Dermatomes, Myotomes And Segmental Innervation Of The limbs Flashcards
What is a dermatomyotome?
• DMT develop in association with a specific neural level of spinal cord (e.g. C6)
• Take nerve supply with them from neural tube as spinal (segmental) nerve (e.g. C6)
Skin and muscle derived from a single dermatomyotome have a common spinal nerve supply
What is a spinal nerve root?
• Roots connect each spinal nerve to a “segment” of cord (the cord is continuous)
• Dorsal (posterior) roots contain afferent / sensory nerve fibers ONLY
– Dorsal root ganglion: cell bodies of the sensory neurons from the periphery
• Ventral (anterior) roots contain efferent / motor and autonomic nerve fibers ONLY
What are spinal nerves?
• Parallel bundles of axons encased in connective tissue
• Mixed (motor & sensory)
• Exist briefly as they pass through the intervertebral
foramen – this marks the division between CNS & PNS
• 31 pairs numbered according to level of vertebral column from which they emerge
What is a foramen and what runs through it
• Differentiate from sclerotomes
• 1 vertebra = 1 vertebral segment
• Spinal cord runs through vertebral foramen (pl. foramina)
• Multiple vertebral foramina = spinal canal
• Spinal (segmental) nerves leave spinal canal via intervertebral foramina
See slide for diff between vertebral and intervertebral
Where does the spinal cord start and end
• Starts at inferior margin of medulla oblongata
• Ends as conus medullaris at L2
• Most spinal cord segments are not vertically aligned with the corresponding vertebrae
• Long roots from inferior segments (lumbar / sacral /
coccygeal) descend in cauda equina to exit at their respective foramina
Where does each group of spinal nerves exit?
• First cervical pair of spinal nerves emerge between
occipital bone and atlas (C1)
• C1-C7 exit above corresponding vertebrae
• Spinal nerve C8 exits between vertebrae C7 and T1
• T1-L5 exit below corresponding vertebrae
• S1-S4 exit via 4 pairs of sacral foramina
• S5 and Co1 exit via sacral hiatus (posterior)
What are rami and what do they supply?
- Mixed spinal nerve divides into rami
- Posterior / dorsal ramus: deep muscles and skin of dorsal trunk (small)
- Anterior / ventral ramus: muscles and skin of the upper and lower limbs and lateral and ventral trunk (large)
- Spinal nerves also give off a meningeal branch
- Re-enters spinal canal through intervertebral foramen
- Supplies vertebrae, ligaments, blood vessels, and meninges
- Rami communicantes: components of the autonomic system
Describe the dorsal/posterior rami
- Divide again into medial and lateral branches
- Supply skin of back in ‘tidy’ segmental manner
- Narrow strip of muscle/skin in line with intervertebral foramen
Describe the ventral/anterior rami
- Complicated!
- Segmental for the trunk
- Enter the plexuses to supply the limbs
Define dermatomes and myotomes
- Myotome = group of muscles supplied by a single spinal nerve (or spinal nerve root)
- Dermatome = area of skin supplied by a single spinal nerve (or spinal nerve root)
Where is there functional overlap between dermatomes
Between adjacent dermatomes but not across axial lines
What are axial lines
- Axial line = junction of two dermatomes supplied from discontinuous spinal levels
- Limbs have anterior and posterior axial lines
- Axial lines mark the centre of either the ventral or dorsal compartments of the limb e.g. anterior and posterior compartments of forearm
How do the limbs rotate in early development?
Upper limbs rotate externally
Lower limbs rotate internally
Describe the arrangement of spinal and peripheral nerves
- Peripheral nerves often contain neurons from several spinal nerves
- Fibres from one spinal nerve can enter multiple peripheral nerves
- The rearrangement occurs in the plexuses
What is the brachial plexus
See slide
How do fibres from the c6 sink nerve enter the musculocutanous nerve
Via the brachial plexus, the dadial Erie and the median nerve
Can multiple spinal nerves form one peripheral nerve ?
Fibres from one spinalnerve can branch out to form many peripheral nerves and fibres from multiple spinal nerves can combine to form a peripheral nerve
What are peripheral nerve territories
- These are not dermatomes
- These are the areas of skin supplied by the peripheral nerves
- Branches of brachial plexus in upper limb
- Branches of lumbosacral plexus in lower limb
- They often overlap sections of multiple dermatomes (consistent with their spinal nerve content)
What is herpes zoster/shingles?
- Viral infection, almost always affects the skin of a single dermatome
- Reactivation of Varicella zoster virus (chickenpox)
- Virus travels through a cutaneous nerve and remains dormant in a dorsal root ganglion after chickenpox
- When host is ‘immunosuppressed’, VZV reactivates and travels through peripheral nerve to skin of a single dermatome
What’s the difference between a motor unit and a myotome?
A motor unit is a motor neuron and the skeletal muscle fibres it innervates
1 spinal nerve (e.g. C6) contains the neurons of many motor units 1 spinal nerve supplies one myotome
What are the actions of myotomes in the upper limb?
C5: shoulder abduction and external rotation plus weak contribution to elbow flexion
C6: elbow flexion / wrist extension / supination /internal rotation of shoulder
C7: elbow extension / wrist flexion / pronation / weak contribution to finger flexion and extension
C8: finger flexion / finger extension / thumb extension / wrist ulnar deviation
T1: finger abduction and adduction
What are the actions of myotomes in the lower limb
L2: hip flexion
L3: knee extension and hip adduction
L4: ankle dorsiflexion
L5: great toe extension /ankle inversion / hip abduction
S1: ankle plantar-flexion/ankle eversion/ hip extension (or L5)
S2: knee flexion (some sources say S1 for this) /great toe flexion
Give an application of Hilton’s law
- Femoral nerve (L 2,3,4)
- Supplies the quadriceps femoris group of muscles
- Action: extends the knee joint
- Supplies the skin overlying the knee joint
What happens when the femoral nerve is injured
• Femoral nerve injury:
– Anaesthesia in cutaneous
distribution of femoral nerve and its branches
• Paralysis in muscles supplied by femoral nerve (distal to site of injury)
