Lecture 3 Dermatomes & Mytomes Flashcards
What is each neural segment called and what do they associate with/give rise to?
Neural level.
-contribute to the dermatomes and myotomes within upper and lower limbs
What contains the precursor cells of the nervous system?
Neural tube
How does the neural tube and somites form?
- at 18 days
- neural plate starts to invaginate forming the neural groove
- eventually the neural folds approach each other where they meet in the midline and fuse forming the neural tube
- by day 24
- the cephalic (head) and caudal (tail) have closed
- from day 20 onwards paired somites appear
What do somites form and how many are there?
- day 30: there are 34-35 pairs of somites
- as soon as they form they start to differentiate
- differentiate into the sclerotome and the dermatomyotome
What does the sclerotome give rise to?
vertebrae and ribs (ventral)
What does the dermatomyotome give rise to?
Dermis and muscle (dorsal)
What are dermatomyotomes?
They develop in association with a specific neural level and take their nerve supply from the neural tube as a spinal nerve.
Why would skin and muscle have a common spinal nerve supply?
The skin and muscle are derived from a single dermatomyotome.
What is a dermatome?
Area of skin that is supplied by a single spinal nerve.
What map do you use to know the anatomical distributiom of dermatomes?
Foerster Dermatome Map
e.g. T4/T5:nipples, T10:umbilicus, L1:groin
Where does the axon of a nerve arise from?
The axon hillock.
What is the structure of a peripheral nerve?
- each nerve fibre/axon is surrounded by the endoneurium
- perineurium surrounds each fascile (bundle of nerve fibres)
- epineurium surrounds the entire peripheral nerve
What does the endoneurium secrete?
Endoneurial fluid, which surrounds each fibre.
- this increases during nerve irritation or injury.
- this oedema in the nerve can be detected by an MRI
What is the function of the endoneurium and what is it made from?
Consists of an inner sleeve of material called the glycocalyx and a mesh of collgen.
-prevents certain molecules passing from blood into endoneurial fluid
What are vasa nervorum?
Nerves with high metabolic rates contain their own blood vessels in epineurium.
What is a spinal nerve?
A MIXED nerve that carries motor & sensory signals b/w the body and spinal chord.
-they are short and exist briefly as they pass through the intervertebral foramen (marks the difference b/w CNS and PNS)
What is a dorsal root?
Contain afferent, sensory nerve fibres ONLY.
What is a ventral root?
Contain efferent motor, autonomic nerve fibres ONLY.
How many pairs of spinal nerves are there?
31 (each pair leave out either side of the intervertebral foramina and branch into the rami)
What is each vertebra derived from?
2 adjacent somites (above and below)
What does the spinal chord run through?
Vertebral foramen (intervertebral foramina allow spinal nerves to leave)
Where does the spinal chord commence and end?
Inferior margin of medulla oblongata, at base os brain stem, and ends at the conus medullaris at L1/2
Why are the spinal chord segments not in line with the vertebrae?
Differential growth. The growth of the spinal chord is much slower than the rest of the body, so is shorter.
What is inferior/superior meaning?
Inferior is closer to bottom of body
Superior is closer to the top of the body
What is the cauda equina and where is it located?
Below the conus medullaris. Cauda equina (Horse's tail) -long ROOTS from inferioir segments (lumbar/sacral/coccygeal) that descend to reach corresponding foramina
What is the ratio of cervical spinal roots to cervical vertebrae?
8 pairs of roots
7 vertebrae
Therefore the roots emerge above the vertebral body
What is the exception to this rule of cervical roots emerging above the ventral body?
C8 root
Between the C7 & T1 vertebrae
How do spinal roots exit from T1 to L5?
They exit below (inferior to) the vertebrae.
How do the sacral nerve roots exit?
Via the sacral foramina.
-the S5 and 1st coccygeal nerve exits by the sacral hiatus.
What are rami, and the two types?
After emerging from intervertebral foramen, each spinal nerve divides into rami.
Posterior/dorsal rami: deep muscles/skin of dorsal trunk
Anterior/ventral rami: muscles and skin of upper & lower limbs and lateral/ventral trunk
Which type of rami is shorter?
Dorsal/posterior
What is a meningeal branch?
Branches off from spinal nerve and re-enters spinal canal via intervertebral foramen.
-supplies the vertebrae, ligaments, blood vessels and meninges.
What is the meninges?
