Musculocutanous nerve

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Musculocutaneous nerve

Roots: C5-C7
Origin: Terminal branch of the lateral cord of the brachial plexus.

Course

Arises in the axilla, inferior to pectoralis minor.
Pierces the coracobrachialis muscle.
Descends in the anterior compartment of the arm, between biceps brachii and brachialis.
Near the elbow it continues as the lateral cutaneous nerve of the forearm (lateral antebrachial cutaneous nerve).

Motor supply

It supplies the flexor muscles of the arm:

Coracobrachialis - flexes and adducts the arm at the shoulder
Biceps brachii - flexes the elbow and powerfully supinates the forearm
Brachialis - primary flexor of the elbow

Sensory supply

Skin over the lateral (radial) side of the forearm, from elbow toward the wrist, via its terminal lateral cutaneous nerve of forearm.

Important clinical features

Injury causes:

Weakness of elbow flexion and forearm supination
Reduced or absent biceps reflex (mainly C5-C6)
Sensory loss over the lateral forearm

It may be injured in penetrating trauma of the axilla/upper arm, coracoid-region surgery, or brachial plexus lesions. The key sensory autonomous zone is a narrow strip along the radial forearm. Localization in Clinical Neurology, 8e describes this as the typical sensory-loss territory in musculocutaneous lesions.

A concise memory aid: “Musculocutaneous = muscles in the front of arm + skin of lateral forearm.”

The TeachMeAnatomy overview summarizes its C5-C7 lateral-cord origin, arm course, and motor and sensory distribution.

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This page is about brachial plexus injuries and how to identify the injured part from the posture, weak muscles, and sensory loss.

1. First understand the brachial plexus

The brachial plexus is the nerve network supplying the upper limb.

Roots → Trunks → Divisions → Cords → Branches

Roots: C5, C6, C7, C8, T1
Trunks:
- Upper trunk = C5-C6
- Middle trunk = C7
- Lower trunk = C8-T1
Cords, named according to their relation to the axillary artery:
- Lateral cord
- Medial cord
- Posterior cord

Mnemonic: R T D C B
“Ready To Drink Cold Beer.”

The terminal nerves are:

Cord	Main terminal branches
Lateral cord	Musculocutaneous nerve + lateral root of median nerve
Medial cord	Ulnar nerve + medial root of median nerve
Posterior cord	Axillary nerve + radial nerve

The median nerve is formed by a lateral root and medial root, so it receives fibers from both lateral and medial cords.

2. Upper trunk injury: C5-C6

This is called Erb-Duchenne palsy or Erb palsy.

How does it occur?

Usually due to excessive separation of the head/neck from the shoulder. Examples:

A difficult delivery with shoulder dystocia
Fall or trauma causing the shoulder to be pulled down while the neck is forced to the opposite side

The page’s example of violent separation of neck and shoulder causing injury at the upper trunk is correct.

Which nerves/functions are mainly affected?

C5-C6 fibers contribute strongly to:

Suprascapular nerve: supraspinatus and infraspinatus
Axillary nerve: deltoid and teres minor
Musculocutaneous nerve: biceps, brachialis, coracobrachialis
Part of radial nerve supply

Therefore, the patient loses or weakens:

Shoulder abduction
Lateral rotation of arm
Elbow flexion
Forearm supination

Why is the posture called “waiter’s tip”?

The arm lies:

Adducted: deltoid and supraspinatus are weak, so abduction is lost.
Medially rotated: lateral rotators such as infraspinatus are weak; medial rotators act unopposed.
Elbow extended: elbow flexors, especially biceps and brachialis, are weak.
Forearm pronated: biceps cannot supinate, so pronators act unopposed.

So remember:

Erb palsy = “waiter’s tip” posture: arm by side, adducted, medially rotated, elbow extended, forearm pronated.

The book’s question asks the effect of upper-trunk injury except. The answer marked, supination of the forearm, is correct because supination is lost or weakened, not produced.

A clinical review describes upper plexus injury as prominent weakness in deltoid, biceps, and other proximal muscles after forced shoulder depression, with the classic waiter’s-tip position. Clinical anatomy review

3. Lower trunk injury: C8-T1

This is called Klumpke palsy.

How does it occur?

The lower trunk is injured when the arm is suddenly pulled upward, such as:

Catching something while falling
Excessive traction on an abducted arm
Difficult breech delivery
Compression from a cervical rib or thoracic outlet pathology

Which muscles are most affected?

C8-T1 fibers mainly supply:

Intrinsic muscles of the hand, especially through ulnar and median nerves
Thenar muscles
Interossei
Lumbricals

The key muscle on your page is opponens pollicis.

Why opponens pollicis?

It is a thenar muscle supplied by the recurrent branch of the median nerve, mainly C8-T1. It opposes the thumb, allowing the thumb tip to touch the fingertips. This is necessary for fine grip, writing, buttoning, and picking up small objects.

