A gap
20 nm wide.
What Crosses the Gap?
Before reading the science — explore the 3D model and think. Tap each structure in the simulation, then answer the discovery questions below.
Explore every structure in 3D — rotate, zoom, click. Identify the presynaptic knob, vesicles, cleft, receptors and more before reading on.
Anatomy of a Synapse
A synapse is a junction between two neurones. It is not a physical connection — it is a precisely engineered chemical relay station.
The electrical impulse cannot cross the synaptic cleft. Instead, it triggers the release of neurotransmitters from synaptic vesicles. These chemical molecules diffuse across the 20nm gap and bind to receptor proteins on the postsynaptic membrane, generating a new electrical impulse in the next neurone.
How the Signal Crosses
Seven steps convert an electrical impulse into a chemical signal and back again. Open the 3D model alongside this page to see each step in full detail.
An electrical action potential travels along the presynaptic neurone and reaches the synaptic knob. The electrical signal cannot cross the cleft on its own.
The action potential causes calcium ion channels to open. Ca²⁺ floods into the synaptic knob — this is the essential trigger for vesicle release.
Ca²⁺ causes synaptic vesicles to move to and fuse with the presynaptic membrane — a process called exocytosis.
Thousands of neurotransmitter molecules pour into the 20nm synaptic cleft and diffuse toward the postsynaptic membrane.
Molecules diffuse across the cleft by concentration gradient — from high concentration (cleft) to low (postsynaptic membrane).
Neurotransmitters bind to complementary receptor proteins on the postsynaptic membrane, opening ion channels and generating a new action potential.
Neurotransmitters are either broken down by enzymes (e.g. acetylcholinesterase) or reabsorbed by reuptake into the presynaptic neurone. This resets the synapse for the next signal.
The Chemical Messengers
Neurotransmitters are the chemical molecules that carry signals across the synaptic cleft. Different neurotransmitters have different effects — some excite the next neurone, some inhibit it.
After binding, it is broken down by the enzyme acetylcholinesterase to stop the signal.
Low dopamine is linked to Parkinson's disease. Some drugs hijack this pathway.
Antidepressants called SSRIs work by blocking serotonin reuptake — keeping more in the cleft longer.
Prepares the body for immediate action — the classic fight-or-flight response.
Anti-anxiety medications work by enhancing GABA activity.
Acetylcholine is the neurotransmitter at every neuromuscular junction in your body. The poison curare (used on arrow tips) blocks ACh receptors → muscles cannot contract → paralysis. The nerve agent sarin blocks acetylcholinesterase → ACh stays in the cleft → muscles in constant contraction. Understanding synapses saves lives.
Apply & Think Critically
Apply your knowledge. One question is a misconception trap. The open questions require genuine biological reasoning.
Explain in detail: what symptoms would this patient experience, and why? Use your knowledge of synaptic transmission in your answer.
Predict the effect of this drug on synaptic transmission. What benefits and potential risks might it have? Evaluate whether this is a good strategy.
Critical Thinking & Creativity
Two sets of higher-order questions. Write your thinking — the AI tutor responds with Socratic guidance, not just answers. Your goal is to build understanding, not to guess right.
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Ahmad reviews open questions personally and gives additional feedback in class.