MPP block 2 lecture 1

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Questions and Answers

What is the primary role of calcium in muscle contraction?

It acts as a catalyst for ATP hydrolysis during muscle contraction.

It facilitates the attachment of actin filaments to Z disks.

It directly initiates the conformational change in myosin.

It causes a conformational change in the troponin complex. (correct)

How does acetylcholine influence muscle contraction?

It enhances the binding of troponin to tropomyosin.

It activates voltage-gated calcium channels directly.

It serves as a neurotransmitter that initiates muscle action potential. (correct)

It directly stimulates myosin ATPase activity.

Which ion channel type is primarily involved in the depolarization of the muscle fiber membrane?

Potassium channels.

Calcium channels.

Chloride channels.

Sodium channels. (correct)

What is the impact of Myasthenia Gravis on muscle function?

It leads to the blockage of acetylcholine receptors at the neuromuscular junction. Signup and view all the answers

What structural component limits the stretch of the sarcomere?

Titin. Signup and view all the answers

What is the primary role of Acetylcholine (ACh) at the neuromuscular junction?

To act as a neurotransmitter that triggers depolarization. Signup and view all the answers

In excitation-contraction coupling, what is the consequence of calcium (Ca²⁺) binding to troponin?

It facilitates the cross-bridge cycle and allows muscle contraction to proceed. Signup and view all the answers

Which ion is primarily responsible for the depolarization of the muscle membrane?

Sodium (Na⁺) Signup and view all the answers

What effect does Myasthenia Gravis have on muscle signal transmission?

It results in reduced functional ACh receptors, impairing signal transmission. Signup and view all the answers

What role do voltage-gated sodium channels play in muscle physiology?

They enhance depolarization, propagating the action potential throughout the muscle fiber. Signup and view all the answers

Which component directly interacts with ryanodine receptors during the excitation-contraction coupling process?

Calcium ions (Ca²⁺) Signup and view all the answers

In what way does the impairment of ACh receptors in Myasthenia Gravis manifest clinically?

As muscle weakness that worsens with activity. Signup and view all the answers

What is the function of nicotinic acetylcholine receptors (nAChRs) at the motor endplate?

To allow the influx of sodium ions, initiating action potentials. Signup and view all the answers

How does potassium (K⁺) contribute to muscle action potentials?

It restores the resting membrane potential following depolarization. Signup and view all the answers

Which statement correctly describes the role of calcium (Ca²⁺) in muscle contraction?

Ca²⁺ binds to troponin, allowing cross-bridge formation between actin and myosin. Signup and view all the answers

What directly causes the exposure of myosin-binding sites on actin during muscle contraction?

Ca²⁺ binding to troponin C Signup and view all the answers

Which statement about the roles of Ca²⁺ in muscle contraction is correct?

Ca²⁺ binds to troponin to initiate the contraction process. Signup and view all the answers

In which phase does ATP bind leading to the detachment of myosin from actin?

Cross-bridge detachment Signup and view all the answers

What characterizes the A-band of the sarcomere during muscle contraction?

It remains constant in length. Signup and view all the answers

What is the correct order of the cross-bridge cycle steps?

Attachment, Power Stroke, Detachment, Reactivation Signup and view all the answers

How does myasthenia gravis affect muscle contraction?

It reduces the binding of acetylcholine to receptors. Signup and view all the answers

Which structure defines the boundary of the sarcomere?

Z-line Signup and view all the answers

What occurs to the I-band during muscle contraction?

It shortens as actin filaments slide past myosin. Signup and view all the answers

Which component of the sarcomere covers actin's myosin-binding sites when the muscle is at rest?

Tropomyosin Signup and view all the answers

What is primarily responsible for the power stroke during contraction?

Release of ADP and Pi from myosin Signup and view all the answers

What is the primary role of dystrophin in muscle cells?

Anchors the cytoskeleton to the sarcolemma and aligns muscle fibers Signup and view all the answers

In the context of excitation-contraction coupling, what triggers the release of Ca²⁺ from the sarcoplasmic reticulum?

The opening of RyR channels by DHP receptors Signup and view all the answers

What is the effect of myasthenia gravis on neuromuscular transmission?

Interferes with signaling by blocking nicotinic acetylcholine receptors Signup and view all the answers

Which ion is primarily responsible for depolarization during excitation at the neuromuscular junction?

Sodium ions (Na⁺) Signup and view all the answers

What compensatory mechanism occurs in muscle cells to restore Ca²⁺ levels during repolarization?

Action of SERCA pumps Signup and view all the answers

Which component is primarily involved in the release of acetylcholine at the neuromuscular junction?

Voltage-gated calcium channels Signup and view all the answers

How does the presence of high levels of magnesium in sarcoplasm affect muscle contractions?

Inhibits excessive calcium release from the sarcoplasmic reticulum Signup and view all the answers

What aspect of neuromuscular junction physiology is primarily affected in Duchenne muscular dystrophy?

Loss of attachment of the cytoskeleton to the sarcolemma Signup and view all the answers

What role do terminal cisternae play in skeletal muscle physiology?

They host ion channels essential for excitation-contraction coupling Signup and view all the answers

What initiating event occurs at the motor endplate of a muscle cell that leads to muscle contraction?

Binding of acetylcholine to nicotinic receptors Signup and view all the answers

What is the primary function of T-tubules in muscle fibers?

