5.6 Disorders of the Peripheral Nervous System Flashcards
A bundle of nerve fibres is called a…
Nerve fascicle/bundle
Outline the components of a peripheral nerve (be careful with the wording…)
- Axon
- Schwann cell
- Endodeurium
- Perineurium
- Epineurium
What is the function of Schwann cells?
- Myelinates axons & supports unmyelinated PNS axons
- Central role in axon development/maintenance
Where is the endoneurium. What does it do?
- Surrounds each axons and Schwann cell
- Rich in components that promote axonal growth
What is the name of the CT sheath that surrounds a nerve fascicle?
Perineurium.
What is the name of the dense CT that surrounds all nerve fascicles in a peripheral nerve?
Epineurium.
What symptoms might we see with a loss of motor function of a peripheral nerve?
- Weakness
- Areflexia
- Atrophy (if chronic)
What symptoms might we see with a loss of sensory function of a peripheral nerve?
Loss of somatosensation (e.g. pain, temp, numbness)
Describe some “+” symptoms of peripheral neuropathy
Hyperexcitability (burning, tingling/”pins and needles”)
Describe some “-“ symptoms of peripheral neuropathy
- Weakness of muscle
- Loss of somatosensation
What is the name of the category of peripheral nerve disorders that affect a single nerve?
Mononeuropathy
What is the name of the category of peripheral nerve disorders that affect multiple nerves, usually symmetrically? Are these deficits distal or proximal?
Polyneuropathy. Usually distal deficits.
Describe mononeuritis multiplex
- Individual nerves affected
- In a haphazard fashion (asymmetrical)
What is polyradiculopathy? Does it cause proximal or distal deficits? Are symptoms symmetric or asymmetric?
- Peripheral neuropathy affecting multiple nerves and nerve roots
- Proximal AND distal deficits (makes sense)
- Usually symmetric symptoms
List the three targets of peripheral nerve disorders
- Cell body
- Axon
- Neuromuscular junction
Give an example of a peripheral axonopathy that can present as either demyelinating or axonal.
Guillian-barre syndrome
What components of an axon are damaged in a peripheral demyelinating axonopathy?
- Schwann cells
- Myelin sheath
Why might synchrony be lost within a nerve fibre as a result of demyelinating disease?
- Saltatory conduction is no longer uniform
- Impulses arrive at different times to the end of their respective axons
- :(
List the two kinds of peripheral axonopathy
- Demyelinating
- Axonal
Give an example of a peripheral neuropathy that affects neuromuscular junctions
Myasthenia Gravis
Describe the proximal changes that occur during peripheral nerve degeneration
- Cell body swells
- Nucleus moves to periphery
- Chromatolysis (Nissl bodies; less secretory)
Describe the distal changes that occur during peripheral nerve degeneration
- Denervation (pulling away from effector cell)
- Dying back phenomenon
- Myelin breakdown
- Macrophages clear
- Effector cell atrophy
In full, describe the damaging and regrowth of a peripheral nerve (successful)
- Nerve is damaged
- Denervation, dying back, chromatolysis
- Eventually, neuron kicks itself back into gear:
- Regrows Nissl bodies (Secretory function), and proximal axons begins sprouting in all directions
- Wise Schwann cells guide young axon into endoneurium tube
- Great success
How long does it take for a peripheral nerve to regrow?
~3 months
Describe two ways in which peripheral nerve regeneration can go wrong
- New axon is not properly guided into endoneurium tube (partial recovery of sensory/motor function)
- Neuroma (tangling), may lead to phantom pain
What factors affect the peripheral nerve repair outcomes?
- Proximity to soma (closer = worse)
- Restoring contact with effector
- Type of injury
Describe the different kinds of traumatic peripheral nerve injuries
- Neuropraxia (transiently blocked nerve conduction; honeymooners’, ischaemia, cold). All components remain intact.
- Axonotmesis (Axon & myelin locally damaged but nerve sheath is partially/fully intact)
- Neurotmesis (axon & myelin & nerve sheath all damaged)
True or false: axonotmesis has a longer term recovery than neuropraxia
- True
- Axon and myelin must regenerate, so takes longer (but, of course, peri and epineurium are retained)
Axonotmesis, neurotmesis and neuropraxia are three types of nerve injury. Rank them from mildest to most severe.
Neuropraxia (mild)
Axonotmesis (mild-moderate)
Neurotmesis (severe)
What is the prognosis of axonotmesis?
- Neurotransmission interrupted until target innervation is restored
- Longer term
What is the prognosis of neurotmesis?
Long term partial/complete loss of motor/sensory functions.
Give some examples of injuries that can cause axonotmesis
Nerve crush, stretch injury
Give some examples of injuries that can cause neurotmesis
Nerve transection (sharp injuries, neurotoxins)
What IS motor neuron disease? What ISN’T it?
- Gradually progressing degeneration of motor neurons
- Minimal or no cognitive dysfunction
- Minimal or no sensory dysfunction
What are typically the first and last symptoms of motor neuron disease?
- First: weakness in hands or legs
- Last: failure of respiratory muscles
What is the typical age range of MND diagnosis?
50-60 years old
Death typically occurs within _ years of diagnosis of MND
2 years
Describe two clinical findings/features of motor neuron disease
- Amyotrophy (muscle wasting due to lack of LMN input)
- Lateral sclerosis (loss of lateral corticospinal tract)
Which groups of motor neurons are spared of MND?
- Autonomic nerves (fully motor, e.g. sphincter control)
- Extraocular muscles
What are the two main types of MND, and their relative proportion of all cases?
- Genetic (10%)
- Sporadic (90%)
Which of sporadic/genetic MND affects older/younger people? At what age is genetic MND typically diagnosed.
Sporadic: typically late-age
Genetic: younger (20-80); avg 45 years old
What occurs in spinal muscular atrophy? What is the result of this?
- Recessive mutation: improper RNA movement and splicing
- Progressive loss of LMNs:
- Weakness, flaccid paralysis, atrophy, areflexia, hypotonia
Name some different hypotheses for the cause of MND
- Genetic cause
- Excitotoxicity
- Free radicals (SOD-1)
- Impaired axonal transport
- Lack of neurotrophic support
List some argument for and against the genetic theory of MND
- Mutations have been linked to various forms of MND (such as SMA)
- Only a minority (10%) of MND cases have a genetic/familial cause
Describe the theory of SOD-1 specific MND
- SOD-1 is an enzyme
- The theory was that, if SOD-1 (pacman) is dysfunctional, he cannot gobble the free radicals (ghosts)
Outline the excitotoxicity hypothesis of MND
- Glial glutamate transporters (e.g. astrocytes) lost
- More glutamate remains in synapse
- Nerve cells may die from glutamate poisoning
How do we categorise the clinical features of MND? What are they?
Diminished function:
- Weakness
- Atrophy
Flaccid paralysis
- Hyporreflexia
- Hypotonia
Excessive function:
- Pathological fasciculations
- Hyperreflexia
- Hypertonia
- Spastic paralysis
LMN lesions cause “diminished function” symptoms. What is the exception?
Pathological fasciculations: spontaneous, grossly visible, contractions of small muscle groups
DO we see weakness in UMN lesions, LMN lesions, or both?
Both
Do we see fasciculations in UMN lesions, LMN lesions, or both?
LMN
Do we see atrophy in UMN lesions, LMN lesions, or both?
Both (although typically more prominent in LMN due to denervation)
Effect of UMN vs LMN lesions on reflexes
UMN: hyperreflexia
LMN: Hyporeflexia
Effect of UMN vs LMN lesions on tone
UMN: Hypertonia
LMN: Hypotonia
