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Concept
Inhibitory postsynaptic potential
Summary
An inhibitory postsynaptic potential (IPSP) is a kind of synaptic potential that makes a postsynaptic neuron less likely to generate an action potential. IPSPs were first investigated in motorneurons by David P. C. Lloyd, John Eccles and Rodolfo Llinás in the 1950s and 1960s. The opposite of an inhibitory postsynaptic potential is an excitatory postsynaptic potential (EPSP), which is a synaptic potential that makes a postsynaptic neuron more likely to generate an action potential. IPSPs can take place at all chemical synapses, which use the secretion of neurotransmitters to create cell to cell signalling. Inhibitory presynaptic neurons release neurotransmitters that then bind to the postsynaptic receptors; this induces a change in the permeability of the postsynaptic neuronal membrane to particular ions. An electric current that changes the postsynaptic membrane potential to create a more negative postsynaptic potential is generated, i.e. the postsynaptic membrane potential becomes more negative than the resting membrane potential, and this is called hyperpolarisation. To generate an action potential, the postsynaptic membrane must depolarize—the membrane potential must reach a voltage threshold more positive than the resting membrane potential. Therefore, hyperpolarisation of the postsynaptic membrane makes it less likely for depolarisation to sufficiently occur to generate an action potential in the postsynaptic neurone.
Depolarization can also occur due to an IPSP if the reverse potential is between the resting threshold and the action potential threshold. Another way to look at inhibitory postsynaptic potentials is that they are also a chloride conductance change in the neuronal cell because it decreases the driving force. This is because, if the neurotransmitter released into the synaptic cleft causes an increase in the permeability of the postsynaptic membrane to chloride ions by binding to ligand-gated chloride ion channels and causing them to open, then chloride ions, which are in greater concentration in the synaptic cleft, diffuse into the postsynaptic neuron.
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