Conducting action potentials deep into the muscle fiber. Signup and view all the answers

How do dihydropyridine receptors (DHPR) contribute to muscle contraction?

They detect action potentials and interact with ryanodine receptors. Signup and view all the answers

What initiates the exposure of myosin-binding sites on actin filaments during muscle contraction?

Binding of calcium ions to troponin C. Signup and view all the answers

Which subunit of the troponin complex directly binds calcium ions?

Troponin C (TnC) Signup and view all the answers

What role does tropomyosin play in muscle contraction?

It prevents actin-myosin interaction when muscle is relaxed. Signup and view all the answers

In the context of excitation-contraction coupling, what is the function of the sarcoplasmic reticulum (SR)?

To store and release calcium ions for muscle contraction. Signup and view all the answers

What is the impact of myasthenia gravis on muscle contraction?

Inhibits the binding of acetylcholine to receptors at the neuromuscular junction. Signup and view all the answers

What process occurs after calcium ions bind to troponin C during muscle contraction?

Troponin moves tropomyosin away from myosin-binding sites on actin. Signup and view all the answers

Study Notes

T-Tubules and Muscle Contraction

T-tubules are invaginations of the sarcolemma that transmit action potentials deep into muscle fibers, promoting synchronized contraction.

Dihydropyridine Receptors (DHPR) and Ryanodine Receptors (RyR)

DHPRs are voltage-sensitive receptors on T-tubules that detect action potentials, leading to a conformational change.
RyRs are channels on the sarcoplasmic reticulum that release calcium ions (Ca²⁺) into the cytosol when activated by DHPRs.

Calcium Ions (Ca²⁺) in Muscle Contraction

Released Ca²⁺ binds to troponin C, initiating the cross-bridge cycle by exposing myosin-binding sites on actin.

Troponin Complex

Composed of three subunits:
- Troponin C (TnC) binds Ca²⁺, triggering conformational changes.
- Troponin I (TnI) inhibits actin-myosin interaction in the absence of Ca²⁺.
- Troponin T (TnT) anchors to tropomyosin, aiding in contraction regulation.
Ca²⁺ binding to TnC shifts tropomyosin away from actin binding sites, allowing myosin interaction.

Tropomyosin

A regulatory protein that blocks myosin-binding sites on actin when the muscle is relaxed, preventing contraction.

Myosin

A motor protein that binds to actin filaments, generating force through the cross-bridge cycle with ATP.

Sarcoplasmic Reticulum (SR) Functions

The SR is the intracellular store of Ca²⁺, which is pumped back by the SERCA pump during relaxation to end contraction.

Cross-Bridge Cycling Process

The cycle, essential for muscle contraction, occurs in several stages:
- Resting State: Myosin heads are energized; tropomyosin covers actin binding sites.
- Calcium Release: Ca²⁺ binds to TnC, exposing myosin-binding sites.
- Cross-Bridge Formation: Myosin head attaches to actin.
- Power Stroke: Myosin head pivots, pulling actin closer to the sarcomere center.
- Cross-Bridge Detachment: Binding of ATP to myosin head causes detachment.
- Reactivation of Myosin: ATP hydrolysis re-cocks the myosin head for another cycle.

Sarcomere Structure

Key components include:
- Z-line: Boundary defining the sarcomere.
- Actin: Thin filaments anchored to the Z-line.
- Myosin: Thick filaments in the center.
- Tropomyosin and Troponin: Regulate actin-myosin interactions.
During contraction, Z-lines move closer, I-band shortens, H-zone disappears, and A-band remains constant.

Preload Principle

Preload refers to the initial stretching of the muscle before contraction, affecting sarcomere length and subsequent contractile force.

Dystrophin and Sarcoplasm

Dystrophin anchors thin and thick filaments to the cytoskeleton, aligning Z-disks with adjacent myofibrils.
Sarcoplasm contains high levels of Mg²⁺, phosphates, and myoglobin, with abundant mitochondria for ATP production.

Key Players in Excitation-Contraction Coupling

Acetylcholine (ACh): Neurotransmitter initiating depolarization at the neuromuscular junction (NMJ).
Nicotinic ACh Receptors (nAChRs): Ligand-gated channels that open upon ACh binding, allowing Na⁺ influx.
Sodium Ions (Na⁺): Primary drivers of depolarization in muscle fibers.
Voltage-Gated Sodium Channels: Amplify depolarization to propagate action potentials.

Neuromuscular Junction (NMJ) Steps

Action potential reaches motor neuron terminal.
Ca²⁺ influx triggers ACh release into the synaptic cleft.
ACh binds to nAChRs, causing Na⁺ influx and sarcolemma depolarization.
Action potential travels down T-tubules, activating DHPR and RyR, leading to Ca²⁺ release and muscle contraction.

Clinical Correlations

Duchenne Muscular Dystrophy: Caused by dysfunction of dystrophin, leading to muscle fiber necrosis and wasting.
Myasthenia Gravis: An autoimmune disorder that attacks ACh receptors at the NMJ, resulting in muscle weakness and fatigue.

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Description

Explore the intricate mechanisms of muscle contraction, including the roles of T-tubules, Dihydropyridine receptors, and the troponin complex. This quiz delves into how calcium ions enhance muscle fiber activity and regulate contractions. Test your understanding of these critical physiological concepts.