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Semester 2 - MSK > 6. Dermatomes, myotomes and segmental innervation of the limbs > Flashcards

6. Dermatomes, myotomes and segmental innervation of the limbs Flashcards

1
Q

What is a neural level?

A

The CNS is segmented throughout its length with each neural segment being known anatomincally as a neural level.

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2
Q

What does the neural tube contain?

A

Precursor cells of the CNS

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3
Q

How is the neural tube formed?

A

In the 18 day embryo, under the influence of the notochord, the neural ectoderm starts to invaginate to form the neural groove.

Gradually the neural folds approach each other in the midline where they form to form the neural tube, starting central lung and progressing towards the cephalic and caudal ends

By the 24 day stage, the cephalic (head) and caudal (tail) ends of the neural tube have closed

Whilst the formation of the neural tube occurs, from 20 days onwards, paired somites begin to appear on either side of the neural tube

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4
Q

What is a somite?

A

In the developing vertebrate embryo, somites are masses of mesoderm distributed along the two sides of the neural tube.

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5
Q

What does the dorsal somite differentiate into?

A

Dermatomyotome which is dorsal and forms the dermis (via the dermatomes) and the muscle tissue (via the myotomes)

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6
Q

What does the ventral somite differentiate into?

A

Sclerotome, which is ventral and gives rise the vertebrae and ribs.

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7
Q

Dermatomyotomes develop in association with….

A

A specific neural level of the spinal cord

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8
Q

When dermatomyocytes develop in association with a specific neural level of the spinal cord, what do they take with them and what do they form?

A

They take their nerve supply with them from the neural tube

They form spinal nerves

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9
Q

What do the skin and muscle derived from a single dermaomyocyte have in common?

A

A common spinal nerve supply

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10
Q

What is a dermatome?

A

Area of skin that is supplied by a single spinal nerve.

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11
Q

What is a myotome?

A

Group of muscles supplied by a single spinal nerve

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12
Q

What are the 3 landmarks of dermatomes?

A 
T4/T5 = nipples
T10 = umbilicus
L1 = groin
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13
Q

What is the typical structure of a neurone?

A

Cell body, dendrites and an axon

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14
Q

What does the axon arise from?

A

axon hillock

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15
Q

From which component of a neuron are most signals received?

A

Dendrites

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16
Q

What are axons covered with?

A

endoneurium

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17
Q

What does the endoneurium contain?

A

Inner sleeve of material called the glycocalyx and a mesh of collagen

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18
Q

What is the structure of a peripheral nerve?

A

Axons covered in endoneurium and bundled together to form fascicles

Each Fascicle is covered in perineurium and bundled together with blood vessels (vasa nervorum) to form a nerve

Nerve is covered by epineurium

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19
Q

why do nerves require their own blood vessels?

A

Nerves have high metabolic requirements and hence contain their own blood vessels within the epineurium

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20
Q

What does the endoneurium prevent?

A

Within the endoneurium, individual nerve fibres are surrounded by endoneurial fluid.

It prevents certain molecules from crossing from the blood into the endoneurial fluid

During nerve irritation/ injury, the amount of endoneurial. Fluid increases which can be detected with MRI

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21
Q

What is a spinal nerve?

A

a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body

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22
Q

What does the dorsal nerve root transmit?

A

Sensory fibres from the sensory receptors to the spinal cord - afferent root

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23
Q

What does the dorsal root ganglion contain?

A

Cell bodies of the sensory neurons from the periphery

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24
Q

What does the ventral nerve root carry?

A

The motor and autonomic fibres that are leaving the spinal cord - efferent root.

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25
Q

How many spinal nerves are there and how long are they?

A

There are 31 pairs of spinal nerves that are numbered according to the level of the
vertebral column from which they emerge.

They are short and exist briefly as they pass through the intervertebral foramen.

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26
Q

What is each vertebra derived from?

A

Parts of two adjacent somites

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27
Q

What structure does the spinal cord run through in each vertebra?

A

The vertebral foramen.

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28
Q

What do multiple vertebral foramina together form?

A

The spinal canal

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29
Q

How do spinal nerves leave the spinal canal?

A

Through gaps between the vertebrae known as intervertebral foramina

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30
Q

How is a vertebra structured?

A

Contains a vertebral body and a vertebral arch and a hole

Many stack together and the hole becomes a tube - the spinal canal- for the spinal cord to run through

From a side view there are gaps called intervertebral foramina for the spinal nerves to run through

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31
Q

Where does the spinal cord commence and end?