Deformity: claw hand

Loss of interossei and medial lumbricals leads to:

Hyperextension at metacarpophalangeal joints
Flexion at interphalangeal joints

This produces clawing, especially of the ring and little fingers.

Horner syndrome

If the T1 sympathetic fibers are also injured, there may be Horner syndrome on the same side:

Ptosis: drooping eyelid
Miosis: constricted pupil
Anhidrosis: reduced facial sweating

Klumpke palsy = C8-T1 = intrinsic hand weakness + claw hand ± Horner syndrome.

4. Lateral cord injury

The lateral cord is formed by the anterior divisions of:

Upper trunk: C5-C6
Middle trunk: C7

Thus it carries mainly C5-C7 fibers.

Branches of the lateral cord

Lateral pectoral nerve
Musculocutaneous nerve
Lateral root of median nerve

Important correction regarding pectoral muscles

Your page says lateral-cord injury causes paralysis of pectoralis minor. That is not the best anatomical answer.

Lateral pectoral nerve, from the lateral cord, mainly supplies pectoralis major.
Medial pectoral nerve, from the medial cord, supplies pectoralis minor and also contributes to pectoralis major.

So, in a lateral-cord injury, the important affected pectoral muscle is mainly pectoralis major, especially its clavicular head.

This agrees with Gray’s Anatomy for Students: the lateral pectoral nerve arises from the lateral cord and innervates pectoralis major, while pectoralis minor is primarily supplied by the medial pectoral nerve. Gray’s Anatomy for Students, p. 89.

Other consequences of lateral cord injury

Because the musculocutaneous nerve is affected:

Weak elbow flexion
Weak forearm supination
Reduced biceps reflex
Sensory loss over lateral forearm

Because the lateral contribution to median nerve is affected:

Some median-nerve weakness can occur, but the medial root may preserve part of median function.

5. Musculocutaneous nerve

This is the central nerve you originally asked about.

Origin and root value

Origin: lateral cord of brachial plexus
Roots: C5-C7

Course

Follow it as if you were tracing it in a dissection:

It begins in the axilla from the lateral cord.
It pierces coracobrachialis.
It travels down the front of the arm between:
- Biceps brachii
- Brachialis
Near the elbow, it emerges from deep fascia.
It continues as the lateral cutaneous nerve of forearm.

Motor supply

It supplies all three major muscles in the anterior compartment of arm:

Muscle	Main action
Coracobrachialis	Flexes and adducts arm at shoulder
Biceps brachii	Flexes elbow and supinates forearm
Brachialis	Main flexor of elbow

Sensory supply

Through its terminal branch, the lateral cutaneous nerve of forearm, it supplies the skin of the:

Lateral/radial aspect of the forearm, from the elbow toward the wrist.

A useful way to remember it:

Musculo-cutaneous nerve = muscles of front of arm + skin of lateral forearm.

Testing it

Ask the patient to:

Flex the elbow against resistance
Supinate the forearm against resistance

Also test:

Biceps reflex, mainly C5-C6, carried by the musculocutaneous nerve
Light touch/pinprick over lateral forearm

Localization in Clinical Neurology describes the musculocutaneous nerve as arising from the lateral cord, piercing coracobrachialis, supplying coracobrachialis, biceps, and brachialis, and ending as the lateral cutaneous nerve of forearm. Localization in Clinical Neurology, 8e, p. 89.

6. Posterior cord injury and latissimus dorsi

The page asks: “Paralysis of latissimus dorsi is most commonly caused by injury to which cord?”

Answer: posterior cord.

Why?

The thoracodorsal nerve arises from the posterior cord and supplies latissimus dorsi.

Latissimus dorsi actions:

Extension of arm
Adduction of arm
Medial rotation of arm
Helps climbing, pulling, swimming, and using crutches

So posterior-cord lesions may affect:

Axillary nerve: deltoid weakness
Radial nerve: wrist/finger extension weakness
Thoracodorsal nerve: latissimus dorsi weakness
Upper/lower subscapular nerves: subscapularis and teres major weakness

7. Axillary brachial plexus block

An axillary block is an anesthetic technique performed in the axilla to numb the forearm and hand.

Why can musculocutaneous nerve be spared?

The musculocutaneous nerve leaves the plexus early and quickly enters coracobrachialis. Therefore, an anesthetic injected around the axillary artery may not reach it adequately.

If the lateral forearm still has sensation after an axillary block, the likely spared nerve is:

Musculocutaneous nerve, via its terminal lateral cutaneous nerve of forearm.

This is why the musculocutaneous nerve may need a separate block when complete forearm anesthesia is required.

C8-T1 fibers and flexor carpi radialis

The page asks: in an axillary injury involving C8-T1, which muscle is spared?

Answer: flexor carpi radialis.

