Problem 71 - Objective Biology for NEET

Question

Rate of heartbeat is determined by (a) SA node (b) AV node (c) Purkinje fibres (d) Papillary muscles

Step-by-Step Solution

Answer

The rate of heartbeat is determined by the \(a)\) SA node. This is because it's the heart's natural pacemaker and it initiates each heartbeat, regulating the heart rate.

Step 1 Understanding SA node

The SA node, or Sinoatrial node, is often called the heart's natural pacemaker. It initiates each heartbeat and regulates the rate of contractions. Therefore, it plays a key role in controlling the heart rate.

Step 2 Understanding AV node

The AV node, or Atrioventricular node, receives signals from the SA node and passes them along to the ventricles. However, it does not initiate the heartbeat nor control the rate of the heartbeat

Step 3 Understanding Purkinje fibers

Purkinje fibres are responsible for carrying the electrical signals from the AV node to the ventricles of the heart, causing them to contract and pump blood. They do not control the rate of heartbeat.

Step 4 Understanding Papillary muscles

The Papillary muscles are located in the ventricles of the heart. They play a role in preventing the backflow of blood, and are not responsible for controlling the rate of the heartbeat.

Key Concepts

SA Node

The SA Node, or Sinoatrial Node, is often referred to as the heart's natural pacemaker. Located in the right atrium, it is responsible for initiating each heartbeat. This node generates electrical impulses that trigger the heart muscle to contract. By doing so, it regulates the rhythm of the heart, ensuring it beats consistently and efficiently.

Projections from the nervous system also influence the activity of the SA node. This means factors like stress, physical activity, or relaxation can affect heart rate.

• The SA node usually sets the heart rate at around 60 to 100 beats per minute.
• Without the SA node's pacemaking ability, the heart would not have a primary pace to follow.

The SA node is crucial in maintaining the synchronized contractions needed for effective blood circulation.

AV Node

The AV Node, or Atrioventricular Node, acts as a bridge for electrical impulses moving from the atria to the ventricles. Positioned centrally between these chambers, the AV node plays a crucial role in the heart's electrical conduction system.

One of its key functions is to delay the signal it receives from the SA node. This brief pause allows the atria to empty their blood fully into the ventricles before they contract.

• The AV node is secondary to the SA node in controlling the heart's rhythm.
• If the SA node fails, the AV node can take over, though it sets a slower heart rate.

Instead of initiating signal production, its primary responsibility is to ensure coordinated contraction and efficient blood flow.

Purkinje Fibers

Purkinje Fibers are specialized conductive fibers located within the heart's ventricles. They operate at the tail end of the cardiac conduction system, rapidly transmitting electrical impulses from the AV node throughout the ventricles.

The rapid transmission allows for a swift and synchronized contraction of the ventricles, ensuring that blood is effectively pumped out of the heart and into circulation.

• They do not regulate heartbeat initiation or rate but ensure proper conduction.
• The unique structure of these fibers supports quick impulse propagation.

Purkinje Fibers are essential for maintaining the heart’s efficient blood-pumping mechanism and supporting its muscular action.

Papillary Muscles

Papillary Muscles are found in the ventricles of the heart, attached to the heart valves via chordae tendineae. Their key function is to help keep the heart valves closed during ventricular contraction, preventing the backflow of blood.

While they are active participants in heart function, they do not influence heart rhythm or rate.

• These muscles contract during ventricular systole, pulling on the heart valves to stabilize them.
• Proper functioning is essential for preventing blood regurgitation into the atria.

Overall, papillary muscles contribute to the efficiency of heart operation, focusing on valve management rather than rhythm control.