Dermatomes, myotomes, segmental innervation of the limbs

Question 1

what does the sclerotome form and where is it found

Answer 1

vertebrae and ribs

found ventrally

Question 2

what does the dermatomyotome form and where is it found

Answer 2

dermis and muscle

dorsal

Question 3

how does the DMT form

Answer 3

develop in association with a specific neural level of spinal cord
take nerve supply with them from neural tube as spinal segmental nerve

skin and muscle derived from a single dermamyotome have a common spinal nerve supply

Question 4

spinal nerve roots

Answer 4

roots connect each spinal nerve to a segment of cord which is continuous

Question 5

dorsal roots contain

Answer 5

afferent/ sensory nerve fibres only

Question 6

dorsal root ganglion

Answer 6

cell bodies of the sensory neurons from the periphery

Question 7

ventral roots contain

Answer 7

efferent/ motor and autonomic nerve fibres only

Question 8

where does the spinal cord run through

Answer 8

vertrebral foramen which join to form the spinal canal

Question 9

where do spinal nerves leave the spinal canal

Answer 9

intervetrebral foramina

Question 10

where does the spinal cord start and end

Answer 10

inferior margin of medulla oblongata and ends at the conus medullaris at L2

Question 11

where do the first pair of spinal nerves emerge

Answer 11

emerge between occipital bone and atlas (C1)

Question 12

where do the cervical spinal nerves emerge

Answer 12

c1 - c7 exit above corresponding vertebrae

Question 13

where does the c8 spinal nerve emerge

Answer 13

between c7 and t1

Question 14

where do the T1-L5 spinal nerves emerge

Answer 14

below corresponding vertebrae

Question 15

where do S1 - S4 exit

Answer 15

Via 4 pairs of sacral foramina

Question 16

where does s5 and Co1 exit

Answer 16

sacral hiatus

Question 17

what does the posterior/ dorsal ramus supply

Answer 17

deep muscles and skin of dorsal trunk
divides again into medial and lateral branches
supply skin of back in ‘tidy’ segmental manner
narrow strip of muscle/ skin in line with intervetrebral foramen

Question 18

what does the anterior/ ventral ramus supply

Answer 18

muscles and skin of the upper and lower limbs and lateral and ventral trunk

enter the plexuses to supply the limbs

Question 19

what is the meningeal branch supply

Answer 19

re-enters spinal canal through intervetrebral foramen

supplues vetebrae, ligaments, blood vessels and meningies

Question 20

where does the brachial plexus supply

Answer 20

upper limb

Question 21

where does the lumbo-sacral plexus supply

Answer 21

lower limb

Question 22

axial line

Answer 22

junction of two dermatomes supplied from discontinuous spinal levels

they mark the centre of either the ventral or dorsal compartments of the limb

Question 23

how do the upper limbs rotate

Answer 23

lateral rotation away from the pre-axial border

Question 24

how do lower limbs rotate

Answer 24

medial rotation towards the pre-axial border

Question 25

in the upper limb what marks the pre/post-axial border

Answer 25

cephalic vein

basilic vein

Question 26

in the lower limb what marks the pre/post-axial border

Answer 26

long saphenous vein

short saphenous vein

Question 27

peripheral nerve territories

Answer 27

these are the areas of the 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)

Question 28

What is the difference between a myotome and motor unit

Answer 28

A myotome is a group of muscles supplied by a spinal nerve (which contains multiple motor
neurons), whereas a motor unit is a single motor neuron and the skeletal muscle fibres it
innervates.

Question 29

what does the C5 myotome do

Answer 29

shoulder abduction and external rotation

Question 30

C6 myotome

Answer 30

elbow flexion / wrist extension / supination

Question 31

c7 myotome

Answer 31

elbow extension / wrist flexion / pronation

Question 32

c8 myotome

Answer 32

finger flexion / finger extension

Question 33

t1 myotome

Answer 33

finger abduction and adduction

Question 34

l2 myotome

Answer 34

hip flexion

Question 35

l3 myotome

Answer 35

knee extension

Question 36

l4 myotome

Answer 36

ankle dorsiflexion

Question 37

l5 myotome

Answer 37

great toe extension

Question 38

s1 myotome

Answer 38

ankle plantar-flexion

Question 39

s2 myotome

Answer 39

great toe flexion

Question 40

what is hiltons law

Answer 40

The nerves crossing a joint supply:
• The muscles acting on it
• The skin over the insertion of those muscles
• The joint itself

Question 41

dermatome

Answer 41

A dermatome is an area of skin supplied by a single spinal nerve.

Question 42

Why do the areas of skin that constitute the dermatomes often differ from those supplied by
the peripheral nerves?

Answer 42

A dermatome is an area of skin supplied by a single spinal nerve. A peripheral nerve,
however, often contains fibres from multiple spinal nerves.
In the limbs, the spinal nerve fibres are distributed via the brachial (upper limb) and
lumbosacral (lower limb) plexuses into multiple different peripheral nerves. Fibres from one
spinal nerve can enter multiple different peripheral nerves, and peripheral nerves can
contain fibres from multiple spinal nerves.

Question 43

State four factors that contribute to the stability and mobility of the vertebral column

Answer 43

• The thickness and compressibility of the intervertebral discs
• The shape and orientation of the intervertebral facet joints
• The tone of the back muscles
• The resistance of the ligaments of the vertebral column.