Anterior Arm Anatomy and Function

• The arm connects the shoulder to the elbow
• It is separated into two compartments by the humerus: the anterior (flexor) and posterior (extensor) compartments
• The brachial fascia surrounds the arm and sends out lateral and medial intermuscular septa, which separate the compartments

Flexor Compartment Contents

• Biceps brachii
• Coracobrachialis
• Brachialis
• Brachial artery/vein
• Musculocutaneous nerve (motor to flexors)
• Median nerve
• Proximal ulnar nerve

Biceps Brachii

• The biceps brachii has two heads
• The long head originates at the supraglenoid tubercle within the shoulder joint capsule
• The short head originates at the tip of the coracoid process
• The two muscle bellies fuse in the lower anterior compartment of the arm
• A flattened tendon crosses the elbow joint and attaches to the posterior aspect of the radial tuberosity
• The bicipital aponeurosis is an aponeurotic sheet that arises from the tendon as it crosses the elbow joint
• It runs medially across the brachial artery and merges with the forearm's deep fascia and adheres to the ulna's periosteum, offering a secondary attachment
• After piercing the coracobrachialis, both heads of the biceps are supplied by the musculocutaneous nerve
• While technically a shoulder flexor, the biceps' more powerful actions affect the forearm bones
• The biceps brachii is a strong elbow flexor and a powerful forearm supinator

Extensor Compartment Contents

• Triceps brachii
• Terminal branch of profunda brachii
• Radial nerve (motor to extensors)
• Distal ulnar nerve courses to the posterior side.
• The median and ulnar nerves do not give off branches in the arm

Triceps Brachii

• The triceps brachii is the only muscle within the posterior compartment responsible for extending the arm
• It has 3 heads that merge into a single tendon that inserts onto the ulna's olecranon process
• The radial nerve innervates all three heads
• Except for gravity, the triceps is the only effective elbow joint extensor
• The triceps contributes to the shoulder's quadrangular and triangular spaces, enabling posterior passage of major structures for supply to the arm and forearm's posterior compartments

Cubital Fossa

• Anterior to the elbow joint, there is an inverted triangular region
• An imaginary line connecting the medial and lateral epicondyles of the humerus creates the base
• The brachioradialis defines the lateral boundary
• The pronator teres defines the medial boundary
• The roof is composed of the skin, superficial fascia, and the bicipital aponeurosis
• The median cubital vein is superficial to the aponeurosis
• The brachial artery is deep
• Starting from the lateral (radial) aspect and moving medially, the removal of the cubital fossa's roof reveals these structures:
  • The tendon of the biceps
  • The brachial artery, which divides into the radial and ulnar arteries as it approaches the elbow
  • Medial to the brachial artery lies the median nerve

Forearm

• Similar to the arm, the forearm contains anterior (flexor) and posterior (extensor) compartments
• To position the forearm correctly in anatomical terms, it needs to be supinated

Muscles of the Flexor Compartment

• The flexor muscles of the forearm are classified into superficial and deep groups

The Superficial Group

• The median nerve innervates the superficial group, except for the flexor carpi ulnaris which is innervated by the ulnar nerve

The Deep Group

• Primarily located beneath the superficial group
• Mainly innervated by the median nerve

Flexor Digitorum Profundus

• The four flexor digitorum profundus tendons travel into the hand under the flexor retinaculum
• This muscle has unique innervation
• The ulnar nerve innervates the medial portion
• The median nerve innervates the lateral component
• These muscles facilitates wrist and digit flexion by flexing them

Nerves and Vessels of the Flexor Compartment

• The brachial artery enters the forearm through the cubital fossa and splits into the radial and ulnar arteries
• The median and ulnar nerves traverse the forearm's flexor compartment and supply all its muscles
• The medial aspect of the compartment marks the meeting point of the ulnar nerve and ulnar artery
• The radial nerve enters the forearm and rapidly divides into superficial and deep branches
• The deep branch crosses into the extensor compartment
• The superficial branch does not have muscular or cutaneous branches within the forearm, only cutaneous distribution to the dorsum of the hand

