Excitation-Contraction in Cardiac Muscle

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Questions and Answers

What structure in the muscle fiber is primarily responsible for the transmission of action potentials?

Sarcoplasmic reticulum

Myofibrils

Sarcolemma

Transverse tubules (correct)

How does the action potential travel along the muscle fiber?

Across the sarcolemma only

Via diffusion in the interstitial fluid

Through the cytoplasm

Along the transverse tubules (correct)

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

Generate ATP

Conduct action potentials (correct)

Facilitate oxygen transport

Store calcium ions

In which part of the muscle fiber does the action potential primarily propagate?

Transverse tubules Signup and view all the answers

Which of the following statements is true regarding the action potential in muscle fibers?

It travels through the transverse tubules. Signup and view all the answers

Who is the writer associated with the lecture in Block 1.2?

Hassan Haraba Signup and view all the answers

What role does Zainab Adel Alali serve in the context of the lecture?

Reviewer Signup and view all the answers

What does the abbreviation 'Doctor explanation' refer to in the context of the lecture?

Content delivered by a medical professional Signup and view all the answers

Which of the following is indicated as deleted in the notes for the lecture?

References Signup and view all the answers

What information can be found on pages 221-223 of the notes?

Excitation-Contraction details Signup and view all the answers

How much greater is the heart blood flow during exercise compared to resting conditions?

3 to 4 times Signup and view all the answers

Which part of the heart is indicated as receiving blood supply from the coronary arteries?

Subendocardial portion of the left ventricle Signup and view all the answers

What are the primary arteries responsible for supplying blood to the heart's inner surface?

Coronary arteries Signup and view all the answers

During exercise, which of the following physiological changes occurs concerning blood flow?

It increases by 3 to 4 times Signup and view all the answers

Where are the branches of the coronary arteries primarily found?

In the inner surface of the heart Signup and view all the answers

What are the end products of glycogen breakdown in glycolysis?

Pyruvic acid and lactic acid Signup and view all the answers

What percentage of energy used by muscles for sustained, long-term contraction comes from fats?

70% - 90% Signup and view all the answers

Which of the following correctly represents the process of reconstituting ATP and phosphocreatine?

ADP + E → ATP Signup and view all the answers

Which method is an excellent measure of the chemical energy liberated by the heart?

Rate of oxygen consumption Signup and view all the answers

What reactions occur during glycolysis to energize muscle contractions?

ADP + energy → ATP Signup and view all the answers

What is the primary role of ATP in muscle contraction?

It binds to myosin, allowing detachment from actin. Signup and view all the answers

What happens to the myosin head when an ATP molecule binds to it?

It undergoes a conformational change that causes detachment. Signup and view all the answers

Which of the following statements is true regarding the effect of ATP on myosin during muscle contraction?

ATP binding to myosin facilitates its detachment from actin. Signup and view all the answers

What is the consequence of ATP not being present for the myosin head?

Myosin will remain tightly bound to actin. Signup and view all the answers

In the context of muscle contraction, what is the significance of the ATP re-binding process?

It is essential for the continuation of muscle contraction. Signup and view all the answers

What is the primary ion responsible for muscle activation during phase 2 of the action potential?

Ca⁺⁺ Signup and view all the answers

How long does the membrane remain depolarized during the plateau phase?

0.2 seconds Signup and view all the answers

What occurs at the beginning of phase 1 of the action potential?

Efflux of K⁺ ions Signup and view all the answers

What causes the plateau phase of the action potential?

Balance between Ca⁺⁺ and Na⁺ inflow Signup and view all the answers

What happens to Ca⁺⁺ and Na⁺ channels during the repolarization phase?

They close, ceasing the influx Signup and view all the answers

Study Notes

Excitation-Contraction Lecture Notes

The lecture covered excitation-contraction coupling in cardiac muscle.
Steps in excitation-contraction were outlined, including events between action potential initiation and final contraction/relaxation.
The role of calcium ions in regulating cardiac muscle fiber contraction and relaxation was discussed.
Extracellular calcium ions also play a role in the contraction strength of the heart.
Other calcium sources affecting excitation-contraction coupling were explained, along with how intracellular calcium concentration affects cardiac contraction strength.
The lecture addressed how cardiac and skeletal muscles gain strength, and how the heart is supplied with adequate blood.

