Postsynaptic potential (PSP)

A postsynaptic potential (PSP) is a change in the membrane voltage of the receiving neuron after neurotransmitter binds to its receptors. In Anatomy and Physiology I, it is the signal that can push the neuron toward or away from firing an action potential.

What is postsynaptic potential (PSP)?

A postsynaptic potential (PSP) is the change in membrane potential that happens in the postsynaptic neuron after a neurotransmitter binds to receptors on its membrane. In Anatomy and Physiology I, this is the electrical response that follows chemical communication at a synapse.

PSPs happen after an action potential reaches the presynaptic neuron’s axon terminal and triggers neurotransmitter release into the synaptic cleft. The neurotransmitter then diffuses across the gap and binds to receptors on the postsynaptic cell. That receptor binding opens or closes ion channels, changing the electrical charge across the postsynaptic membrane.

There are two main kinds of PSPs. An excitatory postsynaptic potential (EPSP) makes the membrane less negative, usually by allowing positive ions in or reducing the outflow of positive charge. That moves the neuron closer to threshold, which is the point where an action potential can start. An inhibitory postsynaptic potential (IPSP) makes the membrane more negative or stabilizes it farther from threshold, which makes firing less likely.

PSPs are graded, not all-or-none. That means a small amount of neurotransmitter may produce a small voltage change, while more neurotransmitter or more receptor activation may create a larger one. This is different from an action potential, which follows the all-or-none rule once threshold is reached.

A neuron often receives many PSPs at once. It adds them up at the axon hillock, the part of the neuron that acts like the decision point for firing. If the total effect of the PSPs brings the membrane to threshold, the neuron generates an action potential. If inhibitory inputs outweigh excitatory ones, the neuron stays quiet.

A simple way to picture it is this: the synapse does not automatically make the next neuron fire. It only changes the odds. PSPs are the local voltage shifts that determine whether the receiving neuron is being pushed toward firing or held back from it.

Why postsynaptic potential (PSP) matters in Anatomy and Physiology I

Postsynaptic potentials are the bridge between chemical signaling and electrical activity in the nervous system. If you are tracing how a message moves from one neuron to the next, PSPs are the step that explains why a neurotransmitter can either encourage or suppress a response.

This term also connects several big ideas in Anatomy and Physiology I: synapses, membrane potential, ion channels, threshold, and homeostasis. When you can explain PSPs, you can explain why the same nervous system uses both excitatory and inhibitory signaling instead of sending every message as a simple yes/no switch.

PSPs matter for reading diagrams and describing nervous system function. If a figure shows neurotransmitter release at a synapse, you should be able to say what happens next on the postsynaptic side and predict whether the membrane becomes more likely or less likely to fire. That shows up in labeling exercises, short-answer questions, and lab-style visuals of neurons.

This concept also helps you make sense of why some drugs, toxins, and neurotransmitters have such noticeable effects. Anything that changes receptor activity, ion movement, or the balance of EPSPs and IPSPs can change how excitable a neuron is. That is the core logic behind many nervous system case questions in A&P.

How postsynaptic potential (PSP) connects across the course

Action Potential

A PSP is the event that can lead to an action potential, but it is not the same thing. PSPs are graded voltage changes in the postsynaptic membrane, while an action potential is the all-or-none spike that happens only if threshold is reached. If you are tracing neuron communication, PSPs come first and help decide whether the action potential will happen next.

Synapse

A PSP happens at a synapse, specifically on the postsynaptic side after neurotransmitter release. The synapse is the junction where the signal crosses from one cell to another. Without the synapse, there would be no neurotransmitter binding and no postsynaptic voltage change to measure or describe.

Neurotransmitter

The neurotransmitter is what triggers the PSP by binding to receptors on the postsynaptic membrane. Different neurotransmitters can have different effects depending on the receptor they bind to, so the chemical signal does not always produce the same voltage change. That is why the receptor matters as much as the neurotransmitter itself.

Calcium Ions

Calcium ions act upstream of the PSP because they enter the presynaptic axon terminal and help trigger neurotransmitter release. Calcium does not usually create the postsynaptic voltage change directly, but without calcium influx, the neurotransmitter would not be released efficiently. So calcium is part of the chain that makes the PSP possible.

Is postsynaptic potential (PSP) on the Anatomy and Physiology I exam?

A quiz question may give you a synapse diagram and ask what happens after neurotransmitter binds, and the correct move is to identify the postsynaptic potential as the membrane voltage change in the receiving neuron. In a labeling item, you may need to tell whether the PSP is excitatory or inhibitory based on whether it moves the membrane toward or away from threshold. In a short-answer question, you might trace the sequence from action potential at the axon terminal, to neurotransmitter release, to PSP, to possible firing at the axon hillock. If you get a case about a drug or toxin that changes signaling, PSP is the step you use to explain why the postsynaptic neuron becomes more or less excitable.

Postsynaptic potential (PSP) vs Action Potential

These are related but not the same. A postsynaptic potential is a graded change in membrane voltage in the receiving neuron after neurotransmitter binding, while an action potential is the rapid, all-or-none electrical signal that occurs if threshold is reached. PSPs can be small or large and can be excitatory or inhibitory. Action potentials, once triggered, follow one standard pattern.

Key things to remember about postsynaptic potential (PSP)

• A postsynaptic potential is the voltage change in the postsynaptic neuron after neurotransmitter binds to receptors.

• PSPs can be excitatory or inhibitory, so they can bring the neuron closer to threshold or move it farther away.

• PSPs are graded signals, which means their size can vary based on how strong the synaptic input is.

• The neuron adds up multiple PSPs at the axon hillock to decide whether to fire an action potential.

• In Anatomy and Physiology I, PSPs are the next step after synaptic neurotransmitter release, so they connect chemical signaling to electrical activity.

Frequently asked questions about postsynaptic potential (PSP)

What is postsynaptic potential (PSP) in Anatomy and Physiology I?

A postsynaptic potential is the change in membrane voltage in the receiving neuron after a neurotransmitter binds to its receptors. It can be excitatory or inhibitory, depending on which ion channels are affected. In A&P I, PSPs are the electrical response that follows chemical signaling at a synapse.

Is a postsynaptic potential the same as an action potential?

No. A PSP is a graded change in voltage, while an action potential is an all-or-none spike that happens only if threshold is reached. PSPs can help trigger an action potential, but they are not the spike itself.

What causes an EPSP or IPSP?

An EPSP happens when the postsynaptic membrane becomes less negative and moves closer to threshold. An IPSP happens when the membrane becomes more negative or is stabilized farther from threshold. The exact effect depends on the receptor and ion channels involved.

How do I identify a PSP on a neuron diagram or in a lab question?

Look for the postsynaptic side of the synapse, where neurotransmitter receptors are located. If the question describes a voltage change after neurotransmitter binding, that change is the PSP. If it asks whether the neuron is more likely to fire, you are usually being asked to interpret the PSP as excitatory or inhibitory.