Excitation-Contraction in Cardiac Muscle

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.