Brachial Plexus Basics: Muscles and Nerves Explained

Today we'll explore the anatomy of the brachial plexus, a complex network of nerves that supplies the entire upper limb.The brachial plexus originates from spinal cord segments C5 through T1. These segments are located in the lower cervical and upper thoracic regions of the spine.These nerve roots exit the intervertebral foramina, which are openings between adjacent vertebrae. Each root carries both sensory and motor fibers.These roots combine to form three trunks. The upper trunk is formed by C5 and C6, the middle trunk is a continuation of C7, and the lower trunk is formed by C8 and T1.Each trunk then divides into anterior and posterior divisions. This separation is based on the developmental pattern of the limb, with posterior divisions supplying the extensor compartments and anterior divisions supplying the flexor compartments.The divisions then reorganize to form three cords named for their relationship to the axillary artery. The posterior cord is formed from all three posterior divisions. The lateral cord is formed from the anterior divisions of the upper and middle trunks. The medial cord is a continuation of the anterior division of the lower trunk.The cords are named based on their position relative to the axillary artery. The posterior cord lies behind the artery, the lateral cord is on the outer side, and the medial cord is on the inner side of the artery.These cords will ultimately give rise to the terminal branches that innervate the muscles and skin of the upper limb. The specific branches and their distributions will be covered in the next section.To summarize, the brachial plexus has a complex organization that proceeds from roots, to trunks, to divisions, to cords, and finally to terminal branches. This intricate arrangement allows for coordinated innervation of the entire upper limb.Now let's examine the five major terminal nerves that emerge from the brachial plexus.The musculocutaneous nerve originates from spinal segments C5 through C7.It passes through the coracobrachialis muscle.The musculocutaneous nerve innervates the anterior muscles of the arm, including the biceps brachii and brachialis.The median nerve arises from segments C6 to T1.It travels through the arm, forearm, and passes through the carpal tunnel to reach the hand.The median nerve supplies most of the anterior compartment muscles of the forearm and the thenar muscles of the thumb.The ulnar nerve originates from segments C8 and T1.It passes behind the medial epicondyle of the humerus, which is why hitting your 'funny bone' causes a tingling sensation.The ulnar nerve innervates most of the intrinsic muscles of the hand, providing fine motor control.The radial nerve is derived from all segments contributing to the brachial plexus, C5 through T1.It follows a unique spiral path around the humerus through the spiral groove.The radial nerve supplies all muscles in the posterior compartment of the arm and forearm, controlling wrist and finger extension.Finally, the axillary nerve originates from segments C5 and C6.It wraps around the surgical neck of the humerus.The axillary nerve innervates the deltoid and teres minor muscles, which are crucial for shoulder movement.Let's summarize the five major terminal nerves of the brachial plexus with their origins, pathways, and innervation patterns.These five nerves branch from the brachial plexus to innervate the entire upper limb, providing both motor and sensory functions. Understanding their pathways is crucial for clinical assessment of upper limb injuries.The brachial plexus innervates muscles in logical patterns, creating functional groups throughout the upper limb.Understanding these patterns helps us predict muscle function and diagnose nerve injuries.The musculocutaneous nerve supplies the anterior compartment of the arm, innervating three key muscles.These muscles - the biceps brachii, brachialis, and coracobrachialis - are primarily responsible for elbow flexion.The median nerve runs along the anterior aspect of the arm and forearm, innervating most of the forearm flexor muscles.It supplies muscles that control wrist flexion, forearm pronation, and most importantly, the thenar muscles that enable precise thumb movements.The ulnar nerve travels along the medial side of the arm, passing behind the medial epicondyle of the humerus.It innervates the hypothenar muscles, interossei, and medial lumbricals, which are essential for fine motor control and finger abduction and adduction.The radial nerve courses through the spiral groove of the humerus, supplying all the posterior compartment muscles of the arm and forearm.It innervates the triceps for elbow extension, and the forearm extensors that control wrist and finger extension.The axillary nerve wraps around the surgical neck of the humerus to supply the deltoid muscle.The deltoid is crucial for shoulder abduction and is the primary muscle that lifts the arm away from the body.The innervation pattern of the brachial plexus follows a logical organization that ensures coordinated movement of the upper limb.Each major nerve supplies specific muscle groups with related functions, creating an elegant system that enables the complex movements of our arms and hands.Brachial plexus injuries can occur through various mechanisms, including trauma, compression, or inflammation.Erb's palsy results from injury to the upper trunk of the brachial plexus, affecting the C5 and C6 nerve roots.This injury produces the characteristic 'waiter's tip' position, with the arm internally rotated and pronated. The patient cannot abduct or externally rotate the shoulder, or flex the elbow.Klumpke's palsy results from injury to the lower trunk of the brachial plexus, affecting the C8 and T1 nerve roots.This injury causes a claw hand deformity with weakness of the intrinsic hand muscles. The patient has difficulty with fine motor movements, finger abduction, and thumb opposition.Thoracic outlet syndrome involves compression of the brachial plexus as it passes between the clavicle and first rib.This compression commonly affects the lower trunk, causing pain and paresthesia in the ulnar nerve distribution. The condition may be caused by an extra cervical rib, hypertrophied scalene muscles, or abnormal clavicle position.Saturday night palsy occurs when the radial nerve is compressed against the humerus in the spiral groove.This classically happens when a person falls asleep with their arm draped over a chair, compressing the nerve. The result is wrist drop, with inability to extend the wrist and fingers.Early recognition of these injury patterns is crucial for proper treatment and rehabilitation. The timing of intervention significantly affects outcomes, with better results when treatment is initiated promptly.Now we'll examine how to clinically assess brachial plexus function through specific motor and sensory tests.Motor testing allows us to assess the function of each major terminal nerve through specific movements.For the musculocutaneous nerve, we test elbow flexion using the biceps test. Ask the patient to flex their elbow against resistance.Weakness in this movement suggests musculocutaneous nerve injury.For the median nerve, we test thumb opposition by having the patient touch their thumb to their little finger.We also check the OK sign, where the patient pinches their thumb and index finger together.For the ulnar nerve, we test finger abduction and adduction. Ask the patient to spread their fingers apart and then resist them being pushed together.The ulnar nerve controls most of the intrinsic muscles of the hand, including those that spread the fingers.Froment's sign tests the adductor pollicis. The patient grips paper between thumb and index finger.For the radial nerve, we test wrist and finger extension. Radial nerve injury causes wrist drop.Ask the patient to extend their wrist and fingers against resistance. This tests the extensor muscles of the forearm.For the axillary nerve, we test shoulder abduction. This primarily tests the deltoid muscle.Ask the patient to abduct their arm to ninety degrees against resistance. Weakness indicates axillary nerve injury.In addition to motor testing, sensory testing is essential for complete brachial plexus assessment.Each major nerve has a specific sensory distribution pattern. Here we see the median nerve supplying the lateral palm and first three and a half digits.The ulnar nerve supplies the medial hand, including the little finger and medial half of the ring finger.The radial nerve supplies the posterior forearm and dorsal aspect of the hand, particularly over the first three digits.Sensory testing includes assessment of light touch with cotton wool, pain with pinprick, temperature sensation, and proprioception.When assessing brachial plexus function, always test both motor and sensory components, compare with the unaffected side, and document specific deficits.The pattern of deficits helps localize the lesion within the brachial plexus, guiding diagnosis and treatment.