve (C5).
3. Nerve to subclavius (C5, C6).
BRANCH FROM THE TRUNK
1. Suprascapular Nerve.
BRANCEHS FROM THE LATERAL CORD
1. Lateral pectoral Nerve. ((C5, C6).
2. Musculocutaneous – (C5,C6, C7)
3. Lateral root of median nerve (C5, C6, C7).
BRANCHES FROM THE MEDIAL CORD
1. Medial pectoral nerve
2. Medial cutaneous nerve of arm
3. Medial cutaneous nerve of forearm
4. Ulnar nerve
5. Medial root of median nerve
POSTERIOR CORD BRANCHES
1. Axillary nerve (C5, C6)
2. Upper subscapular nerve (C5,C6)
3. Thoracodorsal nerve (C7,C8).
4. ower subscapular nerve (C5,C6)
5. Radial nerve (C5-T1).
Dorsal Scapular Nerve:-
It arises from the root, it has a root value of C5 it supplies the levator scapulae, rhombodieus major and minor. It accompanies the dorsal scapula-artery
Nerve to Subclavius:-
It is a small branch arising from the junction between C5 and C6 to supply the subclavius muscle; it is sometimes included as a branch from the trunk.
Long thoracic nerve
It arises from the root of C5, C6 and C7. It then descends posterior to the roots of the brachial plexus. It runs along the anterior surface of the serratus anterior muscle up to its lower border. It gives up branches to each digitation of the serratus anterior muscle. Sometimes the branch from the 7th root is absent and sometimes when present it joins the nerve at the surface to serratus anterior.
This is the only branch from the upper trunk with root value of C5, C6. It is a large branch descending anterior to the brachial plexus. It enters the supraspinous fossa through the suprascapular notch which is converted to a canal by the transverse scapular ligament. It supplies the supraspinatus, and it is accompanied by the suprascapular artery through the spinoglenoid notch to enter the infraspinatus fossa to supply the infraspinatus muscle.
DISTRIBUTION OF THE BRANCHES OF THE LATERAL CORD
Lateral pectoral Nerve:-
It arises just above the pectoralis minor. It crosses the axillary artery and pierces the clavipectoral fascia to supply the clavicular head of the pectoralis major. It gives off a loop or ramus that passes across the axillary artery to the medial pectoral nerve. This ramus accompanies the medial pectoral nerve to supply the pectoral minor.
It is the terminal branch of the lateral cord, arising just below the pectoralis minor lying laterally to the axillary artery. It pierces the coracobrachialis muscle which it supplies and then passes between the biceps brachii and brachialis muscles up to the lower lateral part of the arm, here it continue into the forearm as the lateral cutaneous nerve of forearm, it gives up muscular branches that supplies the bicep brachii and brachialis muscles, it also sends articular branches to the elbow joint and a small branch which accompanies a nutrient artery into the humerus.
Lateral root of median Nerve
This terminal branch joins with medial root of median nerve anterior or sometimes lateral to the axillary artery. The median nerve supplies most of the flexor muscle of the forearm and the muscles of the thenar compartment
DISTRIBUTION OF BRANCHES OF THE MEDIAL CORD
Medial pectoral nerve
It arises from the medial cord with root value (C8, T1) just behind the pectoral minor which it pierces and supplies and end up to supply the sternocostal head of pectoral major muscle.
MEDIAL CUTANEOUS NERVE OF ARM
It arises below the pectoralis minor and runs along the medial border of axillary vein and in the arm it runs along the medial border of the basillic vein up to the lower medial half of the arm, here it supplies the skin of that region.
Medial cutaneous nerve of forearm
It runs between the axillary artery and vein in the axilla and between the brachial artery and basillic vein in the arm. It passes into the forearm and gives off an anterior and posterior branch that supplies the anteromedial and posteromedial halves of the forearm.
Medial root of median nerve
This terminal branch joins with the lateral root of median nerve to form the median nerve.
It is the terminal branch of the medial cord, root value C8, T1 but sometimes it is joined by fibers of C7 which arises from the lateral cord. It runs along the medial border of the axillary artery up to the medial aspect of the brachial artery to the middle of the arm were it pierces the medial intermuscular septum to enter the extensor compartment of the arm. It passes behind medial epicondyle of the humerus to enter the forearm where it supplies the flexor carpi ulnaris and the medial head of flexor digitorum profundus. It enters the hand to supply all the intrinsic muscle of the hand and the muscles of the hypothenar compartment.
It arises from posterior cord, with root value C5 & C6. It passes posterior to the axillary artery running along the anterior surface of the subscapularis muscle. It accompanies the posterior circumflex humeral artery through the quadrangular space. It gives off a branch to the teres minor and continues round the surgical neck of the humerus to supply the deltoid muscle. It also gives off an articular branch to the shoulder joint and a cutaneous branch that supply the skin at the lower border of the deltoid. This branch is known as the lower lateral cutaneous nerve of arm.
