MSK 4 - Dermatomes, Myotomes + Segmental Innervation of Limbs

Question 1

Segmentation is a fundamental design of biological systems, the CNS is segmented throughout - what is each neural segment called?

Answer 1

• A neural level (e.g.: C6)

Question 2

From which somites do the vertebrae/ribs and dermis/muscle differentiate from?
What do dermamyotomes develop in association with?

Answer 2

Vertebrae + Ribs = Sclerotome
Dermis + Muscle = Dermamyotome

Sclerotome + DMT are formed from differentiation of somites on either side of the neural tube.

DMT’s develop in association with specific neural level of spinal cord + take nerve supply with them from neural tube as a spinal nerve. Skin + Muscle derived from 1 DMT have a common spinal nerve supply as a consequence

Question 3

Outline the structure of a nerve (from in to out)

Answer 3

• Single axon wrapped in myelin sheath , surrounded by endoneurium.
• Axons bunched into fascicles, perineurium surrounding fascicles.
• Connective tissue + blood vessels in between separate fascicles
• Epineurium surrounds entire nerve, with spinal nerves running underneath it.

Question 4

What type of nerve fibres do the dorsal (posterior) and ventral (anterior) roots contain in a spinal cord section?
What are the “spinal nerves” + how many are there?

Answer 4

Dorsal = afferent/sensory nerve fibres
Ventral = efferent/motor + autonomic nerve fibres

• Spinal nerves are bundles of mixed (motor + sensory) axons which exist briefly and pass through the intervertebral foramen, marking the division between the PNS + CNS
• 31 pairs numbered according to the level of the vertebral column they appear from.

Question 5

What is each vertebra derived from?
Where does the spinal cord run through?
Where does the spinal cord start + finish?
Where do long roots from segments inferior to the finish descend in?

Answer 5

• Parts of 2 adjacent somites, e.g.: vertebra C2 is derived from sclerotome C2 + C3
• The vertebral foramina (the spinal canal)
• Spinal cord starts at inferior margin of medulla oblongata + ends at conus medullaris (at L2 level)
• Inferior segments descend in as a part of the cauda equina and exit at their respective foramina.

Question 6

Which spinal nerves exit above corresponding vertebrae?

Which spinal nerves exit below corresponding vertebrae?

Answer 6

• C1-C7 spinal nerve exit above corresponding vertebrae
• C8 exists between C7 + T1
• T1-L5 exit below corresponding vertebrae
• S5 + Co1 exit via sacral hiatus (posteriorly)

Question 7

What 3 things do mixed spinal nerves divide into?

What do these divisions supply?

Answer 7

1) The posterior/dorsal ramus - supplies deep muscles and skin of dorsal trunk
2) Anterior/ventral ramus - supplies muscles and skin of upper and lower limbs, and ventral trunk
3) Meningeal branch - re-enters spinal canal through intervertebral foramen, supplying vertebrae, ligaments, blood vessels + meninges.

NB: be able to label spinal nerve divisions - see slide 15

Question 8

What do the dorsal + ventral rami divide again into?

What do these divisions supply?

Answer 8

• Dorsal rami further divides into medial + lateral branches. They supply the skin of the back in a ‘tidy’ segmental manner (i.e.: dermatomes)
• Ventral rami enter plexuses to supply the limbs (brachial plexus for upper limbs, lumbro-sacral plexus for lower limbs).

Question 9

Define “dermatome” and “myotome”

Answer 9

Myotome = group of muscles supplied by single spinal nerve

Dermatome = area of skin supplied by single spinal nerve

Question 10

What are anterior and posterior axial lines of limbs and what is their function?
What are pre-axial and post-axial borders?

Answer 10

• Limbs have A + P axial lines which are junctions of two dermatomes. There is functional overlap between adjacent dermatomes, but not across these axial lines - which mark the centre of ventral or dorsal limb compartments.
• Boundaries of dorsal and ventral compartments on limbs marked by superficial veins.

Question 11

Fibres from one spinal nerve can enter multiple peripheral nerves - give an example of this.
Where does the rearrangement of these nerves occur?

Answer 11

• Fibres from C6 spinal nerve enters the musculocutaenous, radial and median nerve (this concept is key) - via the brachial plexus
• Rearrangement occurs in the plexuses

Question 12

Can fibres from multiple spinal nerve combine to form a peripheral nerve? If yes, give an example.

What is the outcome of this?

Answer 12

Yes - e.g.: spinal nerves from C5,6 + 7 combine to form musculocutaenous nerve

• Outcome of this are peripheral nerve territories (these are NOT dermatomes), but areas of skin supplied by peripheral nerve. Branches of brachial plexus for upper limb + lumbro-sacral for lower. They often overlap sections of multiple dermatomes.

Question 13

Summarise the 2 key concepts of peripheral and spinal nerves

Answer 13

1) Within each peripheral nerve there may be fibres from multiple spinal nerves
2) Fibres from spinal nerves may enter multiple peripheral nerves

Question 14

What is the clinical relevance of dermatomes?

Answer 14

• Viral infections - affect the skin of a single dermatome. E.g.: VZV can reactivate and travel through peripheral nerve to the skin of a single dermatome (typically T2)

Question 15

What is a motor unit?
How many neurones of motor units does a spinal nerve contain?
How many spinal nerves supply 1 myotome?

Answer 15

• A motor unit = a motor neuron and the skeletal muscle fibres it innervates.
• 1 spinal nerve contains neurones of many motor units
• 1 spinal nerve supplies 1 myotome

Question 16

What movements of the upper limb are supplied by these myotomes …

• C5, C6, C7, C8 + T1

Answer 16

C5 = shoulder abduction + external rotation
C6 = elbow flexion/wrist extension/supination 
C7 = elbow extension/wrist flexion/pronation 
C8 = finger flexion/finger extension 
T1 = finger abduction/finger adduction

Question 17

What movements are the lower limb are supplied by these myotome …

• L2, L3, L4, L5, S1 + S2

Answer 17

L2 = hip flexion
L3 = knee extension
L4 = anke dorsiflexion
L5 = great toe extension 
S1 = ankle plantar-flexion
S2 = great toe flexion

Question 18

What is Hilton’s Law?

Answer 18

The nerves crossing a joint supply …

1) The muscles acting on it
2) The skin over the insertion of those muscles
3) The joint itself