Why?

Flexor carpi radialis is supplied by the median nerve but predominantly receives C6-C7 fibers.
A C8-T1 lesion mainly affects lower-trunk/medial-cord functions and intrinsic hand muscles, not a C6-C7 muscle such as flexor carpi radialis.

8. Medial cord continuation

The ulnar nerve is the direct terminal branch of the medial cord.

The medial cord also gives:

Medial pectoral nerve
Medial cutaneous nerve of arm
Medial cutaneous nerve of forearm
Medial root of median nerve

So:

Medial cord → ulnar nerve
Lateral cord → musculocutaneous nerve
Posterior cord → axillary and radial nerves

9. “Thumb is adducted and laterally rotated”: median nerve injury

This refers to the ape-thumb deformity seen in a median nerve lesion.

The recurrent branch of the median nerve supplies the thenar muscles:

Abductor pollicis brevis
Opponens pollicis
Superficial head of flexor pollicis brevis

When these are paralyzed:

Thumb cannot be properly abducted and opposed
Adductor pollicis, supplied by the ulnar nerve, acts unopposed
The thumb tends to lie adducted and in the plane of the palm

This is different from a claw hand, which is more associated with ulnar or lower-trunk lesions.

10. Long thoracic nerve injury

Root value and supply

Roots: C5-C7
Supplies: serratus anterior

The long thoracic nerve is vulnerable because it runs a long, relatively superficial course along the lateral chest wall.

What does serratus anterior do?

It:

Holds the scapula firmly against the chest wall
Protracts the scapula, as in punching
Helps rotate the scapula upward during elevation of the arm

Injury causes winging of scapula

When serratus anterior is weak, the medial border of scapula projects backward, especially when the patient pushes against a wall.

Long thoracic nerve injury → serratus anterior paralysis → winged scapula.

Functional effects

Weak pushing and punching
Difficulty elevating the arm above shoulder level
Arm elevation above 90 degrees becomes difficult because upward rotation of the scapula is impaired

A nuance: the arm can usually abduct to about 90 degrees through supraspinatus and deltoid. Full elevation overhead needs scapular upward rotation by serratus anterior and trapezius.

11. Radial nerve injury

Important relation

The radial nerve runs in the radial/spiral groove on the posterior humerus.

Therefore, a classic cause is:

Midshaft fracture of humerus → radial nerve injury

Main deficit: wrist drop

Radial nerve supplies wrist and finger extensors. If injured:

Wrist cannot extend
Wrist hangs in flexion due to unopposed flexors
This is called wrist drop

Depending on the level of injury, there may also be:

Weak finger extension
Weak thumb extension
Sensory loss over part of the dorsolateral hand, especially the first dorsal web space

Final exam table

Lesion	Roots / nerve	Main posture or deficit
Upper trunk injury	C5-C6	Erb palsy, waiter’s tip
Lower trunk injury	C8-T1	Klumpke palsy, claw hand ± Horner syndrome
Lateral cord injury	C5-C7	Musculocutaneous weakness, lateral forearm sensory loss, pectoralis major involvement
Long thoracic nerve injury	C5-C7	Winged scapula, weak pushing
Radial nerve injury	Often midshaft humeral fracture	Wrist drop
Musculocutaneous nerve injury	C5-C7	Weak elbow flexion/supination, lateral forearm sensory loss
Median nerve injury	Variable roots	Loss of thumb opposition, ape-thumb posture
Ulnar nerve / medial cord involvement	Mainly C8-T1	Intrinsic hand weakness, clawing

Three high-yield memory lines

C5-C6: Erb = waiter’s tip.
C8-T1: Klumpke = claw hand.
Musculocutaneous: elbow flexion + supination + lateral forearm sensation.

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Shoulder abduction and related nerve injuries

This page tests one idea repeatedly:

To raise the arm fully overhead, the shoulder joint and scapula must work together.

A patient may fail to lift the arm because of injury to:

Supraspinatus
Deltoid
Serratus anterior
Trapezius
Or the nerves supplying them.

1. First, distinguish the movements

Abduction

Moving the arm away from the body in the frontal plane, such as making a “T” shape.

Flexion/elevation

Moving the arm forward and then overhead.

Scapular upward rotation

For the arm to go fully above the head, the scapula must rotate upward. This enlarges the available space under the acromion and lets the humerus elevate safely.

Scapular protraction

Moving the scapula forward around the chest wall, as in pushing a wall or punching.

2. The sequence of arm abduction

The page’s central table is high yield:

Range of abduction	Principal muscle	Nerve
0-15°	Supraspinatus	Suprascapular nerve
15-90°	Deltoid	Axillary nerve
Above 90°	Serratus anterior + trapezius	Long thoracic nerve + spinal accessory nerve

A common clinical anatomy summary confirms that supraspinatus initiates abduction to 15°, deltoid predominantly abducts from 15° to 90°, and serratus anterior plus trapezius permit abduction above 90°. Shoulder-abductor anatomy

A. First 15°: supraspinatus

Muscle

Supraspinatus is one of the four rotator-cuff muscles.

