MSK 15 - The Brachial Plexus Flashcards
What are the 5 sections the brachial plexus is divided into?
1) Roots
2) Trunks
3) Divisions
4) Cords
5) Branches (of terminal nerves)
What forms the roots of the brachial plexus?
What are the 5 roots?
Where do the roots run?
- Mixed spinal nerves exit each neural level of the SC and divide into an anterior + posterior rami. Roots are the anterior rami of spinal nerves
- C5, C6, C7, C8, T1
- Roots pass between anterior + middle scalene muscles and enter base of the neck.
What 3 trunks do the roots converge to form at the base of the neck?
1) Superior trunk - formed from C5 + C6 roots.
2) Middle trunk - direct continuation of C7
3) Inferior trunk - combination of C8 + T1
What do the trunks of the brachial plexus divide into?
What do these divisions then form?
- Each of the 3 trunks divide into their own anterior + posterior divisions. These leave the neck passing into axilla.
- Once they enter the axilla they combine together to form cords (lateral, posterior + medial)
What trunk divisions are the 3 cords (lateral, posterior + medial) formed from?
1) Lateral = anterior divisions of the superior + middle trunk.
2) Posterior = posterior divisions of superior, middle + inferior trunks
3) Medial = anterior division of inferior trunk
What are the 5 major branches that the cords give rise to?
1) Musculo-cutaneous
2) Axillary
3) Median
4) Radial
5) Ulnar
Which terminal branch nerves are formed from posterior divisions + which from anterior divisions?
Which compartments do these branches supply?
- Axillary + radial from posterior cords. Supply extensor (posterior) compartments of upper limb.
- Musculo-cutaneous, ulnar and median from anterior cords. Supply flexor (anterior) compartments of upper limb.
What do axons from a single spinal nerve always supply?
- The specific dermatome or myotome of that spinal nerve, however they may travel in multiple different peripheral nerves to get there
Each muscle takes nerve supply from more than 1 SC segment, what in the brachial plexus allows for this?
- Convergence of fibres within the plexus, which occurs at the trunk (from the roots)
Each spinal cord segment supplies both flexors and extensors, what in the brachial plexus allows for this?
- Divergence of fibres within the plexus, which occurs at divisions.
- Anterior divisions supply flexors, posterior supply extensors
What trunks are more distal + proximal muscles supplied by?
- More distal muscles by inferior trunks. E.g.: superior trunk supplies shoulder, middle trunk supplies elbow + inferior trunk supplies hand. (the muscles within these areas)
What occurs within the brachial plexus at the level of the cords in order to distribute fibres to limb compartments?
What do each of the terminal nerves run into?
- Re-assortment at the level of the cords
1) MSC nerve laterally into anterior arm
2) Axillary + radial posteriorly into extensors
3) Ulnar medially down the upper limb
4) Median runs down the centre of upper limb
What pattern does the sensory innervation from the brachial plexus of the upper limb follow?
- A loop-like pattern, looping around the hand at C6/C7/C8
- Just learn your dermatomes bro
What is the characteristic “M” shape in the brachial plexus that allows us to orientate where we are?
- The “M” shape is formed by the MSC, median + ulnar nerves - lies superficial to axillary artery.
How do upper brachial plexus injuries occur?
Which dermatomes/myotomes are affected, and therefore what movements?
What type of palsy does this result in?
- Excessive increase in the angle between neck and shoulder, e.g.: trauma or during child birth (brachial plexus being stretched)
- C5/C6 roots, therefore loss of sensation to these dermatomes. C5 = shoulder abduction/external rotation. C6 = elbow flexion/wrist extension/supination
- All muscles supplied by C5/6 are paralysed, so limb hangs by side in internally rotated and abbducted position - Erb’s palsy.
