Neurons and Action Potentials Study Pack
Kibin's free study pack on Neurons and Action Potentials includes a 6-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 27, 2026
Neurons and Action Potentials Study Guide
Trace the full life cycle of an action potential — from resting membrane potential and the sodium-potassium pump to depolarization, repolarization, afterhyperpolarization, and saltatory conduction — so you can confidently explain every phase and the refractory period on your next exam.
Key Takeaways
- •Neurons maintain a resting membrane potential of approximately –70 mV, established by the sodium-potassium ATPase pump and the selective permeability of the membrane to potassium ions.
- •An action potential is an all-or-nothing electrical signal triggered only when membrane depolarization reaches the threshold of approximately –55 mV.
- •The rising phase of an action potential results from rapid opening of voltage-gated sodium channels, allowing Na⁺ to rush into the cell and drive the membrane potential toward +30 mV.
- •Repolarization occurs when voltage-gated sodium channels inactivate and voltage-gated potassium channels open, allowing K⁺ to exit the cell and restore the negative membrane potential.
- •A brief hyperpolarization (afterhyperpolarization) follows repolarization because potassium channels close slowly, temporarily making the membrane more negative than the resting potential.
- •Action potentials propagate along an axon without losing amplitude; in myelinated axons, this propagation is saltatory — the signal jumps between nodes of Ranvier, dramatically increasing conduction velocity.
- •The refractory period — divided into absolute and relative phases — prevents backward propagation of the signal and limits the maximum firing frequency of a neuron.
Neuron Structure and the Resting Membrane Potential
Before a neuron can transmit a signal, it must maintain a stable electrical state across its membrane — a condition called the resting membrane potential that depends on ion distribution and specialized transport proteins.
Functional Anatomy of a Neuron
- •The cell body (soma) contains the nucleus and metabolic machinery; dendrites receive incoming signals; the axon conducts output signals away from the soma.
- •Axons are often wrapped in myelin, a lipid-rich insulating sheath produced by Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system.
- •Gaps in the myelin sheath called nodes of Ranvier are the only sites where voltage-gated ion channels are densely concentrated in myelinated axons.
Establishing the Resting Membrane Potential
- •The resting membrane potential of a typical neuron is approximately –70 mV, meaning the inside of the cell is negatively charged relative to the outside.
- •This charge difference arises because the membrane at rest is far more permeable to K⁺ than to Na⁺; potassium leaks outward through ungated leak channels, carrying positive charge with it.
- •The sodium-potassium ATPase pump actively transports 3 Na⁺ out of the cell and 2 K⁺ into the cell per cycle, consuming ATP to maintain steep concentration gradients for both ions.
- •Large, negatively charged proteins trapped inside the cell also contribute to the negative interior charge.
Threshold, Graded Potentials, and the All-or-Nothing Principle
Not every stimulus a neuron receives is strong enough to generate a full action potential; the membrane first integrates smaller, local voltage changes before deciding whether to fire.
Graded Potentials
- •A graded potential is a small, local change in membrane voltage caused by the opening of ligand-gated or mechanically gated ion channels at dendrites or the soma.
- •Unlike action potentials, graded potentials decay in amplitude with distance and can be either depolarizing (excitatory postsynaptic potentials) or hyperpolarizing (inhibitory postsynaptic potentials).
- •Multiple graded potentials can summate — either simultaneously (spatial summation) or in rapid succession (temporal summation) — at the axon hillock.
Threshold and the All-or-Nothing Rule
- •The axon hillock is the site of action potential initiation because it has the highest density of voltage-gated sodium channels in the neuron.
- •If summated depolarization raises the membrane potential to approximately –55 mV, this threshold is reached and an action potential fires.
- •The all-or-nothing principle states that once threshold is met, the action potential always reaches the same peak voltage regardless of how much the stimulus exceeded threshold; a stimulus below threshold produces no action potential at all.
- •Stimulus intensity is encoded not by the size of individual action potentials but by their frequency — stronger stimuli cause the neuron to fire more action potentials per second.
Phases of the Action Potential
An action potential unfolds as a precise, time-ordered sequence of ion channel openings and closings that produce a characteristic voltage spike lasting roughly 1–2 milliseconds.
Depolarization: The Rising Phase
- •At threshold, voltage-gated sodium channels open rapidly; the electrochemical gradient drives Na⁺ into the cell, making the interior progressively more positive.
- •This influx is self-reinforcing: depolarization opens more voltage-gated sodium channels, which causes more depolarization — a positive feedback loop that drives the membrane potential to approximately +30 mV.
- •At the peak, the interior of the cell is briefly positive relative to the exterior.
Repolarization: The Falling Phase
- •Voltage-gated sodium channels inactivate on their own shortly after opening, closing an inactivation gate and halting Na⁺ entry regardless of the membrane voltage.
- •Voltage-gated potassium channels, which open more slowly than sodium channels, are now fully open; K⁺ exits the cell down its electrochemical gradient, restoring the negative interior charge.
- •The membrane potential rapidly returns toward –70 mV as positive charge leaves the cell.
Afterhyperpolarization
- •Voltage-gated potassium channels do not close immediately when the resting potential is restored; they remain open briefly, allowing continued K⁺ efflux.
- •This drives the membrane potential slightly below the resting value to around –80 mV — a state called afterhyperpolarization or the undershoot.
- •The sodium-potassium ATPase pump works continuously to restore the original ion concentration gradients after repeated firing.
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Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.
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What is the resting membrane potential of a typical neuron?
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Resting Membrane Potential
Explain how a neuron maintains its resting membrane potential of approximately –70 mV. What ions and structures are responsible for creating and sustaining this electrical state?
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