Exam 2: Brachial Plexus

Dorsal nerve roots

• Input: incoming/afferent dendrites supplying sensory information with the neuron cell body located in an adjacent dorsal root ganglion

• Sensory neurons transmit information to the spinal cord from muscle, joints, skin

Ventral nerve roots

• Output: outgoing/efferent axons supplying motor commands to muscles

Sensory end organs/inputs: Cutaneous Mechanoreceptors (Skin - touch, pressure)

• Merkel Discs: Shape, edges, texture (object recognition)

• Meissner Corpuscles: Light touch, movement (grip control)

• Pacinian Corpuscles: Deep pressure, vibration (tool use)

• Ruffini Endings: Skin stretch (hand shaping)

Sensory end organs/inputs: Proprioceptors: Muscles, tendons, joint position sense

• Muscle Spindles: Muscle length & stretch

• Golgi Tendon Organs: Muscle tension & force regulation

• Joint Receptors: Joint position & movement

These are why you can close your eyes & touch your nose. Sensors tell the brain exactly where they are in 3D space.

High speed feedback loop!

Sensory end organs/inputs: Thermoreceptors & Nociceptors (free nerve endings)

• Temperature (heat & cold)

• Pain (mechanical, thermal, chemical)

Spinal Nerve Roots

Ventral (output) and dorsal (input) nerve roots join to form a spinal nerve root. Spinal nerve roots contain both sensory and motor fibers.

31 pairs of spinal nerve roots:

• 8 cervical nerve roots vs. 7 cervical vertebrae

• Suboccipital nerve (C1) leaves the spinal cord between the occipital bone and the atlas (C1)

• C1-C7 exit above the corresponding vertebrae

• C8 spinal nerve root exits the spinal cord between C7 and T1

• 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal

The rest of the spinal nerves after C8 exit below the corresponding vertebrae.

Dermatomes

• Area of skin supplied by dorsal root/input of a spinal nerve.

• Vary in pattern from one individual to another and overlap one another.

• Used to identify location/level of nerve lesion

• Nerve root compression or SCI

• Dysfunction presents as diminished or altered sensation in the corresponding dermatome.

Dermatome screen: C5

Lateral upper arm

Dermatome screen: C6

Radial forearm to thumb

Dermatome screen: C7

Middle finger

Dermatome screen: C8

Small pinky finger

Dermatome screen: T1

Medial forearm

Myotomes - Motor

• Identifies muscle/group of muscles innervated by the ventral root/output of a spinal nerve.

• Dysfunction presents as weakness, atrophy, or paralysis in the corresponding myotome.

Both dermatomes and myotomes are used to identify high level lesions.

Myotome screen: C2-4

Shoulder shrug

Myotome screen: C5

Shoulder abduction

• Muscles: middle deltoid, supraspinatus (+trapezius, SA)

• chicken wing; lift shoulders to side

Myotome screen: C5-C6

Elbow flexion

• Bicep curl

• C6 flexes elbow w/ biceps, but extends the wrist, pulling the hand back.

Myotome screen: C6

Wrist extension (pulls hand back)

Myotome screen: C7

Wrist flexion (GAY) + Elbow Extension

• Extending arm to reach something

• Tricep extension: straighten arm, but flexes wrist

Myotome screen: C8

Thumb abduction

• Gets fingers ready to grasp

Myotome screen: T1

Finger abduction & adduction

Plexus: an intermingling of ventral rami that form peripheral nerves carrying motor, sensory and sympathetic nerve fibers.

• Only the ventral rami innervate limb muscles.

• Each ventral ramus of a spinal nerve root either forms a plexus or continues as an individual named nerve.

• 4 major plexuses are formed by the ventral rami: Cervical, brachial, lumbar, sacral

Cervical (C1-C4) plexus

diaphragm and neck muscles + sensory innervation to neck

Brachial (C5-T1) plexus

upper extremity muscles + sensation

Lumbar (T12-L4) plexus

pelvis, thigh

Sacral (L4-S4) plexus

posterior thigh, lower leg and foot

Brachial plexus – Anatomy and palpation: Trunks

Lie in the neck

Brachial plexus – Anatomy and palpation: Divisions

• adjacent to the subclavian artery (continues as axillary artery)

• Palpate behind the midpoint of the clavicle to feel the pulse

Brachial plexus – Anatomy and palpation: Cords

• extend from the midpoint of the clavicle to the coracoid process

• Named based on anatomical relationship to the axillary artery

• Palpate inferior to the lateral third of the clavicle

Brachial plexus: Terminal Nerves (5)

1. Musculocutaneous (C5, C6, C7)

2. Axillary (C5, C6)

3. Radial nerve (C5-T1)

4. Median nerve (C5-T1)

5. Ulnar nerve (C8-T1)

Brachial plexus: Injuries divided into 3 trunks

1. Upper trunk (Erb-Duchene C5-C6) = Waiter’s tip posture

2. Extended upper trunk (Erb-Duchene C5-C7)

3. Lower trunk (Dejerine-Klumpke C8-T1) = Neck & trunk separation

Brachial plexus: Vulnerable to?

