Arm

The arm, or “upper arm” in common usage, is the region of the upper limb that extends from the shoulder to the elbow joint and connects inferiorly to the forearm through the cubital fossa. It is divided into 2 fascial compartments (anterior and posterior) and houses numerous structures, including the humerus; the brachial and profunda brachii arteries; the basilic, cephalic, and brachial veins; the radial, median, ulnar, and musculocutaneous nerves; and the flexor and extensor muscles of the arm.

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Bones of the Arm: Humerus

The humerus can be divided into 3 parts: the proximal end, the shaft, and the distal end.

Proximal end

  • Head of the humerus: articulates with the glenoid
  • Anatomical neck
  • Surgical neck: 
    • Frequent site of fracture 
    • A fracture here may injure the axillary nerve and/or posterior circumflex artery.
  • Greater tubercle: site of attachment of the supraspinatus, infraspinatus, and teres minor rotator cuff tendons
  • Lesser tubercle: site of attachment of the subscapularis rotator cuff tendon
  • Intertubercular sulcus: separates the tubercles and contains the tendon of the long head of the biceps brachii

Shaft

  • Deltoid tuberosity: site of insertion of the deltoid muscle
  • Radial or spiral groove: The radial nerve runs in this groove and may be injured secondary to pressure or trauma, with resultant wrist drop.
  • Medial and lateral supracondylar ridges

Distal end

  • Trochlea: medial aspect of distal humerus
  • Capitulum: lateral to the trochlea and articulates with the radial head
  • Medial epicondyle: larger of the epicondyles; the ulnar nerve passes posterior to the medial epicondyle in the ulnar groove
  • Lateral epicondyle: common site of pain with “tennis elbow”
  • Olecranon, coronoid fossa, and radial fossa: depressions that allow for movement of the elbow during flexion and extension
Distal end of the humerus

Anterior view of the distal end of the humerus

Image by BioDigital, edited by Lecturio

Muscles of the Anterior Compartment of the Arm

  • The anterior compartment of the arm is also called the flexor compartment, because its muscles are in charge of flexing the forearm toward the upper arm.
  • Comprises 3 muscles: biceps, brachialis, and coracobrachialis
  • All 3 muscles are innervated by the musculocutaneous nerve and supplied by the brachial artery.
  • To recall the muscles of the anterior compartment of the arm, remember “BBC”:
    1. Biceps
    2. Brachialis
    3. Coracobrachialis
Cross-section of the arm

Cross-section of the arm, featuring the anterior and posterior compartments. Note the brachial fascia enclosing all of the muscles and forming the lateral and medial intermuscular septa.

Image by Lecturio.
Table: Anterior compartment of the arm: flexors
MuscleOriginInsertionInnervationFunction
Biceps brachii
  • Long head: supraglenoid tubercle
  • Short head: coracoid process
Radial tuberosity and forearm fascia via bicipital aponeurosisMusculocutaneous nerve (C6)
  • Supinates forearm, and once supinated flexes elbow joint
  • Flexes shoulder
BrachialisLateral and medial surface of distal half of humerusCoronoid process and tuberosity of ulnaMusculocutaneous nerve (C6)Flexes elbow joint
CoracobrachialisCoracoid processMiddle 3rd of humerusMusculocutaneous nerve (C6)Flexes/adducts shoulder joint

Muscles of the Posterior Compartment of the Arm

  • Also called the extensor compartment because its muscles are in charge of extending the elbow
  • The posterior compartment of the arm comprises 2 muscles: 
    • Triceps brachii
    • Anconeus
      • In some sources considered the “fourth head” of the triceps
      • In some sources considered part of the posterior compartment of the forearm
  • Both muscles are innervated by the radial nerve and supplied by the deep brachial artery.
Table: Posterior compartment of the arm: extensors
MuscleOriginInsertionInnervationFunction
Triceps brachii
  • Long head: infraglenoid tubercle
  • Lateral head: superior to radial groove
  • Medial head: inferior to radial groove
OlecranonRadial nerve (C7, C8)
  • Extensor of elbow joint
  • Long head also extends and externally rotates the shoulder joint
AnconeusInserts on the posterior aspect of lateral epicondyleLateral surface of the olecranonRadial nerve (C7, C8)Assists in extension of the elbow and stabilizes the joint

Vessels of the Arm

Table: Topographic location of the vessels of the arm
Medial bicipital grooveLateral bicipital groove
LocationBetween the biceps brachii muscle and the triceps brachiiBetween the biceps brachii muscle and the brachial muscle
ArteriesBrachial arteryRadial collateral artery
Veins
  • Basilic vein
  • Brachial veins
  • Radial collateral vein
  • Cephalic vein
Nerves
  • Ulnar nerve
  • Median nerve
  • Medial cutaneous nerve of the forearm
Radial nerve

