Infraclavicular part of the plexus and its cords
The anatomy of the infraclavicular part of the brachial plexus is based on the formation of three major nerve cords. These cords surround the axillary artery and are named according to their position relative to it. Long branches arise from each of the cords, heading towards the muscles and skin of the upper extremity.
Key cords and the nerves they form:
- Posterior cord (fasciculus posterior). Gives rise to two major nerves: the axillary nerve (n. axillaris) and the radial nerve (n. radialis).
- Lateral cord (fasciculus lateralis). It gives rise to the musculocutaneous nerve (n. musculocutaneus) as well as the lateral root of the median nerve (radix lateralis n. mediani).
- Medial cord (fasciculus medialis). It is the source of the ulnar nerve (n. ulnaris), the medial root of the median nerve (radix medialis n. mediani), and the medial cutaneous nerves of the arm and forearm (nn. cutanei brachii et antebrachii mediales).
Topography and branches of the axillary nerve
One of the most important nerves originating from the posterior cord is the axillary nerve (n. axillaris). It is formed by fibers arising from the C₅ and C₆ spinal roots.
To exit the axillary cavity and travel to the structures it innervates, this nerve passes through a specific anatomical opening: the quadrangular space (foramen quadrilaterum).
Within its area of responsibility, the axillary nerve performs two main functions:
- Motor innervation. The nerve controls the deltoid muscle (m. deltoideus) and the teres minor muscle (m. teres minor).
- Sensory innervation. The superior lateral cutaneous nerve of the arm (n. cutaneus brachii lateralis superior) branches off the axillary nerve. This branch provides sensation to the skin on the lateral surface of the arm, directly over the deltoid muscle (regio deltoidea).
Cutaneous innervation of the arm (brachium)
The skin of the arm receives sensory innervation from several nerves, each responsible for a strictly defined segment or surface. Knowledge of these zones is critical for the topographic diagnosis of nerve injuries.
| Skin area | Nerve (source) | Roots |
|---|---|---|
| Superior lateral surface (regio deltoidea) | n. cutaneus brachii lateralis superior (from n. axillaris) | C₅–C₆ |
| Inferior lateral surface | n. cutaneus brachii lateralis inferior (from n. radialis) | C₅–C₆ |
| Posterior surface | n. cutaneus brachii posterior (from n. radialis) | C₅–C₈ |
| Medial surface | n. cutaneus brachii medialis | C₈–Th₁ |
| Axillary region | nn. intercostobrachiales | Th₂ (± Th₃) |
Cutaneous innervation of the forearm (antebrachium)
Similarly to the arm, the skin of the forearm is innervated from various sources. The branches of the brachial plexus are distributed across three main surfaces of the forearm.
- Lateral surface of the forearm is innervated by the lateral cutaneous nerve of the forearm — n. cutaneus antebrachii lateralis. This nerve is the terminal branch of the musculocutaneous nerve (n. musculocutaneus) and carries fibers from the C₅–C₆ roots.
- Medial surface is supplied by branches of the medial cutaneous nerve of the forearm — n. cutaneus antebrachii medialis. It originates directly from the medial cord of the plexus (C₈–Th₁ fibers).
- Posterior surface is supplied by the posterior cutaneous nerve of the forearm — n. cutaneus antebrachii posterior. This is one of the sensory branches of the radial nerve (n. radialis), including fibers from the C₅ to C₈ roots.
The plexus also gives off branches to the hand (manus), particularly to its palmar surface (facies palmaris), where a complex system of sensory innervation is formed.
Briefly: answers to topical questions
Which nerves arise from the posterior cord of the brachial plexus?
Both short and long branches arise from the posterior cord of the brachial plexus (fasciculus posterior). The following nerves are formed from this cord:
- Radial nerve (n. radialis)
- Axillary nerve (n. axillaris)
- Thoracodorsal nerve (n. thoracodorsalis)
The axillary nerve passes through the quadrangular space, and the thoracodorsal nerve innervates the latissimus dorsi muscle.
Which nerves innervate the skin of the palmar surface of the hand?
The skin of the palmar surface of the hand is innervated by branches of the median and ulnar nerves. The boundary between their innervation zones runs along the axis of the 4th digit.
- Median nerve (n. medianus) — innervates the radial 2/3 of the palm, the palmar surface of the 1st, 2nd, and 3rd digits, and the radial half of the 4th digit.
- Ulnar nerve (n. ulnaris) — innervates the ulnar part of the palm, the palmar surface of the 5th digit, and the ulnar half of the 4th digit.
Which branches arise from the supraclavicular part of the brachial plexus?
Short branches arise from the supraclavicular part of the brachial plexus (pars supraclavicularis), innervating the muscles of the shoulder girdle. These include:
- Dorsal scapular nerve (n. dorsalis scapulae)
- Suprascapular nerve (n. suprascapularis)
- Long thoracic nerve (n. thoracicus longus)
- Subclavius nerve (n. subclavius)
- Subscapular nerves (nn. subscapulares)
- Thoracodorsal nerve (n. thoracodorsalis)
- Lateral and medial pectoral nerves (nn. pectorales lateralis et medialis)
Which nerves are formed from the medial cord of the brachial plexus?
Long branches extending to the free upper extremity are formed from the medial cord of the brachial plexus (fasciculus medialis). These include:
- Medial cutaneous nerve of the arm (n. cutaneus brachii medialis)
- Medial cutaneous nerve of the forearm (n. cutaneus antebrachii medialis)
- Ulnar nerve (n. ulnaris)
- Medial root of the median nerve (radix medialis n. mediani)
From which spinal roots is the radial nerve formed?
The radial nerve (n. radialis) is the largest branch of the brachial plexus and the main motor nerve for the posterior muscle group of the arm and forearm. It arises from the posterior cord of the plexus and includes fibers from the following spinal roots:
- C5
- C6
- C7
- C8
- Th1
Which muscles does the musculocutaneous nerve innervate?
The musculocutaneus nerve (n. musculocutaneus) is formed from the C5–C7 roots. The provided sources explicitly state that it innervates the biceps brachii muscle (m. biceps brachii).
Class and exam questions
The infraclavicular part consists of three cords: the posterior cord (fasciculus posterior), the lateral cord (fasciculus lateralis), and the medial cord (fasciculus medialis).
The infraclavicular part consists of three cords: the posterior cord (fasciculus posterior), the lateral cord (fasciculus lateralis), and the medial cord (fasciculus medialis).
Which muscles are innervated by the axillary nerve?
The axillary nerve (n. axillaris) innervates the deltoid muscle (m. deltoideus) and the teres minor muscle (m. teres minor).
Where does the lateral cutaneous nerve of the forearm originate?
The lateral cutaneous nerve of the forearm (n. cutaneus antebrachii lateralis) is a branch of the musculocutaneous nerve (n. musculocutaneus).
How is the median nerve formed?
The median nerve (n. medianus) is formed by the union of two roots: the lateral root (from fasciculus lateralis) and the medial root (from fasciculus medialis).
What else to study on this topic
- Formation of the trunks of the supraclavicular part
- Motor innervation of the forearm muscles
- Diagram of the sensory innervation of the palmar surface of the hand (facies palmaris)
- Topography of the quadrilateral space (foramen quadrilaterum)
- Course and branching of the radial nerve (n. radialis)
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