Which of the following causes increase in conduction velocity of impulse through hea ?
**Core Concept**
The heart's conduction system relies on the coordinated electrical activity of cardiac myocytes, which is influenced by various factors that can either increase or decrease conduction velocity. Conduction velocity is primarily determined by the properties of the cardiac myocytes, including the presence of gap junctions and the density of sodium channels.
**Why the Correct Answer is Right**
The increase in conduction velocity of the impulse through the heart is primarily due to the presence of gap junctions, specifically connexin-43, which form electrical synapses between adjacent cardiac myocytes. These gap junctions allow for the rapid exchange of ions, enabling the efficient transmission of the action potential. Additionally, the density of sodium channels, particularly the fast sodium channels (Nav1.5), plays a crucial role in determining the conduction velocity, as they are responsible for the initial rapid depolarization of the cardiac myocytes.
**Why Each Wrong Option is Incorrect**
**Option A:** The autonomic nervous system can modulate heart rate and contractility but does not directly affect conduction velocity.
**Option B:** The presence of fibrosis in the heart can actually decrease conduction velocity by disrupting the normal electrical pathways.
**Option C:** The use of certain medications, such as beta-blockers, can decrease conduction velocity by reducing the automaticity of the sinoatrial node.
**Clinical Pearl / High-Yield Fact**
The presence of gap junctions and the density of sodium channels are critical determinants of conduction velocity in the heart. Abnormalities in these structures or function can lead to arrhythmias, such as Wolff-Parkinson-White syndrome, which is characterized by an accessory electrical pathway that can cause rapid conduction of impulses.
**Correct Answer:** C. The density of sodium channels, particularly the fast sodium channels (Nav1.5).