Brachial plexus (Plexus brachialis) is a network of nerves that supplies most of the upper limb. It usually forms from the anterior rami, or front branches, of spinal nerves C5 to T1. It extends from the lower neck, behind the clavicle or collarbone, into the axilla, the armpit region.
Where is the brachial plexus located?
In the neck, the roots pass between the anterior and middle scalene muscles, which run from the cervical vertebrae to the upper ribs. The roots and trunks lie above the first rib, close to the subclavian artery, the vessel carrying blood towards the upper limb. The subclavian vein lies in front of the anterior scalene muscle, separate from this interval.
The supraclavicular part of brachial plexus lies above the clavicle and includes the roots and trunks. The divisions pass behind the clavicle. The infraclavicular part of brachial plexus lies below it and includes the cords and their branches.
In the axilla, the cords surround the axillary artery, the continuation of the subclavian artery beyond the outer border of the first rib. They lie deep to pectoralis minor, a chest muscle extending from the ribs to the scapula, or shoulder blade.
What are the roots, trunks and divisions?
The sequence is roots, trunks, divisions, cords and branches. These terms describe how nerve fibres join, separate and regroup on their way to the limb. Here, a root means an anterior ramus of a spinal nerve, not a root attaching directly to the spinal cord.
The usual arrangement is:
- Roots: anterior rami of C5, C6, C7, C8 and T1; C identifies cervical nerves and T identifies thoracic nerves.
- Superior trunk: formed by the union of C5 and C6.
- Middle trunk: the continuation of C7.
- Inferior trunk: formed by the union of C8 and T1.
- Divisions: each trunk separates into an anterior division and a posterior division, directed towards the front and back muscle groups of the limb respectively.
The divisions redistribute fibres before they form the cords. As a result, a named nerve leaving the plexus usually contains fibres from more than one spinal nerve level. Contributions from neighbouring spinal nerves can vary.
How are the cords arranged around the axillary artery?
The cords are bundles formed by the divisions. Their names describe their position relative to the second part of the axillary artery, the segment behind pectoralis minor, rather than their position relative to the whole body.
- Lateral cord: formed by the anterior divisions of the superior and middle trunks; it lies on the outer side of the artery.
- Medial cord: the continuation of the anterior division of the inferior trunk; it lies on the inner side of the artery.
- Posterior cord: formed by the posterior divisions of all the trunks; it lies behind the artery.
The branches of lateral cord of brachial plexus include the musculocutaneous nerve, lateral pectoral nerve and lateral root of the median nerve. The branches of medial cord of brachial plexus include the ulnar nerve, medial pectoral nerve, medial cutaneous nerves of the arm and forearm, and medial root of the median nerve.
The branches of posterior cord of brachial plexus include the axillary and radial nerves, thoracodorsal nerve and subscapular nerves. The medial and lateral roots unite to form the median nerve.
Which main nerves leave the brachial plexus?
Terminal branches are the major nerves that continue from the cords into the limb. Their territories overlap at the regional level, but each has a characteristic distribution. Motor fibres carry signals to muscles; sensory fibres carry information from skin, joints and other tissues.
| Nerve | Origin | Main motor territory | Main skin territory |
|---|---|---|---|
| Musculocutaneous nerve | Lateral cord | Front of the arm | Outer side of the forearm |
| Axillary nerve | Posterior cord | Deltoid and teres minor muscles at the shoulder | Skin over the lower deltoid region |
| Radial nerve | Posterior cord | Back of the arm and forearm | Parts of the back of the limb and hand |
| Median nerve | Lateral and medial cords | Most muscles at the front of the forearm and some hand muscles | Much of the thumb-side palm and fingers |
| Ulnar nerve | Medial cord | Some forearm muscles and most small hand muscles | Little-finger side of the hand |
The median and ulnar nerves share the supply of the front of the forearm and hand. The radial nerve chiefly supplies muscles that straighten the elbow, wrist and fingers.
Which branches arise before the cords?
The supraclavicular branches of brachial plexus arise from roots or trunks rather than from the cords. They supply muscles around the shoulder blade and upper chest.
- The dorsal scapular nerve usually arises from C5 and supplies the rhomboid muscles and part of levator scapulae, which position the scapula.
- The long thoracic nerve arises from C5 to C7 and supplies serratus anterior, which holds the scapula against the chest wall.
- The suprascapular nerve arises from the superior trunk and supplies supraspinatus and infraspinatus, muscles that move and stabilise the shoulder.
- The subclavian nerve arises from the superior trunk and supplies subclavius, the muscle beneath the clavicle.
What does the brachial plexus do?
The plexus carries motor signals for shoulder, arm, forearm and hand movements. It also carries sensation from most of the upper limb, including touch and information about joint position.
Its nerves contain sympathetic fibres, which are fibres involved in involuntary control of blood vessels and sweating. The plexus does not supply every patch of upper-limb skin: the intercostobrachial nerve, arising from T2 outside the plexus, supplies part of the inner upper arm.
How can you explore the brachial plexus in 3D?
In the 3D model next to this article, rotate and zoom towards the lower neck and axilla, then select and isolate the brachial plexus. Fade or hide selected structures, or pull parts apart, to adjust which structures remain in view. Switch layers or cut a section along the sagittal, coronal or transverse plane to explore the region from another perspective.