Session 2, lecture 1. Dermatomes & Myotomes

Question 1

Outline the embryological development of dermatomes and myotomes

Answer 1

1. During formation of the neural tube from fusion of the neural folds ( starting central lung progressing towards cephalic and caudal ends) , from 20 days onwards , paired somites begin to appear on either sides of the neural tube.
2. Day 30- there are around 34-35 pairs of somites. They begin to differentiates.
3. Each somite differentiates into the sclerotome ( which is central) and gives rise to the vertebrae and ribs. And the dermatomyotome which is dorsal and forms the dermis( via dermatomes) and the muscle tissue ( via myotomes )

Question 2

Skin and muscle derived from a single dermatomyotome will have different spinal nerve supply or common spinal nerve supply ?

Answer 2

Common spinal nerve supply

Question 3

Define dermatome

Answer 3

An area of skin that is supplied by a single spinal nerve

Question 4

What is a spinal nerve ?

Answer 4

A mixed nerve that carries motor , sensory and autonomic signals between the body and the spinal cord. The dorsal nerve root transmits sensory fibres from sensory receptors to the spinal cord - it is an afferent nerve root. In contrast , the ventral nerve root is an efferent root and carries motor and autonomic fibres that are leaving the spinal cord.

Spinal nerves are short and exist briefly as they pass through the intervertebral column.and are numbered according to the level of vertebral column from which they emerge from.

Eg C1,C2

Question 5

How many spinal nerves are there ?

Answer 5

31 pairs

```
8 Cervical C1-C8
12 thoracic ( T1-T12)
5 Lumbar ( L1-L5)
5 Sacral
1 coccygeal
~~~

Question 6

What does the scelerotome differentiate into ?

Answer 6

Into the vertebrae and in the thoracic region the ribs too.

Each vertebrae is derived from two parts of TWO adjacent somites.

Question 7

Where does the spinal cord run through ?

Answer 7

The vertebral foramen.

Question 8

What is spinal canal ?

Answer 8

Multiple vertebral foramens which form the spinal canal. Where the spinal cord runs through.

Question 9

How do spinal nerves leave the spinal canal ?

Answer 9

Through gaps between the vertebrae known as intervertebral foramina.

Question 10

Where does the spinal cord commence and end ?

Answer 10

The spinal cord commences at the inferior margin of the medulla oblongata ( bass of the brain stem ).

And ends at the conus medullaris at L2.

Question 11

Is the spinal cord shorter than the vertebral column ?

Answer 11

Yes , it is much shorter than the vertebral column because it ends at the level L1/L2. And because of this k most spinal cord segments are not vertically aligned with their corresponding vertebrae.

Question 12

What is below the conus medullaris which continues instead of the spinal cord ?

Answer 12

Cauda equina

Question 13

How many pairs of cervical spinal roots are there ? And how many cervical vertebrae are there ? And due to the difference , do the cervical spinal roots emerge above or below the corresponding vertebral body ?

Answer 13

There are 8 pairs of cervical spinal roots and only 7 cervical vertebrae.

Therefore , the cervical spinal roots emerge above the corresponding vertebral body with the exception of C8 cervical spinal root which emerges between C7/T1 ( thus below C7 and above T1).

Question 14

What is the difference between how the spinal nerve roots emerge in the cervical spine vs the rest of the spine ?

Answer 14

Cervical spine - all spinal nerve roots emerge from above their corresponding vertebrae ( because there are 8 cervical spinal roots and 7 cervical vertebra). Apart from C8 which emerges inferior to the C7 vertebrae and above T1.

Rest of the the spine - from the first thoracic vertebrae (T1) : each corresponding nerve root exits inferior to the vertebral body. For example , L3 spinal nerve root exits inferior to the L3 lumbar vertebrae.

Question 15

We mentioned previously that spinal nerves exit the spinal canal through the intervertebral foramina , there is an exception with the sacral nerve roots and the coccygeal nerve roots. What is this exception ?

