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Excitatory synapses make a postsynaptic neuron more likely to fire an action potential; inhibitory synapses make it less likely. The distinction is functional, not simply a matter of whether the membrane voltage rises or falls. The postsynaptic receptor determines which ions can move, the ion gradients set that synapse’s reversal potential, and the reversal potential’s position relative to spike threshold determines whether the input promotes or suppresses firing.
Glutamate is commonly excitatory in the brain, while GABA is commonly inhibitory there and glycine is especially important for inhibition in the spinal cord. Those are useful rules of thumb, not universal labels: the same transmitter can have different effects when it acts on different receptors, in a different developmental state, or under different ionic conditions.
What “excitatory” and “inhibitory” mean
A chemical synapse converts a presynaptic neuron’s electrical signal into a local postsynaptic potential (PSP). A PSP is graded: its size depends on factors such as the amount of transmitter released, receptor conductance, membrane resistance, and the postsynaptic cell’s voltage at that moment.
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Researchers call a PSP excitatory when it increases the probability that the postsynaptic neuron will generate an action potential. The resulting event is an excitatory postsynaptic potential (EPSP). A PSP is inhibitory when it decreases the probability of an action potential; that event is an inhibitory postsynaptic potential (IPSP).
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This probability-based definition is the one emphasized by Purves and colleagues in Neuroscience (2nd edition, 2001). It avoids a common mistake: defining excitation as any depolarization and inhibition as any hyperpolarization. A depolarizing current can still inhibit a neuron if it clamps the membrane away from the spike threshold and drains excitatory current through a process called shunting.
Excitatory and inhibitory synapses compared
| Feature | Excitatory synapse | Inhibitory synapse |
|---|---|---|
| Effect on spike probability | Raises the likelihood of a postsynaptic action potential | Lowers the likelihood of a postsynaptic action potential |
| Postsynaptic event | EPSP | IPSP |
| Typical ion conductance | Often a nonselective cation conductance, allowing positive charge to enter or redistribute | Often chloride conductance; some inhibitory receptors increase potassium conductance |
| Common reversal-potential relationship | Drives the membrane toward a voltage nearer to or above spike threshold | Keeps the membrane below threshold or shunts incoming excitation |
| Common transmitter examples | Glutamate in many brain circuits | GABA in many brain circuits; glycine prominently in the spinal cord |
| Typical anatomical arrangement | Frequently on dendrites and dendritic spines; exact placement depends on the cell and circuit | May be on dendrites, the soma, or other strategic locations; placement determines how strongly it controls nearby excitation |
| How inputs combine | EPSPs add across space and time | IPSPs subtract from excitation or reduce its effect through shunting |
These are typical patterns rather than definitions. The receptor and the postsynaptic cell’s ionic environment decide the actual effect in a particular synapse.
Why the reversal potential matters more than voltage direction
When transmitter-gated channels open, the membrane voltage moves toward the channels’ reversal potential—the voltage at which the net current through that conductance would be zero. Whether this movement is excitatory or inhibitory depends on where that reversal potential lies relative to the voltage required to trigger an action potential in that cell.
Purves describes the key rule this way: the factor distinguishing excitation from inhibition is the reversal potential of the postsynaptic potential in relation to the action-potential threshold. A conductance with a reversal potential above threshold can push the membrane toward firing; one with a reversal potential below threshold can hold it away from firing or weaken another input that is trying to reach threshold.
A textbook excitatory example
In the worked example from Neuroscience, a glutamatergic synapse has a reversal potential of approximately 0 mV. The illustrative resting potential is about −60 mV and the action-potential threshold about −40 mV. Opening the synaptic conductance therefore drives the membrane upward from rest toward threshold, producing an EPSP.
Those three values are teaching values for that example, not universal constants. Real resting potentials, thresholds, and reversal potentials vary among neurons, synapse types, developmental stages, and recording conditions.
Why a voltage change alone can mislead
Suppose an inhibitory chloride conductance has a reversal potential slightly more positive than the cell’s resting voltage but still well below spike threshold. Opening it might depolarize the cell by a few millivolts. Nevertheless, the conductance can be inhibitory because it prevents the membrane from moving farther toward threshold and diverts current from simultaneous EPSPs.
