2.1 Dermatomes And Myotomes

Question 1

Describe the development of the neural tube

Answer 1

At day 18, the notochord signals the development of the neural tube from the invagination of the ectoderm to from the neural groove. The neural folds gradually fuse in the midline and progress towards the cephalic and Caudal ends. Paired somites (primitive segments) are formed on either side of the neural tube.

Question 2

Describe the development of the somites

Answer 2

Day 30 there are 34/35 pairs of somites. Each somite differentiates into the sclerotome (ventral and forms vertebrae and ribs) and the dermatomyotome (dorsal, forms the dermis via the dermatomes and the muscle tissue via the myotomes).

Question 3

What does the skin and muscle from a single dermatomyotome have in common?

Answer 3

Dermatomyotome are derived from a single somite alongside a sclerotome. They therefore develop in association with a specific neural level of the spinal cord and take their nerve supply with them from the neural tube as a spinal nerve. The skin and muscle derived from a single dermatomyotome therefore have a common spinal nerve supply.

Question 4

What is a dermatome?

Answer 4

Area of skin supplied by a single spinal nerve

Question 5

What is the Foerster Dermatome map?

Answer 5

A map of the body that shows the anatomical distributions of dermatomes.

Question 6

What is a neuron/neurone/nerve cell?

Answer 6

An electrically excitable cell that receives, processes and transmits information through chemical and electrical signals.

Question 7

Describe the structure of a neurone

Answer 7

Dendrite, cell body, and an axon.

Dendrites are thin and arise from the cell body, receiving signals.
Cell body contains the nucleus and transmits the signal to the axon hillock where it propagates down the axon.
The axon is a specialised cellular extension that can be up to 1m long. Often myelinated. Signal is transmitted down the axon to the synapse.

Question 8

What is a nerve fibre?

Answer 8

An enclosed bundle of axons and their supporting cells (Schwann cells) in the peripheral nervous system. Each axon is surrounded by the endoneurium, with nerve fibres surrounded by endoneurium fluid.

Question 9

What is the endoneurium?

Answer 9

A connective tissue that surrounds individual axons and their associated cells. Consists of inner glycocalyx and a mesh of collagen.
Function is to stop certain molecules from crossing from the blood into the endoneurium fluid.

Question 10

Describe the structure of a peripheral nerve.

Answer 10

Axons, surrounded by endoneurium are bundled together to form fascicles. Each fascicle is surrounded by a perineurium. Fascicles are then grouped together and surrounded by epineurium. Nerves contain their own blood vessels within the epineurium called vasa nervorum.

Question 11

What is a spinal nerve?

Answer 11

A mixed nerve that carrier motor, sensory, and autonomic signals between the body and the spinal cord. The are short and pass through the intervertebral foramen.
There are 31 pairs of spinal nerves that are numbered according to the level of the vertebral column from which they emerge.

Question 12

What is the difference between the dorsal nerve root and the ventral nerve root?

Answer 12

Dorsal nerve root transmits sensory fibres from sensory receptors to the spinal cord. It is an afferent root.

Ventral nerve root carries autonomic and motor fibres AWAY from the spinal cord. It is an efferent root.

Question 13

What forms the vertebral segments?

Answer 13

Each vertebra is derived from parts of 2 adjacent somites.

Question 14

What is the spinal canal?

Answer 14

Where multiple vertebral foramina come together. The spinal cord runs through the vertebral foramen.

Question 15

What is the conus medullaris?

Answer 15

The tapered end of the spinal cord. It occurs near the lumbar vertebral levels L1 And L2.

Question 16

Describe the location of the spinal cord

Answer 16

The spinal cord runs vertically within the spinal canal. It commences at the inferior margin of the medulla oblongata, at the base of the brain stem and ends at the conus medullaris at L2. The spinal cord is shorter than the vertebral canal.

Question 17

What is the cauda Equina?

Answer 17

Located below the conus medullaris, the cauda equina starts at L2 and ends at Col1. It is a bundle of spinal nerves that arise from the spinal cord.

Question 18

Describe the different spinal cord segments and cauda equina

Answer 18

From top to bottom:
In the spinal cord
Cervical nerves (C1-C8)
Thoracic nerves (T1-T12)
Lumbar Nerves (L1-L2)
In the cauda equina 
Lumbar Nerves (L3-L5)
Sacral Nerves (S1-S5)
Coccygeal Nerve (1 pair)

Question 19

Describe the relationship between the cervical and thoracic spinal nerves exiting the spinal canal and the vertebrae

Answer 19

8 pairs of cervical spinal roots, but only 7 cervical vertebrae. C1 to C7 spinal roots emerge above the corresponding vertebral body. C8 emerges beneath the C7 vertebrae, above the T1 vertebrae.
From T1, each spinal nerve root exits inferiorly to the vertebral body.

Question 20

How do sacral nerve roots exit the vertebral canal?

Answer 20

S1 - S4 exit vis the sacral foramina. S5 and the first coccygeal nerve roots exit via the sacral hiatus

Question 21

What are rami?

