Most synapses are chemical synapses.
The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal.
The action potential causes the release of the neurotransmitter.
The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell.
Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell.
Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential.
Postsynaptic potentials fall into two categories:
Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold.
Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold.
After release, the neurotransmitter
May diffuse out of the synaptic cleft
May be taken up by surrounding cells
May be degraded by enzymes
Unlike action potentials, postsynaptic potentials are graded and do not regenerate.
Most neurons have many synapses on their dendrites and cell body.
A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron.
If two EPSPs are produced in rapid succession, an effect called temporal summation occurs.
Summation of postsynaptic potentials
Threshold of axon of
Membrane potential (mV)
(a) Subthreshold, no
(b) Temporal summation
E1 + E2
(c) Spatial summation
E1 + I
(d) Spatial summation