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MSK Revision > 4 - Dermatomes and Myotomes > Flashcards

4 - Dermatomes and Myotomes Flashcards

1
Q

In development, the CNS is split into neural segments known as ……… ………..

A

Neural levels

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2
Q

The …….. ……. contains the precursor cells of the nervous system

A

Neural tube

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3
Q

On either side of the neural tube, what structures develop as primitive segments of the body?

A

Paired somites

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4
Q

What two structures does a somite differentiate into?

A
  • Sclerotome
  • Dermomyotome
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5
Q

The sclerotome forms on the ……. side of the somite

A

Ventral

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6
Q

The dermomyotome forms on the …….. side of the somite

A

Dorsal

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7
Q

What structures are formed from the sclerotome?

A

The vertebrae and ribs

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8
Q

What two structures are derived from the dermomyotome?

A

The dermatiome - dermis of the skin

The myotome - muscle tissue

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9
Q

What is the significance (in terms of nerve supply) of the dermomyotome forming both the dermis and the muscle tissue?

A
  • Dermomyotomes develop in association with their specific neural level of the spinal cord
  • The skin and muscle derived from one dermomyotome have the same spinal nerve supply
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10
Q

What is a dermatome?

A

An area of skin supplied by a single spinal nerve

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11
Q

Spinal nerves are mixed nerves, what does this mean?

A

They contain motor, sensory and autonomic signals between the body and the spinal cord

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12
Q

What type of fibre is found in the dorsal nerve root?

A
  • Sensory fibres
  • Travel afferently from sensory receptors to the spinal cord
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13
Q

What type of fibre is found in the ventral nerve root?

A
  • Motor and autonomic fibres
  • Efferent root (leaving the spinal cord)
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14
Q

How many pairs of spinal nerves are there?

A

31

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15
Q

Spinal nerves are short, they only exist briefly as they pass through what structure?

A

The intervertebral foramen

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16
Q

Describe the development of structures from the sclerotome.

A
  • The sclerotome differentiates into the vertebrae
    • Each vertebra is derived from parts of two adjacent somites
  • In the thoracic region it also forms the ribs
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17
Q

The spinal cord runs through holes in the vertebrae called the ……. ………. which make up the spinal canal.

A

Vertebral foramina

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18
Q

Where does the spinal cord commence and terminate?

A
  • Commences at the inferior margin of the medulla oblongata
  • Ends at the conus medullaris at L1/L2
  • Below the conus medullaris is the cauda equina
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19
Q

Why are most of the spinal cord segments not directly aligned with their corresponding vertebrae?

A
  • The spinal cord grows at a slower rate in development than the vertebral canal, so the spinal cord is shorter than the spine
  • The spinal cord ends at L1/L2, all the lower spinal nerves project to their corresponding vertebrae in the cauda equina
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20
Q

There are eight cervical spinal roots but only seven cervical vertebrae. What does this mean for relationship between the spinal nerves and vertebrae in the cervical spine?

A
  • The cervical spinal roots emerge above their corresponding vertebral bodies
  • The exception is the 8th spinal root which emerges between C7 and T1
  • Inferior to T1, the spinal nerve roots emerge inferior to the vertebrae
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21
Q

Spinal nerves divide into two rami. What structures are supplied by the dorsal (posterior) ramus?

A

Deep muscles and skin of the dorsal trunk

22
Q

Spinal nerves divide into two rami. Which structures are supplied by the ventral (anterior) ramus?

A
  • Muscles and skin of the lateral and ventral trunk and upper and lower limbs
  • The ventral rami are much larger than the dorsal rami
23
Q

Each spinal nerve gives off a tiny ………… branch that re-enters the spinal canal. What does this supply?

A
  • Meningeal branch
  • Supplies the vertebrae, ligaments of the vertebral column, blood vessels and meninges
24
Q

What specialised structures branch from the spinal nerves between T1 and L2 and what are they composed of?

A
  • The sympathetic rami that allow the formation of the paravertebral chain
  • The spinal nerve gives off a white ramus comminicans (preganglionic - myelinated) that enters the paravertebral ganglion, these neurones synapse with postganglionic (unmyelinated) neurones and leave the ganglion via the grey ramus communicans
25
Q

The posterior (dorsal) rami divide into …….. and ……… branches. They supply the skin of the back in a segmental manner.

