3 - Dermatomes and Myotomes

Question 1

Label this diagram of the lumbar spine vertebrae

Answer 1

Question 2

Label this sagittal view of the lumbar spinal vertebrae

Answer 2

Question 3

What is each neural segment referred to?

Answer 3

Neural level

Question 4

How does the dermatome form embryologically?

Answer 4

• Day 30 the somites have formed about 34-35 pairs and these differentiate
• Skin and muscle from a single dermatomyotome are innervated by the same spinal nerve

Question 5

What is a

• dermatome
• myotome

-

Answer 5

Dermatome: Area of skin supplies by a single spinal nerve

Myotome: Group of muscles suplied by a single spinal nerve

Question 6

What does the endoneurium consist of?

Answer 6

Mesh of collagen and glycocalyx

Question 7

What happens to a nerve during irritation/injury?

Answer 7

The endoneurial fluid in the endoneurium increases. This oedema can be viewed on MRI

Question 8

What are the blood vessels of nerves?

Answer 8

Vaso nervorum found in the epineurium

Question 9

What is a spinal nerve?

Answer 9

• Mixed nerve that carries motor, sensory and autonomic.
• Caused by joining of dorsal and ventral roots.
• There are 31 and they are very short as they pass through intervertebral foramen and quickly become a peripheral nerve

Question 10

What type of nerves run through the ventral root?

Answer 10

Autonomic and motor

Question 12

What is a spinal canal?

Answer 12

Multiple vertebral foramina

Question 13

What fills the spinal canal?

Answer 13

- Spinal Cord: Inferior margin of medulla oblongata to conus medullaris at L2

- Cauda Equina: Below L2, spinal roots of L,S,Coccygeal region

Question 14

Where do each of the spinal nerves originate from?

Answer 14

Coccygeal only has one pair of spinal nerves

Question 15

What are rami?

Answer 15

When the spinal nerve leaves the intervertebral foramen it splits into:

- posterior/dorsal rami: deep muscles and skin of dorsal trunk

- anterior/ventral ramus: muscles and skin of upper and lower limbs and ventral/lateral trunk

- small meningeal branch: Reenters spinal canal and supplies vertebrae, ligaments, blood vessels and meninges

Question 16

What does the posterior rami split into?

Answer 16

Medial and lateral branches that supply the narrow strip of skin and muscle in line with the intervertebral foramen

Question 17

What is an axial line?

Answer 17

Junction of two dermatomes supplied from discontinous spinal levels.

Mark the centre of the ventral or dorsal limb in embryo

No functional overlap like with continuous spinal levels

Question 18

What does the ventral rami split into?

Answer 18

• Segmental on the trunk but then splits for the upper and lower limbs

- Brachial (C5-T1) and lumbo-sacral (L1-S5) plexus

Question 19

Why is the area of anesthesia smaller than you expect with a spinal injury using the dermatome map?

Answer 19

There is a functional overlap of adjacent spinal nerves, e.g C5 will encroach on C4 and C6

Question 20

What are axial borders?

Answer 20

• At the cephalic and caudal margin of the limb bud and mark the posterior and anterior compartments of the limb bud

Question 21

What happens to the spinal nerves in the limbs?

Answer 21

Anterior rami split into plexuses to supply limbs. They rearrange and one spinal nerve can be in multiple peripheral nerves

Question 22

What spinal nerves supply the musculocutaneous nerve?

Answer 22

C5, 6, 7 for the lateral forearm

Question 23

What is Herpes Zoster?

Answer 23

• Shingles where chicken pox virus travels through cutaneous nerve and remains dormant in dorsal root ganglion
• When host is immunosuppressed varicella zoster virus travels via peripheral nerve to affect a single dermatome of skin

Question 24

What is a motor unit?

Answer 24

A motor neuron and the skeletal muscle fibre it innervates

Question 25

What are the myotomes of the upper limb?

Answer 25

Question 26

What are the myotomes of the lower limb?

Answer 26

Question 27

What spinal nerves does the peripheral femoral nerve contain?

Answer 27

L2,3 and 4 but the nerves still only supply their own dermatome when they pass through it

Question 28

If you are stabbed in femoral nerve what will happen?

Answer 28

• Any myotomes supplied by L2, 3 and 4 will be paralysed
• Therefore cannot hip flex, hip adduct, ankle dorsiflex

Question 29

What is a neural level?

Answer 29

The lowest level of fully intact sensation and motor function!!!