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The brachial plexus is a network of five spinal nerves that originates in the neck, passes through the axilla (armpit), and supplies the skin and musculature of the upper limb. It is divided into five parts: roots (the front branches, called anterior rami, of the five spinal nerves), trunks (the three thick bundles that form as the roots join), divisions (each trunk splits into a front and a back part), cords (the divisions regroup as three bundles, named by their position around the axillary artery, the main artery of the armpit) and branches (the five terminal nerves that carry on into the limb).
The nerves that contribute to the brachial plexus are the C5 (cervical nerve 5), C6, C7, C8 and T1 (thoracic nerve 1).
The terminal branches that are the outputs of the brachial plexus are the musculocutaneous nerve, the axillary nerve, the radial nerve, the median nerve and the ulnar nerve. These are also known as the peripheral nerves of the upper limb.
Diagram - The Brachial Plexus. The spinal roots are colour coded to make it easier to see which of the roots contribute to which terminal branches. It is important to note that the median nerve is anterior to the radial and axillary nerves, which helps when identifying the nerves in a specimen.
SimpleMed original by Dr. Maddie Swannack
Trying to remember which spinal nerves contribute to which peripheral nerve can be challenging. The brachial plexus story and corresponding hand signs can be handy tools to help.
The story is: “3 musketeers were assassinated by 5 rats, 4 mice and 2 unicorns” (This order can also be remembered by the acronym MARMU, made from the first letters of Musculocutaneous, Axillary, Radial, Median and Ulnar).
The first letters of the key words (Musketeers, Assassinated, Rats, Mice and Unicorns) follow the order of the peripheral nerves, and the numbers match the hand signs.
For the hand signs, the fingers on one hand each represent an individual spinal nerve (with the thumb being C5 and the little finger being T1). This means the story can be attached to specific finger positions, which helps you remember which spinal nerves go into which peripheral nerve (see diagram and explanation below).
The first position is “3 Musketeers” - counting to 3 on the hand (thumb, index and middle finger). This represents the Musculocutaneous nerve, which has the nerve roots of C5, C6 and C7.
The second position is “assassinated” - shown by the finger gun (thumb and index finger). This represents the Axillary nerve, which has the nerve roots C5 and C6.
The third position is “by 5 Rats”, which is all 5 fingers (counting the thumb). This is for the Radial nerve, which has the nerve roots C5, C6, C7, C8, and T1.
The fourth position is “4 Mice”, which is the index, middle, ring and little fingers. This is for the Median nerve (Median and Mice both start with M), which has the nerve roots C6, C7, C8 and T1.
The fifth position is “and 2 Unicorns”, which is the ring and little fingers. This is for the Ulnar nerve, which has the nerve roots C8 and T1.
Diagram - The ‘Brachial Plexus Story’. It is a method for remembering which spinal nerves contribute to which peripheral nerves with the aid of a story and hand position
SimpleMed original by Dr. Maddie Swannack
The musculocutaneous nerve is supplied by the spinal nerve roots C5, C6 and C7.
It carries sensation from the skin of the radial side of the forearm, through its branch the lateral cutaneous nerve of the forearm.
It supplies motor function to perform elbow flexion through innervation of the anterior segment of the upper arm - the BBC muscles of biceps brachii, brachialis (which lies under the biceps) and coracobrachialis (a narrow muscle high on the inner side of the upper arm).
The path of the musculocutaneous nerve starts by piercing coracobrachialis. It then runs down the front of the upper arm between the biceps and brachialis, and comes out on the lateral side of the biceps tendon at the elbow.
The axillary nerve is supplied by the spinal nerve roots C5 and C6.
It carries sensation from the skin of the ‘Regimental badge’ area on the lateral aspect of the shoulder (around the insertion of the deltoid muscle, the point where it attaches to the humerus).