A

Commences at the inferior margin of the medulla oblongata, at the base of the brain stem

And ends at the conus medullaris at L2

32
Q

Why does the spinal cord not completely fill the vertebral canal and what does this result in?

A

Due to differential growth between the spinal cord and the vertebral column the spinal cord is shorter than the vertebral canal

Results in misalignment of spinal cord segments and their corresponding vertebrae.

33
Q

What is the region below the conus medullaris called?

A

The cauda equina (horses tail)

34
Q

What are the spinal nerves going downwards?

A 
Cervical = 8
Thoracic = 12
Lumbar = 5
Sacral = 5
Coccygeal = 1
35
Q

Where do spinal nerves exit the spinal canal relative to the vertebral body?

A

There are 8 pairs of cervical nerves but only 7 cervical vertebrae

  • C1 to C7 spinal nerves exit above the vertebral body
  • C8 exits between the C7 and T1 vertebrae (no C8 vertebra)
36
Q

How are TLSC nerves arranged

A

From T1 down, each corresponding nerve root exists inferior to the vertebral body

E.g. L3 spinal nerve root exists inferior to L3 lumbar vertebra and superior to L4 lumbar vertebra

S1-S4 exit via four pairs of sacral foramina

37
Q

From where do the S5 and first coccygeal nerve roots exit?

A

Via the sacral hiatus (posterior)

38
Q

What do each spinal nerve split into?

A

After emerging through the intervertebral foramen, each spinal nerve divides into rami

  • Dorsal/posterior rami
  • Ventral/anterior rami
  • small meningeal branch
39
Q

What does the posterior/ dorsal Ramus supply?

A

Supplies the deep muscles and skin of the dorsal trunk

40
Q

What does the anterior/ ventral Ramus supply?

A

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

41
Q

What is the size difference between ventral and dorsal rami?

A

Dorsal are much smaller

42
Q

What are meningeal branches?

A

Tiny branches that are given off by spinal nerves to re-enter the spinal canal through the intervertebral foramen

This supplies the vertebrae, ligaments of the vertebral column, blood vessels and meninges

43
Q

What are meninges?

A

membranes that cover the brain and spinal cord

44
Q

Which spinal nerves give off a white ramus communicans and where does this enter?

A

The spinal nerves from T1-T2

The white ramus communicans (preganglionic sympathetic nerve) enters the sympathetic trunk via the adjacent paravertebral ganglion

45
Q

Which spinal nerves receive a grey ramus communicans and from where?

A

All of the spinal nerves

Receive a grey ramus communicans from their adjacent paravertebral ganglion (sympathetic chain)

46
Q

What do grey rami communicans contain?

A

Post-ganglionic sympathetic nerve fibres

Composed of largely of unmyelinated neurons

47
Q

What do the posterior/ dorsal rami divide again into and what is their pattern of innervation? ??

A

Divides into medial and lateral branches

  • supply the skin and muscle of the back in a ‘tidy’ segmental manner
  • Each dorsal ramus supplies a narrow strip of muscle and skin in line with the intervertebral foramen
48
Q

What is the pattern of distribution of the ventral rami? Nerves from which spinal cord segment enter the limbs?

A

Remain segmental for the anterior trunk (T2-T12)

  • C5-T1 enter upper limbs, supply skin and muscles there
  • L1-S5 enter lower limbs, supply skin and muscles there
49
Q

What do the nerves of the ventral rami entering the limbs form?

A 
Brachial plexus (C5-T1)
Lumbosacral plexus (L1-S5)
50
Q

Why are the ventral rami from C4 and T2 adjacent to each other?

A

Because the anterior rami from C5-T1 enter the brachial plexus to supply the upper limb, they do not have a cutaneous distribution on the trunk. Hence the dermatomes for C4 and T2 are adjacent to each other on the anterior trunk

51
Q

Which parts of the limbs do the nerves of the anterior rami supply?

A

Both the ventral and dorsal surfaces of the upper and lower limbs.

52
Q

Why is the area of loss of sensation smaller than what you would imagine with single spinal nerve damage?

A

There is functional overlap between adjacent dermatomes

53
Q

When does a functional overlap not occur between adjacent dermatomes?

A

Function overlap does not extend over a axial line.

54
Q

What is an axial line?

A

Defined as the junction of two dermatomes supplied from discontinuous spinal levels.

55
Q

What axial lines are present in the limbs?

A

Anterior and posterior axial lines
- axial lines mark either the ventral or dorsal compartments of the limb

(in the lower limbs, ventral side is the posterior side of the leg and vice versa)

56
Q

What are the axial borders?

A

The axial borders are at the cephalic and caudal

margin of the limb bud and mark the borders of the anterior and posterior compartments of the limb.