Nerve

Suprascapular nerve, mainly C5-C6.

Function

Starts shoulder abduction.
Stabilizes the head of humerus in the glenoid cavity while the arm begins moving.

If it is injured

The patient has difficulty initiating abduction. Once the arm has been passively moved a little away from the body, the deltoid may take over and continue the movement.

Clinical question on the page

A patient cannot abduct the arm initially, but after about 30° the movement becomes smooth.

Answer: Supraspinatus lesion, commonly due to suprascapular nerve injury or a supraspinatus tendon tear.

Drop-arm test

The patient abducts the arm, usually with help, and is asked to lower it slowly. If the arm drops suddenly, this suggests a significant rotator cuff tear, especially a supraspinatus tear. It is not simply a nerve test.

B. 15° to 90°: deltoid

Muscle

Deltoid, especially its middle fibers.

Nerve

Axillary nerve, C5-C6.

Function

Main abductor of the arm from 15° to 90°.

If the axillary nerve is injured

The patient has:

Weak or absent abduction from 15° to 90°
Flattened shoulder contour due to deltoid wasting
Sensory loss over the “regimental badge” area, on the lateral upper arm

First question at the top of the page

The upper limb is hanging and there is loss of sensation on the lateral side of arm.

Answer: Axillary nerve injury.

The lateral upper-arm sensory patch is supplied by the upper lateral cutaneous nerve of arm, a branch of the axillary nerve.

Common causes

Surgical neck fracture of humerus
Anterior shoulder dislocation
Compression in the quadrangular space

C. Above 90°: serratus anterior and trapezius

The humerus alone cannot continue moving efficiently above 90°. The scapula must rotate upward.

Two muscles work as a force couple:

Serratus anterior pulls the inferior angle of scapula forward and laterally.
Upper and lower trapezius rotate the scapula upward and stabilize it.

So:

Above 90° abduction requires scapular upward rotation by serratus anterior and trapezius.

A useful detail is the scapulohumeral rhythm. During full elevation of the arm, roughly:

120° comes from the glenohumeral joint
60° comes from upward rotation of the scapula

3. Serratus anterior and long thoracic nerve

Serratus anterior

Nerve supply

Long thoracic nerve, C5-C7.

Mnemonic: SALT

Serratus
Anterior
Long
Thoracic

Main actions

Holds scapula against the thoracic wall
Protracts scapula, as in punching
Assists upward rotation of scapula for raising arm overhead

Long thoracic nerve injury

Causes

Axillary surgery
Breast surgery or axillary lymph-node dissection
Chest tube insertion near the lateral chest wall
Direct trauma
Traction injury, such as carrying a heavy backpack or sports injury

Findings

Winging of scapula
Weak pushing/punching
Difficulty lifting arm above shoulder level
Difficulty combing hair or reaching overhead

What does “winged scapula” mean?

The medial border and inferior angle of the scapula lift away from the chest wall and become prominent.

How do you test it?

Ask the patient to push against a wall with both hands.

In serratus anterior paralysis, the affected medial border of scapula protrudes backward prominently during the wall-push test.

Long thoracic nerve injury → serratus anterior paralysis → medial scapular winging.

Your page’s statements are therefore correct:

“Patient cannot move scapula forward” means loss of scapular protraction, suggesting serratus anterior paralysis.
“Winged scapula is caused by paralysis of which muscle?”
Answer: Serratus anterior.

Textbook material similarly identifies long thoracic nerve palsy as a cause of medial winging due to serratus anterior dysfunction. Miller’s Review of Orthopaedics, 9e, p. 1205.

4. Trapezius and spinal accessory nerve

Trapezius

Nerve supply

Motor: spinal accessory nerve, cranial nerve XI
Proprioceptive/sensory fibers: C3-C4 cervical nerves

Important correction: C3-C4 are usually described as proprioceptive fibers rather than true cutaneous sensory supply to trapezius.

Main actions by parts

Part	Important action
Upper fibers	Elevate scapula, as in shrugging
Middle fibers	Retract scapula
Lower fibers	Depress scapula and assist upward rotation
Upper + lower together	Upwardly rotate scapula for arm elevation

Spinal accessory nerve injury

Typical setting

The nerve runs superficially in the posterior triangle of the neck. It can be injured during:

Neck surgery
Cervical lymph-node biopsy
Penetrating trauma to the neck

Findings

Weak shoulder shrug
Shoulder droop
Trapezius wasting
Difficulty abducting/elevating the arm above 90°
Altered scapular position and movement

Page question

A man has difficulty shrugging his shoulders. Which nerve is damaged?

Answer: Spinal accessory nerve.

