MD137: Skeletal Muscle Function II Quiz

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Questions and Answers

What initiates the release of intracellular Ca2+ during excitation-contraction coupling?

Electrical discharge at the muscle (correct)

Contraction of the motor neuron

Chemical signal from the muscle fiber

Inhibition of Na+ channels

What is the role of the motor end plate in muscle contraction?

Forms a barrier between nerve and muscle

Increases potassium ion concentration

Stimulates the motor neuron

Serves as the activation site for the muscle fiber (correct)

What occurs during the excitatory postsynaptic potential (EPSP) at the neuromuscular junction?

Hyperpolarization of the muscle fiber

Release of acetylcholine into the bloodstream

Activation of voltage-gated Cl- channels

Depolarization that does not reach threshold for action potential (correct)

What is the primary consequence of the electrical discharge at the neuromuscular junction?

Chemical events triggering muscle action Signup and view all the answers

Which of the following best describes the function of the neuromuscular junction?

Connection point between motor neuron and skeletal muscle fiber Signup and view all the answers

What triggers the opening of calcium channels in the sarcoplasmic reticulum during muscle contraction?

Action potentials generated by motor neurons Signup and view all the answers

How does muscle relaxation occur after a contraction?

Ca2+-ATPase pumps calcium back into the sarcoplasmic reticulum Signup and view all the answers

What role do cross-bridges play in muscle contraction?

They connect the actin and myosin filaments to produce force Signup and view all the answers

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

30% Signup and view all the answers

Which sequence of events correctly describes excitation-contraction coupling?

Action potentials occur, calcium is released, and then cross-bridges form Signup and view all the answers

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

They increase the levels of acetylcholine in the synaptic cleft. Signup and view all the answers

What substance is used in cosmetic surgery to inhibit acetylcholine release?

Botulinum toxin Signup and view all the answers

How does curare affect muscle contractions?

It blocks acetylcholine receptors. Signup and view all the answers

What happens to calcium ions during muscle relaxation?

Calcium ions are pumped back into the sarcoplasmic reticulum. Signup and view all the answers

In Myasthenia gravis, which mechanism leads to the symptoms of grave muscle weakness?

Attack on acetylcholine receptors by the immune system. Signup and view all the answers

Which process involves the opening of calcium release channels in muscle fibers?

Excitation contraction coupling Signup and view all the answers

What is the primary role of the transverse tubules in muscle fibers?

They conduct action potentials. Signup and view all the answers

What characterizes the sarcoplasmic reticulum in muscle cells?

It acts as a storage compartment for calcium ions. Signup and view all the answers

Study Notes

Skeletal Muscle: Structure & Function II

MD137: Principles of Physiology lecture covers the steps of electrical activation of muscle contraction, excitation-contraction coupling, and the neuromuscular junction. Clinical relevance is also discussed.

The Neuromuscular Junction (NMJ)

The NMJ is where a motor neuron stimulates a muscle fiber.
This area shows somatic motor neuron axon and skeletal muscle fibers with neuromuscular junctions (NMJs)

Excitation-Contraction Coupling

A physiological mechanism where an electrical discharge at the muscle initiates chemical events that release intracellular Ca²⁺ and cause muscle action.

Steps of Excitation-Contraction Coupling

Neurotransmitter Release: Acetylcholine (ACh) is released from the somatic motor neuron.
ACh Binding: ACh binds to nicotinic ACh receptors, opening ligand-gated channels.
Sodium Influx: Sodium (Na⁺) diffuses into the muscle fiber, creating a depolarizing stimulus.
Action Potential Generation: An action potential is produced.
Action Potential Propagation: Action potentials are conducted along the transverse tubules (T-tubules).
Calcium Release: Action potentials open voltage-gated calcium (Ca²⁺) channels. Ca²⁺ is released from the sarcoplasmic reticulum (SR).
Calcium Binding: Ca²⁺ binds to troponin, allowing actin-myosin cross-bridges to form.
Muscle Contraction: Cross-bridges bind, rotate, and generate force.
Calcium Removal: Ca²⁺ is actively pumped back into the sarcoplasmic reticulum (SR) by the Ca²⁺-ATPase pump.
Muscle Relaxation: No more Ca²⁺ is available, so the muscle fiber relaxes.

