Autoimmune Neuromuscular Junction Disorder: L20-21
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Questions and Answers

What is the role of autoantibodies to MuSK in myasthenia gravis?

  • They enhance the clustering of AChRs at the neuromuscular junction.
  • They disrupt the synthesis of acetylcholine.
  • They enhance the release of calcium from the presynaptic membrane.
  • They interfere with the clustering of AChRs at the neuromuscular junction. (correct)

    • Which condition is associated with autoantibodies to presynaptic voltage-gated calcium channels (VGCC)?

  • Myasthenia Gravis
  • Neuromyotonia
  • Lambert-Eaton myasthenic syndrome (correct)
  • Multiple sclerosis

    • What effect do autoantibodies that bind to potassium channels cause in autoimmune neuromyotonia?

  • Blockage of acetylcholine synthesis.
  • Continuous or hyper-excitability of the motor nerve. (correct)
  • Decreased endplate potential at the postsynaptic membrane.
  • Increased calcium release at the neuromuscular junction.

    • What distinguishes depolarizing blockers from nondepolarizing blockers in the context of the neuromuscular junction?

    <p>Depolarizing blockers lead to desensitization of the end plate.</p> Signup and view all the answers

    What is the consequence of autoantibodies to presynaptic voltage-gated calcium channels (VGCC) at the neuromuscular junction?

    <p>Disruption of acetylcholine release leading to transmission failure.</p> Signup and view all the answers

    What triggers the release of neurotransmitter at the neuromuscular junction?

    <p>Influx of Ca2+</p> Signup and view all the answers

    Which type of receptor does acetylcholine bind to on the muscle fiber at the motor end-plate?

    <p>Nicotinic acetylcholine receptor</p> Signup and view all the answers

    What is the primary effect of the autoantibodies in myasthenia gravis?

    <p>Blockage of acetylcholine binding</p> Signup and view all the answers

    What happens to unbound acetylcholine in the synaptic cleft?

    <p>It is hydrolyzed by acetylcholinesterase</p> Signup and view all the answers

    What type of diseases are associated with the neuromuscular junction?

    <p>Neuromuscular junction disorders</p> Signup and view all the answers

    What ion influx causes depolarization of the muscle fiber after acetylcholine binds to its receptor?

    <p>Sodium ions (Na+)</p> Signup and view all the answers

    What is the primary mechanism of action of curare?

    <p>Competes with acetylcholine for binding to nicotinic receptors</p> Signup and view all the answers

    Which subunits constitute the adult form of the nicotinic AChR in muscles?

    <p>α1, β1, δ, and ε</p> Signup and view all the answers

    Which agent is classified as a non-depolarizing muscle relaxant?

    <p>Pancuronium</p> Signup and view all the answers

    What results from the influx of sodium ions into the muscle fiber?

    <p>Muscle fiber depolarization</p> Signup and view all the answers

    What symptom is most likely to occur first when paralysis from NMJ blocking agents sets in?

    <p>Finger paralysis</p> Signup and view all the answers

    What is one of the effects of competitive non-depolarizing agents like tubocurarine?

    <p>Flaccid paralysis</p> Signup and view all the answers

    Which of the following agents has less histamine release than curare?

    <p>Pancuronium</p> Signup and view all the answers

    What effect do acetylcholinesterase inhibitors have on acetylcholine levels?

    <p>Increase its availability</p> Signup and view all the answers

    Which of the following is NOT a use for acetylcholinesterase inhibitors?

    <p>Increasing muscle tone</p> Signup and view all the answers

    What effect does a-bungarotoxin have at the neuromuscular junction?

    <p>Causes a postsynaptic block</p> Signup and view all the answers

    What is the primary application of Edrophonium in a clinical setting?

    <p>Diagnosis of myasthenia gravis</p> Signup and view all the answers

    What occurs during a myasthenic crisis?

    <p>Insufficient neuromuscular stimulation</p> Signup and view all the answers

    What distinguishes Physostigmine from Edrophonium?

    <p>It is used for treating glaucoma</p> Signup and view all the answers

    Which of the following statements about the cholinergic hypothesis in dementia is true?

    <p>Enhancing central cholinergic function can improve cognition</p> Signup and view all the answers

    What is the mechanism of action of Botox?

    <p>It cleaves specific SNARE proteins</p> Signup and view all the answers

    Which AChE inhibitor can produce a pseudo-irreversible effect?

    <p>Rivastigmine</p> Signup and view all the answers

    What impact does Edrophonium have during a cholinergic crisis?

    <p>Induces a depolarizing block</p> Signup and view all the answers

    Which AChE inhibitor was introduced first for the treatment of dementia?

    <p>Tacrine</p> Signup and view all the answers

    What is a potential cause of cholinergic crisis?

    <p>Nerve gas exposure</p> Signup and view all the answers

    Which symptom is NOT associated with cholinergic crisis?

    <p>Tachycardia</p> Signup and view all the answers

    What is the primary action of atropine in treating cholinergic toxicity?

    <p>Blocking acetylcholine receptors</p> Signup and view all the answers

    What type of inhibitors are organophosphates classified as?

    <p>Irreversible acetylcholinesterase inhibitors</p> Signup and view all the answers

    Which of the following effects is typically seen with nerve gas poisoning?

