Potencial de ação cardíaco e contração do coração – Fisiologia Humana

Summary

This script delves into the functioning of the human heart's muscular system, highlighting the similarities and differences between cardiac and skeletal muscles. It explains the three main components of the heart muscle: atrial, ventricular, and specialized conducting fibers. The script discusses the unique structure of cardiac cells, including intercalated discs that facilitate ion transfer for action potential propagation. It also describes the phases of the cardiac action potential, emphasizing the role of calcium in muscle contraction and the slower conduction speed compared to nerve fibers. The importance of calcium channels and the refractory period in the heart's rhythmic contractions is also covered.

Takeaways

• 💓 The heart muscle, like skeletal muscles, contracts to perform its function of circulating blood throughout the body.
• 🔍 There are three main components of the heart muscle: the atrial muscle, the ventricular muscle, and the specialized excitatory and conductive fibers.
• 🌟 Atrial and ventricular muscles contract similarly to skeletal muscles, but the duration of contraction is much longer.
• 🚀 Conductive fibers have little contractile capacity but are crucial for controlling the heart's rhythmic contractions.
• 🔗 The heart muscle fibers form a network with extensive interconnections, allowing for the propagation of action potentials across the heart.
• 💔 The heart has two functional units: the atrial and the ventricular, separated by fibrous tissue to prevent the mixing of action potentials.
• 🕒 The atrial and ventricular units contract slightly out of sync, which is essential for the functioning of the heart's pump mechanism.
• 📈 The cardiac action potential involves rapid sodium influx, slow calcium channels, and potassium efflux, resulting in a plateau phase that lasts longer than in skeletal muscles.
• 🔄 The plateau phase is crucial for the prolonged duration of ventricular contractions, which is up to 15 times longer than in skeletal muscles.
• 🛠️ Calcium plays a dual role in cardiac contraction: it promotes interaction between actin and myosin filaments and sustains the plateau phase of the action potential.
• 🚫 The refractory period prevents the heart muscle from being re-excited too quickly, ensuring orderly cardiac function.
• 🏃 The speed of action potential conduction in the heart is slower than in nerve fibers, at about 0.3 to 0.5 meters per second.

Q & A

How does the heart muscle function to circulate blood throughout the body?

The heart muscle, or myocardium, contracts to propel blood through the vascular system. It is composed of atrial and ventricular muscles, which contract similarly to skeletal muscles, and specialized excitatory and conductive fibers that control the rhythm of contraction.

What are the three main components of the heart muscle?

The three main components of the heart muscle are the atrial muscle, the ventricular muscle, and the specialized excitatory and conductive fibers.

How do the contractions of atrial and ventricular muscles differ from skeletal muscles?

While the atrial and ventricular muscles contract similarly to skeletal muscles, the duration of their contraction is much longer, which is essential for the efficient circulation of blood.

What is the role of the specialized fibers in the heart muscle?

The specialized fibers, known as the conductive system, have a limited capacity for contraction but are crucial in conducting the action potential and controlling the heart's rhythm.

What are intercalated discs and why are they important in cardiac muscle?

Intercalated discs are junctions between cardiac cells, or cardiomyocytes, that allow the passage of ions, which is essential for the propagation of the action potential and the coordinated contraction of the heart.

How do the sinoatrial (SA) and atrioventricular (AV) nodes contribute to the heart's function?

The SA and AV nodes form two functional units within the heart. The SA node initiates the heartbeat, and the AV node ensures that the atria contract slightly before the ventricles, which is vital for the heart's pumping action.

What is the resting membrane potential of a cardiomyocyte, and how does it change during a heartbeat?

The resting membrane potential of a cardiomyocyte is approximately -85 mV. During a heartbeat, it changes as follows: it rapidly depolarizes to about +20 mV, then plateaus due to the opening of slow calcium channels, and finally repolarizes as potassium channels open and calcium channels close.

Why do ventricular contractions last longer than those in skeletal muscles?

Ventricular contractions last longer due to the plateau phase caused by the slow calcium channels, which remain open longer, allowing for a sustained interaction between actin and myosin filaments and a more prolonged contraction.

What is the refractory period in cardiac muscle, and why is it important?

The refractory period is a time when the cardiac muscle cannot be depolarized again after an action potential. This ensures that the heart does not contract too frequently and allows for a complete relaxation phase, which is essential for efficient blood pumping.

How does the action potential in cardiac muscle cells lead to contraction?

The action potential in cardiac muscle cells causes the release of calcium from the sarcoplasmic reticulum, which then diffuses and interacts with the myofibrils, leading to contraction. Additionally, calcium from the extracellular space enters through T-tubules, enhancing the calcium release and contraction force.

What is the role of sodium-calcium exchangers in the final phase of the cardiac action potential?

In the final phase of the cardiac action potential, sodium-calcium exchangers pump calcium back into the sarcoplasmic reticulum and out of the cell, helping to restore the resting membrane potential and prepare the cell for the next contraction.