MSK 4 - Dermatomes, Myotomes + Segmental Innervation of Limbs Flashcards
Segmentation is a fundamental design of biological systems, the CNS is segmented throughout - what is each neural segment called?
- A neural level (e.g.: C6)
From which somites do the vertebrae/ribs and dermis/muscle differentiate from?
What do dermamyotomes develop in association with?
Vertebrae + Ribs = Sclerotome
Dermis + Muscle = Dermamyotome
Sclerotome + DMT are formed from differentiation of somites on either side of the neural tube.
DMT’s develop in association with specific neural level of spinal cord + take nerve supply with them from neural tube as a spinal nerve. Skin + Muscle derived from 1 DMT have a common spinal nerve supply as a consequence
Outline the structure of a nerve (from in to out)
- Single axon wrapped in myelin sheath , surrounded by endoneurium.
- Axons bunched into fascicles, perineurium surrounding fascicles.
- Connective tissue + blood vessels in between separate fascicles
- Epineurium surrounds entire nerve, with spinal nerves running underneath it.
What type of nerve fibres do the dorsal (posterior) and ventral (anterior) roots contain in a spinal cord section?
What are the “spinal nerves” + how many are there?
Dorsal = afferent/sensory nerve fibres Ventral = efferent/motor + autonomic nerve fibres
- Spinal nerves are bundles of mixed (motor + sensory) axons which exist briefly and pass through the intervertebral foramen, marking the division between the PNS + CNS
- 31 pairs numbered according to the level of the vertebral column they appear from.
What is each vertebra derived from?
Where does the spinal cord run through?
Where does the spinal cord start + finish?
Where do long roots from segments inferior to the finish descend in?
- Parts of 2 adjacent somites, e.g.: vertebra C2 is derived from sclerotome C2 + C3
- The vertebral foramina (the spinal canal)
- Spinal cord starts at inferior margin of medulla oblongata + ends at conus medullaris (at L2 level)
- Inferior segments descend in as a part of the cauda equina and exit at their respective foramina.
Which spinal nerves exit above corresponding vertebrae?
Which spinal nerves exit below corresponding vertebrae?
- C1-C7 spinal nerve exit above corresponding vertebrae
- C8 exists between C7 + T1
- T1-L5 exit below corresponding vertebrae
- S5 + Co1 exit via sacral hiatus (posteriorly)
What 3 things do mixed spinal nerves divide into?
What do these divisions supply?
1) The posterior/dorsal ramus - supplies deep muscles and skin of dorsal trunk
2) Anterior/ventral ramus - supplies muscles and skin of upper and lower limbs, and ventral trunk
3) Meningeal branch - re-enters spinal canal through intervertebral foramen, supplying vertebrae, ligaments, blood vessels + meninges.
NB: be able to label spinal nerve divisions - see slide 15
What do the dorsal + ventral rami divide again into?
What do these divisions supply?
- Dorsal rami further divides into medial + lateral branches. They supply the skin of the back in a ‘tidy’ segmental manner (i.e.: dermatomes)
- Ventral rami enter plexuses to supply the limbs (brachial plexus for upper limbs, lumbro-sacral plexus for lower limbs).
Define “dermatome” and “myotome”
Myotome = group of muscles supplied by single spinal nerve
Dermatome = area of skin supplied by single spinal nerve
What are anterior and posterior axial lines of limbs and what is their function?
What are pre-axial and post-axial borders?
- Limbs have A + P axial lines which are junctions of two dermatomes. There is functional overlap between adjacent dermatomes, but not across these axial lines - which mark the centre of ventral or dorsal limb compartments.
- Boundaries of dorsal and ventral compartments on limbs marked by superficial veins.
Fibres from one spinal nerve can enter multiple peripheral nerves - give an example of this.
Where does the rearrangement of these nerves occur?
- Fibres from C6 spinal nerve enters the musculocutaenous, radial and median nerve (this concept is key) - via the brachial plexus
- Rearrangement occurs in the plexuses
Can fibres from multiple spinal nerve combine to form a peripheral nerve? If yes, give an example.
What is the outcome of this?
Yes - e.g.: spinal nerves from C5,6 + 7 combine to form musculocutaenous nerve
- Outcome of this are peripheral nerve territories (these are NOT dermatomes), but areas of skin supplied by peripheral nerve. Branches of brachial plexus for upper limb + lumbro-sacral for lower. They often overlap sections of multiple dermatomes.
Summarise the 2 key concepts of peripheral and spinal nerves
1) Within each peripheral nerve there may be fibres from multiple spinal nerves
2) Fibres from spinal nerves may enter multiple peripheral nerves
What is the clinical relevance of dermatomes?
- Viral infections - affect the skin of a single dermatome. E.g.: VZV can reactivate and travel through peripheral nerve to the skin of a single dermatome (typically T2)
What is a motor unit?
How many neurones of motor units does a spinal nerve contain?
How many spinal nerves supply 1 myotome?
- A motor unit = a motor neuron and the skeletal muscle fibres it innervates.
- 1 spinal nerve contains neurones of many motor units
- 1 spinal nerve supplies 1 myotome