• Trauma: childbirth, motorcycle accidents

• Susceptible to compression between the anterior and middle scalenes and underneath the pectoralis minor

Upper trunk and extended upper trunk lesion (Erb-Duchene C5-6 and C5-7)

• “Waiter’s tip posture”: limp hanging arm

• Paralysis of abduction and external rotation in the shoulder, making it pinned to the body/internally rotated

• Lacking elbow flexion and supination

• C7 involvement causes complete loss of wrist extension (Note: Wrist extension is mostly part of C6 but also partly involves C7 fibers)

• Finger, wrist flexors, and hand intrinsics NOT involved.

Lower trunk (Dejerine-Klumpke C8-T1)

• "Claw Hand" deformity: The hand appears clawed due to paralysis of the intrinsic hand muscles (interossei = abduction/adduction, lumbricals = flexion @ MCP and ICP joints).​

• Imbalance of flexors and extensors​

• Wrist may be in an extended or neutral position​

• Sensory Loss: Reduced sensation along the inner forearm and hand (C8-T1 dermatomes).​

• Proximal muscles spared

Musculocutaneous nerve (C5-7): primary motor functions

Elbow flexion & forearm supination

• Biceps brachii, brachialis, coracobrachialis

Musculocutaneous nerve (C5-7): Sensory area

Lateral forearm

Musculocutaneous nerve (C5-7): Functional deficits

Weakness in elbow flexion (biceps atrophy)

Musculocutaneous nerve (C5-7): Functional tasks

Elbow flexion: lifting objects, hand to mouth

Forearm supination: turn palm up, turn doorknob

Shoulder flexion: reach forward, arm adduction

Axillary nerve (C5-6): primary motor functions

Shoulder abduction/initial lift & stabilization

• Middle deltoid, teres minor

Axillary nerve (C5-6): sensory areas

Lateral shoulder/regimental badge area

Axillary nerve (C5-6): functional tasks

Arm abduction: lift arm away from body

Flexion/extension: move arm forward/back

Axillary nerve (C5-6): functional deficit

Limited shoulder abduction & extension (beyond movement initiated by rotator cuff)

Radial nerve (C5-T1): primary motor functions

elbow, wrist, finger extension

• triceps & all major extensors of wrist & digits

Radial nerve (C5-T1): sensory areas

posterior arm, forearm, dorsum of hand/radial side

Radial nerve (C5-T1): functional tasks

• Wrist extension: lift hand up to release grasp

• Finger & elbow extension: straighten fingers to open hand, straighten arm

• Forearm supination: rotate palm up

• Thumb extension/abduction: move thumb away from palm

Radial nerve (C5-T1)/extensors: deficits

Wrist drop

Median nerve (C5-T1): primary motor functions

Forearm pronation, wrist flexion, precision pinch

• Most forearm flexors & thumb thenar muscles

Median nerve (C5-T1): sensory areas

• Palmar thumb

• Index, middle, radial half of ring finger

Median nerve (C5-T1): functional tasks

• Forearm pronation: turn palm down

• Pinching & thumb opposition: tripod/pencil grasp

• Finger & wrist flexion

Median nerve (C5-T1): deficits

Loss of thumb opposition (‘O’ position) & Carpal tunnel

Ulnar nerve (C8-T1): primary motor functions

Intrinsic hand muscle control (fine motor coordination)

• FCU, FDP digits 4+5, intrinsic hand muscles, hypothenar muscles

• Also DOESN’T innervate any upper arm muscles

Ulnar nerve (C8-T1): sensory areas

Ulnar half of hand (palmar & dorsal side, since nerve splits & goes anterior & posterior)

Ulnar nerve (C8-T1): functional tasks

• things that make clawing annoying

Power grip: strong grasp, holding cup ☕

Pinch grip: pinching a cheerio in-between thumb and index, hold thin paper 🤌

Finger manipulation: spread fingers/abduction; bring fingers together/adduction (typing, playing piano) 👋

