🫀 Physiology intermediate Lesson 2 of 4 4 min read

Membrane Potentials and the Nerve Impulse

How neurons build and use electrical charge across their membranes, from the resting potential and the sodium-potassium pump to the action potential, saltatory conduction, and synaptic transmission.

Reading level

What you'll learn

Explain how the resting membrane potential is established and maintained.
Describe the role of the sodium-potassium pump and ion channels in generating charge.
Sequence the phases of an action potential from threshold through repolarization.
Explain saltatory conduction and the steps of chemical synaptic transmission.

Overview

Nerve and muscle cells are excitable: they can store electrical charge across their membranes and release it in a controlled burst to send signals. That stored charge is the membrane potential, and the traveling burst is the action potential, the nerve impulse. This lesson explains how a neuron builds its charge at rest, fires an impulse, and passes the message to the next cell.

The Resting Membrane Potential

A resting neuron is electrically polarized: the inside is negative relative to the outside, typically about negative 70 millivolts. Two features create this.

First, ions are unevenly distributed. Sodium (Na+) is concentrated outside the cell and potassium (K+) inside, while large negatively charged proteins are trapped inside. Second, the membrane is selectively permeable: at rest it is far more permeable to potassium than to sodium, so potassium leaks outward and leaves the interior negative.

These gradients would run down without maintenance. The sodium-potassium pump prevents that, actively pumping three sodium ions out for every two potassium ions in, using ATP. It restores what leaks and keeps the gradients ready to power an impulse.

Reaching Threshold

A stimulus that makes the inside less negative causes depolarization. Small stimuli produce small local changes that fade. But if depolarization reaches threshold, near negative 55 millivolts, it triggers a full action potential. Below threshold, nothing propagates; at or above it, the impulse fires completely. This is the all-or-none principle: the action potential does not grow larger with a stronger stimulus. Instead, the nervous system codes stimulus intensity as the frequency of impulses.

The Action Potential

Once threshold is crossed, a rapid, stereotyped sequence unfolds.

Phase	Ion movement	Effect on voltage
Depolarization	Voltage-gated Na+ channels open; sodium rushes in	Rises toward and past 0 mV (peak near +30 mV)
Repolarization	Na+ channels inactivate; voltage-gated K+ channels open; potassium exits	Falls back toward negative
Hyperpolarization	K+ channels close slowly; slight overshoot below rest	Briefly more negative than resting
Return to rest	Pump and leak channels restore balance	Back to about negative 70 mV

During and just after the spike, the neuron passes through a refractory period when it cannot easily fire again. This ensures impulses travel in one direction, down the axon, and limits how rapidly they can repeat.

Conduction Along the Axon

An action potential at one point depolarizes the neighboring membrane to threshold, regenerating the impulse further along. On bare (unmyelinated) axons this happens continuously and is relatively slow.

Many axons are wrapped in myelin, a fatty insulating sheath with periodic gaps called nodes of Ranvier. Voltage-gated channels cluster at the nodes, so the action potential regenerates only there and effectively jumps from node to node. This is saltatory conduction, and it makes myelinated fibers dramatically faster and more energy efficient than unmyelinated ones.

Synaptic Transmission

Where a neuron meets its target lies a synapse, usually with a narrow gap between the cells. Most synapses are chemical and work in steps:

The action potential reaches the axon terminal and depolarizes it.
Voltage-gated calcium channels open, and calcium enters the terminal.
Calcium triggers vesicles to fuse with the membrane and release neurotransmitter into the gap.
Neurotransmitter diffuses across and binds receptors on the target cell.
Binding opens channels there, either depolarizing the target (excitatory) or making it more negative (inhibitory).

The signal is then terminated as neurotransmitter is broken down, taken back up, or diffuses away, readying the synapse for the next impulse.

Clinical Relevance

Membrane physiology underlies much of clinical medicine. Multiple sclerosis destroys myelin, so saltatory conduction fails and impulses slow or stall, producing weakness, numbness, and visual loss. Local anesthetics such as lidocaine block voltage-gated sodium channels, preventing depolarization from reaching threshold so pain impulses cannot form. Abnormal blood potassium shifts the resting potential and can make the heart dangerously unexcitable or overexcitable, which is why potassium is watched closely in emergency and nursing care. And because ion pumps require ATP, loss of oxygen or fuel to a tissue quickly collapses these gradients, one reason nerve and cardiac cells are so vulnerable to ischemia.

Going deeper advanced

Extra depth for when you're ready — expanded automatically in Advanced mode.

From the Nernst equation to Goldman-Hodgkin-Katz

The Nernst equation gives the equilibrium potential for a single ion from its concentration gradient, predicting roughly +60 mV for sodium and about -90 mV for potassium at body temperature. The resting membrane sits near potassium's equilibrium because it is far more permeable to potassium than to sodium, but not exactly at it. The Goldman-Hodgkin-Katz equation generalizes this by weighting each permeant ion's equilibrium potential by its relative permeability, which is why opening sodium channels during an action potential swings the potential toward the sodium equilibrium value.

Refractory periods and synaptic summation

During the absolute refractory period no stimulus can trigger another spike because voltage-gated sodium channels are inactivated rather than merely closed; in the relative refractory period a stronger-than-normal stimulus can fire because some channels have recovered while potassium conductance remains elevated. These periods cap firing frequency and enforce one-way propagation. At the postsynaptic membrane, individual excitatory (EPSP) and inhibitory (IPSP) potentials are graded and add together through spatial summation across many synapses and temporal summation of rapid inputs, and the neuron fires only if their net effect reaches threshold at the axon hillock.

Key terms

Resting membrane potential: The steady voltage across a resting neuron's membrane, typically about negative 70 millivolts inside relative to outside.
Sodium-potassium pump: An active transporter that moves three sodium ions out and two potassium ions into the cell per cycle, using ATP to maintain ion gradients.
Depolarization: A shift of the membrane potential toward zero or positive values, usually caused by sodium entering the cell.
Repolarization: The return of the membrane potential toward its negative resting value, driven mainly by potassium leaving the cell.
Threshold: The critical level of depolarization, near negative 55 millivolts, that must be reached to trigger an action potential.
Action potential: A rapid, all-or-none reversal and recovery of membrane potential that travels along an axon as a nerve impulse.
Saltatory conduction: Rapid impulse propagation in which the action potential jumps between nodes of Ranvier on a myelinated axon.
Synapse: The junction where a neuron communicates with another cell, typically by releasing neurotransmitter across a small gap.
Neurotransmitter: A chemical messenger released from a neuron that binds receptors on the target cell to pass along a signal.

Check your understanding

6 questions · answers reveal instantly.

1.What primarily maintains the ion gradients that underlie the resting membrane potential?
2.During the rising phase of an action potential, which ion movement causes depolarization?
3.The 'all-or-none' property of the action potential means that:
4.Saltatory conduction increases conduction speed because the impulse:
5.At a chemical synapse, the arrival of an action potential at the axon terminal triggers the entry of which ion to cause neurotransmitter release?
6.During repolarization, the membrane potential returns toward its resting value mainly because:

Citations & References

Links open publicly available educational and peer-reviewed sources.