Lecture 3 - Dermatomes and myotomes Flashcards
How many pairs of somites
34-35 pairs of somites (Day 30)
Sclerotome
Differentiated somite
Ventral
Gives rise to vertebrae and ribs
Dermatomyotome
Differentiated somite
Dorsal
Forms the dermis - dermatome
Forms the muscle - myotome
Why does the skin and muscle have the same common spinal nerve supply
Derived from the same dermatomyotome
Dermatome
Area of skin that is supplied by a single spinal nerve
T4/T5
Nipples
T10
Umbilicus
L1
Groin
Posterior (dorsal) rami
After emerging from the intervertebral foramina each nerve root divides into rami
Into medial and lateral branches that supply the skin and deep muscles of the back in a segmental manner
Each dorsal ramus supplies a narrow strip of muscle and skin in line with the intervertebral foramen
Anterior (ventral) rami
Complicated distribution
Segmental for the skin and muscle of the trunk
Supply both the anterior (ventral) and posterior (dorsal) skin of the upper and lower limb
C5-T1 - upper limbs (brachial plexus)
L1- S5 - lower limbs (lumbosacral plexus)
C5 - T1
Brachial plexus
Upper limbs
No cutaneous distribution on the trunk
L1-S5
Lumbosacral plexus
Lower limbs
Adjacent dermatomes in anterior trunk
C4 and T2
Functional overlap
Overlap between adjacent dermatomes
When a spinal nerve is damaged, the area that loses sensation (anaesthesia) is smaller than expected due to the overlap
Overlap does not extend across an axial line
Hilton’s Law
Nerve supplying the joint capsule also supplies th muscles moving the joint and the overlying skin
Neural level (clinical)
Lowest level of fully intact sensation and motor function
What does the neural tube consist of
Precursor cells for the nervous system
Neurone
Electrically excitable cell that receives, processes and transmits information via chemical and electrical signals
Nerve
Enclosed bundle of axons and associated cell bodies e.g Schwann cells in the peripheral nervous system
Endoneurium
Encloses each axon
Made of glycocalyx and a collagen mesh
Contains endoneurial fluid
Prevents certain molecules from the blood crossing
Nerve irritation or injury
Endoneurial fluid increases
Oedema detected by MRI
Fascicles
Bundles of axons enclosed by perineurium
Perineurium
Covers fascicles
Epineurium
Covers group of fascicles
Contains nerve blood supply- vasa nervorum
Spinal nerve
Mixed nerve
- motor
- sensory
- autonomic signals
Exit vertebrae via intervertebral foramina
Dorsal nerve root
Afferent
Sensory fibres from the sensory receptors towards the spinal cord
Ventral nerve root
EFFERENT
Carries motor or autonomic fibres that are leaving the spinal cord
Spinal cord
Origin- inferior margin of the medulla oblongata (base of brain stem)
Up to L1/L2 level - Conus Medullaris
Cauda Equina
Continuation of spinal cord after L2
Where do the cervical spinal roots emerge from in relation to the cervical vertebrae
7 vertebrae and 8 nerves
First 7 cervical spinal roots run above the vertebrae
The 8th cervical spinal root runs below C7 (C7/T1)
Where does the 5th sacral and 1st coccygeus nerve root exit from
Sacral hiatus
After the spinal nerve exits the intervertebral foramen, what does it do?
Splits into:
- Dorsal rami
- Ventral rami
- Meningeal branch
- receives grey ramus communicans
Meningeal branch
Supplies the:
- vertebrae
- vertebral ligaments
- blood vessels
- meninges
Grey ramus communicans
- Joins spinal nerve once exits intervertebral foramen
- From adjacent paravertebral ganglion of the sympathetic trunk
- Contains postganglionic nerve fibres of ANS (unmyelinated)
Axial line
Junction of 2 dermatomes supplied from discontinuous spinal levels
Axial borders
At the cephalic and caudal margins of the limb bud
Mark the border of the anterior and posterior compartments of the limb
Preaxial
Upper limb - Rotates and lies laterally (thumb is lateral)
Lower limb - Rotates and lies medially (Big toe is medial)
Peripheral nerves
Contain different contributions of axons from different spinal nerves
Map pain
Myotomes
Group of muscles supplied by a single spinal nerve
Motor unit
Motor neurone and skeletal muscle fibre it innervates
