Brachial plexus

Definitions & Key takeaways

The brachial plexus is a network of nerves that originate in the spinal cord in the neck and pass down through the shoulder area to sensory and motor nerve supply to the shoulder area, and the arm, the forearm, and the hand. It originates from spinal roots C5, C6, C7, C8, and T1, which combine to form three trunks. Trunks divide into 6 divisions, because each trunk splits into an anterior and a posterior division. Divisions regroup into three cords, which finally, give rise to five terminal branches of the brachial plexus that are the musculocutaneous nerve, the axillary nerve, the radial nerve, the median nerve, and the ulnar nerve. Injury to any of these nerves can cause paralysis or other problems with movement and sensation. The brachial plexus can be injured during birth when the baby's head or shoulders get stuck in the birth canal. This can stretch or tear the nerve fibers. Other causes of injury include heavy lifting, car accidents, and falls.

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Introduction0:00–1:44

The brachial plexus may look difficult to draw, but here’s a shorthand way to draw the brachial plexus really fast. Now that the nerves are all drawn, let’s begin labeling the brachial plexus, starting with the cervical and thoracic nerves - C5, C6, C7, C8, & T1.
The brachial plexus is also divided into roots, trunks, divisions, cords, and terminal branches. The order can be remembered using the acronym “Remember To Drink Cold Beer.” Then to remember the terminal branches, you can use the word “MARMU.” Which stands for the musculocutaneous, AXILLARY, radial, median, and ulnar nerves respectively.
Next, we can start labeling the collateral nerves starting with those that branch off the roots. The dorsal scapular nerve branches off of the C5 root, and the long thoracic nerve arises from the C5, C6, and C7 roots.
The superior trunk has two collateral branches, the suprascapular and subclavius nerves. And the cords have a total of 7 collateral branches.
The lateral cord gives rise to the lateral pectoral nerve. The POSTERIOR cord gives rise to the upper, middle, and lower subscapular nerves.
And the medial cord gives rise to the medial pectoral, medial cutaneous nerve of the arm, and the medial cutaneous nerve of the forearm.

Nervous system1:44–3:08

The nervous system is divided into the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system, which is further divided into the somatic and the autonomic nervous systems.
Broadly speaking, the nervous system is split into an afferent and an EFFERENT division. The afferent division brings sensory information from the outside into the central nervous system, and includes visual receptors, auditory receptors, and touch receptors.
On the other hand, the EFFERENT division brings motor information from the central nervous system to the periphery, ultimately resulting in contraction of skeletal muscles to trigger movement through the somatic nervous system, as well as contraction of the smooth muscles to trigger activity of the glands and organs through the autonomic nervous system.
Now, the peripheral nervous system consists of nerves, which are enclosed bundles of fibers that connect the central nervous system to every other part of the body.
Cranial nerves arise from the brain and exit from the skull and innervate the head and neck, and spinal nerves exit various regions of the spinal cord and innervate the rest of the body.
There are 31 pairs of spinal nerves, which are grouped into eight pairs of cervical nerves, twelve pairs of thoracic nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one pair of coccygeal nerves.

