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