Arterial supply of the arm

The brachial artery:

  • Begins at the inferior border of the teres major as a continuation of the axillary artery
  • Runs primarily between the biceps and brachialis muscles
  • Supplies the anterior compartment
  • Has 1 major branch, the deep brachial artery
    • Also known as the profunda brachii artery
    • Supplies the posterior compartment
    • Runs in the radial groove with the radial nerve
Arteries of the arm

Arteries of the arm

Image by BioDigital, edited by Lecturio

Venous drainage of the arm

  • Basilic vein:
    • Begins distally in the hand
    • Travels on the medial aspect of the upper arm
    • Combines with the brachial veins to form the axillary vein
  • Cephalic vein:
    • Begins distally in the hand
    • Travels proximally on the antero-lateral aspect of the arm
    • Enters the shoulder through the deltopectoral groove (the groove between the deltoid and pectoralis muscles), where it empties into the axillary vein

Nerves of the Arm

NerveOriginBranchesFunctions
Musculo-cutaneousTerminal branch of lateral cord of brachial plexus (C5–C7)
  • Branch to the:
    • Coracobrachialis
    • Biceps
    • Brachialis
    • Humerus
    • Elbow joint
  • Terminates as the lateral antebrachial cutaneous nerve
  • Motor supply of the anterior compartment of the arm
  • Sensory supply of the skin of the lateral forearm
MedianUnison of medial and lateral roots of brachial plexus (C5–T1)
  • Branches in the forearm:
    • Muscular branches to pronator teres, palmaris longus, flexor digitorum superficialis, and flexor carpi radialis
    • Anterior interosseous nerve
    • Cutaneous nerve of the palm
  • Terminates as 2 common palmar digital nerves between the index and middle and ring and middle fingers
  • Motor supply to the flexor muscles in the forearm (except flexor carpi ulnaris and the ulnar head of flexor digitorum profundus) and muscles of the thenar eminence and 2 radial lumbricals
  • Sensory supply of the palmar and distal dorsal aspects of the lateral 312 digits, adjacent palm, thumb, radial half of 2nd digit, and central palm
UlnarTerminal branch of the medial cord of the brachial plexus (C8, T1)
  • Articular branches to the elbow joint
  • Muscular branches to the flexor carpi ulnaris and ulnar part of the flexor digitorum profundus
  • Dorsal sensory branch in the hand
  • Deep terminal branches to the intrinsic hand muscles
  • Dorsal and palmar cutaneous branches
  • Motor supply to the flexors of the forearm and some of the muscles of the hand (intrinsic)
  • Sensory supply to the anterior aspect of 112 ulnar fingers, medial palm, and 1 and a half of the ulnar fingers dorsally
RadialPosterior cord of the brachial plexus (C5–T1)
  • Divides at the elbow into:
    • Deep branch, the posterior interosseous nerve, which supplies the wrist extensor and long thumb muscles
    • Superficial branch, which supplies doral skin of the hand
  • Provides sensory branches for the posterior arm
  • Motor supply to the posterior arm muscles (triceps, anconeus, brachioradialis) and the posterior forearm muscles
  • Sensory supply:
    • Inferior lateral cutaneous nerve of arm
    • Posterior cutaneous nerve of the arm
    • Posterior cutaneous nerve of the forearm
    • Superficial branch of the radial nerve

Clinical Relevance

The following injuries are important conditions related to the upper arm:

  • Proximal humerus fractures: classic patient is an older woman with osteoporosis who has a low-impact fall onto an outstretched hand (FOOSH). May cause damage to radial nerve causing decreased function of deltoid and teres minor muscles, with loss of sensation in lateral shoulder.
  • Mid-shaft humerus fractures: may result in injury to the radial nerve within the radial groove. The radial nerve innervates the extensor muscles, and this injury may result in wrist drop.
  • Supracondylar humerus fracture: typically from a FOOSH. If the proximal fragment is displaced anterolaterally, then the radial nerve can be damaged; if it is displaced anteromedially, the median nerve and brachial artery can be injured, possibly resulting in inability to flex wrist or pronate the forearm and/or loss of pulses in the radial and ulnar arteries.
  • Anterior shoulder dislocation: the most common dislocation at the shoulder, where the humeral head has been moved to a position in front of the joint; can damage the axillary nerve, and (rarely) the axillary artery.
  • Tear of the long head of the biceps brachii tendon: Complete tear of the tendon of the long head of the biceps brachii results in a “Popeye muscle” secondary to prominence of the mid-portion of the biceps.
  • Saturday night palsy: compressive neuropathy of the radial nerve secondary to prolonged pressure on the nerve; may also result in wrist drop.

References

Drake, R.L., Vogl, A.W., & Mitchell, A.W.M. (2014). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA:  Churchill Livingstone.

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