Cardiac Muscle

What are the features of cardiac muscle?

• Striated/Syncytial

• Found in heart

• Propels blood through the circulatory system

• Is involuntary

• Has intrinsic activity

• Regulated by the autonomic nervous system

• Affected by hormones

What are the three types of modified muscle cells? What function does each have?

1.) Pacemaker Cells

• Small pale, few organelles

• Function to initiate electrical impulses

2.) Conductors

• Short, broad, oriented end to end, few lateral connections

• Transmit the electrical signals

3.) Contractile Myocardial cells

• Generate the contractions

Describe the pathway of the electrical signal through the heart.

1.) Sinoatrial (SA) Node: The electrical signal begins at the SA node, located in the right atrium. It acts as the heart's natural pacemaker, generating electrical impulses that initiate each heartbeat.

2.) Atrial Contraction: The impulse spreads through the atrial muscle cells, causing the atria to contract and push blood into the ventricles.

3.) Atrioventricular (AV) Node: The signal then reaches the AV node, located at the junction between the atria and ventricles. Here, there is a brief delay, allowing the ventricles time to fill with blood from the atria before they contract.

4.) Bundle of His: From the AV node, the electrical signal travels down the Bundle of His (atrioventricular bundle), which branches into the right and left bundle branches that run along the interventricular septum.

5.) Purkinje Fibers: The signal then spreads into the Purkinje fibers, which extend throughout the ventricles. This rapid conduction allows for simultaneous contraction of the ventricular muscle.

6.) Ventricular Contraction: As the Purkinje fibers transmit the signal, the ventricles contract, pumping blood out of the heart—right ventricle to the lungs via the pulmonary artery and left ventricle to the rest of the body via the aorta.

What is the importance of the connective tissue in the bundle of his?

• Delays signal transmission from the AV node through bundle of his, this brief pause allows the ventricles to fill with blood prior to contracting

What separates the atria and the ventricles?

a layer of non-conducting connective tissue (meaning it does NOT have gap junctions)

What do we note in the transverse and longitudinal sections of cardiac muscle?

Transverse Section

• Single central nucleus

• Lots of CT

Longitudinal Section

• Central nucleus

• Striated

• Short-branched fibers

• Abundant CT between fibers

• Intercalated discs

What is the intercalated disk?

• Structures found between adjacent cardiac cells, have gap junctions allowing passage of ions between cells, allowing rapid electrical communication

Describe the action potential you would see in a pacemaker cell.

1.) Pacemaker Potential

• Decreased permeability to K+

• Increased permeability to Ca2+

2.) Action Potential

• Once threshold is reached, voltage gated calcium channels open, rapidly depolarizing the cell membrane

3.) Repolarization

• Ca2+ channels close, K+ channels open, allowing K+ ions to exit the cell, bringing membrane potential back to resting level

What is the resting potential in a pacemaker cardiac cell?

- 50 mv

Describe the action potential you would see in a contractile cell.

1.) RAPID Depolarization

• Rapid influx of sodium ions and decreased K+ permeability

2.) Initial repolarization

• After reaching peak of depolarization, closure of Na+ channels, brief efflux of K+ leading to small drop in membrane potential

3.) Maintained Depolarization (Plateau phase)

• Calcium ions enter the cell, balance efflux of K+ which creates a plateu

4.) Repolarization

• Slowly closing Ca channels close, efflux of K+ helps repolarize the membrane

What is the physiological significance of the plateau phase in the contractile cardiac muscle cells?

• Prolonged plateau prevents heart from undergoing tetanus, or a sustained contraction, which would now allow the heart to pump

What is notable about the timing of action potentials in contractile muscle cells, compared to those in nerves and skeletal muscle cells?

Cardiac Contractile Cells

• Last around 100 ms

• Have plateau phase allowing sustained contraction to effectively pump blood, prevents premature contractions and ensure heart has time to fill with blood

Nerves and other Muscle Cells

• Lasts 1-2 ms

• Require rapid signaling, do not have plateau phase, so repolarize more quickly and have shorter action potentials

Why is mechanical summation not possible in cardiac muscle cells?

• Prolonged action potential, preventing stimulation until fully relaxed, so there is a very long absolute refractory period

• Ensures the chambers can fill properly without risk of overcontraction

What sets the heart rate in the body?

SA node

How might the slope of the pacemaker potential affect heart rate?

• Sympathetic NS: can increase heart rate by opening more Na+ channels (Increases slope = increases depolarization)

• Parasympathetic NS: can decrease heart rate by opening more K+ channels (Decreases slope = slows depolarization)

• Warmth: speeds up ion movements and increases heart rate

• Cooling: slows down ion movement and decreases heart rate

How might the threshold change the heart rate? Can the body do this on its own?

• Higher Threshold = decreases heart rate

• Lower Threshold = increased heart rate

No but drug companies can

How does the minimum membrane potential alter heart rate?

• PNS secreted ACh which opens more K+ channels, causing hyperpolarization, makes it harder for pacemaker cell to reach threshold

• Decreases slope of pacemaker potential, longer time to fire AP

• Lowers HR

How does excitation contraction coupling work in cardiac muscle?

1.) Action Potential: An electrical impulse is generated by the SA node.

2.) Depolarization: Sodium channels open; sodium (Na⁺) enters the cell.

3.) Calcium Entry: AP stimulates the calcium channels to open; calcium (Ca²⁺) enters from the extracellular fluid. (Voltage gated Ca2+ channels)

4.) Calcium Release: Calcium triggers release of more calcium from the sarcoplasmic reticulum (SR). (This is through calcium regulated calcium channels)

5.) Troponin Binding: Calcium binds to troponin, exposing myosin-binding sites on actin.

6.) Cross-Bridge Formation: Myosin heads attach to actin.

7.) Contraction: Myosin pulls actin, shortening the muscle fiber.

8.) Relaxation: Calcium is pumped back into the SR, leading to muscle relaxation.

Where do each of the following source calcium in the body?

• Skeletal

• Smooth

• Cardiac

Skeletal

• Intracellular Calcium from SR

Smooth

• Extracellular Calcium

Cardiac

• Intracellular from SR AND extracellular

Why might the heart rely on two mechanisms to get calcium to its cells?

Therefore the heart has several mechanisms to keep the heart beating

What are some Factors Affecting Stroke Volume?

1.) Starling's Law of the Heart

• Increasing stretch due to increased diastolic filling leads to an increased force of contraction

• At rest, cardiac muscle fibers are not at optimal length

2.) Faster Ventricular Contraction and Relaxation

• Adrenaline from sympathetic NS and adrenal gland affects the pacemaker potential and contractile force for any given end diastolic volume

• Increased Ca permeability, increased Ca from SR results in increased cross bridge formation, so faster contraction and more tension

• Faster Ca reuptake = faster relaxation

What is the best length for cardiac muscle fibers?

• The Frank-Starling law explains this mechanism: the more the heart muscle is stretched during filling (due to increased EDV), the stronger the contraction will be, up to a certain point.

• This helps the heart pump out the amount of blood it receives. For example, if more blood enters the heart (as seen with exercise or increased venous return), the heart will stretch more and contract harder, thus pumping out more blood to meet the body’s needs.

Can the AV node generate its own action potentials?

Yes, the AV node can also initiate action potentials; however, they do so at a slower rate and therefore, if the SA node is functioning properly, its action potentials usually override those that would be produced by other tissues.

• AV node can take over if needed but this indicates a health issue