Problem 8 - Entrance Test

* Voltage-gated Na+ channels are crucial for the initiation and propagation of action potentials. When a neuron reaches threshold potential, these channels rapidly open, allowing a massive influx of Na+ ions into the cell. This influx causes the rapid depolarization phase of the action potential, driving the membrane potential from negative to positive. * Blocking these channels means that even if a stimulus reaches the threshold, the necessary rapid influx of Na+ cannot occur. Without this rapid depolarization, an action potential cannot be generated or propagated. Let's analyze the options: A. Blocking Na+ channels would prevent Na+ influx, but it wouldn't directly affect the resting membrane potential (which is primarily maintained by K+ leak channels and the Na+/K+ pump). It would lead to inability to excite, not hyperexcitability. Na+ exiting the cell is not the primary mechanism of depolarization. B. This is correct. The depolarization phase of the action potential is entirely dependent on the opening of voltage-gated Na+ channels and the subsequent rapid influx of Na+ ions. If these channels are blocked, the neuron cannot depolarize sufficiently to generate an action potential. C. The repolarization phase is primarily mediated by the efflux of K+ ions through voltage-gated K+ channels. While Na+ channel blockage impacts depolarization, it doesn't directly cause prolonged repolarization due to Na+ accumulation. In fact, if an action potential can't be generated, there's no repolarization phase to prolong. D. Saltatory conduction relies on the rapid depolarization at the Nodes of Ranvier, which is dependent on voltage-gated Na+ channels. Blocking these channels would inhibit saltatory conduction, not enhance it. Increased K+ permeability would typically lead to hyperpolarization or stabilization, making it harder to reach threshold.