Dermatomes, Myotomes And Segmeetnal Innervation Of The Limbs Flashcards
Embryological segmentation and folding of the neural plate
Segmentation during embryo development contributes to the dermatomes and myotomes that form in the upper and lower limbs
Neural plate at day 21 in development dips down with the surrounding tissue forming over it to form the Neural groove
On day 22 the neural groove is closing to form the neural tube
On day 24 somites develop on the posterior ridge lining the neural tube
Day 30 there are 34-35 pairs of somites
Somites differentiates into the sclerotome (ventral) which gives rise to the vertebrae and the ribs - and dermatomyotome which is dorsal and forms the dermis (dermatome) and the muscle tissue (myotomes)
Dermatomyotomes (DMT)
A dermatome is an area of skin that is supplied by a single spinal nerve
Dermatomyotomes develop in association with a specific neural level of the spinal cord and take their nerve supply with them from the neural tube as a spinal (segmental) nerve e.g. C6.
The skin and muscle derived from a single dermatomyotome therefore have a common spinal nerve supply.
Nerve structure
A typical neuron consists of a cell body, dendrites and an axon
Each nerve contains many axons, within the nerve each axon is surrounded by a layer of connective tissue called the endonuerium (inner sleeve of material called glycocalyx and collagen, has properties analogous to the BBB)
Bundles of axons grouped together forms a fasicle which is surrounded by the perineurim
Bundles of fasicle come together with Bv (called vasa nervorum) to form the spinal nerve surrounded by epineurium
Spinal nerve roots
Spinal nerve = a mixed nerve that contains motor, sensory and autonomic signals between body and spinal cord (dorsal root = afferent and sensory, ventral root = efferent and motor)
Tissue surrounding the spinal cord (working in to out):
Pia mater covers the entire spinal cord and dorsal and ventral roots
Arachnoid mater forms a circle around whole spinal cord (doesn’t directly contact the spinal cord like pia mater does
Dura mater sits on top of the arachnoid mater
vertebral segments
Differentiate from sclerotomes
- Each vertebra is derived from parts of two adjacent somites
- Spinal cord runs through vertebral foramen (pl. foramina)
- Multiple vertebral foramina = spinal canal
- Spinal (segmental) nerves leave spinal canal via intervertebral foramina
The spinal cord
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 as spinal cord is shorter than the vertebral canal
Long roots from inferior segments (lumbar / sacral / coccygeal) descend in cauda equina to exit at their respective foramina
Spinal nerves
8 pairs of cervical spinal roots and only 7 cervical vertebrae
First cervical pair of spinal nerves emerge between occipital bone and atlas (C1)
C1-C7 exit above corresponding vertebrae (e.g. C6 nerve will exit through the inferior part of the C5 vertebrae (therefore superior to C6))
Spinal nerve C8 exits between vertebrae C7 and T1
T1-L5 exit below corresponding vertebrae (e.g. T7 nerve will exit through the inferior part of the T7 vertebrae (therefore inferior to T7))
S1-S4 exit via 4 pairs of sacral foramina
S5 and Co1 exit via sacral hiatus (posterior)
Rami (Ramus)
After emerging through the intervertebral foramen each spinal nerve divides into rami
Dermatomes and axial lines
There is functional overlap between adjacent dermatomes (e.g. C5 and C6)
But not across an axial line
Axial line = junction of two dermatomes supplied from
discontinuous spinal levels (e.g. C5 and T1)
Limbs have anterior and posterior axial lines
Axial lines mark the centre of either the ventral or dorsal compartments of the limb
E.g. If you injure dermatome of C5 then numbness wont be present over the whole area of C5 as some functional overlap from neighbouring dermatomes form C4 and C6, but not from T1 or T2 as it doesn’t cross the axial line
Rotation of limbs and Pre-axial and Post-axial borders
The developing upper and lower limbs rotate in opposite directions and to different degrees. The upper limb rotates externally (laterally) through 90° on its longitudinal axis; and the lower limb rotates internally (medially) through almost 90°
As the upper limb rotates externally (laterally), the pre-axial border comes to lie on the lateral side of the arm and the post-axial border comes to lie on the medial side of the arm. Hence the thumb lies laterally.
As the lower limb rotates internally (medially), the pre-axial border comes to lie on the anteromedial aspect of the limb and the post-axial border on the posterior (or sometimes the posterolateral) aspect of the limb. Hence, the great toe lies medially
Boundaries of dorsal (posterior) and ventral (anterior) compartments Marked by superficial veins - Upper limb: Cephalic vein (pre-axial), Basilic vein (post-axial)
Comparing spinal nerves and peripheral nerves
the axons from a single spinal nerve can follow multiple different routes through the plexus and therefore emerge in several different peripheral nerves.
E.g. the C6 spinal nerve root, some of its axons emerge in the median nerve. For the C7 spinal nerve root, you will see that some of its axons also emerge in the median nerve as well
If you compare this with the dermatome map you will see that this overlaps the C6 and C7 territories. The axons that originated in the C6 spinal nerve and travelled in the median nerve are supplying the skin that is in both the C6 dermatome and the median nerve territory (i.e. the palmar surface and nailbed of thumb and index finger).
The axons that originated in the C7 spinal nerve and travelled in the median nerve are supplying the skin that is in both the C7 dermatome and the median nerve territory (palmar surface and nailbed of middle finger). This illustrates how the dermatomes and the peripheral nerve territories overlap.
Outcome - 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)
Myotomes
Group of muscles supplied by a single spinal nerve (or spinal nerve root)
1 spinal nerve contains the neurons of many motor units
1 spinal nerve supplies 1 myotome
C5: shoulder abduction and external rotation
C6: elbow flexion / wrist extension / supination
C7: elbow extension / wrist flexion / pronation
C8: finger flexion / finger extension
T1: finger abduction and adduction
L2: hip flexion L3: knee extension 4: ankle dorsiflexion L5: great toe extension/ankle inversion S1: ankle plantar-flexion/ankle eversion S2: great toe flexion
Applicaiton if Hiltons law
Femoral nerve (L 2,3,4) Supplies the quadriceps femoris group of muscles Action: extends the knee joint Supplies the knee joint and skin over insertion of quadriceps femoris
Myotome for knee extension is L3
i.e. muscle fibres extending the knee are L3
Dermatome overlying the anterior knee (and insertion of the quadriceps femoris) is L3
Clinical relevance of spinal cord and dermatomes and myotomes
Peripheral nerve injury
Knowledge of the muscles and skin supplied by a peripheral nerve has relevance when examining a patient with a possible peripheral nerve injury.
For example, a patient with a femoral nerve injury (e.g. a stab wound) in the inguinal region (groin) is likely to have anaesthesia in the cutaneous distribution of the femoral nerve (both the anterior femoral cutaneous nerve and the saphenous branch) as well as paralysis of the muscles that are supplied by the femoral nerve distal to the site of injury (Note: any muscles supplied proximally to the injury will be spared).
Spinal cord injury:
When you are assessing a patient for spinal cord injury, you are looking to determine the clinical neural level of the injury.
The neural level is the lowest level of fully intact sensation and motor function
