Muscle Physiology: Excitation-Contraction Coupling

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Questions and Answers

What primarily causes the rapid depolarization of the myocyte during Phase 0?

Influx of Na+ ions (correct)

Closure of potassium channels

Efflux of K+ ions

Opening of calcium channels

What is the effect of the L-type calcium channels during Phase 2 of the myocyte action potential?

They facilitate calcium influx. (correct)

They close sodium channels.

They allow K+ efflux.

They cause rapid repolarization.

During which phase does the myocyte experience a plateau where no further action potential can be generated?

Phase 2 (correct)

Phase 3

Phase 0

Phase 1

Which channels are responsible for initial rapid repolarization in Phase 1?

Fast potassium voltage-gated channels Signup and view all the answers

How does the resting membrane potential (RMP) remain stable during Phase 4?

Equilibrium of potassium flow Signup and view all the answers

What indicates the threshold is reached in Phase 0?

Opening of all sodium VGC Signup and view all the answers

What structural feature connects adjacent cardiomyocytes?

Gap junctions Signup and view all the answers

Which type of action potential is characterized by a shorter duration compared to others in the heart?

Atrial APs Signup and view all the answers

Which current is primarily associated with Phase 3 slow repolarization?

Slow K+ voltage-gated current Signup and view all the answers

What happens to the membrane potential during initial rapid repolarization?

It becomes less negative. Signup and view all the answers

During which phase of an action potential does rapid depolarization occur?

Phase 0 Signup and view all the answers

What is a distinguishing feature of Purkinje cell action potentials compared to ventricular action potentials?

Unstable phase 4 Signup and view all the answers

What role do ion pumps play in the action potential of cardiac myocytes?

Establish and maintain ion gradients Signup and view all the answers

Which cells in the heart are responsible for setting the heart rate through spontaneous depolarization?

Pacemaker cells Signup and view all the answers

What is the outcome of electrical events occurring at the sarcolemma of a cardiac myocyte?

Flow of ions through channels Signup and view all the answers

Which phase of the action potential corresponds to repolarization in cardiac myocytes?

Phase 3 Signup and view all the answers

What is partial tetanus characterized by?

Increased peak muscle tension that oscillates Signup and view all the answers

Which factor contributes to increased force development in skeletal muscle contraction?

Increased engagement of myosin with actin Signup and view all the answers

How does the arrangement of actin and myosin filaments contribute to muscle force production?

Optimal overlap enhances cross-bridge interactions Signup and view all the answers

What prevents tetanic contractions in cardiac myocytes?

Long electrical refractory period Signup and view all the answers

Which statement is true regarding the differences between skeletal and cardiac myocytes?

Skeletal muscle can exhibit sustained tetanic contractions Signup and view all the answers

What is a key structural difference between skeletal and cardiac muscle cells?

Cardiac muscle cells only have one nucleus Signup and view all the answers

What primarily triggers the release of calcium from the sarcoplasmic reticulum into the cytoplasm during excitation-contraction coupling?

Opening of L-type calcium voltage-gated channels Signup and view all the answers

Which of the following statements accurately describes the function of calcium in muscle contractions?

Excessive calcium leads to twitch fusion in skeletal muscle Signup and view all the answers

In skeletal muscle, what happens when action potentials occur too rapidly?

Muscle twitches can lead to sustained contractions Signup and view all the answers

What role does calcium play in the cross-bridge cycle?

Calcium binds to troponin, enabling myosin binding sites on actin Signup and view all the answers

What occurs immediately after the myosin head binds to actin?

Initiation of the power stroke Signup and view all the answers

What restores the myosin head to its original position after a power stroke?

Binding of a new ATP molecule Signup and view all the answers

What happens to tropomyosin when calcium levels decrease in the cytoplasm?

It covers the myosin binding sites on actin Signup and view all the answers

Which sequence correctly describes the order of events during the cross-bridge cycle?

Calcium binding, power stroke, detachment, ATP binding Signup and view all the answers

What is required to maintain continuous activation of the cross-bridge cycle?

Constant high levels of calcium and ATP Signup and view all the answers

What is the initial effect of depolarization of the sarcolemma in skeletal myocytes?

Opening of voltage-gated sodium channels Signup and view all the answers

What initiates the depolarization process in automatic cells?

Calcium influx through L-type calcium channels Signup and view all the answers

Which phase is characterized by the 'funny current' that allows for spontaneous depolarization?

Phase 4 Signup and view all the answers

What effect does parasympathetic nervous system activation have on automatic cell action potentials?

Increases K⁺ conductance leading to hyperpolarization Signup and view all the answers

What happens during Phase 3 of the automatic cell action potential?

Potassium channels open causing efflux Signup and view all the answers

Which channel is specifically mentioned as being open during hyperpolarization?

Funny current sodium channel Signup and view all the answers

What is the main result of positive charge leakage in automatic cells?

Spontaneous depolarization between action potentials Signup and view all the answers

What primarily governs the heart rate according to automatic cell action potentials?

The cell that depolarizes most frequently Signup and view all the answers

How does the resting membrane potential differ in automatic cells compared to cardiomyocytes?

