🎓 Prepared by students from Boğaziçi University

What is a Synapse and Neurotransmitter?

A synapse is the junction where two neurons meet, allowing them to communicate via chemical signals. Neurotransmitters are the molecular messengers that carry signals across the synaptic cleft, enabling nerve impulses to propagate and drive thought, movement, and sensation.

Short answer

A synapse is the gap between neurons filled by neurotransmitters—chemical molecules released by the presynaptic neuron that bind to receptors on the postsynaptic neuron, transmitting the signal.

Neurotransmitter Release and Reception

1
Action potential arrives at axon terminal
Electrical signal reaches the presynaptic neuron's axon terminal.
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2
Calcium influx
Voltage-gated Ca²⁺ channels open, calcium rushes in.
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3
Vesicle fusion
Synaptic vesicles fuse with the presynaptic membrane.
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4
Neurotransmitter release
Neurotransmitters are released into the synaptic cleft.
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5
Receptor binding
Neurotransmitters bind to postsynaptic receptors.
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6
Postsynaptic response
Receptors trigger ion channels; signal continues or stops.

Step-by-step worked examples

Acetylcholine is released at the neuromuscular junction. How does it cause muscle contraction?

Acetylcholine binds to nicotinic receptors on muscle cell membrane
→ Na⁺ ions flow in → membrane depolarizes
→ Action potential triggers muscle contraction

Serotonin levels are low. What mood changes might occur?

Low serotonin → fewer receptors activated in mood-regulating brain regions
→ reduced signaling in limbic system
→ depression, anxiety, poor sleep

GABA is released at a synapse. What type of signal does it produce?

GABA binds to inhibitory receptors
→ Cl⁻ ions flow in or K⁺ flows out
→ Hyperpolarization occurs
→ Postsynaptic neuron is less likely to fire (inhibitory)

Flashcards

Quick quiz

Q1.Where is neurotransmitter stored in the presynaptic neuron?

Correct answer: B. Synaptic vesicles are membrane-bound organelles that store and release neurotransmitters.

Q2.What causes the postsynaptic neuron to depolarize after an excitatory signal?

Correct answer: C. Excitatory neurotransmitters open Na⁺ channels, allowing Na⁺ to flow in and depolarize the membrane.

Q3.Which ion's influx into the presynaptic terminal triggers neurotransmitter release?

Correct answer: C. Voltage-gated Ca²⁺ channels open during depolarization, and Ca²⁺ influx triggers vesicle fusion.

Q4.What does GABA do at its synapse?

Correct answer: B. GABA is an inhibitory neurotransmitter that hyperpolarizes the membrane and reduces firing likelihood.

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Common mistakes

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Synapses are physical connections between neurons. — Correct: Synapses are gaps (synaptic clefts) — neurons don't physically touch.

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Neurotransmitters always excite the postsynaptic neuron. — Correct: Some are excitatory (Na⁺), others inhibitory (Cl⁻/K⁺).

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Neurotransmitters are recycled immediately. — Correct: Some are reuptaken, some degraded, some diffuse away.

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All synapses are chemical. — Correct: Some are electrical (gap junctions) or mixed.

FAQ

What is a synapse and how does it work?

A synapse is the space between two neurons. The presynaptic neuron releases neurotransmitters that cross the synaptic cleft and bind to receptors on the postsynaptic neuron, passing the signal.

What are the main types of neurotransmitters?

Monoamines (dopamine, serotonin), amino acids (GABA, glutamate), peptides, and acetylcholine are major types.

How does reuptake affect neurotransmitter action?

Reuptake is the recycling of neurotransmitters back into the presynaptic terminal, ending the signal and conserving neurotransmitter supply.

Why are some synapses excitatory and others inhibitory?

It depends on the neurotransmitter type and postsynaptic receptor — Na⁺ channels = excitatory, Cl⁻/K⁺ = inhibitory.