    <p>Respiratory failure</p> Signup and view all the answers

    What is the function of oximes in the context of cholinergic crisis treatment?

    <p>Regenerating acetylcholinesterase activity</p> Signup and view all the answers

    Which of the following medications is a reversible anticholinesterase?

    <p>Neostigmine</p> Signup and view all the answers

    Which symptom occurs due to the overstimulation of muscles, organs, and glands in cholinergic crisis?

    <p>Areflexia</p> Signup and view all the answers

    Study Notes

    Learning Outcomes

    • Students should be able to provide examples of drugs that impact the neuromuscular junction
    • Students should be able to describe the effects of these drugs
    • Students should be able to describe the modes of action of these drugs

    The Neuromuscular Junction (NMJ)

    • The NMJ is a specialized form of synaptic transmission
    • It facilitates communication between neurons and skeletal muscle

    Peripheral Nervous System

    • Autonomic NS:
      • Regulates smooth muscle activity
      • Regulates exocrine gland activity
      • Regulates some endocrine activity
      • Regulates cardiac tissue activity
      • Regulates metabolic activities
      • Functions involuntarily
      • Regulated by the brain stem centers
    • Somatic NS:
      • Activates skeletal muscle contraction
      • Controls voluntary body movements
      • Regulated by corticospinal tracts and spinal reflexes

    Acetylcholine Synthesis and Degradation

    • Acetylcholine is synthesized via the enzyme choline acetyltransferase
    • Acetylcholine is degraded by acetylcholinesterase (AChE)
    • The byproducts are choline and acetate

    Nicotinic Receptors

    • Nicotinic receptors are ligand-gated ion channels
    • Binding of two acetylcholine molecules is required for activation
    • Nicotinic receptors are composed of five subunits (a pentamer)
    • Different subunit types exist (α, β, γ, δ, and ε)
    • NM receptors only contain a1 and β1 subtypes
    • NN receptors contain a2 to a10 and β2 to β4 subtypes

    Acetylcholine (ACh) and Receptor

    • Acetylcholine binds to a specific binding site
    • The acetylcholine binding site is located on an alpha helix
    • This binding prompts a conformational change
    • The structural change can open a gate allowing ions to pass through the receptor

    Major Events in Neuromuscular Transmission

    • Motor neuron depolarization triggers an action potential that travels to the NMJ
    • Depolarization of the axon terminal releases calcium ions (Ca2+)
    • Calcium triggers the release of acetylcholine (ACh)
    • Acetylcholine diffuses to the post-synaptic ACh receptor
    • Binding of ACh to the ACh receptors opens sodium channels causing sodium influx
    • Depolarization of the sarcolemma travels through t-tubules
    • Depolarization releases calcium from the sarcoplasmic reticulum - muscle contraction
    • Unbound acetylcholine is hydrolyzed (inactivated) by acetylcholinesterase (AChE)

    Neuromuscular Junction Disorders

    • Myasthenia gravis:
      • Autoantibodies target the nicotinic ACh receptors on the neuromuscular junction
      • This causes a reduction in ACh receptors function
      • Symptoms include muscle weakness
    • Lambert-Eaton myasthenic syndrome:
      • Autoantibodies target the presynaptic voltage-gated calcium channels (VGCC)
      • Less ACh is released at the NMJ, thus less muscle contraction
      • Symptoms include muscle weakness and fatigue
    • Neuromyotonia (Isaac's syndrome):
      • Autoantibodies target the presynaptic voltage-gated potassium channels (VGKCs)
      • Continuous and repeated muscle contractions can occur

    Drugs Acting on the NMJ

    • Competitive (non-depolarizing) Blocking Agents (e.g., tubocurarine):
      • Compete with acetylcholine for binding to receptor sites
    • Depolarizing Blocking Agents (e.g., succinylcholine):
      • Bind to receptors and induce sustained depolarization
    • Drugs Affecting Axonal ACh (e.g., hemicholinium):
      • Interfere with the uptake of choline, a precursor for ACh synthesis
    • Drugs Affecting ACh Storage (e.g., vesamicol):
      • Prevents the storage of ACh in vesicles

    Cholinesterase Inhibitors

    • Used to treat myasthenia gravis, and reverse non-depolarizing muscle paralysis
    • Increase the availability of ACh, partially overcoming decreased receptor availability
    • Some examples include Edrophonium, Neostigmine, Physostigmine

    Nerve Gas Poisoning

    • Nerve gas poisoning results from the overstimulation of muscles, organs, and glands
    • Symptoms include ataxia, slurred speech, areflexia, generalized convulsions, and respiratory failure
    • Atropine is an antidote that binds to ACh receptors and prevents acetylcholine binding

    Organophosphates

    • These are irreversible cholinesterase inhibitors
    • The phosphorous in organophosphates covalently binds to a serine hydroxyl on cholinesterase enzymes

    Botox (Botulinum Toxin)

    • Botox inhibits neurotransmitter release
    • Results in muscle paralysis
    • Used for medical, cosmetic reasons

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    Neuromuscular Junction - BIOL2048/2049 PDF

    Description

    This quiz explores the role of autoantibodies in various neuromuscular junction disorders, including myasthenia gravis and autoimmune neuromyotonia. Test your knowledge on how these autoantibodies affect neurotransmitter release and muscle response. Understand the differences in drug mechanisms and the involvement of various receptors.

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