Basics: neuroscience and psychophysics

6 Action Potentials

Learning Objectives

Distinguish between action potentials and graded potentials

Understand that action potentials are all-or-nothing events—size doesn’t matter, rate does

Understand that both the rate and timing of action potentials have a random component

The Membrane Potential

The electrical state of the cell membrane can have several variations. These are all variations in the membrane potential. A potential is a distribution of charge across the cell membrane, measured in millivolts (mV). The standard is to compare the inside of the cell relative to the outside, so the membrane potential is a value representing the charge on the intracellular side of the membrane based on the outside being zero, relatively speaking.

Illustration of a lipid membrane with a voltmeter and an electrode stuck through the membrane.
Figure 6.1. Measuring Charge across a Membrane with a Voltmeter A recording electrode is inserted into the cell and a reference electrode is outside the cell. By comparing the charge measured by these two electrodes, the transmembrane voltage is determined. It is conventional to express that value for the cytosol relative to the outside. (credit: OpenStax Anatomy and Physiology,

Action Potentials vs. Graded Potentials

Resting membrane potential describes the steady-state of the cell, which is a dynamic process that is balanced by ion leakage and ion pumping. Without any outside influence, it will not change. To get an electrical signal started, the membrane potential has to change.

Each neuron, and each section of each neuron, has a specific distribution of ion channels in the cell membrane that determines how the membrane potential can change. Axons have the right distributions and types of Na+ and K+ channels to support action potentials, which are all-or-nothing events (Figure 6.2). In dendrites, on the other hand, and in specialized sensory neurons like the photoreceptors in the retina, graded potentials happen instead of action potentials (Figure 6.3). Unlike action potentials, graded potentials can be different sizes and can be positive or negative. Therefore, graded potentials are analog signals while action potentials are digital signals.

a plot showing the rise and fall of membrane potential during an action potential, with depolarization, repolarization, and hyperpolarization phases labeled
Figure 6.2. Stages of an Action Potential Plotting Voltage. Measured across the cell membrane against time, the events of the action potential can be related to specific changes in the membrane voltage. (1) At rest, the membrane voltage is -70 mV. (2) The membrane begins to depolarize when an external stimulus is applied. (3) The membrane voltage begins a rapid rise toward +30 mV. (4) The membrane voltage starts to return to a negative value. (5) Repolarization continues past the resting membrane voltage, resulting in hyperpolarization. (6) The membrane voltage returns to the resting value shortly after hyperpolarization. (credit: OpenStax Anatomy and Physiology, Section 12.4 The Action Potential)


Voltage trace from a cell showing blocks of time during which membrane potential decreases or increases.
Figure 6.3. Graded potentials are temporary changes in the membrane voltage, the characteristics of which depend on the size of the stimulus. Some types of stimuli cause depolarization of the membrane, whereas others cause hyperpolarization. It depends on the specific ion channels that are activated in the cell membrane. (credit: OpenStax Anatomy and Physiology, Section 12.5 Communication Between Neurons)

For the unipolar cells of sensory neurons—both those with free nerve endings and those within encapsulations—graded potentials develop in the dendrites that influence the generation of an action potential in the axon of the same cell. This is called a generator potential. For other sensory receptor cells, such as taste cells or photoreceptors of the retina, graded potentials in their membranes result in the release of neurotransmitters at synapses with sensory neurons. This is called a receptor potential.

A postsynaptic potential (PSP) is the graded potential in the dendrites of a neuron that is receiving synapses from other cells. Postsynaptic potentials can be depolarizing or hyperpolarizing. Depolarization in a postsynaptic potential is called an excitatory postsynaptic potential (EPSP) because it causes the membrane potential to move toward threshold. Hyperpolarization in a postsynaptic potential is an inhibitory postsynaptic potential (IPSP) because it causes the membrane potential to move away from threshold.

Cartoon of a neuron with a box around the base of the axon indicating that's where the voltage traces, which swoop up and down in a panel on the bottom of the figure, are measured
Figure 6.4. Postsynaptic Potential Summation. The result of summation of postsynaptic potentials is the overall change in the membrane potential. At time A, several different excitatory postsynaptic potentials add up to a large depolarization. At time B, a mix of excitatory and inhibitory postsynaptic potentials result in a different end result for the membrane potential. (credit: OpenStax Anatomy and Physiology, Section 12.5 Communication Between Neurons)

Variability in Action Potential Rates and Times

Given the complex mechanisms that generate action potentials, it is no wonder that there is a bit of randomness in the process. A sensory neuron will not respond in the exact same way every time a stimulus is present. On average, a more intense stimulus will result in stronger graded potentials and, after summation, a higher rate of action potentials. But on each trial (each time the stimulus is applied), the rate will not be exactly the same, and the timing of all the spikes in the spike train will not be the same. Sometimes, there are systematic differences. For example, neurons adapt to stimuli, and respond more weakly after repeated trials. On top of this, there are also random differences. In order to support perception, neural networks must evolve efficient strategies for making perceptual decisions even in the presence of sensory noise.


OpenStax, Anatomy and Physiology Section 12.4 Action Potential
Provided by: Rice University.
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OpenStax, Anatomy and Physiology Section 12.5 Communication Between Neurons
Provided by: Rice University.
Access for free at
License: CC-BY 4.0



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Introduction to Sensation and Perception Copyright © 2022 by Students of PSY 3031 and Edited by Dr. Cheryl Olman is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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