Extensor Compartment Muscles

• The extensor (posterior) section of the musculature is separated into superficial and deep groups

Superficial Group

• The superficial group extends the wrist and digits
• It's muscles are innervated by the radial nerve

Deep Group

• The deep extensors lie beneath the superficial group
• All muscles, except the supinator, form long tendons that cross the wrist and enter the hand

Vessels and Nerves of the Extensor Compartment of the Forearm

• No major artery extends from the arm into the forearm's extensor compartment
• However, a short trunk of the ulnar artery forms in the anterior (flexor) compartment: the common interosseous artery
  • This splits subsequently into anterior and posterior interosseous arteries
• Both supply structures inside the extensor compartment
• The two interosseous arteries anastomose at the pronator quadratus' upper border and continue into the wrist, joining the dorsal carpal arch
• Two forearm extensor muscles, the brachioradialis and extensor carpi radialis longus, are supplied by the radial nerve as the nerve divides into superficial and deep branches at the elbow

The Dorsum of the Hand

• The dorsal interosseous muscles are mainly innervated on the dorsal side of the hand
• All tendons crossing the wrist joint are surrounded by synovial sheaths and are bound to the dorsal aspect by the extensor retinaculum
• The radial and ulnar nerves supply cutaneous innervation, there are no intrinsic muscles to be innervated

The Palm

• Deep fascia divides the fatty superficial fascia into loculi
• These fibrous septa attach down to the skin
• The deep fascia is connected to a sheet of fibrous tissue called the palmar aponeurosis in the palm's center
• The palmar aponeurosis covers the palm from the thenar and hypothenar eminences

Flexor Retinaculum

• Flexor retinaculum keeps the long flexor tendons in close proximity to the wrist and palm
• It is an osseofibrous carpal tunnel through which the median nerve and flexor tendons pass
• A separate tunnel is created by a septum that runs to the underlying trapezium for the flexor carpi radialis tendon

Thenar eminence

• It creates the raised region between the wrist and the base of the thumb
• It is comprised of: the abductor pollicis brevis, flexor pollicis brevis and the opponens pollicis
• The median nerve supplies all three thenar muscles

Hypothenar Eminence

• Lies at the base of the little finger between the wrist
• Contains 3 muscles: abductor digiti minimi, flexor digiti minimi and the opponens digiti minimi
• All three are innervated by the ulnar nerve

Nerves and Vessels in the Hand

• A muscular branch of the median nerve emerges from under the flexor retinaculum and supplies the musculature of the thenar eminence
• Pressure or irritation of the median nerve as it runs superficial to the long flexor tendons within the carpal tunnel causes "carpal tunnel syndrome"
• Carpal tunnel syndrome causes declines in sensory and muscular functions
• The ulnar artery and nerve pass lateral to the flexor retinaculum
• They split into deep and superficial branches with the nerve dividing

Ulnar Nerve Branches

• The superficial branch then innervates the palmaris brevis and supplies palmar digital nerves to both sides of the 5th finger and remaining half of the 4th finger
• The deep branch:
  • Provides motor function to muscles of the hypothenar eminence
  • Innervates the medial two lumbrical muscles
  • Innervates all the interossei
  • Both heads of the adductor pollicis are also innervated

Radial and Ulnar Arteries and the Palmar Arches

• The radial artery passes onto the dorsum of the hand from the anterior compartment through the anatomical snuff box then to the palm by going between the two heads of the adductor pollicis
• The deep palmar branch of the ulnar artery anastomoses here
• the radial artery is the major contributer to the deep palmar arch
• Before passing onto the dorsum of the hand in the anatomical snuff box the radial artery gives off a superficial palmar branch
• The superficial palmar is completed or helped by anastomosis with the ulnar artery
• The ulnar artery is said to be the major contributor to the superficial palmar arch

Explore the anatomy and function of the anterior arm, focusing on its muscles, nerves, and blood vessels. Test your knowledge of the structures that define the anterior compartment, their innervation, and potential clinical implications. Review common injuries and lesions affecting this region.