Lecture 42: Neuromuscular junction

Question 1

What does the motor unit consist of?

Answer 1

• LMN in ant. horn, ventral grey matter
• Its axon
• NMJ
• All connected muscle fibers (of the same type)

Question 2

What are the two patterns of motor unit recruitment?

Answer 2

Spatial recruitment - Activate more motor units to produce greater force

Temporal recruitment - Hz of activation of muscle fiber contractions

Question 3

What is wallerian degeneration?

Answer 3

Degeneration of the segment of an axon distal to nerve injury that destroys its continuity

Begins a few days after injury (some lag) and is completed after 12-14 days (NB for EMG recordings)

Question 4

What are fibrillation potentials of the muscles?

Answer 4

Occur in any muscle fibre that is not innervated

• Lost their innervation (dennervated)
• Muscle has been damaged (necrosis, fiber splitting)

Question 5

What are fasiculation potentials of the muscle?

Answer 5

• Random discharge of a motor unit
• Spontaneous firing maybe generated anywhere along LMN
• Can occur in healthy ind. and chornic neurogenic disorders

Question 6

What is the hallmark characteristic of diseases affecting the motor unit?

Answer 6

WEAKNESS

Question 7

How can diseases affecting the motor unit be described?

Answer 7

• Acute/subacute or chronic (inherited/genetic)
• +/- sensory symptoms (if nerve root, plexus, peripheral nerve)
• Muscle atrophy
• Hyporeflexia or areflexia
• Fasiculations and fibrillations (only on EMG)
• Fluctuations, fatigability
• No central (UPN) signs

Question 8

Write some notes on motor neuron disease:

Answer 8

• Most common form is amytropic lateral sclerosis
• PROGRESSIVE WEAKNESS (W/O sensory symptoms)
• Mixture of UPN and LMN signs ie
• > Fasiculations, fibrillations, atrophy
• > Spasticity, hyperreflexia

Eventually, resp. insufficiency

(Impacts the neural pathway)

Question 9

Write some notes on Guillian Barre Syndrome:

Answer 9

• Acute inflammatory demyelinating polyradiculoneuropathy
• (possibly molecular mimicry, unknown)
• Immune response that eventually targets own nerves.
• > Days/weeks after anecdotal illnesss
• Progressive weakness +/- sensory impairment
• Areflexia
• Eventually reaches nadir, followed by slow, gradual movements

(Impacts the axon)

Question 10

Write some notes on myasthenia gravis:

Answer 10

• Autoimmune, AChR autoantibodies
• Occular symptoms +/- generalised weakness are the common presentation
• Fluctuating symptoms, fatigable element

(Impacts the NMJ receptors)

Question 11

How can MG be treated?

Answer 11

ACh esterase inhibitor, steroids and other immunotherapy

Question 12

What is necrotizing autoimmune myopathy?

Answer 12

• Autoimmune, anti-HMG-CoA reductase antibodies
• Associated with statin use. (increases HMG-CoA reductase levels and HMG-CoA reductase protein)
• These remain high even after statin discontinued, as does autoantibody

Question 13

What are the symptoms and treatment of necrotizing autoimmune myopathy?

Answer 13

• Subacute, progressive weakness, proximal muscles more impacted (beevor sign, supine - lift head off table and belly button lifts up towards head b/c lack of stabliizers)
• Requires treatment with immunotherapy i.e steroids because suspending statins does not work.

(Impacts myofibrils)

Question 14

What is FSHD? What causes it?

Answer 14

Fascioscapulohumeral muscular dystrophy

Overexpression of DUX4 gene due to hypomythalation of chromosome 4q
= Gain of function mutation causing disease

Muscle weakness and wasting