It is less stable in automatic cells. Signup and view all the answers

Study Notes

Skeletal Muscle Excitation-Contraction Coupling

Acetylcholine binds to nicotinic receptors on the sarcolemma, initiating depolarization.
Depolarization of the sarcolemma opens voltage-gated sodium channels, causing an action potential.
The action potential opens voltage-gated calcium channels (L-type) which allow a small amount of calcium ions into the cell.
Calcium influx triggers the release of a large amount of calcium from the sarcoplasmic reticulum (SR) through ryanodine receptors.
T-tubules facilitate the propagation of the action potential deeper into the muscle cell, contributing to calcium release from the SR.
Increased cytosolic calcium binds to troponin, causing a conformational change.
Troponin's change shifts tropomyosin off the myosin binding sites on actin, allowing the myosin heads to bind.
The myosin heads bind to actin and undergo the cross-bridge cycle, which generates muscle force.
Once the action potential ceases, calcium is removed from the cytoplasm by active transport pumps, returning it to the SR and the extracellular fluid.
Decreased cytosolic calcium levels allow tropomyosin to reposition, blocking myosin binding to actin, resulting in muscle relaxation.

Cross-Bridge Cycle Events

At rest, myosin heads are attached to ADP and inorganic phosphate and are positioned to interact with actin, but tropomyosin blocks the interaction.
Calcium binding to troponin-C removes the inhibition, allowing myosin to bind to actin.
Once bound, the myosin head releases ADP and inorganic phosphate, changing its conformation from 90° to 45°, stretching the myosin S2 region.
The recoil of the S2 region generates the power stroke, pulling the actin filament along.
The myosin head is then in the rigor state, remaining bound to actin until a new ATP molecule binds.
ATP binding to the myosin head causes its detachment from actin and hydrolysis of ATP resets the myosin head to its original 90° conformation.

Factors Affecting the Strength of Skeletal Muscle Contraction

Frequency of action potentials: Increased frequency leads to "tetany" as individual muscle twitches summate, resulting in a sustained contraction.
Overlap of actin and myosin: Greater overlap between actin and myosin filaments increases the potential for cross-bridge formation and, therefore, the strength of contraction.

Skeletal vs. Cardiac Myocytes

Similarities:*
Both skeletal and cardiac muscle are striated.
Both exhibit a parabolic isometric length-tension relationship, reaching peak force at optimum resting length.
Both possess T-tubules and calcium ATPase pumps for calcium removal.
Differences:*
Cardiac myocytes do not undergo tetanic contractions due to their long refractory period.
Cardiac myocytes form a syncytium, interconnected by gap junctions and desmosomes, which enables coordinated contraction.
T-tubules play a less significant role in cardiac muscle excitation-contraction coupling; they are larger but fewer in number.
Cardiac muscle cells are uninucleated and have abundant mitochondria.

Cardiomyocyte Histology

Each cardiomyocyte contains a single nucleus and many mitochondria.
Cardiac myocytes are branched and connected to each other via gap junctions located at the intercalated disks.
This interconnected network allows for the rapid transmission of electrical signals throughout the heart, facilitating synchronized contraction.
The triad structure in cardiac myocytes differs from skeletal muscle, with SR cisterns extending radially from the T-tubules instead of circumferentially.

The Cardiomyocyte - Electrical Events

Four Major Types of Action Potentials in the Heart

Atrial: Distinct phases of depolarization, plateau, and repolarization, but shorter than ventricular APs, allowing for faster contraction cycles.
Ventricular: Distinct phases of depolarization, plateau, and repolarization.
Purkinje Cell: Similar to ventricular APs but with a slightly unstable phase 4, giving them the ability to spontaneously generate action potentials in abnormal conditions.
Automatic Cell: Pacemaker cells (SA and AV nodes) have an unstable phase 4 and spontaneously depolarize due to the "funny current" (If). They govern the heart rate through rhythmic action potentials.

Action Potential Phases

Phase 4: Resting membrane potential (RMP) - Potassium leak channels (iK1) are open and potassium flow is at equilibrium.
Phase 0: Rapid depolarization - Sodium voltage-gated channels (VGC Na+) open, allowing sodium influx.
** Phase 1:** Initial rapid repolarization - Sodium VGC close and fast potassium VGC open, allowing potassium efflux.
** Phase 2:** Plateau - L-type calcium VGC open, allowing calcium influx, and "slow" outward potassium channels open for potassium efflux.
** Phase 3:** Slow repolarization - Calcium VGC close and slow potassium VGC open, allowing potassium efflux

Automatic Cell Action Potential Phases

** Phase 4:** Resting membrane potential (RMP) - The "funny current" (If) conducts sodium and potassium and is open during hyperpolarization.
Phase 0: Depolarization - L-type calcium channels open, allowing calcium influx.
Phase 3: Repolarization - Potassium channels open, allowing potassium efflux, repolarizing the cell.

Automatic Cell Action Potential Modulation by the Nervous System

Sympathetic nervous system stimulation increases the rate of depolarization and heart rate by increasing sodium and calcium permeability.
Parasympathetic nervous system stimulation decreases the rate of depolarization and heart rate by:
- Increasing potassium conductance, leading to hyperpolarization and slower depolarization.
- Decreasing calcium influx, slowing down depolarization in phase 0.

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Description

This quiz explores the process of excitation-contraction coupling in skeletal muscle. It covers the role of acetylcholine, action potentials, calcium signaling, and the cross-bridge cycle in muscle contraction. Test your understanding of these key concepts in muscle physiology.