Peripheral Nerves and Dermatomes

Question 1

What is the general structure of a neuron?

Answer 1

• Cell body
• Axon
• Axon terminal

Question 2

Why are the shapes of different neuron types important?

Answer 2

-Their shapes relates to their function

Question 3

What is the structure of peripheral nerves?

Answer 3

• Similar to an electric cable with an outer protective cover to help bundle together nerve cell axons.
• Peripheral nerves contain axons from two functionally distinct types of nerve cells: sensory and motor.

Question 4

What types of axons may be in peripheral nerves?

Answer 4

• Myelinated

- Unmyelinated

Question 5

What do myelinated axons have that unmyelinated don’t?

Answer 5

• Have a series of schwaan cells lined up along axon, each having a wrapped coating of myelin insulating the axon.
• Unmyelinated axons are encased by schwaan cells cytoplasm but no wrapped coating of myelin surrounding the axons.

Question 6

What is the size difference between myelinated and unmyelinated axons in micrometres?

Answer 6

• Myelinated = large (5-20)

- Unmyelinated = small (0.2-1.5)

Question 7

What is the difference in AP conduction rate between myelinated and unmyelinated axons in m/s?

Answer 7

• Myelinated = fast (5-120)

- Unmyelinated = slow (0.5-2)

Question 8

What are the differences in function between myelinated and unmyelinated axons?

Answer 8

• Myelinated = touch, vibration, motor

- Unmyelinated = pain, thermal (hot or cold)

Question 9

What can be seen at histological level if a peripheral nerve is sectioned?

Answer 9

• Separated into fasicles (bundles of axons from individual cells). Housed in tough connective tissue called epineurieum.
• Perineurium and blood vessels also visible
• If look at closer then myelinated sheaths of axons can be seen.

Question 10

What are sensory afferents classed as?

Answer 10

-Pseudounipolar

Question 11

Where are cell bodies of sensory neurons housed?

Answer 11

-In dorsal root ganglia

Question 12

What sort of receptros may sensory neurons encounter?

Answer 12

• Mechanoreceptors = mechanical deflection, touch
• Thermoreceptors = hot/cold
• Nociceptors = noxious (pain)
• Special sensory (only in cranial nerves) = vision taste, olfaction

Question 13

What matter composes the spinal cord?

Answer 13

• Grey matter (composed of lots of functionally distinct neurons)
• White matter (composed of axons that send info up to brain/down from brain)

Question 14

Are dorsal and ventral horns found in grey or white matter?

Answer 14

-Grey matter

Question 15

What are the spinal cord segments?

Answer 15

-C8
-T12
-L5
-S5
-C1
Total 31 pairs

Question 16

What does the cervical section of SC innervate?

Answer 16

-Innervates skin and muscle around the neck, chest and most of upper limb.

Question 17

What does the thoracic section of SC innervate?

Answer 17

-Mostly trunkal area, and distinct region of upper limb

Question 18

What does lumbar section of SC innervate?

Answer 18

-Mostly lower limb

Question 19

What does sacral and coccygeal section of SC innervate?

Answer 19

-Genitalia and skin around anus

Question 20

What are some of the plexuses in the body and what spinal cord sections make them up?

Answer 20

• Cervical = C1-C5
• Brachial = C5-T1
• Lumbosacral = T12-S5

Question 21

What is a dermatome?

Answer 21

-An area of skin innervated by a particular level/segment of spinal cord and its corresponding spinal nerve.

Question 22

What is a myotome?

Answer 22

-Region of muscle supplied by a particular level/segment of the spinal cord and by its corresponding spinal nerves.

Question 23

Why can dermatome and myotome maps be clinically significant?

Answer 23

• Allow docs to evaluate extent and level of spinal injury.

- Helps look at effects on voluntary, reflex movements and sensory responses. (important in ASIA scale)

Question 24

What is the pathway for voluntary movement?

Answer 24

• Starts with activation of cells in motor cortex.
• Upper motor neuron travels down to lower motor neuron (SC) at most appropriate spinal segment to innervate the distinct muscle group.

Question 25

What is the reflex pathway?

Answer 25

• No upper motor neuron component
• Muscles stretched to causes sensory neurons to generate electric signal into ventral SC to send electric signal back to muscle. (monosynaptic reflex arc)

Question 26

How can spinal reflexes be induced and give examples as to where this could be done?

Answer 26

-Tap on tendon for appropriate muscle group eg:
biceps jerk reflex
triceps jerk reflex
ankle jerk reflex
rectus abdominus reflex

Question 27

Why is the lower motor neuron important and what are some examples of problems with it?

Answer 27

• Important for voluntary and reflex movements
• Motor neuron disease
• Lesions of motor axons

Question 28

What can happen as result of upper motor neuron pathology and give examples of problems.?

Answer 28

• No voluntary movements, reflex movements still available
• Stroke
• Lesions of axons

Question 29

Describe the process of carrying out the ASIA scales tests?

Answer 29

• Test different reflexes and ability to initiate voluntary movement, allows doctor to get appreciation of level of damage.
• Test by: light touch and pin prick test on left and right sides on key sensory points at every level and assess results.

Question 30

What is a question that we should ask patients while doing ASIA scale?

Answer 30

-Ask about patients awareness of their bodies in space: proprioception.