Infraclavicular Part 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 spatial relationship to it. Long branches destined for the muscles and skin of the upper limb arise from each cord.
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). Gives rise to the musculocutaneus nerve (n. musculocutaneus) and the lateral root of the median nerve (radix lateralis n. mediani).
- Medial cord (fasciculus medialis). Serves as 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 spinal nerve roots C₅ and C₆.
To leave the axillary cavity and reach its target structures, this nerve passes through a specific anatomical landmark — the quadrangular space (foramen quadrilaterum).
In its innervation territory, the axillary nerve performs two primary 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 supplying a strictly defined segment or surface. Knowledge of these zones is critical for the topographic diagnosis of nerve injuries.
| Skin Area | Nerve (Source) | Roots |
|---|---|---|
| Upper lateral surface (regio deltoidea) | n. cutaneus brachii lateralis superior (from n. axillaris) | C₅–C₆ |
| Lower 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 multiple sources. 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 roots C₅–C₆.
- 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 (fibers C₈–Th₁).
- Posterior surface is supplied by the posterior cutaneous nerve of the forearm — n. cutaneus antebrachii posterior. This is a sensory branch of the radial nerve (n. radialis), containing fibers from roots C₅ to C₈.
The plexus also sends branches to the hand (manus), particularly to its palmar surface (facies palmaris), where a complex sensory innervation network is formed.