Learning Objectives

Students should be able to describe the steps in excitation-contraction coupling processes.
They should understand the events occurring between action potential initiation and muscle fiber contraction/relaxation.
Students should know the role of calcium ions in regulating cardiac muscle contraction and relaxation.
The effect of extracellular calcium ions on contraction strength was also discussed.
Other calcium sources involved in these processes were detailed.
Students should also describe how cardiac and skeletal muscles gain strength and the blood supply to the heart.

Action Potential (AP) in Cardiac Muscle

The AP in cardiac muscle fibers is 105 mV.
AP is due to voltage-activated fast Na+ channels (phase 0), K+ Channels (phase 1), L-type Ca++ channels (phase 2), and closing of Calcium and sodium channels (phase 3), and K+ channel efflux (phase 4).
Various channels play different roles.
The plateau phase of the AP is important.

Refractory Period (RP)

The refractory period is the interval during which cardiac muscles cannot transmit impulses.
Absolute RP lasts 0.25-0.30 seconds for ventricles and 0.15 seconds for atria.
Relative refractory period is approximately 0.05 seconds, during which the muscle is harder to excite than normal.

Advantages of Long Plateau

The long plateau in cardiac muscle contraction is important for maintaining long contractions, in contrast to skeletal muscle contractions.
The prolonged period prevents excessively fast re-excitation in the ventricles, allowing for proper filling of the heart

Heart Muscle and Tetanus

Heart muscle can't exhibit tetanus.
Contraction proceeds into relaxation before generating the next one, allowing uninterrupted flow of blood

Syncytial Nature of Cardiac Muscle Fibers

Cardiac muscle fibers are interconnected.
Gap junctions facilitate rapid transmission of action potentials.

Contractile Proteins

Thick filaments (myosin)
Thin filaments (actin, tropomyosin, troponin)

Calcium Role in Cardiac Muscle Contraction

Calcium ions play a crucial role in cardiac muscle contraction by inhibiting the inhibitory effect of troponin and tropomyosin on actin filaments.
The mechanism involves calcium ions binding to troponin and moving tropomyosin, exposing actin binding sites.
This leads to actin binding to myosin and contraction.

The Walk-Along Theory of Contraction

Myosine ATPase hydrolyzes ATP to energize contraction.
Binding to actin, ADP release, and power stroke lead to myosin filaments sliding over actin filaments.
New ATP binding causes detachment of myosin from actin.

ATP Hydrolysis

ATP hydrolyzes into ADP and phosphate, energizing myosin heads.
Myosin-actin binding, ADP dissociation, and bending of myosin head leads to actin filament pulling.
ATP attaching to myosin detaches it from actin, allowing the cycle to repeat.

Excitation-Contraction Coupling

Mechanism by which action potentials lead to muscle contraction.
Action potential travels into transverse tubules (TTs).
Voltage-dependent calcium channels open, allowing calcium influx.
Calcium release channels open, releasing calcium into sarcoplasm.
Calcium initiated contraction of muscles.

Extracellular Ca++ Role in Heart Contraction Strength

Extracellular Ca++ significantly contributes to cardiac muscle contraction strength.
Cardiac muscle's sarcoplasmic reticulum (SR) is less well-developed compared to skeletal muscle.

Coronary Blood Supply

Coronary arteries supply blood to the heart.
Small arteries penetrate deeper to supply the nutritive mass within the muscle.

Venous Blood

Most venous blood from the left ventricle passes through the coronary sinus.
Small anterior cardiac veins also drain venous blood.
Venous blood is pumped into the right atrium of the heart.

Normal Coronary Blood Flow

Resting coronary blood flow averages 225 ml/min.
Increased blood flow occurs during exercise to meet increased nutrient demand.

Subendocardial Portion of the Left Ventricle

Pressure in this area is slightly greater than the aorta during systole. Blood flow primarily occurs during diastole.
Coronary blood flow in the subendocardium will not be drastically influenced by systole.

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Description

This quiz focuses on the process of excitation-contraction coupling in cardiac muscle. It covers the steps involved from action potential initiation to muscle contraction and relaxation, with particular emphasis on the role of calcium ions. Understanding these concepts is vital for grasping how the heart functions and maintains adequate blood supply.