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Conversely, an inhibitory potassium conductance can drive the membrane more negative and produce a clearly visible hyperpolarizing IPSP. Both effects are inhibitory, but only one requires a large downward voltage deflection.
How excitatory synapses produce EPSPs
Glutamate is the main excitatory amino-acid transmitter in many brain circuits. At a typical glutamatergic contact, transmitter binds postsynaptic receptors that open nonselective cation channels. The resulting conductance changes the local membrane current and usually moves the voltage toward a reversal potential near 0 mV, creating an EPSP from a negative resting potential.
Excitatory contacts are often classified anatomically as Type I synapses. They commonly occur on dendritic shafts and dendritic spines, where they receive signals from other neurons. A spine can compartmentalize the local chemical event, while the dendrite conducts the resulting voltage and current toward the cell body and action-potential trigger zone. The exact impact still depends on distance, dendritic properties, and the other conductances active in the cell.
An EPSP is not an action potential. It is local and graded, and it weakens as it spreads. A single EPSP may be too small to matter at the trigger zone, while many appropriately timed or positioned EPSPs can bring the membrane to threshold.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsHow inhibitory synapses produce IPSPs
GABA is a principal inhibitory transmitter in the brain, and glycine is a principal inhibitory transmitter in the spinal cord. Their receptors commonly increase chloride conductance. Other inhibitory receptors increase potassium conductance. In either case, the resulting reversal potential and conductance change reduce the chance that the postsynaptic membrane will reach spike threshold.
Hyperpolarizing inhibition
If the inhibitory reversal potential is more negative than the membrane voltage, opening the channels drives the membrane downward. The resulting hyperpolarizing IPSP increases the distance to threshold, so a larger excitatory input is needed to fire.
Shunting inhibition
In shunting inhibition, the membrane voltage may change little. The newly opened inhibitory conductance lowers the effective membrane resistance, providing a pathway through which excitatory current leaks. An EPSP arriving at the same time is therefore smaller at the trigger zone, even if the inhibitory synapse produces no obvious hyperpolarizing trace.
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Shunting is why “inhibitory equals hyperpolarizing” is an incomplete rule. A neuron can be held near its resting voltage yet become substantially harder to excite.
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An IPSP can even be slightly depolarizing and remain inhibitory when its reversal potential is below the action-potential threshold. The membrane moves upward, but the inhibitory conductance still limits the voltage range available for excitation or shunts concurrent EPSPs.
Neurotransmitters are not intrinsically excitatory or inhibitory
It is convenient to memorize “glutamate excites” and “GABA inhibits,” but the transmitter molecule alone does not determine the outcome. The postsynaptic receptor’s ion selectivity is decisive. The chloride and potassium gradients, the cell’s membrane voltage, and the developmental state of the neuron also matter.
For example, two cells exposed to the same transmitter can respond differently if they express different receptor types or maintain different ion concentrations. The same receptor can also have a different effect if ion gradients change. Therefore, a transmitter should be described as commonly excitatory or inhibitory in a stated circuit, not as universally one or the other.
How EPSPs and IPSPs add up
Neurons continuously combine many graded PSPs. The decisive calculation occurs near the action-potential trigger zone, usually close to the axon initial segment, where voltage-dependent channels can convert a sufficiently large depolarization into an all-or-none spike.
Spatial summation
Spatial summation occurs when signals from different synapses overlap in time. EPSPs arriving on several dendrites can add together. An IPSP near the soma or along a pathway carrying those EPSPs can subtract from them or shunt their current. Distance and dendritic filtering mean that equal-sized local PSPs do not necessarily have equal effects at the trigger zone.
Temporal summation
Temporal summation occurs when one synapse is activated repeatedly before its previous PSP has fully decayed. Closely spaced EPSPs can build a larger depolarization. Closely spaced IPSPs can maintain an inhibitory conductance and prevent that build-up. The membrane time constant and the synapse’s kinetics determine how much overlap occurs.
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The net decision
- A presynaptic action potential reaches the axon terminal.
- Transmitter is released into the synaptic cleft.
