23. The brachial plexus

Question 1

Where do most nerves supplying the upper limb arise from?

Answer 1

The brachial plexus

Question 2

What is the brachial plexus formed from?

Answer 2

The union of the anterior rami of the C5-T1 spinal nerves

Question 3

Where does the brachial plexus commence and where does it terminate?

Answer 3

Commences in the neck and terminates in the axilla

Question 4

What five parts is the brachial plexus divided into?

Answer 4

Roots, trunks, divisions, cords and branches (no functional difference)
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Question 5

What types of roots enter and exit the spinal cord at each ventral level? What do they converge to form?

Answer 5

Dorsal (sensory) roots enter the spinal cord and ventral (motor and autonomic) roots leave the spinal cord

These roots converge to form a mixed nerve called a spinal nerve at the intervertebral foramen

Question 6

What do spinal nerves divide into?

Answer 6

Each divides into an anterior and a posterior ramus

Question 7

What do the posterior rami of spinal nerves C5-T1 do?

Answer 7

They innervate the skin and musculature of the intrinsic back muscles and do not enter the brachial plexus

Question 8

Where do the roots of the brachial plexus pass after their formation?

Answer 8

Pass between the anterior and middle scalene muscles (scalenus anterior and scalenus medius) to enter the posterior triangle of the neck

At the base of the neck, the roots converge to form three trunks

Question 9

What three trunks does the brachial plexus form when its roots converge?

Answer 9

 Superior trunk – a combination of C5 and C6 roots.
 Middle trunk – direct continuation of C7 root.
 Inferior trunk – combination of C8 and T1 roots.

Question 10

Where do the trunks from the brachial plexus travel?

Answer 10

• travel inferolaterally across the posterior triangle of the neck
• each of the three trunks divide into anterior and posterior divisions in the triangle
• these divisions leave the posterior triangle and pass into the axilla (armpit)
• here they combine to form three cords, named by their position relative to the axillary artery

Question 11

What are the three cords and what combinations of divisions are they formed from?

Answer 11

 Lateral cord, formed by the union of the:
o Anterior division of the superior trunk
o Anterior division of the middle trunk
 Posterior cord, formed by the union of the:
o Posterior division of the superior trunk
o Posterior division of the middle trunk
o Posterior division of the inferior trunk
 Medial cord, formed by the continuation of the anterior division of the
inferior trunk

Question 12

What are the five major terminal branches of the brachial plexus?

Answer 12

Musculocutaneous, axillary, median, radial, ulnar

Question 13

What do the terminal branches do?

Answer 13

Tend to supply an entire compartment in the upper limb, but there is some overlap between them

Question 14

What are the axillary and radial nerves terminal branches of? What do they supply?

Answer 14

The posterior cord

Supply the extensor (posterior/ dorsal) compartments within the upper limb

Question 15

What are the ulnar, median and musculocutaneous nerves branches of and what is their function?

Answer 15

Formed from the anterior divisions, via the medial and lateral cords

Supply the flexor (anterior/ ventral) compartments within the upper limb

Question 16

What is the origin of the musculocutaneous nerve? Where does it run and which joints does it supply?

Answer 16

C5-C7 from lateral cord

Runs laterally into the anterior arm

Supplies the shoulder, elbow and radioulnar joint

Question 17

What is the origin of the median nerve? Where does it run and which joints does it supply?

Answer 17

C6-T1 - is the median of the lateral and medial cords

Runs down the centre of the limb

Supplies the elbow, radioulnar, wrist and interphalangeal joints

Question 18

What is the origin of the ulnar nerve? Where does it run and what is its function?

Answer 18

C8-T1 from medial cord

Runs medially down the upper limb

Supplies the wrist and interphalangeal joints

Question 19

What is the origin of the axillary nerve? Where does it run and what joints does it supply?

Answer 19

C5-C6 from the posterior cord

Runs posteriorly to supply the extensors

Supplies the shoulder

Question 20

What is the origin of the radial nerve?

Answer 20

C5-T1 from the posterior cord

Runs posteriorly to supply the extensors

Supplies the shoulder, elbow, radioulnar, wrist and interphalangeal joints

Question 21

What is the origin of the long thoracic nerve?

Answer 21

C5-C7, directly from the anterior rami of spinal nerves

Question 22

What is the origin of the lateral pectoral nerve?

Answer 22

C5-7 from the lateral cord

Question 23

Do the anterior divisions supply the flexors or the extensors?

Answer 23

Flexors

Question 24

Do the posterior divisions supply the flexors or the extensors?

Answer 24

Extensors

Question 25

What should be remembered about axons and the dermatomes or myotomes they supply?

Answer 25

You should remember that axons from a single spinal nerve will always supply the
dermatome or myotome of that spinal nerve. However, they may travel in multiple
different peripheral nerves to get there.

Question 26

The more distal muscles are supplied by the more —- cord segments.

