Relation To Action Potentials
Neurons have a resting potential of about -70mV. If the opening of the ion channel results in a net gain of positive charge across the membrane, the membrane is said to be depolarized, as the potential comes closer to zero. This is an excitatory postsynaptic potential (EPSP), as it brings the neuron's potential closer to its firing threshold (about -50mV).
If, on the other hand, the opening of the ion channel results in a net gain of negative charge, this moves the potential further from zero and is referred to as hyperpolarization. This is an inhibitory postsynaptic potential (IPSP), as it changes the charge across the membrane to be further from the firing threshold.
Neurotransmitters are not inherently excitatory or inhibitory: different receptors for the same neurotransmitter may open different types of ion channels.
EPSPs and IPSPs are transient changes in the membrane potential, and EPSPs resulting from transmitter release at a single synapse are generally far too small to trigger a spike in the postsynaptic neuron. However, a neuron may receive synaptic inputs from hundreds, if not thousands, of other neurons, with varying amounts of simultaneous input, so the combined activity of afferent neurons can cause large fluctuations in membrane potential. If the postsynaptic cell is sufficiently depolarized, an action potential will occur. Action potentials are not graded; they are all-or-none responses.
Read more about this topic: Postsynaptic Potential
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