Answer 15

The sacral nerve roots exit through the sacral foramina and the S5 and first coccygeal nerve roots exit via the sacral hiatus.

Question 16

What are rami ?

Answer 16

After the spinal nerve roots emerge through the intervertebral foramen , each spinal nerve divides into rami. Either Posterior ( dorsal ) or Anterior ( ventral ) rami.

Question 17

What does anterior ( ventral) ramus supply ?

Answer 17

They supply the muscles and skin of the upper and lower limb and also the lateral and ventral trunk. ( both the ventral ( anterior ) and dorsal ( posterior ) skin of the upper and lower limbs

Question 18

What does the posterior ( dorsal ) rami supply ?

Answer 18

The deep muscles and skin of the dorsal trunk .

Question 19

What is a difference in structure between the anterior ( ventral ) and posterior ( dorsal ) rami ?

Answer 19

Thr anterior ( ventral ) rami are much larger than the dorsal rami ( posterior )

Question 20

What is a meningeal branch ?

Answer 20

Each spinal nerve also gives off a tiny meningeal branch that re enters the spinal canal through the intervertebral Foramen. This supplies the vertebrae , ligaments of the vertebral column , blood vessels and meninges.

Question 21

Why are C4 and T2 dermatomes adjacent to eachother ?

Answer 21

Because the anterior rami from C5-T1 enter the brachial plexus to supply the upper limb - they do not have cutaneous distribution on the trunk.

Question 22

Define axial line

Answer 22

Defined as the junction of two dermatomes supplied from discontinous spinal levels.

For example , the upper and lower limbs have ventral ( anterior ) and dorsal ( posterior ) axial lines which mark the centre of either ventral or dorsal compartments of the limb.

Question 23

Define axial border

Answer 23

They are at the cephalic and caudal margin of the limb bud and mark the borders of the anterior and posterior compartments of the limb.

Question 24

Where is the pre axial border and post axial border located on the upper limb ?

Answer 24

As the upper limb rotates laterally , the pre axial border comes to lie on the lateral side of the arm and the post axial border comes to lie on the medial side of the arm. Hence the thumb lies laterally.

Question 25

What is the location of the pre axial border and post axial border on the lower limb ?

Answer 25

As the lower limb rotates medially , the pre axial border comes to lie on the antero medial aspect of the limb and the post axial border on the posterior lateral aspect of the Limb. Hence why the great toe lies medially.

Question 26

The pre / post axial borders are marked by superficial veins : which veins mark the pre / post axial borders in the lower and upper limb ?

Answer 26

Upper limb : the pre axial border is marked by the cephalic vein & the post axial border is marked by the basicilic vein.

Lower limb : the pre axial border is marked by the long saphenous vein and the post axial border is marked by the short saphenous vein.

Question 27

The anterior rami of L1-S5 spinal nerve roots enter …

Answer 27

The lumbosacral plexus to supply the muscles and skin of the lower limb.

Question 28

Do Peripheral nerves contain fibres from one spinal nerve or multiple spinal nerves ?

Answer 28

Multiple spinal nerves. For example , the musculocutanous nerve ( periphery nerves ) which contains spinal root fibres from C5,C6 and C7.

Question 29

Define myotome

Answer 29

A group of muscles fibres supplied by a single spinal nerve.

Question 30

What is a motor unite ?

Answer 30

A single motor neurone ( only one axon ) and the skeletal muscle it innervates.

Question 31

L2 L3 , L4 , L5 , S1 , S2 myotomes of the Lower limb cause what movements ?

Answer 31

L2 - Hip extension

L3 - knee flexion

L4 - Dorsioflexion

L5 - Great toe extension

S1 - Ankle plantar flexion ( tiptoeing )

S2 -:great toe flexion

Question 32

C5 , C6 , C7 , C8 and T1 myotomes of the upper limb contribute to what movements ?

Answer 32

C5 - shoulder abduction and external rotation

C6 - elbow flexion , wrist extension & supination

C7 - elbow extension , wrist flexion & pronation

C8 - finger flexion + finger extension

T1- Finger abduction & adduction