MCAT Chemical and Physical Foundations of Biological Systems Flashcards: 4c Electrical Signaling Neurons

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This deck focuses on 4c Electrical Signaling Neurons, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Chemical and Physical Foundations of Biological Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: Which direction does $\text{Na}^+$ move through open channels at rest: into or out of the neuron?

Answer: Into the neuron (down its electrochemical gradient). At resting potential, the electrochemical gradient favors sodium influx due to both concentration and electrical driving forces.

Flashcard 2: Which ions have the highest extracellular concentrations in typical neurons: $\text{Na}^+$ or $\text{K}^+$?

Answer: $\text{Na}^+$ has the higher extracellular concentration. Sodium ions are actively pumped out of the neuron by the Na+/K+ ATPase, establishing a higher concentration in the extracellular fluid compared to potassium.

Flashcard 3: Which ions have the highest intracellular concentrations in typical neurons: $\text{Na}^+$ or $\text{K}^+$?

Answer: $\text{K}^+$ has the higher intracellular concentration. Potassium ions are actively pumped into the neuron by the Na+/K+ ATPase, resulting in a higher intracellular concentration than sodium.

Flashcard 4: If $[\text{Na}^+]_\text{out}$ decreases while $[\text{Na}^+]_\text{in}$ is constant, what happens to $E_{Na}$?

Answer: $E_{Na}$ becomes less positive (moves toward $0$). Decreasing extracellular sodium reduces the concentration gradient, making the equilibrium potential less positive per the Nernst equation.

Flashcard 5: What is the typical sign of the resting membrane potential $V_m$ relative to the outside of the cell?

Answer: Negative inside relative to outside (typically about $-70\ \text{mV}$). The resting membrane potential is negative due to the dominance of potassium leak channels and the electrochemical gradients maintained by ion pumps.

Flashcard 6: What is the primary electrogenic pump that maintains $\text{Na}^+$ and $\text{K}^+$ gradients in neurons?

Answer: $\text{Na}^+/\text{K}^+$ ATPase. The Na+/K+ ATPase actively transports sodium out and potassium in, using ATP to maintain steep ion gradients across the neuronal membrane.

Flashcard 7: What is the net ion movement per cycle of the $\text{Na}^+/\text{K}^+$ ATPase?

Answer: $3\ \text{Na}^+$ out and $2\ \text{K}^+$ in (net $+1$ out). Each cycle hydrolyzes ATP to export three sodium ions and import two potassium ions, contributing to the membrane potential by net positive charge efflux.

Flashcard 8: What type of ion channel primarily sets the resting membrane potential in many neurons?

Answer: $\text{K}^+$ leak channels. Potassium leak channels allow greater passive flux of K+ than other ions at rest, driving the membrane potential close to the potassium equilibrium potential.

Flashcard 9: What is the definition of the equilibrium (Nernst) potential $E_{ion}$ for a given ion?

Answer: The $V_m$ at which net flux of that ion is $0$. The equilibrium potential balances the chemical driving force from concentration gradients with the electrical driving force, resulting in zero net ion movement.

Flashcard 10: State the Nernst equation for an ion of valence $z$ using natural logarithms.

Answer: $E=\frac{RT}{zF}\ln\left(\frac{[\text{out}]}{[\text{in}]}\right)$. The Nernst equation calculates the reversal potential based on temperature, valence, and the ratio of extracellular to intracellular ion concentrations.

Flashcard 11: What is the approximate Nernst equation at $37^\circ\text{C}$ using base-10 logs?

Answer: $E\approx\frac{61\ \text{mV}}{z}\log\left(\frac{[\text{out}]}{[\text{in}]}\right)$. At body temperature, the Nernst equation simplifies to this form for easier computation using base-10 logarithms and millivolt units.

Flashcard 12: If $[\text{K}^+]_\text{out}$ increases while $[\text{K}^+]_\text{in}$ is constant, what happens to $E_K$?

Answer: $E_K$ becomes less negative (moves toward $0$). Increasing extracellular potassium reduces the concentration gradient, shifting the equilibrium potential toward zero according to the Nernst equation.

Flashcard 13: Which direction does $\text{K}^+$ move through open channels at rest: into or out of the neuron?

Answer: Out of the neuron (down its concentration gradient). At resting potential, the concentration gradient drives potassium efflux, outweighing the opposing electrical gradient.

Flashcard 14: What is depolarization of a neuron in terms of membrane potential $V_m$?

Answer: $V_m$ becomes less negative (moves toward $0$ or positive). Depolarization occurs when positive ion influx or negative ion efflux reduces the magnitude of the negative membrane potential.

Flashcard 15: What is hyperpolarization of a neuron in terms of membrane potential $V_m$?

Answer: $V_m$ becomes more negative than resting potential. Hyperpolarization results from increased potassium efflux or chloride influx, enhancing the negative charge inside the neuron.

Flashcard 16: What is the typical threshold membrane potential for initiating an action potential?

Answer: Approximately $-55\ \text{mV}$ (cell-type dependent). Threshold is the voltage at which voltage-gated sodium channels open sufficiently to trigger regenerative depolarization in the action potential.

Flashcard 17: During the rising phase of an action potential, which voltage-gated channel opens first?

Answer: Voltage-gated $\text{Na}^+$ channels. These channels activate at threshold, allowing rapid sodium influx that drives the membrane potential toward the sodium equilibrium potential.

Flashcard 18: What causes the falling phase (repolarization) of the neuronal action potential?

Answer: $\text{Na}^+$ channel inactivation plus opening of voltage-gated $\text{K}^+$ channels. Sodium channel inactivation halts influx, while potassium channel opening promotes efflux, restoring the membrane potential to resting levels.

Flashcard 19: What is the absolute refractory period defined by in voltage-gated $\text{Na}^+$ channels?

Answer: Period when inactivated $\text{Na}^+$ channels cannot reopen. During this period, sodium channels remain in an inactivated state, preventing further action potentials regardless of stimulus strength.

Flashcard 20: What is the relative refractory period primarily due to in an action potential?

Answer: Persistently open $\text{K}^+$ channels and hyperpolarization. Delayed closure of potassium channels causes afterhyperpolarization, raising the threshold for subsequent action potentials.

Flashcard 21: Identify the property that makes action potentials non-decremental along the axon.

Answer: Regenerative opening of voltage-gated channels along the membrane. Local depolarization triggers sequential activation of voltage-gated channels, regenerating the full amplitude of the action potential at each point along the axon.

Flashcard 22: Which change increases action potential conduction velocity more: increased myelination or decreased myelination?

Answer: Increased myelination. Myelination insulates the axon, reducing capacitance and enabling faster saltatory conduction between nodes of Ranvier.

Flashcard 23: In myelinated axons, at which structures are action potentials regenerated?

Answer: Nodes of Ranvier. These unmyelinated gaps concentrate voltage-gated channels, allowing regeneration of the action potential and efficient propagation.

Flashcard 24: What is saltatory conduction?

Answer: Apparent "jumping" of depolarization between nodes in myelinated axons. Myelin prevents ion flux between nodes, allowing passive current spread that depolarizes the next node, speeding conduction.

Flashcard 25: If extracellular $\text{K}^+$ rises (hyperkalemia), what is the immediate effect on resting $V_m$?

Answer: Resting $V_m$ depolarizes (becomes less negative). Elevated extracellular potassium shifts the potassium equilibrium potential less negative, pulling the resting potential toward it via leak channels.