- Postsynaptic receptors open or activate their associated conductances.
- The membrane moves toward each conductance’s reversal potential, creating graded EPSPs or IPSPs.
- PSPs combine across active synapses and across successive events in time.
- The resulting voltage and conductance at the trigger zone are compared with that neuron’s threshold.
- If threshold is reached, voltage-gated channels generate an action potential; if inhibition keeps the net signal below threshold, no spike begins.
Chemical transmission adds a finite delay. A University of Texas electronic textbook gives a typical delay of about 0.5–1 millisecond between presynaptic action-potential initiation and the postsynaptic potential. That delay matters when inputs arrive nearly simultaneously and helps determine whether temporal summation occurs.
For introductory teaching, OpenStax gives roughly 1–2 mV as a representative size for an individual EPSP or IPSP. Actual amplitudes vary substantially with the synapse, neuron, location, and recording conditions, so the value should not be treated as a universal step size or as a fixed number of inputs required to fire.
Where excitatory and inhibitory synapses are located
Excitatory inputs are often found on dendrites and dendritic spines, where they collect signals from many presynaptic cells. Their effect depends on how far the contact is from the soma and trigger zone and on the electrical properties of the intervening dendrite.
Inhibitory contacts can occur on dendrites, the cell body, or other strategic parts of the neuron. A location close to the soma or trigger zone can exert strong control over many excitatory inputs, while a dendritic inhibitory contact can regulate a particular branch or local group of excitatory synapses. There is no single anatomical placement that defines inhibition; circuit architecture and cell type determine the arrangement.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to interpret a synaptic recording
When examining a postsynaptic response, use this sequence rather than relying only on whether the trace points up or down:
- Identify the outcome: Does the input increase or decrease the probability of a spike under the cell’s operating conditions?
- Identify the receptor conductance: Which ions can pass when the receptor is active?
- Estimate the reversal potential: At what voltage would the synaptic current change direction or become zero?
- Compare with threshold: Is that reversal potential on the firing-promoting side or the firing-suppressing side of the action-potential threshold?
- Check for shunting: Does the conductance reduce the size of a coincident EPSP even when the baseline voltage changes little?
- Account for context: Consider membrane potential, ion gradients, receptor distribution, developmental state, and the synapse’s location.
This approach explains why the same response can look different when the membrane is clamped at different voltages and why a small voltage deflection can have a large effect on firing probability.
Common misconceptions
“Every depolarization is excitatory.”
No. A depolarizing conductance is inhibitory when its reversal potential remains below spike threshold and its added conductance shunts excitatory current.
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“Every hyperpolarization is inhibitory, so voltage direction is enough.”
A hyperpolarizing response is usually inhibitory, but voltage direction does not reveal the complete mechanism. Receptor identity, reversal potential, and the timing of other inputs determine the effect.
“One glutamate or GABA molecule has a fixed effect.”
The transmitter’s effect depends on the receptor and ionic gradients in the postsynaptic cell. Glutamate is commonly excitatory and GABA commonly inhibitory in the brain, but those labels describe prevalent circuit roles, not an unavoidable chemical property.
“A single EPSP should make the neuron fire.”
EPSPs are graded local signals. A single event is often insufficient; firing depends on the combined spatial and temporal pattern of excitation and inhibition at the trigger zone.
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“The −60 mV, −40 mV, and 0 mV values apply to every neuron.”
Those numbers belong to a Purves textbook example illustrating how a glutamatergic conductance can move a cell toward threshold. Actual values differ among neurons and experimental conditions.
Why the distinction matters
Excitation and inhibition are the basic arithmetic of neural circuits. Excitatory synapses provide signals that can drive a neuron toward an output, while inhibitory synapses control when, where, and how strongly that drive is expressed. Inhibition can cancel an excitatory signal, narrow the timing window in which inputs can combine, or selectively suppress activity in one dendritic branch without silencing the entire cell.
Thinking in terms of receptor conductance and reversal potential also makes the terminology portable across brain regions and experimental methods. Instead of asking only whether a trace rises or falls, ask whether the synaptic input moves the cell toward or away from its own firing threshold. That is the reliable difference between an EPSP and an IPSP.
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