Question

The electrical impulse generated by the SA node travels through the AV node, Purkinje fibers and ventricular muscle respectively at speeds of (in m/s):

• A. \( 0.5, \ 2 \text{ and } 1 \)
• B. \( 1, \ 0.3 \text{ and } 2 \)
• C. \( 0.05, \ 3 \text{ and } 0.5 \)
• D. \( 0.5, \ 1 \text{ and } 2 \)

Hint:

Functional Design of Cardiac Conduction Velocities: - AV Node ($0.05\text{ m/s}$): Slowest conduction speed. This creates the necessary delay that allows the ventricles to fill completely with blood before they contract. - Purkinje System ($3\text{ m/s}$): Fastest conduction speed. This enables nearly simultaneous contraction of the ventricles, maximizing systolic ejection efficiency.

Answer 1

The Correct Option is C

Concept: The heart's coordinate pumping action relies on a specialized cardiac conduction system that generates and distributes electrical action potentials through the myocardium. The velocity at which these action potentials travel varies significantly across different regions of the heart. This variation is highly functional, tailored to optimize the cardiac cycle's mechanical performance.

Step 1: Analyzing the Atrioventricular (AV) Node conduction velocity.
The electrical impulse begins at the Sinoatrial (SA) Node and spreads across the atria. It then reaches the AV Node. The AV node is designed to introduce a critical time delay (approximately $0.1\text{ seconds}$) in the electrical pathway. This delay gives the atria adequate time to fully contract and finish pumping blood into the ventricles before ventricular contraction begins. To create this delay, the AV node tissue has small-diameter cells with fewer gap junctions, resulting in an exceptionally slow conduction velocity of approximately $0.05\text{ m/s}$.

Step 2: Analyzing the Purkinje Fibers conduction velocity.
After exiting the AV node and the Bundle of His, the impulse enters the Purkinje fiber network. These fibers are large-diameter cells specialized for rapid electrical conduction, rich in gap junctions. Their job is to spread the action potential across the entire endocardial surface of both ventricles almost instantly. This rapid distribution ensures the ventricular muscle cells contract in a highly synchronized wave from the apex upward. The conduction velocity in Purkinje fibers is the fastest in the heart, reaching speeds of $2.0 \text{ to } 4.0\text{ m/s}$ (typically cited around $3\text{ m/s}$).

Step 3: Analyzing the Ventricular Muscle conduction velocity.
Once the impulse leaves the terminal Purkinje fibers, it spreads through the working ventricular myocardium from cell to cell via intercalated discs. This muscular conduction proceeds at an intermediate velocity, generally averaging around $0.3 \text{ to } 0.5\text{ m/s}$.

Step 4: Evaluating the correct value sequence.
Matching our velocity values in the requested sequence (AV Node $\rightarrow$ Purkinje Fibers $\rightarrow$ Ventricular Muscle) yields: \[ \text{AV Node} \approx 0.05\text{ m/s}, \quad \text{Purkinje Fibers} \approx 3\text{ m/s}, \quad \text{Ventricular Muscle} \approx 0.5\text{ m/s} \] This sequence corresponds perfectly to the values listed in Option (C).