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Flashcards in MSK dermatomes Deck (41):

What does sclerotome form (ventral)?

What does dermatomyotome (dorsal) form?

Vertebrae and ribs

Dermis (dermatome) and muscle (myotome)


During what week of gestation are somites paired?

4th week


How do the dermatomyotomes have a nerve supply?

They take their nerve supply with them from neural tube as spinal (segmental) nerve


How many nerves does the skin and muscle supplied from a single dermatomyotome have?

They have a common single nerve supply


What do the spinal nerve roots connect?

What do the dorsal (posterior) roots contain?

What do the ventral (anterior) roots contain?

Each spinal nerve to a 'segment' of cord

Dorsal (posterior) - afferent/sensory nerve fibres ONLY

Ventral (anterior) - efferent/ motor and autonomic nerve fibres ONLY


What is a spinal nerve?

What does each spinal nerve contain?

How many pairs of spinal nerves are there?

How are they numbered?

Parallel bundles of axons encased in connective tissue

Dorsal and ventral root - mixed nerves (motor and sensory)

31 pairs - numbered according to the level of vertebral column from which they emerge


What marks the division between the CNS and PNS?

Intervertebral foramen


What surrounds each axon?

What surrounds each fasicle?

What surrounds each nerve?





What runs through the vertebral foramen?

Where do the spinal nerves leave the spinal canal?

Spinal cord

Intervertebral foramina


Where do dermatomyotome so develop?

In association with a specific neural level of spinal cord


What are the layers of the meninges from outside in?

Dura mater
Pia meter



Where does the spinal cord start and end?

Starts - Inferior margin of medulla oblongata

Ends - conus medullaris at L2


Why are most spinal nerve segments not vertically aligned with corresponding vertebrae?

Spinal nerves are shorter than corresponding column


What happens in the cauda equina?

Where is it?

Long roots from inferior segments descend into it to exit at respective foramina

Located below spinal cord


Where does the first pair of cervical spinal nerves emerge?

Where do C1-C7 exit?

What exists between vertebrae C7 and T1?

Where do T1-L5 exit?

Where do S1-S4 exit?

What exits via sacral hiatus?

Occipital bone and atlas (C1)

Above corresponding vertebrae

Spinal nerve C8

Below corresponding verebrae

Exit via 4 pairs of sacral foramina

S5 and Co1


What does a mixed spinal nerve divide into?

What does the posterior/dorsal Ramus innervate?

What does the anterior/ventral ramus innervate?


Deep muscles and skin of dorsal trunk

Muscles and skin of upper and lower limbs and lateral and ventral trunk


What other branch do spinal nerves give off?

Where does it re-enter spinal canal?

What does it supply?

Meningeal branch

Re enters spinal canal through intervertebral foramen

Supplies vertebrae, ligaments, blood vessels and meninges


What are rami communicantes?

Components of autonomic nervous system


What does the dorsal ramus innervate?

What does it split into?

What does it supply?

What is the narrow strip of muscle in line with?

Deep muscles and skin of dorsal trunk

Medial and lateral branches

Skin of back in tidy segmental manner

Intervertebral foramen


Which nerve originates from the lateral cord of the brachial plexus?

What muscle does it supply?

Lateral pectoral

Pectoralis major


What is the level of injury defined as?

The lowest level of full sensation and function


What two plexuses does the anterior rami form?

What are they connected by?

Lumbar plexus (L1-L4)

Sacral plexus (L4-S4)

Lumbosacral trunk (lumbosacral plexus)


What muscles does the lumbar plexus innervate?

Where does the lumbar plexus form?

Psoas major

Psoas minor

Forms behind psoas muscles, emerges medial and lateral to psoas major


What are the origins and insertions of psoas major and minor?

*Lumbar plexus innervates them*

Psoas major
Origin: T12, L1-L4 and intervertebral discs
Insertion: Lesser trochanter of femur

Psoas minor
Origin: T12, L1
Insertion: Innominate line and iliopectineal eminence (pelvis)
Often absent


What are the major branches of lumbar plexus?

Medial and lateral.

Medial - obturator (L2, 3, 4)
Lumbosacral trunk (L4 and 5)

Femoral (L2, 3, 4)
Iliohypogastric (L1)
Illioinguinal (L1)
Lateral cutaneous nerve of thigh (L2, 3)


Where does the sacral plexus form?

What does it supply?

Within the pelvic cavity
Anterior to piriformis

Pelvic region
Gluteal region
Perineal regio
Lower limb via sciatica nerve


Define Myotome

Define dermatome

A group of muscles supplied by a single nerve

Area of skin supplied by a single spinal nerve


What is a motor unit?

What does one spinal nerve contain in terms of neurones?

A motor neuron and the skeletal muscle fibres it innervates

1 spinal nerve contains the neurone of many motor units


What is an axial line?

What do these lines mark?

Do dermatomes cross them?

Junction of two dermatomes supplied from discontinuous spinal levels

The centre of either pre-axial or post axial compartments of the limb e.g. Anterior and posterior compartments of forearm.

No but there is functional overlap between adjacent dermatomes


What marks the boundaries of the pre axial and post axial compartments?


Cephalic and basilic
Long (great) saphenous and short (small) saphenous


What are the peripheral nerve territories?

NOT dermatomes

Areas of the skin supplied by peripheral nerves e.g. Branches of lumbar plexus


What is shingles?

What happens after chickenpox?

How does it manifest?

Viral infections - affecting the skin of a single dermatome

Reactivation of chickenpox

Virus travels through a cutaneous nerve and remains dormant in dorsal root ganglion after chicken pox.

When host is immunosupressed, varicella zoster virus) reactivated and travels through peripheral nerve to skin of single dermatome.


Why does transection of a single spinal dorsal nerve not lead to anaesthesia of entire dermatomal area?

What is the exception?

As there is considerable overlap between dermatomes

Skin areas that abut onto an axial line


What spinal nerves is the upper limb innervated by?

Anterior primary rami originating from neural levels C5 to T1

These rami form brachial plexus and it is from various parts of this plexus of nerves that the entire neuronal innervation of upper limb is derived


Where does the lower limb receive its motor innervation?

What are they formed from?

From branches of either lumbar or sacral plexus

Formed from anterior primary rami of L2,3,4,5 and S1,2 and 3


What is the difference between myotome and motor unit?

Myotome - muscles innervated by single spinal nerve root

Motor unit - Within a muscle each axon innervates a variable number of muscle fibres. Muscle fibres innervated by a single motor nerve fibre


What spinal nerves stimulate flexion at the elbow?

Which ones stimulate extension?

Flexion = C5 and C6

Extension = C7 and C8


What will a lesion of a spinal nerve root cause?

What will a lesion in a peripheral nerve cause?

Loss of sensation in relevant dermatome and loss of function in relevant myotome

Loss of sensation and muscle weakness in the distribution of the peripheral nerve


Which myotome causes movement of intrinsic muscles of hand?







Flexion = 7,8
Extension = 7,8

Flexion = 5
Extension = 6, 7, 8

Flexion = 5,6
Extension = 7,8

Pronation = 6
Supination = 7,8

Flexion = 6,7
Extension = 6,7


What is the area of skin a peripheral nerve innervates called?

Cutaneous distribution of a peripheral nerve


What is each neural segment known as?

Neural level