Formation of nerves from the medial and lateral cords
The long nerves of the upper extremity originate from the three cords of the brachial plexus. In the axillary region, the following major branches arise from two of them:
Medial cord (fasciculus medialis):
- Medial cutaneous nerve of the arm (n. cutaneus brachii medialis) — provides innervation to the skin of the corresponding area.
- Medial cutaneous nerve of the forearm (n. antebrachii medialis).
- Ulnar nerve (n. ulnaris).
- Medial root of the median nerve (radix medialis n. mediani).
Lateral cord (fasciculus lateralis):
- Lateral root of the median nerve (radix lateralis n. mediani) — joining the medial root, it forms the single trunk of the median nerve.
- Musculocutaneous nerve (n. musculocutaneus) — this nerve is also known by its historical eponym as Casserio's nerve [Casserio].
Branches of the posterior cord: radial nerve
Two major trunks arise from the posterior cord (fasciculus posterior): the radial and axillary nerves.
The radial nerve (n. radialis) is notable for being the largest branch of the brachial plexus. Its topography in the axillary region has several specific features:
- Throughout the subpectoral triangle, the nerve is located strictly posterior to the artery.
- The radial nerve, along with its accompanying artery, lies closely against the tendon of the latissimus dorsi muscle (m. latissimus dorsi).
- Subsequently, this nerve passes into the anterior region of the arm. The median nerve (n. medianus), ulnar nerve (n. ulnaris), and medial cutaneous nerves also head there, to the anterior region.
Topography of the axillary nerve
The second branch of the posterior cord is the axillary nerve (n. axillaris). Its path and relationships with surrounding structures are highly specific:
- Initial location: Initially, the nerve is located on the posterior wall of the axillary fossa, lying posterior and slightly lateral to the artery.
- Course of fibers: The nerve runs in an oblique and lateral direction. Its destination is the quadrangular space (foramen quadrilaterum), which is located near the upper margin of the latissimus dorsi muscle (m. latissimus dorsi).
- Formation of the bundle: The axillary nerve heads into this space not alone, but as part of a neurovascular bundle together with veins and the posterior circumflex humeral artery (a. circumflexa humeri posterior).
- Relation to bone and joint: Having passed through the space, the bundle lies posteriorly against the surgical neck of the humerus, heading into the subdeltoid space. It is important to note that deeper than the nerve itself, beneath a layer of loose connective tissue, lies the lower part of the shoulder joint capsule — the so-called axillary recess (recessus axillaris).
Briefly: answers to topical questions
What forms the walls of the quadrilateral space?
The walls of the quadrangular space are formed by the muscles of the shoulder girdle and the humerus.
The boundaries of the space are formed by the following structures:
- Superior wall — subscapularis muscle (m. subscapularis) and teres minor muscle (m. teres minor).
- Inferior wall — teres major muscle (m. teres major).
- Medial wall — long head of the triceps brachii muscle (caput longum m. tricipitis brachii).
- Lateral wall — surgical neck of the humerus (collum chirurgicum humeri).
Which muscles does the musculocutaneous nerve innervate?
The musculocutaneous nerve innervates the muscles of the anterior fascial compartment of the arm.
Which artery accompanies the radial nerve in the radial groove?
In the radial nerve canal (the canal of the posterior region of the arm), the radial nerve is accompanied by the deep brachial artery (a. profunda brachii).
The contents of this osteofascial canal include the following structures:
- Radial nerve (n. radialis) — the largest branch of the brachial plexus.
- Deep brachial artery (a. profunda brachii) — a major branch of the brachial artery.
- Accompanying veins (vv. profundae brachii) run together with the artery and nerve.
The median nerve is formed by two roots (from the lateral and medial cords) resembling the letter "Y" or a slingshot — radix lateralis and radix medialis.
Class and exam questions
What structures form the branches of the long nerves of the upper limb?
The branches of the long nerves are formed exclusively from the three cords of the brachial plexus: medial, lateral, and posterior.
Which muscle does the radial nerve contact in the subpectoral triangle?
Together with the artery, the radial nerve lies adjacent to the tendon of the latissimus dorsi muscle (m. latissimus dorsi).
What structures accompany the axillary nerve in the quadrangular space?
The axillary nerve passes through the quadrangular space (foramen quadrilaterum) together with veins and the posterior circumflex humeral artery (a. circumflexa humeri posterior).
Where is the recessus axillaris located relative to the axillary nerve?
The axillary recess (recessus axillaris) of the shoulder joint capsule is located deeper than the axillary nerve, beneath a layer of loose connective tissue.
What else to study on this topic
- Boundaries and contents of the quadrangular space (foramen quadrilaterum).
- Topography of the subpectoral triangle and syntopy of its neurovascular bundle.
- Features of the subdeltoid space and the exit of the axillary nerve.
- The surgical neck of the humerus as a zone at risk for injury to the axillary nerve.
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