Brachial plexus3:08–7:12

The brachial plexus is a network of nerves that innervates the shoulder, arm, and hand, by supplying afferent or sensory nerve fibers from the skin, as well as EFFERENT or motor nerve fibers to the muscles, and there’s a brachial plexus on each side of the body.
The plexus is really important because it can get injured during sports injuries, industrial accidents, surgical procedures and other traumatic injuries to the upper limbs.
And the resulting functional deficit can be significant. In terms of anatomy, the brachial plexus is divided into five roots, three trunks, six divisions, three anterior and three POSTERIOR, three cords, and five terminal branches.
Additionally, there are pre-terminal or collateral branches that leave the plexus at various points along its length. The five roots come from the last four cervical nerves - C5, C6, C7, and C8 - as well as the first thoracic nerve or T1.
These five roots give off some collateral branches: the long thoracic nerve - which arises from C5, C6, and C7, and innervates the serratus anterior muscle that draws the scapula laterally - the dorsal scapular nerve from C5 - which innervates the rhomboid muscles that retract the scapula - and a contribution to the phrenic nerve from C5, which gives motor and sensory innervation to the diaphragm to help breathing.
Then, the five roots combine to form three trunks: C5 and C6 merge to form the superior or upper trunk, C7 remains as the middle trunk, and C8 and T1 merge to form the inferior or lower trunk.
The superior trunk gives off another collateral branch - the suprascapular nerve, which gives motor innervation the supraspinatus and infraspinatus muscles, and also gives sensory innervation to the acromioclavicular and glenohumeral joints.
Then, each trunk splits into an anterior and a POSTERIOR division - giving rise to a total of six divisions. The six divisions then regroup with each other to form three cords.
The cords are named for their relationship to the AXILLARY artery, so there’s the lateral cord - formed by the anterior divisions of the superior and middle trunks - the POSTERIOR cord - formed by the POSTERIOR divisions of the superior, middle, and inferior trunks - and the medial cord - formed by the anterior division of the inferior trunk.
The three cords give rise to various collateral branches. The lateral cord gives rise to the lateral pectoral nerve.
The POSTERIOR cord gives rise to the upper, middle, and lower subscapular nerves, which help innervate the muscles that move the scapula.
And the medial cord gives rise to the medial cutaneous nerve of the arm and the medial cutaneous nerve of the forearm - which give sensory innervation to the medial skin of the arm and forearm - as well as to the medial pectoral nerve, which gives motor innervation to the PECTORALIS minor and major muscles.
Then, the three cords give rise to the five terminal branches. The lateral cord divides into two terminal branches, which are the median nerve - which is formed by merging with the medial cord and is made up of contributions from C5, C6, C7, C8, and T1 - and the musculocutaneous nerve, which is made up of contributions from C5, C6, and C7.
The POSTERIOR cord divides into two terminal branches, which are the AXILLARY nerve – which is made up of contributions from C5 and C6 - and the radial nerve, which is made up of contributions from C5, C6, C7, C8, and T1.
Finally, the medial cord gives rise to two terminal branches, which are the median nerve - which is formed by merging with lateral cord - and the ulnar nerve, which is made up of contributions from C8, T1, and occasionally C7.
Erb’s palsy, also known as “waiter’s tip”, is when there’s traction or a tear along the superior roots, and commonly occurs in infancy during labor, but can happen at any age following shoulder trauma.

Erb's palsy7:12–7:47

This affects the deltoid, supraspinatus, infraspinatus, biceps brachii, and supinator muscles, leading to a deficit in abduction, lateral rotation, flexion, and supination of the arm - like a waiter discreetly trying to get a tip.
Klumpke’s palsy occurs when there’s traction or a tear of the inferior roots, and also can occur during a delivery or from a shoulder trauma.

Klumpke's palsy7:47–8:19

It mainly affects the intrinsic hand muscles, and makes the muscles in the hand flex into a claw. Occasional involvement of T1 root can lead to Horner’s syndrome which causes ptosis or drooping of the upper eyelid, miosis or constriction of the pupil, and anhidrosis or the inability to sweat.
A winged scapula occurs when there’s lesion of the long thoracic nerve, usually due to a stab wound or during damage during an AXILLARY node dissection after mastectomy.

Winged scapula8:19–8:50

This leads to weakness of the serratus anterior muscle, which causes the scapula to protrude or “wing out” from a person’s back when in action because it’s not anchored to the thoracic cage.
When this happens it’s really hard for a person to abduct the arm. The median nerve runs on the medial part of the arm.