3 layers that enclose the spinal chord and brain. (pia mater, dura mater, arachnoid)
- meningitis: meninges cause the pain
Which nerves give off a white ramus communicans and what are they?
Spinal nerves T1-L2.
Pre-ganglionic sympathetic nerve.
They give off a white ramus communicans which enters the sympathetic trunk via paravertebral ganglion (ganglion of sympathetic chain)
Which nerves receive a grey ramus communicans and what are they?
All spinal nerves.
-postganglionic sympathetic nerve fibres, unmyelinated
They receive grey ramus communicans from their adjacent paravertebral ganglion.
What do the posterior rami divide into?
Medial and lateral branches.
-these supply a strip of skin/muscle of the back in line with the intervertebral foramen.
What is the anterior rami distribution?
- distribution remains segmental for the skin/muscles of the trunk
- C5-T1 and L1-S5 enter the limb buds
- the anterior rami that supply the limbs immediately enet the brachial plexus (C5-T1) or the lumbosacral plexus (L1-S5)
What is strange about the segments for the front of the trunk?
C4 & T2 are adjacent in the trunk because the segments inbetween these form the limbs.
What parts of the limb do the anterior rami supply?
Supply both the ventral and dorsal skin of upper & lower limbs.
What is a myotome?
Group of muscles supplied by a single spinal nerve.
Why is area of anaesthesia likely to be smaller than expected if a single spinal nerve is damaged?
There is functional overlap between adjacent dermatomes.
-this overlap does not cross the axial line!
What is the axial line?
The junction of 2 dermatomes supplied from discontinuous spinal levels.
(you get ventral and dorsal axial lines)
What are the axial borders?
At the cephalic/caudal margins of the limb.
-mark the borders of the anterior and posterior compartments of the limb
What does the rotation of the limbs do to the post/pre-axial borders of the limbs?
-preaxial border lies on the lateral side of the arm
-postaxial border lies on the medial side of the arm
(arms rotate internally)
-preaxial border lies on the anteromedial aspect of limb
-postaxial border lies on the posterolateral aspect of limb
What does anteromedial mean?
Towards the front, in the middle.
How can you easily recall the posterior/anterior boundaries?
They are marked by superficial veins.
Upper limb:
-Cephalic vein (preaxial)
-basilic vein (postaxial)
Lower limb:
- long saphenous vein (preaxial)
- short saphenous vein (postaxial)
What does a plexus allow to happen?
It allows axons from a single spinal nerve to emerge in several different peripheral nerves.
Is the dermatome map and the peripheral nerve territories the same?
NO! They are different.
Give an example of what plexus’ allow:
They allow the peripheral nerve e.g. median nerve to contain axons from C6,7,8 territories- they overlap.
What are peripheral nerve territories?
Areas of the skin supplied by peripheral nerves.
-they overlap sections of multiple dermatomes
In a peripheral nerve can there fibres from more than one spinal nerve?
Yes. Within each peripheral nerve there can be fibres from more than one spinal nerve.
Can fibres from one spinal nerve enter more than one peripheral nerves?
Yes they can enter many peripheral nerves.
How does shingles present?
In a dermatomal distribution.
- virus from chickenpox lies dormant in dorsal root ganglion.
- when host is immunosuppressed, virus reactivates and travels through a peripheral nerve to the skin of a single dermatome.
What is a motor unit & how does this differ from a myotome?
A single motor neuron and the skeletal fibre it innervates.
Whereas a myotome is supplied by the whole spinal nerve which consists of many motor units.
Does a single myotome include several muscles?
Yes. Because many peripheral nerves originate from a single spinal nerve. Therefore a single myotome usually includes muscle fibres in several muscles.
Why will not all the axons in a peripheral nerve supply the same myotome?
Because they arise from different spinal nerves.
What are the myotomes of the upper limb?
C5: shoulder abduction/external rotation
C6: elbow flexion/wrist extension/supination
C7: elbow extension/wrist flexion/pronation
C8:finger flexion/finger extension
T1: finger adduction/abduction
What are the myotomes of the lower limb?
L2: hip flexion L3: knee extension L4: ankle dorsiflexion L5: great toe extension S1: ankle plantar-flexion S2: great toe flexion
How is Hilton’s Law applied?
Use the fact that nerves supplying the muscles moving the joint, also supply the joint capsule and the skin overlying it.
What do the white and grey ramus communicans do?
They join the spinal nerve to the sympathetic chain which allows nerves to travel superior/inferior to their origin.
