MD137 Skeletal Muscle: Structure & Function II
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Questions and Answers

What role does acetylcholine play in muscle contraction?

  • It activates the Ca2+-ATPase pump for muscle relaxation.
  • It stimulates the release of calcium from the sarcoplasmic reticulum.
  • It directly binds to troponin C to initiate contraction.
  • It produces end plate potentials leading to action potentials. (correct)

Which physiological process occurs first during excitation-contraction coupling?

  • Acetylcholine is released from the motor neuron. (correct)
  • Cross-bridge formation occurs.
  • Calcium is released from the sarcoplasmic reticulum.
  • Action potentials cease.

What percentage of ATP is used by the Ca2+-ATPase pump during muscle relaxation?

  • 70%
  • 30% (correct)
  • 50%
  • 20%

What is the primary function of dihydropyridine receptors (DHPR) during muscle contraction?

<p>Change shape in response to action potentials. (A)</p> Signup and view all the answers

During muscle relaxation, what occurs once calcium levels decrease in the cytosol?

<p>No more calcium is available to bind to troponin C. (B)</p> Signup and view all the answers

What initiates force development in muscle fibers during contraction?

<p>The binding of calcium to troponin C. (C)</p> Signup and view all the answers

What is the temporal sequence of events in excitation-contraction coupling?

<p>Cross-bridge cycling follows calcium release. (B)</p> Signup and view all the answers

What is the main function of the sarcoplasmic reticulum in muscle cells?

<p>To store and release calcium ions. (A)</p> Signup and view all the answers

What is the main energy requirement for cross-bridge cycling during muscle contraction?

<p>70% of ATP. (D)</p> Signup and view all the answers

What happens to calcium levels during muscle relaxation?

<p>Calcium is pumped back into the sarcoplasmic reticulum. (D)</p> Signup and view all the answers

What is the primary effect of Acetycholinesterase inhibitors in the treatment of Myasthenia gravis?

<p>Decrease degradation of acetylcholine (B)</p> Signup and view all the answers

Which substance is responsible for initiating muscle contractions by diffusing out of the Sarcoplasmic Reticulum?

<p>Calcium ions (Ca2+) (A)</p> Signup and view all the answers

What is the primary mechanism of action for Botulism toxin (BOTOX) in muscle contraction?

<p>Inhibits acetylcholine release from the nerve terminal (A)</p> Signup and view all the answers

In myasthenia gravis, the immune system primarily targets which aspect of neuromuscular function?

<p>Acetycholine receptors at the neuromuscular junction (D)</p> Signup and view all the answers

What is the pathway through which action potentials can travel to muscle fibers?

<p>Transverse Tubules (D)</p> Signup and view all the answers

Which treatment method besides Acetycholinesterase inhibitors is mentioned for myasthenia gravis?

<p>Plasmapheresis (B)</p> Signup and view all the answers

Which mechanism ensures that calcium ions are returned to the sarcoplasmic reticulum after muscle contraction?

<p>Ca2+-ATPase pump (B)</p> Signup and view all the answers

What is the incidence of myasthenia gravis in the general population?

<p>1 in 5000 (B)</p> Signup and view all the answers

What is the role of the Ryanodine receptor in muscle contraction?

<p>Releases calcium from the sarcoplasmic reticulum (D)</p> Signup and view all the answers

Which condition is treated using curare, blocking acetylcholine receptors?

<p>Poisoning by snake venom (C)</p> Signup and view all the answers

What is the primary physiological mechanism of excitation contraction coupling in skeletal muscle?

<p>The initiation of intracellular Ca2+ release following an electrical discharge (B)</p> Signup and view all the answers

Which component of the neuromuscular junction directly interacts with the motor neuron?

<p>Motor end plate (C)</p> Signup and view all the answers

What occurs when depolarization at the neuromuscular junction fails to reach the threshold?

<p>A graded potential occurs without muscle activation (A)</p> Signup and view all the answers

What ion's influx is critical for muscle contraction following electrical activation?

<p>Ca2+ (C)</p> Signup and view all the answers

What is the role of excitatory postsynaptic potentials (EPSP) at the neuromuscular junction?

<p>To facilitate the activation of sodium ion channels (C)</p> Signup and view all the answers

Which part of the muscle fiber sarcolemma is specifically involved in the excitation at the neuromuscular junction?

<p>Motor end plate (D)</p> Signup and view all the answers

What is the result of a successful excitation contraction coupling sequence?

<p>Action potential leading to muscle action (C)</p> Signup and view all the answers

How does acetylcholine function at the neuromuscular junction?