It supplies motor function to the deltoid muscle and to teres minor (a small muscle at the back of the shoulder blade). The deltoid performs shoulder abduction, and its front and back fibres help with flexion and extension. Teres minor turns the arm outwards (lateral rotation).
Its path is short. It leaves the axilla through the quadrangular space (a gap in the muscles at the back of the shoulder), then winds round the surgical neck of the humerus (the narrow part of the bone just below its rounded head) to reach the deltoid. This is why it can be damaged by a fracture of the surgical neck or by a shoulder dislocation.
The radial nerve is supplied by the spinal nerve roots C5, C6, C7, C8 and T1.
It carries sensation from the skin of the lateral side of the upper arm, the dorsal side of the forearm, and the radial side of the dorsum (back) of the hand.
It supplies motor function to the posterior segments of the arm and the forearm, performing wrist and elbow extension. The muscles of the posterior forearm also extend the fingers and supinate the forearm (turn the palm upwards), so damage to the nerve in the arm causes wrist drop (the wrist and fingers cannot be extended).
The radial nerve exits the brachial plexus and travels inferiorly, spiralling around the humerus in the radial groove (a shallow groove along the back of the humerus). It then passes anterior to the lateral epicondyle (the bony bump on the outer side of the lower end of the humerus), into the forearm.
The median nerve is supplied by the spinal nerve roots C6, C7, C8 and T1 (some people also have fibres from C5).
It carries sensation from the skin on the palm side of the thumb, index finger, middle finger and radial half of the ring finger, and from the radial side of the palm.
It supplies motor function to most of the muscles of the anterior forearm. The exceptions are flexor carpi ulnaris and the medial half of flexor digitorum profundus (forearm muscles that bend the wrist and the fingers), which the ulnar nerve supplies. In the hand it supplies some of the intrinsic muscles (the small muscles that lie wholly inside the hand), including the thenar eminence (the fleshy base of the thumb). It therefore performs wrist flexion, pronation of the forearm (turning the palm downwards) and some movements of the thumb.
This nerve has no branches in the upper arm, then passes through the cubital fossa (the hollow at the front of the elbow, where the nerve lies medial to the brachial artery). It then passes through the carpal tunnel (the narrow passage at the front of the wrist) into the palm of the hand.
The ulnar nerve is supplied by the spinal nerve roots C8 and T1.
The ulnar nerve supplies cutaneous (skin) sensation to the medial part of the hand and the medial one-and-a-half digits: the little finger and the ulnar half of the ring finger. Skin over the medial forearm is supplied by the separate medial cutaneous nerve of the forearm, rather than the ulnar nerve.
It supplies motor function of the majority of the intrinsic muscles of the hand, including the muscles of the hypothenar eminence (the fleshy base of the little finger). In the forearm, it supplies flexor carpi ulnaris and the medial half of flexor digitorum profundus.
The ulnar nerve has no branches in the upper arm, passing medially down the humerus and then posteriorly, behind the medial epicondyle (the bony bump on the inner side of the lower end of the humerus). At the wrist, the ulnar nerve passes superficial to the flexor retinaculum (the tough band of tissue across the front of the wrist), and enters the hand via Guyon’s Canal (the ulnar canal).
Other Nerves From the Brachial Plexus
Medial Pectoral Nerve - arises from the medial cord of the brachial plexus and supplies both pectoralis major and pectoralis minor (the two muscles of the front of the chest, with pectoralis minor lying under pectoralis major).
Lateral Pectoral Nerve - arises from the lateral cord of the brachial plexus, and supplies pectoralis major only.
Long Thoracic Nerve - originating from the roots of C5, C6 and C7, this nerve supplies the serratus anterior muscle (a muscle on the side of the chest wall that holds the shoulder blade against the ribs). This muscle rotates the scapula upwards so that the arm can be lifted above shoulder level. If this nerve is paralysed, the sign called ‘Winged Scapula’ appears: the scapula sticks out from the back.
Edited by: Dr. Ben Appleby
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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