“Girl unable to comb her hair”

Combing hair needs arm elevation above shoulder level. The most classic answer in a neck-injury context is:

Spinal accessory nerve injury, causing trapezius weakness.

However, serratus anterior paralysis can also make overhead activity difficult. In an MCQ, use the clue:

Neck surgery/neck trauma/weak shrug → spinal accessory nerve
Breast or axillary surgery/winging on wall push → long thoracic nerve

5. Important distinction: two types of scapular winging

Type	Injured nerve	Paralyzed muscle	Typical finding
Medial winging	Long thoracic nerve	Serratus anterior	Medial border becomes prominent, especially during wall push
Lateral winging	Spinal accessory nerve	Trapezius	Shoulder droop and lateral displacement of scapula

This distinction is useful because both patients may have trouble lifting the arm above the head, but the cause is different.

6. Suprascapular nerve

Root value

C5-C6.

Muscles supplied

Supraspinatus
Infraspinatus

Effects of injury

Difficulty initiating abduction due to supraspinatus weakness
Weak external rotation due to infraspinatus weakness
Possible wasting in supraspinous and infraspinous fossae

Remember:

Suprascapular nerve = supra-spinatus + infra-spinatus.

7. Axillary nerve versus upper brachial plexus injury

At the top of the page is another clinical scenario:

A child after forceps delivery cannot abduct the shoulder, cannot actively flex the elbow, and cannot supinate the forearm.

This is not an isolated axillary nerve injury.

The combination tells you that several C5-C6 muscles are affected:

Deltoid: shoulder abduction
Biceps and brachialis: elbow flexion
Biceps: supination

Therefore, the diagnosis is:

Upper trunk brachial plexus injury, C5-C6, also called Erb palsy.

Forceps delivery and shoulder dystocia can stretch the upper trunk. The classic final posture is waiter’s-tip deformity:

Arm adducted
Medially rotated
Elbow extended
Forearm pronated

8. Latissimus dorsi and thoracodorsal nerve

Muscle

Latissimus dorsi

Nerve

Thoracodorsal nerve, usually C6-C8, from the posterior cord of brachial plexus.

Actions

Extends the arm
Adducts the arm
Medially rotates the arm
Helps in climbing, swimming, pulling, and using crutches

The page asks:

Latissimus dorsi is supplied by which nerve?

Answer: Thoracodorsal nerve.

“Cannot scratch the back”

This phrase alone is not a very specific test. Reaching the hand behind the back needs a combination of shoulder extension, adduction, internal rotation, and elbow flexion. Latissimus dorsi weakness may contribute, but identify it most reliably from weak pulling, climbing, adduction, extension, and medial rotation.

9. Other muscles in the lower table

Muscle	Nerve supply	Main clinical action
Trapezius	Spinal accessory nerve, CN XI	Shrugging, retraction, upward rotation of scapula
Latissimus dorsi	Thoracodorsal nerve	Extension, adduction, medial rotation of arm
Levator scapulae	C3-C4 and dorsal scapular nerve, C5	Elevates scapula
Rhomboid major/minor	Dorsal scapular nerve, mainly C4-C5	Retracts and downwardly rotates scapula
Serratus anterior	Long thoracic nerve, C5-C7	Protraction and upward rotation, fixes scapula to chest wall
Deltoid	Axillary nerve, C5-C6	Abduction from 15° to 90°

A modern anatomy summary lists these same associations: axillary nerve to deltoid, suprascapular nerve to supraspinatus/infraspinatus, long thoracic nerve to serratus anterior, dorsal scapular nerve to rhomboids, and spinal accessory nerve to trapezius. Shoulder-muscle innervation

10. How to solve the page’s MCQs

“Patient cannot shrug shoulders”

Spinal accessory nerve injury
Reason: trapezius is paralyzed.

“Patient cannot push/punch or move scapula forward”

Serratus anterior paralysis
Reason: serratus anterior protracts the scapula.

“Winging of scapula”

Usually long thoracic nerve injury causing serratus anterior paralysis, unless the question clearly suggests trapezius palsy.

“Cannot abduct arm initially, then movement becomes smooth”

Supraspinatus lesion or suprascapular nerve injury.

“Cannot abduct from 15° to 90° and has lateral shoulder sensory loss”

Axillary nerve injury.

“Cannot raise arm above 90° after neck operation”

Spinal accessory nerve injury, because trapezius cannot rotate scapula upward.

“Cannot raise arm above 90° after breast/axillary surgery or has clear medial winging”

Long thoracic nerve injury, because serratus anterior is paralyzed.

“Latissimus dorsi paralysis”

Thoracodorsal nerve injury.

