Cardiac Muscle Contraction | Excitation Contraction Coupling | Cardiac Physiology

Summary

This video explains the process of cardiac muscle contraction by first reviewing skeletal muscle contraction. Key similarities include the role of calcium in muscle contraction, where calcium binds to Troponin C and enables actin-myosin interaction. The major difference in cardiac muscle is the origin of the action potential, which comes from the Sinoatrial node, not nerves. Cardiac muscle also experiences a calcium influx through L-type calcium channels, triggering additional calcium release from the sarcoplasmic reticulum. The video also discusses how factors like autonomic nervous system regulation affect cardiac contractility, impacting heart performance and cardiac output.

Takeaways

• 😀 Cardiac muscle contraction is similar to skeletal muscle contraction, with a few key differences.
• 😀 Skeletal muscle contraction starts at the neuromuscular junction, where acetylcholine triggers sodium influx, generating an action potential.
• 😀 The action potential propagates along the membrane and down T-tubules, stimulating calcium release from the sarcoplasmic reticulum via Dihydropyridine and Ryanodine receptors.
• 😀 Calcium binds to Troponin C in both skeletal and cardiac muscles, allowing actin and myosin to interact, resulting in muscle contraction.
• 😀 The process of contraction in skeletal muscles is powered by ATPase activity in myosin, which causes the thin filament to slide, shortening the sarcomere.
• 😀 In cardiac muscle, the impulse for contraction comes from the Sinoatrial node, not from a nerve.
• 😀 Cardiac muscle contraction involves a plateau phase in the action potential, during which calcium enters the cell via L-type calcium channels.
• 😀 Calcium-induced calcium release (CICR) in cardiac muscle amplifies calcium levels from both the extracellular space and the sarcoplasmic reticulum.
• 😀 After contraction, calcium is pumped back into the sarcoplasmic reticulum or out of the cell via a sodium-calcium exchanger, preparing the muscle for the next contraction.
• 😀 The heart's contractility, or inotropism, depends on intracellular calcium concentration, and can be modulated by sympathetic and parasympathetic stimulation.
• 😀 Sympathetic stimulation increases contractility (positive inotropic effect), while parasympathetic stimulation reduces contractility (negative inotropic effect).

Q & A

What is the first step in skeletal muscle contraction?

The first step in skeletal muscle contraction is the arrival of an action potential at the Neuromuscular Junction, where acetylcholine is released and acts on receptors on the muscle membrane.

How does the action potential propagate in skeletal muscle?

The action potential propagates along the muscle membrane and down the T-tubules, which are invaginations of the membrane, eventually triggering a calcium release from the sarcoplasmic reticulum.

What role do the Dihydropyridine Receptors and Ryanodine Receptors play in skeletal muscle contraction?

The Dihydropyridine Receptor, which is a voltage-gated calcium channel, is mechanically coupled to the Ryanodine Receptor on the sarcoplasmic reticulum. When the Dihydropyridine Receptor opens, it causes the Ryanodine Receptor to open, releasing calcium from the sarcoplasmic reticulum into the muscle cell.

What is the function of calcium in skeletal muscle contraction?

Calcium binds to Troponin C, which is part of the thin filaments, causing a conformational change that moves tropomyosin out of the way, allowing myosin to bind to actin. This leads to the sliding of filaments and muscle contraction.

How does muscle relaxation occur after skeletal muscle contraction?

Muscle relaxation occurs when calcium is pumped back into the sarcoplasmic reticulum by the SERCA pump, reducing intracellular calcium levels, which leads to the dissociation of calcium from Troponin C and the relaxation of the muscle.

What is the key difference between skeletal and cardiac muscle contraction?

The key difference is that cardiac muscle contraction is initiated by an electrical impulse from the Sinoatrial (SA) node, not from a nerve. The action potential in cardiac muscle cells also features a plateau phase due to the prolonged influx of calcium ions.

What is the significance of calcium in cardiac muscle contraction?

In cardiac muscle, the calcium influx during the plateau phase of the action potential triggers the release of additional calcium from the sarcoplasmic reticulum, which is essential for muscle contraction through the Calcium-induced Calcium Release mechanism.

What is the function of the L-type calcium channels in cardiac muscle?

L-type calcium channels in cardiac muscle allow calcium ions to enter the cell during the plateau phase of the action potential, which raises the intracellular calcium levels necessary for muscle contraction.

How is calcium removed from the cardiac muscle cell after contraction?

Calcium is removed from the cardiac muscle cell by being pumped back into the sarcoplasmic reticulum via the Sarcoplasmic Endoplasmic Reticulum Calcium ATPase (SERCA) and by being exchanged with sodium through the sodium-calcium exchanger on the plasma membrane.

What factors influence cardiac contractility?

Cardiac contractility is influenced by factors such as the intracellular calcium concentration, which is regulated by calcium influx through L-type calcium channels and calcium uptake into the sarcoplasmic reticulum. Sympathetic stimulation can increase contractility by enhancing calcium influx and storage, while parasympathetic stimulation reduces contractility.