A

Medial and lateral

26
Q

How does the dermatome distribution of the anterior (ventral) rami differ from the posterior (dorsal) rami?

A
  • The distribution remains segmental for the trunk
  • The anterior rami of C5-T1 (brachial plexus) and L1-S5 (lumbosacral plexus) enter the limb buds
    • Means the dermatomes for C4 and T2 are adjacent on the anterior trunk
27
Q

The ………. rami supply both the ventral and dorsal skin of the upper and lower limbs

A

Anterior (ventral)

28
Q

When a single spinal nerve is damage, the area of anaesthesia is smaller than the corresponding dermatome. Why is this?

A

There is functional overlap between adjacent dermatomes (not axross axial lines).

29
Q

What is an axial line? How do these affect loss of sensation when a single spinal nerve is damaged?

A
  • Axial line = the junction of two dermatomes from discontinuous spinal levels
  • Functional overlap (that would normally make the area of anaesthesia smaller) does not extend across axial lines
30
Q

Describe the rotation of the upper and lower limb buds during development.

A
  • The developing upper and lower limbs rotate in opposite directions
  • The upper limb rotates externally (laterally) by 90 degrees - elbow points posteriorly
  • The lower limb rotates internally (medially) by almost 90 degrees - knee points forwards
31
Q

What are the pre-axial and post-axial borders of a limb and where are they found?

A
  • Pre-axial border - cephalic side of a limb bud
  • Post-axial border - caudal end of a limb bud
  • Mark the borders of the anterior and posterior compartments of the limb
32
Q

In the upper limb, the ……. vein marks the pre-axial border and the ……… vein marks the post-axial border.

A
  • Pre-axial border - cephalic vein
  • Post-axial border - basilic vein
33
Q

In the lower limb, the ……. vein marks the pre-axial border and the ……… vein marks the post-axial border.

A
  • Pre-axial border - great saphenous vein
  • Post-axial border - small saphenous vein
34
Q

Describe the relationship between the spinal nerves and peripheral nerves through the brachial plexus.

A
  • The axons from a single spinal nerve follow multiple routes through the plexus and emerge in several different peripheral nerves
  • E.g. the median nerve contains axons from C6-T1
35
Q

Describe the relationship between the limb dermatomes and peripheral nerve territories.

A
  • Peripheral nerves contain fibres from more than one spinal nerve, so the peripheral nerve supplies skin in multiple dermatomes
  • A dermatome is supplied by multiple peripheral nerves because the fibres in a single spinal nerve enter multiple peripheral nerves
36
Q

What is a myotome?

A

A group of muscle fibres supplied by a single spinal nerve

37
Q

What is a motor unit?

A

A single motor neurone and the skeletal muscle fibres it innervates

38
Q

A single …….. nerve sends axons to multiple ………. nerves. These each innervate different muscles, a single myotome usually includes fibres of several muscles.

A
  • Spinal
  • Peripheral
39
Q

Peripheral nerves contain axons from multiple ……….. nerves, so not all the axons within a peripheral nerve supply the same …………….. .

A
  • Spinal
  • Myotome

Only axons from the same spinal nerve can supply the same myotome.

40
Q

What spinal nerve root is responsible for shoulder abduction and external rotation of the upper limb?

A

C5

41
Q

What spinal nerve root is responsible for elbow flexion, wrist extension and supination?

A

C6

42
Q

What spinal nerve root is responsible for elbow extension, wrist flexion and pronation?

A

C7

43
Q

What spinal nerve root is responsible for finger flexion and extension?

A

C8

44
Q

What spinal nerve root is responsible for finger abduction and adduction?

A

T1

45
Q

What spinal nerve root is responsible for hip flexion?

A

L2

46
Q

What spinal nerve root is responsible for knee extension?

A

L3

47
Q

What spinal nerve root is responsible for ankle dorsiflexion?

A

L4

48
Q

What spinal nerve root is responsible for great toe extension?

A

L5

49
Q

What spinal nerve root is responsible for ankle plantarflexion?

A

S1

50
Q

What spinal nerve root is responsible for great toe flexion?

A

S2

51
Q

In the context of classification of a spinal cord injury, what is the patient’s neural level?

A

The lowest spinal level of fully intact sensation and motor function

MSK Revision (6 decks)

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