11. Final memory framework

Learn this as a sequence:

Start - Middle - Finish

Start, 0-15°: supraspinatus, suprascapular nerve
Middle, 15-90°: deltoid, axillary nerve
Finish, above 90°: trapezius plus serratus anterior, spinal accessory plus long thoracic nerves

And for winging:

SALT: Serratus Anterior, Long Thoracic
Shrug = CN XI

That framework is enough to answer most anatomy viva questions and MCQs from this page.

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Upper-limb anatomy revision: wrist, hand, nerves, and important clinical points

This page covers several separate high-yield areas. I will teach them in the order shown and point out a few places where simplified notes can be misleading.

1. Posterior interosseous nerve

The page begins with the posterior interosseous nerve (PIN).

Origin

The radial nerve divides near the lateral side of the elbow.
Its deep branch passes through the supinator muscle.
After passing through supinator, it is called the posterior interosseous nerve.

Supply

The PIN is mainly a motor nerve to the extensor muscles of the forearm, including muscles that extend:

Fingers
Thumb
Wrist, partly

It gives little or no cutaneous sensation. Therefore:

PIN palsy causes motor weakness without a sensory loss.

Injury effects

Weak extension of the fingers at metacarpophalangeal joints
Weak thumb extension
Difficulty extending wrist, though wrist extension may still occur weakly because extensor carpi radialis longus is supplied before the radial nerve becomes PIN.

This produces a finger-drop pattern. If the main radial nerve is injured higher up, the deficit is more extensive and there is typical wrist drop with sensory loss over the dorsum of the hand.

2. Flexor retinaculum of wrist

The flexor retinaculum, also called the transverse carpal ligament, is a strong fibrous band on the front of the wrist.

It stretches across the concavity of the carpal bones and converts it into a tunnel: the carpal tunnel.

Attachments

Side	Attachment
Lateral / radial side	Tubercle of scaphoid and ridge/tubercle of trapezium
Medial / ulnar side	Pisiform and hook of hamate

Memory aid:

Scaphoid and trapezium on the radial side; pisiform and hook of hamate on the ulnar side.

3. Structures related to the flexor retinaculum

This is a frequent MCQ area. Learn it by grouping structures into three categories.

A. Structures passing deep to the flexor retinaculum: carpal tunnel

The carpal tunnel contains:

Median nerve
Four tendons of flexor digitorum superficialis
Four tendons of flexor digitorum profundus
One tendon of flexor pollicis longus

So the standard count is:

9 flexor tendons + 1 median nerve

Mnemonic:

“9 tendons and 1 nerve.”

Important detail

The tendons are surrounded by synovial sheaths:

Ulnar bursa surrounds FDS and FDP tendons.
Radial bursa surrounds flexor pollicis longus.

Clinical importance: carpal tunnel syndrome

Compression of the median nerve beneath the flexor retinaculum can cause:

Tingling/numbness in thumb, index, middle, and radial half of ring finger
Night symptoms
Weakness of thenar muscles in advanced disease
Difficulty with thumb opposition

The palmar cutaneous branch of median nerve arises before the tunnel and travels superficial to it, so central palmar sensation is often relatively spared.

B. Structures in their own compartment within the retinaculum

Flexor carpi radialis tendon

The flexor carpi radialis (FCR) tendon passes deep to the retinaculum but through a separate fibro-osseous compartment, not through the main carpal tunnel.

This distinction is useful in MCQs.

FCR is deep to flexor retinaculum but not a carpal-tunnel content.

C. Structures passing superficial to flexor retinaculum

1. Palmaris longus tendon

Passes superficial to flexor retinaculum.
Inserts into palmar aponeurosis.
It is absent in some people and loss of it causes no important functional deficit.

2. Ulnar nerve and ulnar artery

Pass superficial to the flexor retinaculum.
They enter the hand through Guyon canal or ulnar canal.

Guyon canal lies between:

Pisiform, medially
Hook of hamate, laterally

It contains:

Ulnar nerve
Ulnar artery
Accompanying veins

Therefore:

Median nerve goes through carpal tunnel.
Ulnar nerve and artery go through Guyon canal, superficial to flexor retinaculum.

A StatPearls wrist anatomy review also distinguishes the median nerve in the carpal tunnel from the ulnar neurovascular structures in Guyon canal.

3. Ulnar vessels and ulnar nerve

The note says they pass superficial to flexor retinaculum. Correct.

4. Palmar cutaneous branch of median nerve

This branch also passes superficial to the flexor retinaculum. Thus, it is spared in carpal tunnel syndrome.

4. Why a hook of hamate fracture matters

The hook of hamate forms the lateral boundary of Guyon canal.

A fracture can damage the ulnar nerve, causing:

Weakness of interossei
Weakness of hypothenar muscles
Weakness of adductor pollicis
Reduced sensation over little finger and ulnar half of ring finger, depending on the level of lesion

It is especially relevant in sports involving a racket, bat, golf club, or repeated handle pressure.