57
Q

Where do the pre-axial border and post-axial border come to lie as the upper limb of the embryo rotates laterally ?

A

The pre-axial border comes to lie on the lateral side of the arm

The post-axial border comes to lie on the medial side of the arm

Hence the thumb lies laterally

58
Q

In the upper limb, which veins mark the pre-axial and post-axial border?

A

Cephalic - pre-axial (lateral)

Basilic - post-axial (medial)

59
Q

Where do the pre-axial and post-axial border come to lie as the lower limb rotates medially?

A

The pre-axial border comes to lie on the anteromedial aspect of the limb

The post-axial border on the posterior (or sometimes posterolateral) aspect of the limb

Hence the great toe lies medially

60
Q

Which veins in the lower limb mark the pre-axial border and the post-axial border?

A

Long saphenous vein - pre-axial border

Short saphenous vein - post-axial border

61
Q

What are the boundaries of the dorsal and ventral compartments marked by in the limbs?

A

Superficial veins

62
Q

What is the difference between dermatome regions and cutaneous regions?

A

Dermatome - supplied by spinal nerves
Cutaneous - supplied by peripheral nerves (originally from spinal nerves)

These regions can OVERLAP.

63
Q

What is a plexus and what emerges from the plexus?

A

Complex networks of nerves that divide and merge with other bundles of nerve fibres multiple times.

Anterior (ventral) rami of the spinal nerves
enter each plexus and, eventually, peripheral nerves emerge from the other end

64
Q

What do plexuses allow?

A

Axons from a single spinal nerve can follow multiple different routes through the plexus and therefore emerge in several different peripheral nerves.

65
Q

What do peripheral nerves contain?

A

Often contain neurons from several spinal nerves

66
Q

How do spinal nerves and peripheral nerves relate to each other?

A
  • 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

The clinical consequence of this is that the area of skin supplied by a peripheral nerve does not correspond to the dermatome map as the peripheral nerve contains fibres from multiple dermatomes.
Similarly, a dermatome may be supplied by multiple peripheral nerves because the fibres within a single spinal nerve may enter multiple different peripheral nerves.

67
Q

What are peripheral nerve territories?

A

Areas of the skin supplied by the peripheral nerves - branches of brachial plexus/ lumbosacral plexus

68
Q

Define myotome.

A

A myotome is a group of muscles supplied by a single spinal nerve.

69
Q

Define motor unit.

A

A motor unit is a motor neuron and the skeletal muscle fibres it innervates.

70
Q

describe innervation of peripheral nerves and myotomes

A

As a single spinal nerve often sends axons into multiple peripheral nerves and these peripheral nerves all innervate different muscles, a single myotome usually includes muscle fibres in several muscles, not just one muscle.

Also, because peripheral nerves often contain axons from several spinal nerves, not all of the axons within a peripheral nerve will supply the same myotome; only those axons that travelled in the same spinal nerve can supply the same myotome

71
Q

Which movements in the upper limbs are controlled by which spinal roots?

A

C5: shoulder abduction and external rotation (plus weak contribution to elbow flexion)

C6: elbow flexion/wrist extension/supination /(internal rotation of shoulder)

C7: elbow extension/wrist flexion/pronation / (weak contribution to finger flexion and extension)

C8: finger flexion / finger extension / (thumb extension / wrist ulnardeviation)

T1: finger abduction and adduction

72
Q

Which movements in the lower limbs are controlled by which spinal roots?

A

L2: hip flexion
L3: knee extension (and hip adduction)
L4: ankle dorsiflexion
L5: great toe extension (/ankle inversion / hip abduction)
S1: ankle plantar-flexion/(ankle eversion/ hip extension)
S2: (knee flexion /)great toe flexion

73
Q

What is Hilton’s Law?

A

Any nerve that innervates a muscle producing movement at a joint also innervates the joint itself and the skin overlying the insertion of these muscles.

74
Q

Give an example of Hilton’s law.

A

For example, the femoral nerve has the root values L2, 3 and 4.
It supplies the quadriceps femoris muscle group which extends the knee. It also supplies the skin overlying the insertion of the quadriceps muscles (just below the knee) via its anterior femoral cutaneous branch.

75
Q

What is the anatomical definition of neural level?

A

Level of each neural segment of the spinal cord

76
Q

What is the clinical definition of neural level?

A

Lowest level of fully intact sensation and motor function

77
Q

Give an example of how knowledge of myotomes aids assessing peripheral nerve injury.

A

Patient with a femoral nerve injury 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).

Semester 2 - MSK (35 decks)

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