Median nerve8:50–9:26

At the level of the elbow, the median nerve moves to the anterior forearm; at the forearm, the median nerve gives off branches to innervate the skin and to most flexor muscles in the forearm that pronate the forearm, flex the wrist and fingers, and oppose the thumb.
The median nerve then proceeds distally and travels through the wrist at the carpal tunnel. The median nerve provides sensory fibers to the lateral surface of the palm, as well as the palmar and dorsal surfaces of the thumb, second, third, and lateral half of the fourth finger.
“Pope’s blessing” or “hand of benediction” occurs as a result of prolonged compression or injury of the median nerve at the forearm or elbow.

Pope's blessing9:26–9:45

So when individuals affected try to make a fist, they’re unable to flex the index and middle fingers. The musculocutaneous nerve courses inferiorly in the anterior arm, and it gives motor innervation to the biceps brachii, brachialis, and the coracobrachialis muscles.

Musculocutaneous nerve9:45–10:10

These muscles are responsible for the flexion of the arm at the elbow and supination of the forearm. Distal to the elbow, it provides sensory innervation to the skin of the lateral forearm.

Axillary nerve10:10–10:30

The AXILLARY nerve runs POSTERIOR to the surgical neck of the humerus. It is in charge of the motor innervation of the deltoid and teres minor muscles: moreover, it provides sensory innervation to the skin and joint capsule of the shoulder.

Axillary nerve palsy10:30–10:49

Axillary nerve palsy happens when the AXILLARY nerve is damaged or entrapped, usually following shoulder dislocation or a fracture of the surgical neck of the humerus bone, which causes deltoid muscle weakness and sensory loss below the shoulder.

Radial nerve10:49–11:46

The radial nerve is the largest branch of the brachial plexus. After its origin, the radial nerve moves to the POSTERIOR part of the arm and runs inside a shallow depression of the humerus called the radial groove where it supplies the triceps muscle which extends the elbow.
After passing through the radial groove, the radial nerve moves slightly anteriorly, innervating the brachioradialis muscle.
After this, the radial nerve enters the POSTERIOR compartment of the forearm and gives motor innervation to all the extensor muscles of the forearm, which extend the wrist, supinate the forearm, extend the fingers, and extend and abduct the thumb.
The radial nerve is also in charge of the sensory innervation of the lateral arm, POSTERIOR forearm, dorsal part of the first, second and lateral half of the third fingers, excluding the distal phalanges.
There’s a term - “Saturday night paralysis” which is where an intoxicated person falls asleep with an arm hanging over the back of a chair, and this can compress the radial nerve.

Saturday night paralysis11:46–12:09

So when the person wakes up, they may complain of impaired sensation of the arm and hand, as well as an inability to extend the wrist and fingers - which is called wrist drop.

Ulnar nerve12:09–13:00

The ulnar nerve runs close to the median nerve along the arm: however, at the level of the elbow, the ulnar nerve passes medially through an unprotected area right behind the medial epicondyle.
It then continues distally and enters the hand passing inside the Guyon's or ulnar canal, formed by the superficial palmar carpal ligament.
The ulnar nerve gives motor innervation to the muscles in charge of flexing the wrist and fingers, and to the muscles that help adducting the thumb.
The sensory innervation of the ulnar nerve goes to the skin of the palmar and dorsal medial half of the fourth finger and of the entire pinky finger, as well as the correspondent palmar and dorsal hand.
This is why hitting the “funny bone” - or the spot at the elbow behind the medial epicondyle - makes the pinky finger tingle.
Alright, as a quick recap, the brachial plexus is a network of nerves that supplies sensory and motor nerve fibers to the shoulder, arm, and hand.

Review13:00–13:40

The brachial plexus originates from C5, C6, C7, C8, and T1. These five roots combine to form three trunks.
Each trunk splits into an anterior and a POSTERIOR division - for a total of six divisions, which then regroup with each other and form three cords.
Finally, the cords divide and give rise to five terminal branches, which are the musculocutaneous nerve, the AXILLARY nerve, the radial nerve, the median nerve, and the ulnar nerve.