This question tests understanding of how membrane capacitance affects action potential propagation speed. In electrical signaling, neurons propagate action potentials through sequential depolarization of membrane segments, where the rate depends on how quickly each segment can reach threshold voltage. In the demyelinated segment, increased capacitance from 1.0 to 2.5 μF/cm² means more charge must accumulate to achieve the same voltage change (Q = CV). Since the current from the previous segment remains the same but more charge is needed, it takes longer to depolarize this segment to threshold, slowing propagation. Choice B incorrectly assumes that storing more charge speeds propagation, but capacitance actually acts as a barrier to rapid voltage changes. To approach similar problems, remember that higher capacitance requires more time to charge, analogous to filling a larger bucket with the same water flow rate.

This question tests understanding of how ion concentrations determine membrane potential through the Nernst equation. In electrical signaling, the resting membrane potential of neurons primarily depends on K+ permeability and follows the K+ equilibrium potential (EK). When extracellular K+ increases from 4 mM to 12 mM while intracellular K+ remains at 140 mM, the concentration ratio [K+]out/[K+]in increases from 4/140 to 12/140. According to the Nernst equation, this makes EK less negative (depolarizes) because the natural log of a larger ratio yields a less negative value. Choice B incorrectly predicts hyperpolarization, but increasing external K+ always depolarizes by reducing the K+ concentration gradient. To solve Nernst equation problems, remember that increasing external cation concentration or decreasing internal cation concentration makes the equilibrium potential more positive.

This question tests understanding of synaptic currents and excitatory postsynaptic potentials (EPSPs). When ligand-gated channels with a reversal potential of 0 mV open at a membrane potential of -70 mV, there is a 70 mV driving force for net inward current. Since these channels are permeable to both Na+ and K+, and the reversal potential (0 mV) is between ENa (+60 mV) and EK (-90 mV), the net effect is inward positive current that depolarizes the cell toward 0 mV. The correct answer (B) accurately describes this depolarization as an EPSP. Answer A incorrectly suggests net outward current would occur, which would require the membrane potential to be above the reversal potential. To analyze synaptic currents, always compare the membrane potential to the channel's reversal potential: current flows to drive the membrane toward the reversal potential.

This question tests understanding of saltatory conduction in myelinated axons. Myelin acts as an insulator that increases membrane resistance and decreases capacitance in internodal regions, forcing current to jump between nodes of Ranvier where sodium channels are concentrated. When myelin is damaged, the internodal membrane resistance decreases and capacitance increases, allowing more current to leak across the membrane between nodes. This current leak means less depolarizing current reaches the next node, slowing the rate of depolarization and reducing conduction velocity. Option A correctly identifies these effects, while option B incorrectly suggests reduced resistance speeds conduction, option C misunderstands action potential directionality, and option D confuses axonal and synaptic mechanisms. The principle is that myelin integrity is crucial for maintaining fast conduction velocity through efficient current flow between nodes.

This question tests understanding of driving force and current direction in electrical signaling. The driving force for any ion equals the difference between membrane potential and that ion's equilibrium potential (Vm - Eion). For Na+ at Vm = -40 mV with ENa = +60 mV, the driving force is -40 - (+60) = -100 mV. This negative driving force means Na+ flows inward (into the cell) when channels open, as the electrochemical gradient favors Na+ entry. The current is inward because both the electrical gradient (negative inside attracts positive Na+) and concentration gradient (higher Na+ outside) drive Na+ into the cell. Choice A incorrectly assumes negative Vm pushes Na+ out, but driving force direction depends on the difference between Vm and ENa, not Vm alone. To determine current direction, calculate driving force: negative values produce inward current for cations, positive values produce outward current.

This question tests understanding of excitatory synaptic transmission and equilibrium potentials. In electrical signaling between neurons, neurotransmitter binding opens ligand-gated channels that allow specific ions to flow according to their electrochemical gradients. When channels permeable to both Na+ and K+ open, the membrane potential moves toward a weighted average of their equilibrium potentials (ENa = +60 mV and EK = -90 mV), which lies between these values. Since the resting potential is -70 mV, opening these channels causes net positive charge influx (more Na+ enters than K+ exits), resulting in depolarization. Choice A incorrectly states Na+ exits at rest, but the negative membrane potential and positive ENa drive Na+ inward. To solve similar problems, identify which ions can flow through opened channels and compare the membrane potential to each ion's equilibrium potential to determine current direction.

This question tests understanding of voltage-gated ion channel roles in action potential generation. Electrical signaling in neurons requires rapid Na+ influx through voltage-gated channels to generate the characteristic fast upstroke of an action potential. When these Na+ channels are blocked, the depolarizing current injection can still move the membrane potential positive, but without the positive feedback of Na+ channel opening, the membrane cannot generate the explosive depolarization that defines an action potential. The neuron fails to fire because the rapid inward Na+ current, which normally drives the membrane from threshold (-55 mV) to peak (+30-40 mV), is absent. Choice A incorrectly suggests K+ channels could substitute for Na+ in the rising phase, but K+ channels cause repolarization, not depolarization. To analyze ion channel contributions, remember that Na+ channels are essential for the rising phase while K+ channels drive the falling phase of action potentials.

This question tests understanding of calcium's role in neurotransmitter release. In electrical signaling at synapses, voltage-gated Ca2+ channels at presynaptic terminals open during action potentials, allowing Ca2+ influx that triggers vesicle fusion and neurotransmitter release. When the drug blocks 80% of Ca2+ entry, fewer vesicles fuse with the membrane, releasing less neurotransmitter into the synaptic cleft. This reduced neurotransmitter concentration activates fewer postsynaptic receptors, generating a smaller excitatory postsynaptic potential (EPSP). Choice B incorrectly suggests Ca2+ affects postsynaptic K+ channels, but the drug specifically targets presynaptic Ca2+ channels. To approach synaptic transmission problems, trace the sequence: action potential → Ca2+ entry → vesicle fusion → neurotransmitter release → postsynaptic receptor activation → postsynaptic response.

This question tests understanding of the absolute refractory period in neuronal electrical signaling. During an action potential, voltage-gated Na+ channels undergo a cycle of closed → open → inactivated states, and inactivated channels cannot reopen until the membrane repolarizes sufficiently. The absolute refractory period occurs when most Na+ channels remain inactivated, preventing action potential generation regardless of stimulus strength. A second suprathreshold stimulus during this period fails to elicit an action potential because insufficient Na+ channels are available to generate the rapid depolarization required. Choice D incorrectly suggests Na+ conductance increases after a spike, but channel inactivation actually reduces available conductance. To understand refractory periods, remember that Na+ channel inactivation creates an absolute refractory period (no action potential possible) followed by a relative refractory period (higher threshold required).

The skill being tested is the effect of sodium channel inactivation on action potential duration. In neurons, electrical signaling during action potentials involves sodium channels opening for depolarization and then inactivating, while potassium channels open for repolarization. In this scenario, preventing sodium channel inactivation allows persistent sodium influx, prolonging depolarization as it opposes the repolarizing potassium efflux. This results in a longer-lasting positive membrane potential, as in choice B. Choice A is incorrect because the upstroke requires sodium channel opening, not inactivation, which normally limits duration. For similar toxin effects, compare the altered waveform to normal phases and ion currents. A useful strategy is to recall that channel states (open, inactivated) dictate spike shape, so disruptions predict specific changes like extended plateaus.