<p>It depolarizes the motor end plate and causes muscle contraction (D)</p> Signup and view all the answers

What physiological change occurs after an electrical discharge at a muscle in terms of ionic movement?

<p>Increase in intracellular Ca2+ concentration stimulating contraction (B)</p> Signup and view all the answers

What would happen if there is a malfunction in the synaptic vesicles at the neuromuscular junction?

<p>Impaired communication between the motor neuron and muscle fiber (A)</p> Signup and view all the answers

Flashcards

Excitation-Contraction Coupling

The process by which an electrical signal (action potential) in a motor neuron triggers the release of calcium ions (Ca2+) in a muscle fiber, ultimately leading to muscle contraction.

Neuromuscular Junction (NMJ)

The specialized synapse where a motor neuron communicates with a muscle fiber, transmitting the signal for muscle contraction.

Motor End Plate

The region on the muscle fiber's sarcolemma where the motor neuron's axon terminal makes contact, receiving the signal for contraction.

End Plate Potential (EPP)

The localized depolarization of the muscle fiber membrane at the motor end plate, caused by the release of acetylcholine (ACh) from the motor neuron.

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Acetylcholine (ACh)

The neurotransmitter released by the motor neuron at the NMJ, binding to receptors on the muscle fiber to initiate the contraction process.

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ACh Receptors

Protein molecules on the muscle fiber's sarcolemma that bind to acetylcholine, initiating the depolarization of the muscle membrane.

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Depolarization

A change in the electric potential across a cell membrane, becoming less negative (more positive) due to the influx of positive ions.

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Threshold

The minimum level of depolarization required to trigger an action potential (electrical signal) in a muscle fiber.

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Action Potential (AP)

A rapid electrical impulse that travels down the muscle fiber, initiating the release of calcium ions and ultimately muscle contraction.

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Calcium Ions (Ca2+)

Essential ions released from the sarcoplasmic reticulum of the muscle fiber, triggering the sliding filament mechanism and muscle contraction.

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Dihydropyridine Receptor (DHPR)

Voltage-gated calcium channels located in the transverse tubules of muscle cells. They change shape when an action potential travels down the tubule, triggering calcium release from the sarcoplasmic reticulum.

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Sarcoplasmic Reticulum (SR)

A network of membrane-bound sacs within muscle cells that stores and releases calcium ions (Ca2+).

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Calcium-ATPase Pump

A protein pump located in the SR that actively transports calcium ions back into the SR, causing muscle relaxation.

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Troponin C

A protein that binds to calcium ions, initiating muscle contraction.

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Cross-Bridges

Structures formed between the myosin head of thick filaments and the actin filaments of thin filaments in muscle cells. They generate force during muscle contraction.

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Muscle Relaxation

The process in which muscle fibers return to their resting length after contraction. It is initiated by the removal of calcium ions from the sarcoplasm.

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Muscle Contraction

The shortening of muscle fibers, generating force. This process requires the interaction of actin, myosin, and calcium ions.

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Action Potential

An electrical impulse that travels along the cell membrane. It triggers the release of calcium ions that initiate muscle contraction.

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Acetylcholine

A neurotransmitter released by motor neurons at the neuromuscular junction. It initiates muscle contraction.

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Botulism Toxin (BOTOX)

A toxin that inhibits acetylcholine release from the nerve terminal, weakening muscle contraction. Used to treat focal dystonias, spasticity in cerebral palsy, and for cosmetic purposes.

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Curare

A poison that blocks acetylcholine receptors (AChR) on muscle fibers, preventing muscle contraction.

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Acetylcholinesterase Inhibitors

Drugs that inhibit the enzyme acetylcholinesterase, leading to increased acetylcholine levels in the synaptic cleft. Used to treat Myasthenia gravis.

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Myasthenia Gravis

An autoimmune disease where the immune system attacks acetylcholine receptors (AChR) at the neuromuscular junction, leading to muscle weakness.

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Transverse Tubules

Narrow membranous tunnels that extend from the sarcolemma into the muscle fiber. They conduct action potentials from the surface of the muscle fiber to the sarcoplasmic reticulum.

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Ryanodine Receptor (RyR)

Calcium release channels located on the sarcoplasmic reticulum (SR). They open in response to an action potential, releasing Ca2+ into the cytoplasm for muscle contraction.

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Ca2+-ATPase Pump

An active transport pump located in the sarcoplasmic reticulum (SR) that pumps calcium ions (Ca2+) back into the SR after muscle contraction, leading to relaxation.

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Plasmapheresis

A treatment for autoimmune diseases, such as myasthenia gravis, where plasma containing antibodies against acetylcholine receptors is removed from the blood.