Answer 26

The more distal muscles are supplied by the more inferior cord segments.

Hence, the superior trunk primarily supplies muscles acting on the shoulder; the
middle trunk primarily supplies muscles acting on the elbow and the inferior trunk
primarily supplies the muscles of the hand.

Question 27

Since each cord segment supplies both flexors and extensors, there must also
be —- of fibres in the plexus. This occurs at the —– , where the anterior —- supply the flexors and the posterior —– supply the
extensors.

Answer 27

Since each cord segment supplies both flexors and extensors, there must also
be divergence of fibres in the plexus. This occurs at the divisions, where the
anterior divisions supply the flexors and the posterior divisions supply the
extensors.

Question 28

Where does re-assortment of fibres to distribute to limb compartments occur?

Answer 28

Occurs at the level of the cords in the brachial plexus

Question 29

What is the relationship between the number of spinal nerves contributing to a terminal branch and the number of joints that the muscles it supplies act upon?

Answer 29

Approximately proportional

Question 30

The sensory innervation to the upper limb via the dermatomes follows a ‘—’ pattern

Answer 30

The sensory innervation to the upper limb via the dermatomes follows a ‘looplike’ pattern

If you recall the elongation of the limb bud in the embryo, you can see how this has been generated. It is easier to understand this segmental progression if we adopt the stance of a four-legged animal.

Question 31

When dissecting the brachial plexus, which structure should we look for?

Answer 31

The important structure to look for is an ‘M’ shape. This is formed by the musculocutaneous nerve, medial and lateral heads of the median nerve, and ulnar
nerve, and usually lies superficial to the axillary artery.

Question 32

what can brachial plexus injuries affect?

Answer 32

can affect both motor function and cutaneous sensation

within the upper limb

Question 33

What can traction injuries affect?

Answer 33

Traction injuries may affect either the upper nerve roots or the lower nerve roots of the brachial plexus

Question 34

What are usual causes of upper brachial injury?

Answer 34

Injuries to the upper brachial plexus usually result from an excessive increase in the angle between the neck and shoulder. This may occur in trauma or during the birth of a baby if the shoulders become impacted in the pelvis (SHOULDER DYSTOCIA) and excessive traction is applied to the baby’s neck.

Question 35

Which nerve roots are affected in upper brachial plexus injury and thus, which actions are affected?

Answer 35

The C5 and C6 roots are affected causing sensory alteration in these dermatomes and paralysis of muscles predominantly supplied by these nerve roots.
Actions:
C5: shoulder abduction and external rotation plus weak contribution to elbow flexion
C6: elbow flexion/wrist extension/supination/internal rotation of shoulder

Question 36

In an upper brachial plexus injury, which are the paralysed muscles?

Answer 36

 Deltoid (axillary nerve: C5-6)
 Teres minor (axillary nerve: C5-6)
 Biceps brachii (musculocutaneous nerve: C5-7)
 Brachioradialis (radial nerve: C5-T1)
 Brachialis (musculocutaneous nerve: C5-7)
 Coracobrachialis (musculocutaneous nerve: C5-7)

Question 37

What is the physical effect of an upper brachial plexus injury?

Answer 37

the limb hangs by the side in internal (medial) rotation with an adducted arm and extended elbow. This is called the ‘waiters tip’ position.

Question 38

What are the causes of lower brachial plexus injury?

Answer 38

Injuries to the lower brachial plexus usually occur due to forced hyperextension or hyperabduction, such as when someone falls from a height and grabs onto a tree branch on the way down.
An alternative mechanism of injury is if the baby’s arm is delivered first and traction is applied to the arm to deliver the rest of the baby

Question 39

What is Injury to the lower roots of the brachial plexus called?

Answer 39

Klumpke’s palsy

Question 40

What is Injury to the upper roots of the brachial plexus called?

Answer 40

Erb’s palsy

Question 41

Which nerve roots are affected in lower brachial plexus injury and thus, which actions are affected?

Answer 41

Nerve roots C8 and T1 are affected
Actions affected:
C8: finger flexion / finger extension / thumb extension / wrist ulnar deviation
T1: finger abduction and adduction

Question 42

Which muscles are affected by lower brachial plexus injury?

Answer 42

The paralysis affects the intrinsic muscles of the hand and those flexors within the forearm that are supplied by the ulnar nerve (i.e. flexor carpi ulnaris and the ulnar
half of flexor digitorum profundus). It also affects those muscles supplied by the C8 and T1 fibres within the median and radial nerves, so is not just a straightforward high ulnar nerve injury

Question 43

How does Klumpke’s palsy present?

Answer 43

The pattern of deformity classically presents as a ‘claw hand’ with hyperextension of all of the metacarpophalangeal joints (not just the ring and little finger seen in an ulnar nerve injury), flexion of the interphalangeal joints, abduction of the thumb and wasting of the interossei.