Peripheral and Central Neuropathy

Question 1

What is the normal peripheral nerve?

Answer 1

Collection of nerve fibres and supporting tissue organized into discrete anatomic structure - the nerve

Question 2

What are the supporting elements of the normal peripheral nerve?

Answer 2

Schwann cells - produce myelin
Fibroblasts - produce collagen
Small blood vessels

Question 3

What is the structural unit of a nerve and what does it consist of?

Answer 3

Nerve fibre - single axon arising from a nerve cell body

Question 4

What is the endoneurium?

Answer 4

Loose connective tissue layer surrounding nerve fibres

Schwann cells also present

Question 5

What is the perineurium?

Answer 5

Dense layer of collagenous tissue surrounding bundles or fascicles (nerve fibres collected into these structures)

Question 6

What is the epineurium?

Answer 6

Binds the fascicles into a nerve

Question 7

What is present throughout the layers of a nerve?

Answer 7

Small vessels

Question 8

How great in diameter do nerve axons need to be in order to be myelinated?

Answer 8

Greater than 3 microns

Question 9

What is important to note about non-myelinated axons and Schwann cells?

Answer 9

Non-myelinated axons form an invagination in the cytoplasm of a Schwann cell such that the axon becomes surrounded by the Schwann cell’s cytoplasm

Question 10

What is the mesaxon?

Answer 10

The invagination in the cell’s cytoplasm fuses and this forms the mesaxon

Question 11

How does myelination of an axon occur?

Answer 11

Begins with formation of mesaxon
Myelination is a continuation of this step - mesaxon rotates around the axon multiple times leaving the axon wrapped in spiral layers of Schwann cell cytoplasm and cell membrane
Cytoplasm between the layers of Schwann cell membrane is extruded & membrane layers fuse forming the myelin sheath

Question 12

What is the node of Ranvier?

Answer 12

Discontinuity in the myelin layer between two adjacent Schwann cells - reveals a short segment of the axon

Question 13

What is the function of myelination?

Answer 13

Increases axon conduction velocity because it allows the action potential to travel by jumping from one Node of Ranvier to the next
= saltatory conduction

Question 14

What does demyelination lead to?

Answer 14

Decrease in nerve conduction velocity

Question 15

What is peripheral neuropathy?

Answer 15

Diseases of the peripheral nerves including sensory, motor, autonomic or mixed nerves, dorsal or ventral nerve roots, dorsal root ganglia & cranial nerves (apart from cranial nerves I/II = CNS)
Results from injury to the myelin sheath, the axon or the nerve cell body

Question 16

Name the types of injury to peripheral nerves.

Answer 16

Inflammatory/immunologic
Infectious
Vascular insufficiency 
Metabolic
Hereditary 
Toxic
Traumatic

Question 17

What is mononeuropathy?

Answer 17

Peripheral neuropathy of a single nerve

Question 18

What is mononeuropathy multiplex?

Answer 18

Peripheral neuropathy of a single nerve in multiple sites

Question 19

What is polyneuropathy?

Answer 19

Peripheral neuropathy of multiple nerves

Question 20

What usually causes mononeuropathies?

Answer 20

Entrapment/compression e.g. Carpal Tunnel Syndrome

Trauma

Question 21

What usually causes Mononeuropathy Multiplex?

Answer 21

Inflammatory cause, in particular vasculitis e.g. polyarteritis nodosa (PAN), systemic lupus erythematosis (SLE), hypersensitivity vasculitis

Question 22

Name some causes of polyneuropathy.

Answer 22

Diabetes mellitus
Toxins (chemotherapeutic agents)
Nutritional neuropathies (vit B12 deficiency)
Inherited neuropathies ( Charcot-Marie-Tooth disease)
Immune mediated (inflammatory) neuropathies e.g. Guillain-Barre Syndrome

Question 23

What are the pathological processes behind peripheral neuropathy?

Answer 23

Demyelination without axonal damage (segmental demyelination)
Axonal degeneration
Axonal regeneration

Question 24

Why might segmental demyelination occur?

Answer 24

Schwann cell dysfunction

Damage to myelin sheath itself

Question 25

What happens in segmental demyelination?

Answer 25

Damaged myelin is phagocytosed by Schwann cells and then macrophages
Demyelinated axon segments then become a stimulus for remyelination
Precursor cells in the endoneurium can replaced injured Schwann cells which proliferate and remyelinate the axons

Question 26

What is the difference between remyelinated axons and normal myelinated axons?

Answer 26

Remyelinated segments are shorter & thinner myelin sheath

Question 27

What causes axon degeneration?

Answer 27

Death of the nerve cell body or injury to the axon in a peripheral nerve

Question 28

When does axon regeneration not occur for a degenerated axon?

Answer 28

When there is loss of either motor or sensory neurons