5. Radial artery, ulnar artery, and palmar arches

The hand has two arterial arches. The key is to know which artery mainly forms each one.

Arch	Main artery	Position
Superficial palmar arch	Mainly ulnar artery	More superficial
Deep palmar arch	Mainly radial artery	Deep in palm

Superficial palmar arch

Formed predominantly by the ulnar artery
Usually completed by the superficial palmar branch of radial artery
Gives common palmar digital arteries, which divide into proper digital arteries

Deep palmar arch

Formed predominantly by the radial artery
Completed by the deep branch of ulnar artery
Gives palmar metacarpal arteries and perforating branches

Memory:

Ulnar is superficial. Radial is deep.

The deep arch lies deep to the long flexor tendons and is closer to the metacarpal bases. The superficial arch is more distal and lies in front of the flexor tendons.

6. Clinical relevance of palmar arches

Before taking the radial artery for procedures or cannulation, the clinician should verify that ulnar arterial flow can maintain hand perfusion. The traditional bedside assessment is the Allen test, though its role varies by procedure and setting.

A cut in the palm can damage:

Superficial palmar arch
Common digital arteries
Digital nerves
Flexor tendons

This is why deep palmar injuries require careful assessment of bleeding, sensation, tendon movement, and circulation.

7. Wrist movements: radial and ulnar deviation

The page uses the terms abduction and adduction of wrist.

At the wrist:

Abduction = radial deviation: hand moves toward thumb/radius.
Adduction = ulnar deviation: hand moves toward little finger/ulna.

Ulnar deviation: adduction of wrist

The main muscles are:

Muscle	Nerve	Action
Flexor carpi ulnaris	Ulnar nerve	Flexes wrist and causes ulnar deviation
Extensor carpi ulnaris	Deep branch of radial nerve / PIN	Extends wrist and causes ulnar deviation

These two muscles work together for ulnar deviation because one flexes and one extends. Their shared action is deviation toward the ulna.

Ulnar deviation = FCU + ECU.

Radial deviation: abduction of wrist

Main muscles include:

Flexor carpi radialis
Extensor carpi radialis longus
Extensor carpi radialis brevis
Abductor pollicis longus can assist

Radial deviation = FCR + ECRL/ECRB.

8. The clinical case: inability to adduct the wrist

The page describes a person with a wound near the wrist who cannot perform adduction of wrist, meaning ulnar deviation.

Since ulnar deviation needs:

Flexor carpi ulnaris, supplied by ulnar nerve
Extensor carpi ulnaris, supplied by radial nerve/PIN

injury to either may weaken it. But a wound at the medial wrist commonly injures the ulnar nerve, and this may affect FCU if the injury is proximal enough.

Be precise clinically:

A lesion at the wrist generally does not paralyze FCU, because the branch to FCU leaves the ulnar nerve in the forearm.
A proximal ulnar nerve lesion can weaken FCU and reduce wrist flexion with ulnar deviation.
An injury to ECU or PIN can also weaken ulnar deviation, but it has a different location and pattern.

9. Combined wrist movements

A movement can result from two muscles acting together, even if each muscle has another opposing action.

For example:

FCR flexes wrist and radially deviates it.
ECRL/ECRB extend wrist and radially deviate it.

When FCR and ECRL/ECRB act together, flexion and extension oppose each other, while radial deviation is reinforced.

Likewise:

FCU flexes and ulnarly deviates.
ECU extends and ulnarly deviates.

Together, they produce ulnar deviation.

This is a common concept in anatomy MCQs.

10. Upper-limb nerve and muscle review from the right page

Trapezius

Nerve supply

Motor: spinal accessory nerve, cranial nerve XI
Proprioceptive fibers: C3-C4

Actions

Upper fibers: shrugging or elevation of scapula
Middle fibers: retraction of scapula
Upper and lower fibers together: upward rotation of scapula for overhead elevation

A spinal accessory nerve lesion causes:

Shoulder droop
Weak shrug
Difficulty raising arm above 90°
Trapezius wasting

Latissimus dorsi

Nerve supply

Thoracodorsal nerve, from posterior cord of brachial plexus.

Actions

Extension of arm
Adduction of arm
Medial rotation of arm

It helps in climbing, swimming, pulling, and using crutches.

Levator scapulae

Nerve supply

Dorsal scapular nerve, usually C5
Direct cervical nerves, mainly C3-C4

Main action

Elevates the scapula.

Rhomboid major and minor

Nerve supply

Dorsal scapular nerve, mainly C4-C5.

Main actions

Retract scapula
Downwardly rotate scapula
Fix scapula to thoracic wall

Serratus anterior

Nerve supply

Long thoracic nerve, C5-C7.

Actions

Protracts scapula
Holds scapula against thoracic wall
Works with trapezius to upwardly rotate scapula

Injury

Long thoracic nerve injury causes:

Medial winging of scapula
Difficulty pushing/punching
Difficulty raising arm above shoulder level

Mnemonic:

SALT = Serratus Anterior, Long Thoracic.