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Study Notes

Skeletal Muscle: Structure & Function II

  • Course: MD137: Principles of Physiology
  • Lecturer: Dr K.McCullagh
  • Topic: Describing the steps of electrical activation of muscle contraction, excitation-contraction coupling, and the neuromuscular junction with clinical relevance.

The Neuromuscular Junction (NMJ)

  • The NMJ is where a motor neuron stimulates a muscle fiber.
  • It's the area of the muscle fiber sarcolemma where a motor neuron stimulates it.
  • The NMJ is a connection between the somatic motor neuron (axon) and skeletal muscle fibers.

Excitation-Contraction Coupling

  • The physiologic mechanism where an electrical discharge at a muscle initiates chemical events at the cell surface to release intracellular Ca2+ and produce muscle action.
  • This process has multiple stages:
    • Acetylcholine (ACh) release from the somatic motor neuron.
    • ACh binding to nicotinic ACh receptors. Opening ligand-gated channels.
    • Sodium (Na+) diffuses in, creating a depolarizing stimulus.
    • An action potential is produced.
    • Action potentials propagated along transverse tubules.
    • Action potentials open voltage-gated Ca2+ channels.
    • Release of Ca2+ from the sarcoplasmic reticulum.
    • Ca2+ diffuses out into sarcoplasm.
    • Ca2+ binds to troponin, stimulating contraction.

Structural Features of the Neuromuscular Junction

  • Schwann cell: Surrounds the axon terminal.
  • Synaptic vesicles: Contain ACh.
  • Synaptic cleft: Space between the axon terminal and the muscle fiber.
  • Postjunctional fold: Increase surface area for ACh receptors.
  • Acetylcholine receptors: Nicotinic receptors respond to ACh.
  • Acetylcholinesterase: Degrades ACh and terminates the signal.

Electrical Activity at the Neuromuscular Junction

  • Chemical transmitter release.
  • Inward membrane current
  • Excitatory postsynaptic potential (EPSP).
    • A depolarization below threshold to activate Na+ channels.
    • No action potential occurs.

Clinical Implications

  • Substances affecting ACh transmission impact muscle contraction.
    • Botulinum toxin (Botox): Inhibits ACh release from nerve terminals. Used in some medical cases like Botox.
    • Curare: Blocks ACh receptors, used in indigenous peoples.
    • Acetylcholinesterase inhibitors: Increase ACh at the synapse to compensate for low receptor numbers (e.g., myasthenia gravis.)

Myasthenia Gravis

  • An autoimmune disease where the immune system attacks acetylcholine receptors.
  • This results in a decrease in the number of receptors.
  • Clinical treatments include including Acetylcholinesterase inhibitors, and plasmapheresis (plasma exchange).

Sarcoplasmic Reticulum (SR)

  • A modified endoplasmic reticulum in muscle cells. Stores Ca2+ when the muscle is at rest.
  • Stimulation releases Ca2+ via Ryanodine receptors.
  • At the end of contraction, a Ca2+-ATPase pump actively pumps Ca2+ back into the SR.

Transverse Tubules (T-tubules)

  • Narrow membranous tunnels formed from the sarcolemma.
  • Open to the extracellular environment.
  • Conduct action potentials.

Myofibrils and Sarcoplasmic Reticulum (SR)

  • Myofibrils are the contractile elements of the muscle fiber.
  • SR surrounds myofibrils.

Excitation-Contraction Coupling (detailed process)

  • DHP receptor: voltage-gated calcium channel in the T-tubules.
  • Ryanodine receptor (RyR): Calcium release channels in the SR.
  • Calcium binds to troponin, moving tropomyosin.
  • Cross-bridges form, and the muscle contracts.
  • 70% of ATP is used for the force generation via crossbridge rotation.
  • 30% of ATP is utilized to pump calcium back to SR for relaxation.

Muscle Relaxation

  • Action potentials cease.
  • Ca2+-ATPase pumps Ca2+ back into the SR.
  • No more Ca2+ available to bind to troponin C so no more crossbridges form.

Stimulating a Muscle Contraction

  • ACh released from motor neuron.
  • End-plate potentials; action potentials are produced.
  • Voltage-gated calcium channels (DHPR) change shape; open SR channels.
  • Calcium released binds to troponin C.
  • Cross-bridges form; muscle contracts.

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Description

This quiz covers the essential aspects of skeletal muscle, focusing on the steps of electrical activation of muscle contraction and the role of the neuromuscular junction. Understanding excitation-contraction coupling is crucial for grasping how muscle fibers respond to stimulation. Prepare to explore the mechanisms with clinical relevance for a deeper insight into muscle physiology.

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