Step by Step
Ca2+
Calcium triggers the whole process
When an action potential reaches the axon terminal, voltage-gated calcium (Ca2+) channels open, and calcium flows in. This calcium influx is the essential trigger for everything that follows — without it, no neurotransmitter release occurs.
Rel
Vesicle fusion and neurotransmitter release
Calcium triggers synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft via exocytosis. The neurotransmitter then diffuses across the cleft to reach the postsynaptic cell.
E/I
EPSP vs IPSP — excitatory or inhibitory
Neurotransmitter binding postsynaptic receptors opens or closes ion channels, producing either an EPSP (excitatory postsynaptic potential — depolarizing, via Na+ or Ca2+ entering) or an IPSP (inhibitory postsynaptic potential — hyperpolarizing, via K+ exiting or Cl- entering). Summation of many EPSPs and IPSPs at the axon hillock ultimately determines whether a new action potential fires.
Rmv
Removing the neurotransmitter
Neurotransmitter is cleared from the synaptic cleft through reuptake into the presynaptic terminal (most neurotransmitters), enzymatic degradation (acetylcholine, broken down by acetylcholinesterase), or simple diffusion away from the site.
Nerve agents work by inhibiting acetylcholinesterase, the enzyme responsible for breaking down acetylcholine in the synaptic cleft — without this enzyme functioning normally, acetylcholine accumulates and continuously overstimulates postsynaptic receptors, which is exactly why nerve agent exposure is so dangerous.
Applied Walkthrough
1
A toxicology student is trying to understand why nerve agents are so dangerous, given that they don't directly damage neurons themselves.
2
Ask: what specific step in synaptic transmission do nerve agents actually target? They inhibit acetylcholinesterase, the enzyme responsible for breaking down and clearing acetylcholine from the synaptic cleft after it's done its job.
3
Without functioning acetylcholinesterase, acetylcholine keeps accumulating in the synaptic cleft and continues binding postsynaptic receptors over and over, causing continuous, uncontrolled overstimulation of the postsynaptic cell rather than a normal, brief signal.
4
This illustrates that disrupting even one specific step in the multi-step synaptic transmission process — here, neurotransmitter removal rather than release or binding — can have devastating physiological consequences, since the whole system depends on each step functioning correctly in sequence.
Exam Application
Exams test the correct sequence of synaptic transmission (action potential → Ca2+ influx → vesicle fusion → neurotransmitter release → receptor binding → EPSP or IPSP), the distinction between EPSP (depolarizing, excitatory) and IPSP (hyperpolarizing, inhibitory), and the mechanisms of neurotransmitter removal (reuptake, enzymatic degradation, diffusion).
⚠ Common Trap
The most common trap is forgetting that calcium influx, not the action potential itself, is the direct trigger for vesicle fusion and neurotransmitter release. The action potential's arrival at the terminal is what opens the calcium channels, but it's the calcium itself that directly triggers the release mechanism.
✓ Quick Self-Check
1. What ion triggers neurotransmitter release at the synapse, and where does it enter?
Calcium (Ca2+), entering through voltage-gated channels at the axon terminal.
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2. What happens to synaptic vesicles once calcium enters the terminal?
They fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft via exocytosis.
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3. What is the difference between an EPSP and an IPSP?
An EPSP is excitatory and depolarizing (via Na+ or Ca2+ entering); an IPSP is inhibitory and hyperpolarizing (via K+ exiting or Cl- entering).
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4. What determines whether a postsynaptic neuron ultimately fires a new action potential?
The summation of many EPSPs and IPSPs at the axon hillock.
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5. What enzyme breaks down acetylcholine in the synaptic cleft, and what happens if nerve agents inhibit it?
Acetylcholinesterase; if inhibited, acetylcholine accumulates and continuously overstimulates postsynaptic receptors.
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