2 Neuroscience2.5 Neurochemistry and neurophysiology
2.5.2.1 Chemical synapse organisation
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Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
What a chemical synapse is
A chemical synapse turns electrical activity in the presynaptic terminal into transmitter release. The transmitter then acts on receptors and changes the activity of the receiving cell.
The presynaptic active zone holds the release machinery and calcium channels beside docked vesicles.
The synaptic cleft is the gap between the two membranes. Transmitter diffuses across it. There is no direct cytoplasmic connection.
The postsynaptic density contains receptors and anchoring proteins. These organise the response and support changes in synaptic strength.
The two sides are built differently. This structural asymmetry makes conventional transmission run one way, although retrograde messengers can still regulate the presynaptic terminal.
How the receiving cell combines its inputs
Feature | What it does |
|---|---|
Excitatory postsynaptic response | Makes firing more likely, depending on the membrane conditions at the time |
Inhibitory postsynaptic response | Makes firing less likely, by hyperpolarisation or by shunting excitatory current |
Temporal summation | Closely spaced responses at one input can overlap |
Spatial summation | Responses from different inputs add together across the cell |
Axoaxonic contact | Can regulate release from another terminal |
Postsynaptic potentials are graded. A single presynaptic spike does not automatically make the next cell fire. The cell reaches firing threshold only when its inputs add up.
Synaptic delay comes from calcium entry, vesicle fusion, diffusion and receptor activation. This makes chemical transmission slower than direct electrical coupling.
The response depends on the receptor subtype and the ion gradients. A transmitter's name alone does not tell you whether every target will be excited or inhibited.