Definition

The inhibitory postsynaptic potential (IPSP) is a type of electrical signal that occurs in the postsynaptic neuron in response to the release of inhibitory neurotransmitters by the presynaptic neuron. It is a graded hyperpolarization of the postsynaptic membrane potential, which decreases the likelihood of the postsynaptic neuron firing an action potential.

Mechanism

When an inhibitory neurotransmitter, such as GABA (gamma-aminobutyric acid) or glycine, is released from the presynaptic neuron, it binds to specific receptors on the postsynaptic neuron. These postsynaptic receptors are typically ligand-gated ion channels, allowing the influx or efflux of ions upon activation.

Upon binding of the inhibitory neurotransmitter, the postsynaptic receptors open and allow the influx of negatively charged chloride ions (Cl⁻) or the efflux of positively charged potassium ions (K⁺). The movement of these ions across the postsynaptic membrane leads to a hyperpolarization of the neuronal membrane potential.

Effect

The IPSP has an inhibitory effect on the postsynaptic neuron, as it increases the threshold needed to generate an action potential. By hyperpolarizing the membrane potential, the IPSP counteracts the depolarization caused by excitatory postsynaptic potentials (EPSPs), which are generated by the release of excitatory neurotransmitters.

This inhibitory effect contributes to the overall balance of excitatory and inhibitory inputs, which is essential for proper neuronal function. The integration of IPSPs and EPSPs determines whether the postsynaptic neuron will reach the threshold for action potential initiation.

Summation

The IPSPs can summate with each other and with EPSPs, influencing the postsynaptic neuron’s firing rate. Spatial summation occurs when multiple inhibitory synapses are activated simultaneously, leading to a combined IPSP that may reach the threshold for action potential inhibition. Temporal summation occurs when multiple IPSPs from the same synapse are generated in rapid succession, collectively increasing the strength of inhibition.

Functional Significance

IPSPs play a crucial role in the regulation of neuronal activity and the fine-tuning of neuronal circuits. They contribute to the maintenance of resting membrane potential, preventing excessive excitation and promoting stability within neural networks.