Brachial Plexus PDF

BRACHIAL PLEXUS By Dr Idorenyin Umoh  Introduction The brachial plexus is a network (plexus) of nerves (formed by the anterior ramus of the lower four cervical nerves and first thoracic nerve (C5, C6, C7, C8, and T1). This plexus extends from the spinal cord, th...

BRACHIAL PLEXUS By Dr Idorenyin Umoh  Introduction The brachial plexus is a network (plexus) of nerves (formed by the anterior ramus of the lower four cervical nerves and first thoracic nerve (C5, C6, C7, C8, and T1). This plexus extends from the spinal cord, through the cervicoaxillary canal in the neck, over the first rib, and into the armpit.  The brachial plexus (plexus brachialis) is a somatic nerve plexus formed by intercommunications among the ventral rami (roots) of the lower 4 cervical nerves (C5-C8) and the first thoracic nerve (T1). The plexus, is responsible for the motor innervation of all of the muscles of the upper extremity, with the exception of the trapezius and levator scapula  The brachial plexus is a network of nerve fibres that supplies the skin and musculature of the upper limb. It begins in the root of the neck, passes through the axilla, and runs through the entire upper extremity. The plexus is formed by the anterior rami (divisions) of cervical spinal nerves C5, C6, C7 and C8, and the first thoracic spinal nerve, T1.  The brachial plexus is divided into five parts;  roots,  trunks,  divisions,  cords and  branches  Roots The ‘roots’ refer the anterior rami of the spinal nerves that comprise the brachial plexus. These are the anterior rami of spinal nerves C5, C6, C7, C8, and T1. At each vertebral level, paired spinal nerves arise. They leave the spinal cord via the intervertebral foramina of the vertebral column. Each spinal nerve then divides into an anterior and a posterior ramus..  The roots of the brachial plexus are formed by the anterior rami of spinal nerves C5-T1 the posterior divisions innervate  the skin and  musculature of the intrinsic back muscles. After their formation, these nerves pass between the anterior and medial scalene muscles to enter the base of the neck  Trunks At the base of the neck, the roots of the brachial plexus converge to form three trunks. These structures are named by their relative anatomical location: Superior trunk – a combination of C5 and C6 roots. Middle trunk – continuation of C7. Inferior trunk – combination of C8 and T1 roots. The trunks traverse laterally, crossing the posterior triangle of the neck.  Divisions Each trunk divides into two branches within the posterior triangle of the neck. One division moves anteriorly (toward the front of the body) and the other posteriorly (towards the back of the body). Thus, they are known as  the anterior and posterior divisions. There are three anterior and three posterior nerve fibres. These divisions leave the posterior triangle and pass into the axilla. They recombine into the cords of the brachial plexus.  Cords Once the anterior and posterior divisions have entered the axilla, they combine together to form three cords, named by their position relative to the axillary artery. The lateral cord is formed by: The anterior division of the superior trunk The anterior division of the middle trunk The posterior cord is formed by: The posterior division of the superior trunk The posterior division of the middle trunk The posterior division of the inferior trunk The medial cord is formed by: The anterior division of the inferior trunk. The cords give rise to the major branches of the brachial plexus.  Major Branches In the axilla and the proximal aspect of the upper limb, the three cords give rise to five major branches. These nerves continue into the upper limb to provide innervation to the muscles and skin present. Musculocutaneous Nerve. Roots: C5, C6, C7. Motor Functions: Innervates  the brachialis, biceps brachii and coracobrachialis muscles.  Sensory Functions: Gives off the lateral cutaneous branch of the forearm, which innervates  the lateral half of the anterior forearm, and  a small lateral portion of the posterior forearm.  Axillary Nerve Roots: C5 and C6. Motor Functions: Innervates the teres minor and deltoid muscles. Sensory Functions: Gives off the superior lateral cutaneous nerve of arm, which innervates the inferior region of the deltoid  Median Nerve Roots: C6 – T1. (Also contains fibres from C5 in some individuals). Motor Functions: Innervates most of the flexor muscles in the forearm, the thenar muscles, and the two lateral lumbricals associated with the index and middle fingers. Sensory Functions: Gives off the palmar cutaneous branch, which innervates the lateral part of the palm, and the digital cutaneous branch, which innervates the lateral three and a half fingers on the anterior (palmar) surface of the hand.  Radial Nerve Roots: C5 – T1. Motor Functions: Innervates the triceps brachii, and the muscles in the posterior compartment of the forearm (which are primarily, but not exclusively, extensors of the wrist and fingers). Sensory Functions: Innervates the posterior aspect of the arm and forearm, and the posterolateral aspect of the hand.  Ulnar Nerve Roots: C8 and T1. Motor Functions: Innervates the muscles of the hand (apart from the thenar muscles and two lateral lumbricals), flexor carpi ulnaris and medial half of flexor digitorum profundus. Sensory Functions: Innervates the anterior and posterior surfaces of the medial one and half fingers, and associated palm area.  Practical Relevance: Dissecting the Brachial Plexus When dissecting the upper limb, it can be difficult to recognise what part of the brachial plexus you are at – it can just look like a mass of nerves. The important structure to look for is an ‘M’ shape. This is formed by the musculocutaneous, median, and ulnar nerves, usually superficial to the axillary artery. This shape is usually consistent between cadavers. It can help you get your bearings, and you can work backwards to identify the cords, divisions and branches.  Clinical Relevance: Injury to the Brachial Plexus An intact brachial plexus is vital for the normal function of the upper limb. There are two major types of injuries that can affect the brachial plexus. An upper brachial plexus injury affects the superior roots, and a lower brachial plexus injury affects the inferior roots. Upper Brachial Plexus Injury – Erb’s Palsy Erb’s palsy commonly occurs where there is an excessive increase in the angle between the neck and shoulder, which stretches (or even tears) the nerve roots of C5 and C6. It can occur as a result of result of a difficult birth or shoulder trauma. Nerves affected: Nerves derived from solely C5 or C6 roots: musculocutaneous, axillary, suprascapular and nerve to subclavius. Muscles  Muscles paralysed: Supraspinatus, infraspinatus, subclavius, biceps brachii, brachialis, coracobrachialis, deltoid and teres minor. Motor functions: Movements that are lost or greatly weakened include abduction at shoulder, lateral rotation of arm, supination of forearm, and flexion at shoulder. Sensory functions: Loss of sensation down lateral aspect of arm, which covers the sensory innervation of the axillary and musculocutaneous nerves. The affected limb hangs limply, medially rotated by the unopposed action of pectoralis major. The forearm is pronated due to the loss of biceps brachii. The wrist is weakly flexed due to the normal increased tone of the wrist flexors relative to the wrist extensors. This is position is known as ‘waiter’s tip’, and is characteristic of Erb’s palsy.  Lower Brachial Plexus Injury – Klumpke Palsy A lower brachial plexus injury results from excessive abduction of the arm (e.g. person catching a branch as they fall from a tree). It has a much lower incidence than Erb’s palsy. Nerves affected: Nerves derived from the T1 root – ulnar and median nerves. Muscles paralysed: All the intrinsic hand muscles (the flexor muscles in the forearm are also supplied by the ulnar and median nerves, but are innervated by different roots). The primary symptom is a “claw hand,” caused by the unopposed action of the finger extensor muscles. The lumbrical muscles flex the metacarpophalangeal joints and extend the interphalangeal joints, so their paralysis will cause the opposite: extension of the MCP and flexion of the IP joints. Sensory functions: Loss of sensation along medial side of arm.