4 - Dermatomes and Myotomes

Question 1

In development, the CNS is split into neural segments known as ……… ………..

Answer 1

Neural levels

Question 2

The …….. ……. contains the precursor cells of the nervous system

Answer 2

Neural tube

Question 3

On either side of the neural tube, what structures develop as primitive segments of the body?

Answer 3

Paired somites

Question 4

What two structures does a somite differentiate into?

Answer 4

• Sclerotome
• Dermomyotome

Question 5

The sclerotome forms on the ……. side of the somite

Answer 5

Ventral

Question 6

The dermomyotome forms on the …….. side of the somite

Answer 6

Dorsal

Question 7

What structures are formed from the sclerotome?

Answer 7

The vertebrae and ribs

Question 8

What two structures are derived from the dermomyotome?

Answer 8

The dermatiome - dermis of the skin

The myotome - muscle tissue

Question 9

What is the significance (in terms of nerve supply) of the dermomyotome forming both the dermis and the muscle tissue?

Answer 9

• Dermomyotomes develop in association with their specific neural level of the spinal cord
• The skin and muscle derived from one dermomyotome have the same spinal nerve supply

Question 10

What is a dermatome?

Answer 10

An area of skin supplied by a single spinal nerve

Question 11

Spinal nerves are mixed nerves, what does this mean?

Answer 11

They contain motor, sensory and autonomic signals between the body and the spinal cord

Question 12

What type of fibre is found in the dorsal nerve root?

Answer 12

• Sensory fibres
• Travel afferently from sensory receptors to the spinal cord

Question 13

What type of fibre is found in the ventral nerve root?

Answer 13

• Motor and autonomic fibres
• Efferent root (leaving the spinal cord)

Question 14

How many pairs of spinal nerves are there?

Answer 14

31

Question 15

Spinal nerves are short, they only exist briefly as they pass through what structure?

Answer 15

The intervertebral foramen

Question 16

Describe the development of structures from the sclerotome.

Answer 16

• The sclerotome differentiates into the vertebrae
  
  • Each vertebra is derived from parts of two adjacent somites
• In the thoracic region it also forms the ribs

Question 17

The spinal cord runs through holes in the vertebrae called the ……. ………. which make up the spinal canal.

Answer 17

Vertebral foramina

Question 18

Where does the spinal cord commence and terminate?

Answer 18

• Commences at the inferior margin of the medulla oblongata
• Ends at the conus medullaris at L1/L2
• Below the conus medullaris is the cauda equina

Question 19

Why are most of the spinal cord segments not directly aligned with their corresponding vertebrae?

Answer 19

• The spinal cord grows at a slower rate in development than the vertebral canal, so the spinal cord is shorter than the spine
• The spinal cord ends at L1/L2, all the lower spinal nerves project to their corresponding vertebrae in the cauda equina

Question 20

There are eight cervical spinal roots but only seven cervical vertebrae. What does this mean for relationship between the spinal nerves and vertebrae in the cervical spine?

Answer 20

• The cervical spinal roots emerge above their corresponding vertebral bodies
• The exception is the 8th spinal root which emerges between C7 and T1
• Inferior to T1, the spinal nerve roots emerge inferior to the vertebrae

Question 21

Spinal nerves divide into two rami. What structures are supplied by the dorsal (posterior) ramus?

Answer 21

Deep muscles and skin of the dorsal trunk

Question 22

Spinal nerves divide into two rami. Which structures are supplied by the ventral (anterior) ramus?

Answer 22

• Muscles and skin of the lateral and ventral trunk and upper and lower limbs
• The ventral rami are much larger than the dorsal rami

Question 23

Each spinal nerve gives off a tiny ………… branch that re-enters the spinal canal. What does this supply?

Answer 23

• Meningeal branch
• Supplies the vertebrae, ligaments of the vertebral column, blood vessels and meninges

Question 24

What specialised structures branch from the spinal nerves between T1 and L2 and what are they composed of?

Answer 24

• The sympathetic rami that allow the formation of the paravertebral chain
• The spinal nerve gives off a white ramus comminicans (preganglionic - myelinated) that enters the paravertebral ganglion, these neurones synapse with postganglionic (unmyelinated) neurones and leave the ganglion via the grey ramus communicans

Question 25

The posterior (dorsal) rami divide into ........ and ......... branches. They supply the skin of the back in a segmental manner.