Answer 21

Each spinal nerve divides into rami.
Dorsal ramus = small, supplies deep muscles and skin of the dorsal trunk.
Ventral ramus = larger, supplies muscles, skin of the upper and lower limbs and also the lateral and ventral trunk.

Question 22

What is the purpose of the meningeal branch?

Answer 22

The meningeal branch is forms from the spinal nerve, and renters the spinal canal supplying the vertebrae, ligaments, blood vessels and meninges of the vertebral column.

Question 23

What are the meninges?

Answer 23

the three membranes (the dura mater, arachnoid, and pia mater) that line the skull and vertebral canal and enclose the brain and spinal cord.

Question 24

Describe the relationship between the sympathetic chain and the spinal nerves and rami.

Answer 24

Sympathetic nervous system operates from T1-L2. Each sympathetic spinal nerve gives off white ramus communicans (preganglionic sympathetic nerve) which enters the sympathetic trunk adjacent paravertebral ganglion.
All of the spinal nerves receive a grey ramus communicans (post-ganglionic sympathetic nerve fibres composed largely of unmyelinated axons) from their adjacent paravertebral ganglion.

Question 25

Describe the structure and distribution of the dorsal rami

Answer 25

The dorsal ramus divides again into the medial and lateral branches.
Each dorsal ramus supplies a narrow strip of muscle in line with the intervertebral foramen.

Question 26

Describe the distribution of anterior rami

Answer 26

T2 - T12 anterior rami are segmental for the skin and muscles of the trunk (similar to the dorsal rami)
C5 - T1 enter a complex network called the brachial plexus supplying the muscles and skin of the upper limbs.
L1 - S5 enter the lumbosacral plexus, supplying the lower limbs and groin.
C4 and T2 dermatomes are adjacent to each other on the anterior trunk.

Question 27

What is an axial line?

Answer 27

The junction of two dermatomes supplied from discontinuous spinal levels

Question 28

What is the functional overlap between dermatomes?

Answer 28

If a single spinal nerve is damaged, the loss of sensation will be smaller than that depicted by the dermatomes map due to functional overlap of adjacent dermatomes. Functional overlap does not extend across the axial line.

Question 29

Describe the rotation of the limb buds during development

Answer 29

Upper and lower limbs rotate in different directions and to different degrees.

Upper limbs rotate externally (laterally) through 90 degrees on its longitudinal axis. Future elbows point backwards, extensor muscles on the lateral and posterior aspect of the limb.

Lower limbs rotate internally (medially) through almost 90 degrees. Thus future knees face forward, and the extensor muscles lie on the anterior aspect of the lower limb.

Question 30

What is the difference between axial borders and axial lines?

Answer 30

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

Axial lines = run down centre of limbs in embryo, marking the junction of two dermatomes in discontinuous spinal levels

Question 31

How are boundaries of the dorsal and ventral compartments marked?

Answer 31

Marked by superficial veins.

Upper limb = cephalic vein marks the pre-axial border, basilic vein markets the post-axial border.

Lower limb = long saphenous vein marks the pre-axial border, short saphenous vein marks the post axial border.

Question 32

What is plexus network?

Answer 32

Brachial plexus and lumbosacral plexus are networks of nerves that divide and merge with other bundles of nerve fibres multiple times. Anterior rami of the spinal nerves enter each plexus, and peripheral nerves emerge from the other end. This allows axons from a single spinal nerve tofollow multiple different routes through the plexus and therefore emerge in several different peripheral nerves.

Question 33

Describe the origins of peripheral nerves

Answer 33

Peripheral nerves exit plexus networks. They generally contain fibres from more than one spinal nerve.
The clinical consequences of this is that the area of skin supplied by a single peripheral nerve does not correspond to the dermatome map as the peripheral nerve contains multiple dermatomes.
Similarly a dermatome may be supplied by multiple peripheral nerves because the fibres within a single spinal nerve may enter multiple different spinal nerves.

Question 34

What is a myotome?

Answer 34

The group of muscle fibres supplied by a single nerves.

As a single spinal nerve often sends axons into multiple peripheral nerves, a myotome usually include muscle fibres in several muscles.

can only be supplied by axons that travelled in the same spinal nerve.

Question 35

What is a motor unit?

Answer 35

A single motor neurone (only one axon) that is innervated by skeletal muscle.

Question 36

Describe nerve supply to a joint

Answer 36

Nerves supplying the muscles moving a joint also supply the joint capsule and the skin overlying the insertions of these muscles.

Question 37

how do the spinal nerves leave the spinal canal?

Answer 37

through the intervertebral foramina

Question 38

why is the spinal cord shorter than the vertebral canal?

Answer 38

as the vertebral column grows quicker than the spinal cord, and extends the spinal nerves that are tethered to the interventricular foramina

Question 39

what is herpes zoster?

Answer 39

reactivation of varicellar zoster virus in the dorsal root ganglion of to a cutaneous nerve. When reactivated it travels through the peripheral nerve to skin of a single dermatome

Question 40

what is meant by the clinical neural level of the injury

Answer 40

the lowest level of fully intact sensation and motor function