Upper subscapular nerve
Sometimes it is double and passes to supply the upper fibers of the subscapularis muscle.
Lower subscapular nerve
It is larger than the upper subscapular nerve. It supplies the lower fibres of the subscapularis muscle and in addition, it supplies the teres major muscle.
This large branch arises between the upper and lower subscapular nerves and passes downward being accompanied by subscapular artery a branch of the axillary artery, along the posterior axillary fold to supply the latissimus dorsi muscle.
It is the largest branch of the brachial plexus. It arises below the pectoralis minor posterior to the axillary artery. It runs posterior to the brachial artery. It then accompanies the profunda brachii artery through the spiral groove between the lateral and medial heads of tricep muscle. The radial nerve supplies all the extensor compartment muscles of the arm and forearm, it gives cutaneous branches to the skin of the arm and articular branch to the elbow joint.
APPLIED ANATOMY OF THE BRACHIAL PLEXUS
This is the paralysis that occurs as a result of damage to the upper trunk of the brachial plexus. This area is referred to as Erb’s point which is marked by the convergence of two nerves (the C5 and C6 ventral rami) the divergence of two nerve ( the upper and lower divisions of the upper trunk) and the emergence of two nerves (the suprascapular nerve and nerve to subclavius). Injury to this area could be as a result of accidental fall from a bike where by the victim lands on the neck shoulder angle or cases of breech labor were the inexperience nurse try to pull the head of the emerging baby when the shoulder is fixed within the pelvic cavity. Erb’s paralysis can also be as a result to damage to the C5 and C6 ventral rami at its root. Erb’s paralysis results in damage to the nerves that have root value of C5 and C6 which include:
· Suprascapular nerve
· Nerve to subclavius
· Lateral pectoral nerve
· Axillary nerve
· Musculocutaneous nerve
The damage to these nerves results in the following defects:
1. Loss of abduction of the arm due to the damage to the suprascapular nerve which supply the supraspinatus muscle and the axillary nerve which supply the deltoid muscle.
2. The arm is medially rotated due to paralysis of the lateral rotator muscles of the arm which include the teres minor supplied by the axillary nerve and the infraspinatus muscle supplied by the suprascapular nerve.
3. Loss of flexion at the elbow joint due to paralysis of the biceps brachii muscle as a result of damage to the musculocutaneous nerve.
4. Forearm is pronated due to the unopposed action of pronator teres muscle as a result of paralysis of the biceps.
Due to these defects the arm is placed in an adducted position and medially rotated, the forearm pronated and the palm facing backwards presenting the classical waiters tip position.
This is the paralysis that occurs due to the damage of the lower trunk of the brachial plexus. The lower trunk could be damaged when the victim tries to brake a fall with an out stretched arm, in so doing the weight of the falling body’s pull on the arm results to the tearing of the lower trunk. Lower trunk damage can also occur during breech delivery when the arm is unduly pulled. Damage to this trunk affects the following nerves:
· Medial pectoral nerve
· Medial cutaneous nerve of the arm
· Medial cutaneous nerve of the forearm
· Ulnar nerve
The defects that occur include:
1. Loss of skin sensation on the medial aspect of the arm and forearm.
2. Paralysis of the sternocostal head of pectoralis major muscle due to damage of the medial pectoral nerve.
3. Claw hand formation due to damage of the ulnar nerve.
SATURDAY NIGHT PALSY (SLEEP PARALYSIS)
This paralysis is related to Saturday’s weekend parties were the victim becomes drunk with too much wine so when he sits on a chair he sleeps off but with the arm thrown backwards over the chair’s back rest the undue pressure on the upper arm in that state of drunkenness does compress and damage the radial nerve as it passes through the spiral groove. This results to wrist drop.
This paralysis results due to damage to the radial nerve by ill-fitting crutches which how compress the nerve at the axilla. The effect is wrist drop.
WINGING OF THE SCAPULAR
This defect is as a result of damage to the long thoracic nerve mainly due to heavy weight carried on the shoulder. The long thoracic nerve supplies the serratus anterior muscle so when it is damage it results in the paralysis of the muscle which will result in retraction of the medial border of the scapular especially when the patient tends to push on a wall. The retracted scapular gives the presentation of a wing hence its name.
This syndrome is as a result of compression of the roots of the brachial plexus by the two heads of the scalene muscle as they emerge from the intervertebra foramen.
CERVICAL RIB SYNDROME
This syndrome is as a result of compression of the lower trunk by an abnormal presence of a cervical rib. The defect presents loss of sensation along the medial aspect of the arm and forearm in severe cases it might present the claw hand formation due to damage to the ulnar nerve