Musculocutaneous Nerve and Brachial Vessels
The musculocutaneous nerve arises from the lateral cord of the brachial plexus (C5-T1) and innervates the brachial flexor muscles (coracobrachialis, biceps brachii, and brachialis), while the brachial vessels consist of the brachial artery and accompanying veins that run through the medial aspect of the arm.
Musculocutaneous Nerve
Anatomy and Course
- Originates from the lateral cord of the brachial plexus (C5-T1)
- Typically pierces the coracobrachialis muscle approximately 5-8 cm distal to the coracoid process 1
- Runs between the biceps and brachialis muscles
- Provides motor innervation to three muscles:
- Coracobrachialis
- Biceps brachii
- Brachialis
- Continues distally as the lateral antebrachial cutaneous nerve, providing sensory innervation to the lateral forearm 1, 2
Anatomical Variations
- Communicating branches with the median nerve are common and can be classified into five types 1
- Complete absence of the musculocutaneous nerve occurs in 1.4-15% of cases, with its functions taken over by the median nerve 3, 4
- Variations in motor branches to biceps brachii:
- Type I (most common): A common primary branch that bifurcates to supply both heads
- Type II: Two separate primary branches from the main trunk
- Type III: Similar to type I but with an additional distal motor branch 2
Brachial Vessels
Brachial Artery
- Main arterial supply to the arm
- Continuation of the axillary artery at the lower border of the teres major muscle
- Runs medially in the arm within the medial neurovascular bundle
- Located medial to the humerus in the proximal arm and anterior to it distally
- Terminates at the cubital fossa by dividing into the radial and ulnar arteries 5
Major Branches
- Profunda brachii artery (deep brachial artery)
- Superior and inferior ulnar collateral arteries
- Nutrient artery to the humerus 5
Brachial Veins
- Paired vessels that accompany the brachial artery
- Receive tributaries corresponding to the branches of the brachial artery
- Unite with the basilic vein to form the axillary vein 5
Clinical Significance
Surgical Access
- A 2×2 inch "key window" on the medial surface of the arm, centered 2 inches distal to the axillary fold, provides access to major neurovascular structures including:
- Median, ulnar, and musculocutaneous nerves
- Brachial artery and veins
- Basilic vein
- Profunda brachii artery 5
Vascular Access
- The brachial artery is commonly used for:
- Blood pressure measurement
- Arterial blood sampling
- Flow-mediated dilation studies 6
- Ultrasound imaging of the brachial artery is typically performed above the antecubital fossa in the longitudinal plane 6
Nerve Injury Implications
- Injury to the musculocutaneous nerve results in:
- Weakness in elbow flexion
- Sensory loss on the lateral forearm
- In cases of anatomical variation where the median nerve supplies the brachial flexors, injury to the median nerve would cause unexpected paralysis of elbow flexion 4
Imaging Considerations
- For ultrasound assessment of the brachial artery, longitudinal images are preferred over cross-sectional images for accurate diameter measurement 6
- Clear visualization of both near (anterior) and far (posterior) wall lumen-intima boundaries indicates proper imaging plane 6
Understanding these anatomical structures and their variations is crucial for surgical approaches to the arm, nerve repair procedures, vascular access, and accurate interpretation of clinical findings in upper limb pathologies.