Answer 25

Medial and lateral

Question 26

How does the dermatome distribution of the anterior (ventral) rami differ from the posterior (dorsal) rami?

Answer 26

* The distribution remains segmental for the trunk * The anterior rami of C5-T1 (brachial plexus) and L1-S5 (lumbosacral plexus) enter the limb buds * Means the dermatomes for C4 and T2 are adjacent on the anterior trunk

Question 27

The .......... rami supply both the ventral and dorsal skin of the upper and lower limbs

Answer 27

Anterior (ventral)

Question 28

When a single spinal nerve is damage, the area of anaesthesia is smaller than the corresponding dermatome. Why is this?

Answer 28

There is functional overlap between adjacent dermatomes (not axross axial lines).

Question 29

What is an axial line? How do these affect loss of sensation when a single spinal nerve is damaged?

Answer 29

* Axial line = the junction of two dermatomes from discontinuous spinal levels * Functional overlap (that would normally make the area of anaesthesia smaller) does not extend across axial lines

Question 30

Describe the rotation of the upper and lower limb buds during development.

Answer 30

* The developing upper and lower limbs rotate in opposite directions * The upper limb rotates externally (laterally) by 90 degrees - elbow points posteriorly * The lower limb rotates internally (medially) by almost 90 degrees - knee points forwards

Question 31

What are the pre-axial and post-axial borders of a limb and where are they found?

Answer 31

* **Pre-axial border** - cephalic side of a limb bud * **Post-axial border** - caudal end of a limb bud * Mark the borders of the anterior and posterior compartments of the limb

Question 32

In the upper limb, the ....... vein marks the pre-axial border and the ......... vein marks the post-axial border.

Answer 32

* Pre-axial border - cephalic vein * Post-axial border - basilic vein

Question 33

In the lower limb, the ....... vein marks the pre-axial border and the ......... vein marks the post-axial border.

Answer 33

* Pre-axial border - great saphenous vein * Post-axial border - small saphenous vein

Question 34

Describe the relationship between the spinal nerves and peripheral nerves through the brachial plexus.

Answer 34

* The axons from a single spinal nerve follow multiple routes through the plexus and emerge in several different peripheral nerves * E.g. the median nerve contains axons from C6-T1

Question 35

Describe the relationship between the limb dermatomes and peripheral nerve territories.

Answer 35

* Peripheral nerves contain fibres from more than one spinal nerve, so the peripheral nerve supplies skin in multiple dermatomes * A dermatome is supplied by multiple peripheral nerves because the fibres in a single spinal nerve enter multiple peripheral nerves

Question 36

What is a myotome?

Answer 36

A group of muscle fibres supplied by a single spinal nerve

Question 37

What is a motor unit?

Answer 37

A single motor neurone and the skeletal muscle fibres it innervates

Question 38

A single ........ nerve sends axons to multiple .......... nerves. These each innervate different muscles, a single myotome usually includes fibres of several muscles.

Answer 38

* Spinal * Peripheral

Question 39

Peripheral nerves contain axons from multiple ........... nerves, so not all the axons within a peripheral nerve supply the same ................. .

Answer 39

* Spinal * Myotome Only axons from the same spinal nerve can supply the same myotome.

Question 40

What spinal nerve root is responsible for shoulder abduction and external rotation of the upper limb?

Answer 40

C5

Question 41

What spinal nerve root is responsible for elbow flexion, wrist extension and supination?

Answer 41

C6

Question 42

What spinal nerve root is responsible for elbow extension, wrist flexion and pronation?

Answer 42

C7

Question 43

What spinal nerve root is responsible for finger flexion and extension?

Answer 43

C8

Question 44

What spinal nerve root is responsible for finger abduction and adduction?

Answer 44

T1

Question 45

What spinal nerve root is responsible for hip flexion?

Answer 45

L2

Question 46

What spinal nerve root is responsible for knee extension?

Answer 46

L3

Question 47

What spinal nerve root is responsible for ankle dorsiflexion?

Answer 47

L4

Question 48

What spinal nerve root is responsible for great toe extension?

Answer 48

L5

Question 49

What spinal nerve root is responsible for ankle plantarflexion?

Answer 49

S1

Question 50

What spinal nerve root is responsible for great toe flexion?

Answer 50

S2

Question 51

In the context of classification of a spinal cord injury, what is the patient's neural level?

Answer 51

The lowest spinal level of fully intact sensation and motor function