Deltoid

Nerve supply

Axillary nerve, C5-C6.

Main action

Abduction of shoulder from approximately 15° to 90°.

11. Nerve supply of selected muscles from the table

Some listed structures are not upper-limb muscles, but are common MCQ associations.

Structure	Nerve supply
Geniohyoid	C1 fibers traveling with hypoglossal nerve
Stylohyoid	Facial nerve
Posterior belly of digastric	Facial nerve
Mylohyoid	Nerve to mylohyoid, branch of inferior alveolar nerve from V3
Submandibular gland	Parasympathetic fibers via chorda tympani, facial nerve
Sublingual gland	Parasympathetic fibers via chorda tympani, facial nerve
Sternocleidomastoid	Spinal accessory nerve, with C2-C3 proprioceptive fibers
Sternohyoid	Ansa cervicalis
Omohyoid	Ansa cervicalis
Sternothyroid	Ansa cervicalis
Thyrohyoid	C1 fibers via hypoglossal nerve
Platysma	Facial nerve
Cricothyroid	External laryngeal nerve, branch of superior laryngeal nerve
Inferior pharyngeal constrictor	Mainly vagus nerve via pharyngeal plexus, with a contribution from external laryngeal and recurrent laryngeal nerves depending on part
Diaphragm	Phrenic nerve, C3-C5
Suprarenal gland	Preganglionic sympathetic fibers, mainly via greater splanchnic nerve
Piloerector muscles	Postganglionic sympathetic fibers

12. Autonomic ganglia and why the page lists them

A ganglion is a cluster of neuronal cell bodies outside the central nervous system.

There are two broad types.

A. Sensory ganglia

Contain cell bodies of sensory neurons. They do not contain synapses between sensory neurons.

Examples:

Dorsal root ganglion
Trigeminal ganglion
Geniculate ganglion
Spiral ganglion
Vestibular ganglion

B. Autonomic ganglia

These contain synapses between preganglionic and postganglionic autonomic neurons.

Examples:

Ciliary ganglion
Pterygopalatine ganglion
Submandibular ganglion
Otic ganglion
Superior cervical ganglion
Celiac ganglion
Superior and inferior mesenteric ganglia

13. Important individual ganglia

Ciliary ganglion

Located in the orbit.

Parasympathetic pathway

Preganglionic fibers originate from Edinger-Westphal nucleus.
Travel in oculomotor nerve, CN III.
Synapse in ciliary ganglion.
Postganglionic fibers travel in short ciliary nerves.

Effects

Constriction of pupil: miosis
Accommodation for near vision by contracting ciliary muscle

Superior cervical ganglion

A sympathetic ganglion in the neck.

It sends postganglionic sympathetic fibers to:

Dilator pupillae muscle
Facial sweat glands
Facial blood vessels
Superior tarsal muscle

A lesion can cause Horner syndrome:

Ptosis
Miosis
Anhidrosis

Celiac ganglion

Receives preganglionic sympathetic fibers mainly through the greater splanchnic nerve.

Supplies sympathetic fibers to many foregut organs:

Stomach
Liver
Spleen
Pancreas
Upper duodenum

Inferior mesenteric ganglion

Associated with sympathetic supply to hindgut structures.

14. Hirschsprung disease

The last section relates to ganglion cells in the gut.

What happens?

In Hirschsprung disease, there is failure of neural crest cell migration into a portion of the colon. Therefore, that segment lacks enteric ganglion cells.

Affected plexuses:

Meissner plexus: submucosal plexus
Auerbach plexus: myenteric plexus

The aganglionic segment cannot relax normally, causing functional obstruction.

Consequences

Delayed passage of meconium in a newborn
Abdominal distension
Bilious vomiting
Constipation
Proximal colon dilatation

The affected segment is usually distal colon and rectum. Diagnosis is confirmed by rectal biopsy showing absence of ganglion cells.

15. Final exam-oriented summary

Wrist

Carpal tunnel: median nerve + FDS, FDP, FPL tendons
FCR: in a separate compartment
Ulnar nerve and artery: Guyon canal, superficial to flexor retinaculum
Palmaris longus: superficial to flexor retinaculum

Arteries

Superficial palmar arch: mainly ulnar artery
Deep palmar arch: mainly radial artery

Wrist deviation

Ulnar deviation: FCU + ECU
Radial deviation: FCR + ECRL/ECRB

Scapular muscles

Trapezius: spinal accessory nerve
Serratus anterior: long thoracic nerve
Rhomboids: dorsal scapular nerve
Latissimus dorsi: thoracodorsal nerve
Deltoid: axillary nerve

Autonomic point

Hirschsprung disease: absent ganglion cells in Meissner and Auerbach plexuses.

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