Fine motor skills: pinky & ring finger

Ulnar nerve (C8-T1): deficits

• Claw hand (Klumpke’s)

• loss of finger abduction & adduction

Radial nerve motor innervation: upper arm

Triceps C6-8

Radial nerve motor innervation: Elbow

• Brachioradialis (C5-6)

• Brachialis (small C7 portion ONLY)

• ECRB/L (C6-7)

• Anconeus (C6-8)

Radial nerve motor innervation: Forearm/PIN group

Supinator (C6-7)

Distal extensor (C7-8): extensor carpi ulnaris (ECU)

Thumb extensors (C7-8):

• Extensor pollicis longus (EPL)

• Extensor pollicis brevis (EPB)

• Abductor pollicis longus (APL)

Finger extensors (C7-8):

• Extensor digitorum communis (EDC)

• Extensor digiti minimi (EDM)

• Extensor indicis proprius (EIP)

Why are triceps usually spared in humeral shaft radial nerve injuries?

• The nerve branches that supply the triceps (long, lateral, and medial heads) arise proximal to the typical fracture site, while the radial nerve itself is injured further down in the spiral groove

• The nerve only becomes susceptible to compression here as it passes against the bone

Median nerve loss: Anterior interosseous nerve palsy (AIN)

• Deep motor branch of median n.

• Can’t make perfect ‘O’; end up pressing finger pads together flat, like a pinch

• Person IS opposing, can touch finger tip to finger tip

• Muscle belly in thumb thenar shown

• “OK” sign to see presentation; less round, more angular

Median nerve loss: Carpal tunnel syndrome

• Can’t oppose/touch finger tip to tip

• Loss of thenar muscles & sensation going into finger tips (eye of hands): compression of the median nerve in the wrist, resulting in pain, numbness, tingling, and weakness in the hand, especially in the thumb, index, and middle fingers

• Screening: Try to make OK sign as round as possible; feels clumsy/weak when trying to pick up things

Ulnar nerve loss: High ulnar nerve lesion

• Hand muscles & deep forearm flexors (curl team) both paralyzed = hand looks flatter, less deformed

• BUT WORSE! Lost grip strength AND fine motor control. Hand is useless

Ulnar nerve loss: Low ulnar n. lesion

• Some grip, even though it looks bad

• Nerve signals in forearm muscles are fine, pulling hard against paralyzed hand muscles, yanking fingers into claws

• Curl team yes, balancing team NO

• Visually more dramatic. Little muscles in hand when paralyzed = Flexors (curl team) WINS = CLAW!

• Can still flex/move 4th and 5th fingers

Ulnar Paradox

• A low lesion will typically present with more marked clawing of the little and ring fingers due to the unopposed action of the long flexors

• Whereas: a high lesion around the elbow the clawing is more mild as the long flexors are affected too.

The more proximal (higher) the nerve injury, the greater the functional loss because all distal muscles and sensory areas supplied by that nerve are affected.

EX: Injuries at the level of the brachial plexus affecting whole trunks vs. injuries at the level of the terminal nerves

Within the specific terminal nerves: the pattern of deficit looks different depending on the specific branch of nerve that was injured

EX: AIN palsy and carpal tunnel both are caused by injuries to the median nerve but occur at different branches of the median nerve

A newborn presents with shoulder adduction, internal rotation, elbow extension, and forearm pronation. Wrist extension is preserved. Which nerve roots are affected?

C5, 6 (Erb-Duchenne palsy)

• C5: Shoulder abduction → now shoulder adduction

• C5-6: Elbow flexion → now elbow extension

A client presents to outpatient OT following a mid-shaft humeral fracture. They are unable to extend the wrist or fingers.

1. Which peripheral nerve is most likely injured?

2. In which anatomical plane does the primary deficit occur?

1. Radial nerve

2. Sagittal plane

A 19-year-old female college student while hiking, she slipped and reflexively grabbed a tree branch to break her fall. This caused sudden, forceful hyperabduction (upward traction) of her right arm. She reported difficulty with fine motor coordination and demonstrates a claw hand. She also experienced a loss of sensation or numbness along the medial aspect of the forearm and hand.

1. Which nerve roots and trunk are most likely involved?

2. Based on the sensory loss in the medial forearm, which specific cutaneous nerve has been affected?

1. Ulnar nerve C8 - T1, lower trunk

2. Medial antebrachial cutaneous n.