What is Synaptic Transmission?
Synaptic transmission is the process by which one neuron passes a signal to the next neuron, muscle or gland across a tiny gap called a synapse. It converts an electrical impulse into a chemical message and back.
Synaptic transmission occurs when an action potential triggers release of neurotransmitter from the presynaptic neuron, which diffuses across the synaptic cleft and binds receptors on the postsynaptic cell, generating a new signal.
- 1↓Action potential arrivesThe electrical impulse reaches the presynaptic axon terminal.
- 2↓Calcium channels openVoltage-gated Ca²⁺ channels open and calcium ions flow into the terminal.
- 3↓Vesicle fusionCalcium triggers synaptic vesicles to fuse with the membrane and release neurotransmitter (exocytosis).
- 4↓Diffusion across cleftNeurotransmitter molecules diffuse across the narrow synaptic cleft.
- 5↓Receptor bindingNeurotransmitter binds receptors on the postsynaptic membrane, opening ion channels.
- 6Signal terminationNeurotransmitter is broken down by enzymes or reabsorbed (reuptake), ending the signal.
Step-by-step worked examples
A synapse has a typical synaptic delay of about 0.5 milliseconds. If a signal crosses 4 synapses in a row, what is the total delay?
Delay per synapse = 0.5 ms Number of synapses = 4 Total delay = 0.5 × 4 = 2 milliseconds
One action potential can trigger the release of about 200 synaptic vesicles, and each vesicle contains roughly 5,000 neurotransmitter molecules. How many molecules are released in total?
Total molecules = vesicles × molecules per vesicle Total = 200 × 5,000 = 1,000,000 molecules About one million neurotransmitter molecules flood the synaptic cleft.
Acetylcholine is broken down by acetylcholinesterase within about 1 millisecond of release. If a neuron fires 100 times per second, is there enough time between signals to clear the neurotransmitter?
Time between firings = 1 second / 100 = 10 milliseconds Breakdown time ≈ 1 millisecond Since 1 ms < 10 ms, there is enough time to clear the synapse before the next signal arrives.
Flashcards
Quick quiz
Q1.What directly triggers neurotransmitter release from the presynaptic terminal?
Q2.Where do neurotransmitters travel after release?
Q3.What process releases neurotransmitter from vesicles into the synaptic cleft?
Q4.Which of these ends a synaptic signal?
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Common mistakes
Thinking the electrical signal jumps directly across the synapse. — Correct: At most synapses, the electrical signal is converted into a chemical signal (neurotransmitter) that crosses the gap.
Believing all synaptic signals are excitatory. — Correct: Some synapses are inhibitory (IPSPs), making the postsynaptic neuron less likely to fire.
Assuming neurotransmitter stays in the cleft indefinitely. — Correct: It is quickly cleared by reuptake or enzymes, allowing the synapse to reset for the next signal.
Confusing the presynaptic and postsynaptic neuron. — Correct: The presynaptic neuron releases neurotransmitter; the postsynaptic neuron receives it via receptors.
FAQ
What is synaptic transmission?
Synaptic transmission is how a neuron sends a signal across a synapse to another neuron, muscle or gland, typically by releasing chemical neurotransmitters.
What is an example of synaptic transmission?
At a neuromuscular junction, a motor neuron releases acetylcholine, which binds receptors on a muscle fiber and triggers contraction.
How is synaptic transmission timed?
It typically takes about 0.5–2 milliseconds per synapse — the synaptic delay — from action potential arrival to postsynaptic response.
Why is synaptic transmission important?
It's the basis of all communication in the nervous system, letting neurons form circuits that process sensation, thought, memory and movement.