3 Dermatomes, Myotomes And Segmental Innervation Of The Limbs Flashcards
What are the two parts of a somite?
Sclerotome
Dermatomyotome
What does the sclerotome become?
Vertebrae and ribs
What does the dermatomyotome become?
Dermis (dermatome) and muscle (myotome)
What are dermatomyotomes?
Develop in association with a specific neural level of spinal cord
Take nerve supply with them from neural tube as spinal nerve
Skin and muscle derived from a single dermatomyotome have a common spinal nerve supply
What are spinal nerve roots?
Roots connect each spinal nerve to a segment of cord
Dorsal roots contain afferent/sensory nerve fibres only
Ventral roots contain efferent/motor and autonomic nerve fibres only
What are spinal nerves?
Parallel bundles of axons encased in connective tissue
Mixed (sensory and motor)
Exist briefly as they pass through the intervertebral foramen - marks division between CNS and PNS
How many spinal nerves are there?
31 pairs
What are the derivatives of vertebral segments?
Each vertebra is derived from parts of two adjacent somites
Differentiate from sclerotomes
Where does the spinal cord run?
Through vertebral foramen
Stops at L2 level
What is the spinal canal formed from?
Multiple vertebral foramina
How do spinal nerves leave the spinal canal?
Leave via intervertebral foramina
Where does the spin cord start and end?
Starts at inferior margin of medulla oblongata
Ends as conus medullaris at L2
What occurs to the spinal cord after L2?
Long roots form inferior segments descend in casual equine and exit at their respective foramina
Where do the spinal nerves emerge?
First cervical pair between occipital bone and 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
What does mixed spinal nerve divide into?
Rami
What does the dorsal/posterior ramus supply?
Deep muscles and skin of dorsal trunk
What does the ventral/anterior ramus supply?
Muscles and skin of the upper and lower limbs and lateral and ventral trunk
What is the meningeal branch?
Comes off the spinal nerve
Re-enters spinal canal through intervertebral foramen
Supplies vertebrae, ligaments, blood vessels and meninges
What does the dorsal/posterior rami divide into further?
Medial and lateral branches
Supply skin of back in tidy segmental manner
Narrow structure of muscle/skin in line with intervertebral foramen
Where does the ventral/anterior rami go through and to?
Enters the plexuses to supply the limbs - brachial plexus and lumbo-sacral plexus
Segmental for the trunk
Complicated for the limbs
What is a myotome?
A group f muscles supplied by a single spinal nerve or spinal nerve root
What is a dermatome?
Area of skin supplied by a single spinal nerve or spinal nerve root
What are axial lines?
Junction of two dermatome supplied from discontinuous spinal levels
There is functional overlap between adjacent dermatomes but not across axial lines
Axial lines mark the centre of either the ventral or dorsal compartments of the limb
What are pre-axial and post-axial borders?
Boundaries of dorsal and ventral compartments
Marked by superficial veins
Upper limb: cephalon vein (pre-axial), basilic vein (post-axial)
Lower limb: long saphenous vein (pre-axial), short saphenous vein (post-axial)
Fibres from one spinal nerve…
Can enter multiple peripheral nerves
Where does the rearrangement of nerves occur?
Plexuses
What are peripheral nerve territories?
Areas of skin supplied by the peripheral nerves
Branches of brachial plexus in upper limb
Branches of lumbosacral plexus in lower limb
Often overlap sections of multiple dermatomes
Within each peripheral nerve there may be…
Fibres from more than one spinal nerve
Fibres from one spinal nerve may enter…
More than one peripheral nerve
What is a motor unit?
A motor neuron and the skeletal muscle fibres it innervates
1 spinal nerve supplies one myotome
Action for C5
Shoulder abduction and external rotation
Action for C6
Elbow flexion/wrist extension/supination
Action for C7
Elbow extension/wrist flexion/pronation
Action for C8
Finger flexion/finger extension
Action for T1
Finger abduction and adduction
Action for L2
Hip flexion
Action for L3
Knee extension
Action for L4
Ankle dorsiflexion
Action for L5
Great toe extension
Action for S1
Ankle plantar-flexion
Action for S2
Great toe flexion
