Session 2, lecture 1. Dermatomes & Myotomes Flashcards
Outline the embryological development of dermatomes and myotomes
- 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.
- Day 30- there are around 34-35 pairs of somites. They begin to differentiates.
- 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 )
Skin and muscle derived from a single dermatomyotome will have different spinal nerve supply or common spinal nerve supply ?
Common spinal nerve supply
Define dermatome
An area of skin that is supplied by a single spinal nerve
What is a spinal nerve ?
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
How many spinal nerves are there ?
31 pairs
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8 Cervical C1-C8
12 thoracic ( T1-T12)
5 Lumbar ( L1-L5)
5 Sacral
1 coccygeal
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What does the scelerotome differentiate into ?
Into the vertebrae and in the thoracic region the ribs too.
Each vertebrae is derived from two parts of TWO adjacent somites.
Where does the spinal cord run through ?
The vertebral foramen.
What is spinal canal ?
Multiple vertebral foramens which form the spinal canal. Where the spinal cord runs through.
How do spinal nerves leave the spinal canal ?
Through gaps between the vertebrae known as intervertebral foramina.
Where does the spinal cord commence and end ?
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.
Is the spinal cord shorter than the vertebral column ?
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.
What is below the conus medullaris which continues instead of the spinal cord ?
Cauda equina
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 ?
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).
What is the difference between how the spinal nerve roots emerge in the cervical spine vs the rest of the spine ?
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.
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 ?
The sacral nerve roots exit through the sacral foramina and the S5 and first coccygeal nerve roots exit via the sacral hiatus.
What are rami ?
After the spinal nerve roots emerge through the intervertebral foramen , each spinal nerve divides into rami. Either Posterior ( dorsal ) or Anterior ( ventral ) rami.
What does anterior ( ventral) ramus supply ?
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
What does the posterior ( dorsal ) rami supply ?
The deep muscles and skin of the dorsal trunk .
What is a difference in structure between the anterior ( ventral ) and posterior ( dorsal ) rami ?
Thr anterior ( ventral ) rami are much larger than the dorsal rami ( posterior )
What is a meningeal branch ?
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.
Why are C4 and T2 dermatomes adjacent to eachother ?
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.
Define axial line
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.
Define axial border
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.
Where is the pre axial border and post axial border located on the upper limb ?
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.