Neuromuscular Junctions and Motor End Plates

Neuromuscular junction: The site where a motor neuron stimulates a muscle fiber.
Motor end plate: The area of the muscle fiber's sarcolemma where a motor neuron stimulates it.

The Neuromuscular Junction (Detailed)

Motor neuron action potential: Initiates the process.
Calcium entry: Calcium enters voltage-gated channels.
Acetylcholine release: Acetylcholine is released into the synaptic cleft.
Acetylcholine binding: Acetylcholine binds to receptors on the muscle fiber.
Sodium entry: Sodium enters, generating an action potential.
Muscle action potential: Propagated along the muscle fiber's membrane.
Calcium release: Calcium is released from the sarcoplasmic reticulum.
Muscle contraction: Calcium binding triggers muscle contraction.
Acetylcholine degradation: Acetylcholinesterase breaks down acetylcholine.

Structural Features of the Neuromuscular Junction

Schwann cell: Encloses and isolates the axon terminal.
Synaptic vesicles: Contain acetylcholine (ACh).
Synaptic cleft: The gap between the neuron and muscle fiber.
Postjunctional fold: Increases the surface area of the muscle fiber membrane.
Nicotinic acetylcholine receptors: Molecules that bind to ACh.

Electrical Activity at the Neuromuscular Junction

Chemical transmitter release: Motor axon action potential triggers release of acetylcholine into the synaptic cleft
Inward membrane current: Acetylcholine binding leads to sodium ion inward current, generating an excitatory postsynaptic potential (EPSP).
Depolarization below threshold: EPSP not strong enough to trigger a full action potential.

Electrical Activity at the Neuromuscular Junction (Continued)

Many motor axon action potentials: Repeated signals create numerous EPSPs, each summating to reach the action potential threshold.
Na⁺ channel threshold: Action potential is triggered when sodium channels open above a critical threshold.

Clinical Implications

Substances affecting acetylcholine transmission affect muscle contraction.
Botulism toxin (Botox) inhibits acetylcholine release, used for various medical conditions.
Curare blocks acetylcholine receptors, found in poison arrows.
Acetylcholinesterase inhibitors treat myasthenia gravis (muscle weakness due to insufficient acetylcholine).

Myasthenia Gravis (Grave Muscle Weakness)

An autoimmune disease where the immune system attacks acetylcholine receptors at the neuromuscular junction.
Incidence: 1 in 5000 people.
Treatment: Acetylcholinesterase inhibitors (e.g., neostigmine) and plasmapheresis (plasma exchange).

Acetylcholinesterase Inhibitors

Inhibition mechanism: Prevents breakdown of acetylcholine (ACh).
Result: High ACh concentration in synaptic cleft, compensating for reduced receptor numbers.

Excitation-Contraction Coupling (Summary)

Begins with an action potential.
Leads to calcium release.
Cross-bridging and muscle contraction are activated.
Relaxation occurs through calcium reabsorption.

Transverse Tubules

Narrow membranous tunnels from sarcolemma.
Open to extracellular environment.
Conducts action potentials.

Sarcoplasmic Reticulum (SR)

Stores Ca²⁺ when muscle is at rest.
Releases Ca²⁺ through RyR (ryanodine receptor) channels when stimulated.
Actively pumps Ca²⁺ back into the SR during relaxation.

Myofibrils and Sarcoplasmic Reticulum

Myofibrils contain actin and myosin filaments, involved in muscle contraction.
SR surrounds the myofibrils.
Structures are visualized in the provided image.

Temporal Sequence of Events

Action potential precedes the increase in intracellular calcium (Ca²⁺).
Calcium increase precedes cross-bridge cycling and force generation.

Stimulating a Muscle Contraction

Acetylcholine is released .
End-plate potentials (EPPs) are created.
Action potentials are generated.
Ca2+ channels in transverse tubules (T-tubules) are opened by the altered shape of DHPRs on the T-tubules
Ca2+ release from sarcoplasmic reticulum (SR)

Muscle Relaxation

Action potentials stop.
Ca²⁺-ATPase pumps Ca²⁺ back into the SR.
No more calcium is available for cross-bridge attachment

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Description

Test your knowledge on the structure and function of skeletal muscle, focusing on electrical activation and excitation-contraction coupling. This quiz covers the neuromuscular junction and its clinical relevance, highlighting key processes involved in muscle contraction.