Inhibitory Postsynaptic Potential

A graded postsynaptic voltage response whose receptor-gated conductance and reversal potential reduce the target cell's probability of reaching spike threshold.

Version

v3 · 2026-09-19 · History

Domain-specific #

2073

Domain group

Natural Sciences

Origin domain

Neuroscience

Subdomain

Synaptic Physiology → Neuroscience

Aliases

IPSP, Inhibitory post-synaptic potential

Core Idea

An inhibitory postsynaptic potential (IPSP) is a graded voltage response in a postsynaptic neuron or excitable target cell that reduces the probability of reaching action-potential threshold. It is produced when synaptically released transmitter activates postsynaptic receptors and changes membrane conductance so that the local voltage is driven toward a synaptic reversal potential and/or excitatory current is shunted. The defining outcome is functional inhibition of the postsynaptic cell, not necessarily a downward trace. The authoritative Neuroscience treatment by Purves and colleagues states the key rule: a postsynaptic response is inhibitory when its reversal potential lies below the cell's spike threshold; it may be hyperpolarizing, depolarizing, or nearly voltage-silent while still reducing excitability.

How would you explain it like I'm…

The Hold-Back Message

Brain cells pass messages, and a cell "fires" when it gets pushed enough. Some messages are like a friend gently holding you back, so it's harder to jump. An inhibitory postsynaptic potential is one of those hold-back messages. It makes the cell less likely to fire.

Harder-to-Fire Signal

Nerve cells talk using chemicals sent across tiny gaps called synapses. When enough "go" signals add up, the receiving cell fires a spike. An inhibitory postsynaptic potential is a small change in the receiving cell that makes a spike less likely. It does this by opening little gates (channels) in the cell's skin. These gates can pull the cell away from its firing point or soak up the "go" signals. What makes it count is that it holds the cell back, not which way the voltage moves.

Threshold-Suppressing Synaptic Potential

An inhibitory postsynaptic potential (IPSP) is a small, graded change in a neuron's membrane voltage that makes it less likely to reach the threshold for an action potential. It starts when a transmitter such as GABA or glycine opens receptor channels on the receiving cell. These channels pull the voltage toward a reversal potential that sits below threshold. People often picture an IPSP as the voltage simply dropping, but that's not always true. The voltage can go down, go slightly up, or barely change, and it is still inhibitory if firing becomes less likely. Opening extra channels also "shunts," or leaks away, excitatory current arriving at the same time. What makes it an IPSP is the effect on firing, not the direction the voltage moves.

 

An IPSP is a graded postsynaptic voltage response that lowers the probability of reaching spike threshold. It is produced when transmitter released by the presynaptic cell activates postsynaptic receptors and changes membrane conductance. The voltage is driven toward the synaptic reversal potential, and excitatory current may be shunted as well. The rule is that a response is inhibitory when its reversal potential lies below spike threshold. Fast IPSPs come from GABA_A or glycine receptors, which open anion channels. In many mature neurons the reversal potential is below rest, so these IPSPs hyperpolarize the cell. In other cells or states the reversal potential lies between rest and threshold, so the IPSP depolarizes yet still inhibits. At exactly the reversal potential there is no net voltage change, but the lowered input resistance still blunts concurrent EPSPs. A slower route goes through GABA_B receptors, whose G-protein signaling opens inwardly rectifying K⁺ channels and gives a prolonged IPSP. IPSP is narrower than neurotransmission or inhibition in general, and it is not the same thing as excitation–inhibition balance.

Scope of Application

IPSPs are used wherever synaptic physiology asks how an inhibitory connection changes a postsynaptic cell's excitability. At single synapses, spontaneous or evoked events quantify receptor kinetics, amplitude, latency, reversal, conductance, and release variability. In paired recordings, a presynaptic interneuron is stimulated while the postsynaptic voltage response is measured, linking a defined connection to its inhibitory effect. In extracellularly stimulated preparations, compound IPSPs summarize the synchronized contribution of many inhibitory fibers but no longer identify a single presynaptic unit.

Clarity

The abstraction clarifies why voltage sign is an unreliable classification rule. Three voltages must be distinguished: resting membrane potential, inhibitory reversal potential, and spike threshold. A response from -65 mV toward -75 mV is hyperpolarizing and inhibitory. A response from -65 mV toward -55 mV can be depolarizing yet inhibitory if threshold is -50 mV, because the conductance pulls a more strongly depolarized membrane back toward -55 mV and reduces input resistance.

Manages Complexity

Neurons receive many conductances with different locations, time courses, and reversal potentials. IPSP compresses a large molecular chain—release, diffusion, binding, channel gating, ion flow, cable filtering—into a tractable electrical object with measurable amplitude, rise, decay, reversal, and conductance. This lets experiments compare inhibitory connections without restating every molecular detail, while still preserving the variables needed to explain when the comparison fails.

Abstract Reasoning

The equations license clear predictions. Increasing g_i usually strengthens the shunt and pulls voltage more strongly toward E_i, but voltage amplitude need not grow if V_m is already near reversal. Moving E_i toward threshold weakens hyperpolarizing inhibition and can make the response depolarizing. Moving the synapse farther from the spike-initiation zone can reduce the measured somatic voltage while preserving strong local control of a dendritic excitatory event. Extending receptor kinetics broadens the temporal window during which excitation is suppressed.

Knowledge Transfer

Within neuroscience, the role map transfers intact across preparations. A cat spinal motoneuron, hippocampal pyramidal neuron, cerebellar Purkinje cell, cortical principal neuron, retinal neuron, and spinal sensory neuron differ in transmitters, receptor subunits, geometry, and kinetics, yet each IPSP can be analyzed through g_i(t), E_i, local V_m, spatial propagation, threshold, and firing probability. This is genuine within-domain transfer because the terms retain their electrophysiological meanings.

Relationships to Other Abstractions

Current abstraction Inhibitory Postsynaptic Potential Domain-specific

Parents (1) — outgoing typed relations

• Inhibitory Postsynaptic Potential presupposes Neurotransmission Domain-specific

  Neurotransmission is the single the broader abstraction under strict composition/presupposition.

Hierarchy paths (4) — routes to 4 parentless roots

• Inhibitory Postsynaptic Potential → presupposes Neurotransmission → is a kind of Propagation

Show alternative paths (3)

Neighborhood in Abstraction Space

Inhibitory Postsynaptic Potential sits in a sparse region of the domain-specific corpus (97^th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Neuronal Signaling & Plasticity (14 abstractions)

Nearest neighbors

• Neurotransmission — 0.81
• Spike-Timing-Dependent Plasticity — 0.79
• Synaptic Plasticity — 0.76
• Three-Factor Learning — 0.76
• Central Pattern Generator — 0.76

Computed from structural-signature embeddings · 2026-10-25