Mechanism of Synaptic Transmission Across a Chemical Synapse
Biology · Neural Control and Coordination · NEET
At a chemical synapse the nerve impulse does not jump across as electricity. The action potential reaches the axon terminal, synaptic vesicles fuse with the membrane and release neurotransmitters into the synaptic cleft; these bind to receptors on the post-synaptic membrane, open ion channels, and create a new potential. Memory hook: "Arrive, Fuse, Release, Bind, Open" — the electrical signal turns into a chemical message, then back into an electrical one.
At a chemical synapse the impulse arrives at the pre-synaptic axon terminal, synaptic vesicles fuse and release neurotransmitters into the synaptic cleft, and these bind to receptors on the post-synaptic membrane to open ion channels and generate a new (excitatory or inhibitory) potential.
Your doubts, answered
Does the electrical impulse itself cross the synaptic cleft?
No. This is the most common mistake. At a chemical synapse the pre- and post-synaptic membranes are separated by a fluid-filled synaptic cleft, so the electrical signal cannot jump across. The impulse is converted into a chemical signal: neurotransmitters carry the message across the cleft, then they generate a new electrical potential on the other side.
What is the correct step-by-step order of chemical synaptic transmission?
Follow the NCERT order: (1) an action potential arrives at the axon terminal, (2) it stimulates synaptic vesicles to move toward the membrane, (3) vesicles fuse with the plasma membrane and release neurotransmitters into the synaptic cleft, (4) neurotransmitters bind to specific receptors on the post-synaptic membrane, (5) binding opens ion channels, ions enter, and a new potential (excitatory or inhibitory) is generated in the post-synaptic neuron.
Is a chemical synapse faster or slower than an electrical synapse?
Slower. NCERT clearly states impulse transmission across an electrical synapse is always faster than across a chemical synapse. In an electrical synapse current flows directly. A chemical synapse adds steps (vesicle fusion, neurotransmitter diffusion, receptor binding), so there is a small delay. NEET has directly tested this exact comparison.
Why is transmission at a chemical synapse one-directional?
Neurotransmitters are stored only in the vesicles of the pre-synaptic (axon terminal) side, and receptors are present only on the post-synaptic membrane. So the chemical message can travel only from pre-synaptic to post-synaptic neuron, never backward. This is why a nerve impulse moves in one direction across the synapse.
Where exactly are the neurotransmitter receptors located?
On the post-synaptic membrane. The vesicles and neurotransmitters are on the pre-synaptic (axon terminal) side, but the specific receptors that the released neurotransmitters bind to are on the post-synaptic membrane. Binding there opens ion channels and generates the new potential. This exact location has been asked in multiple NEET years.
⚠️ The NEET trap ✗ Impulse transmission across a chemical synapse is always faster than across an electrical synapse. ✓ An electrical synapse is always faster than a chemical synapse; the chemical synapse is slower because of vesicle fusion, neurotransmitter diffusion and receptor binding. 🧠 Read the speed comparison carefully before ticking any option.
Real NEET questions
NEET 2022
Select the incorrect statement regarding synapses:
A · The membranes of presynaptic and postsynaptic neurons are in close proximity in an electrical synapse.
B · Electrical current can flow directly from one neuron into the other across the electrical synapse.
C · Chemical synapse use neurotransmitters.
D · Impulse transmission across a chemical synapse is always faster than that across an electrical synapse. ✓
Solution: NCERT states that transmission across an electrical synapse is always faster than across a chemical synapse, because electrical current flows directly. A chemical synapse involves neurotransmitter release, diffusion and receptor binding, which adds a delay. So option (d) is the incorrect statement. The other three are all correct NCERT facts.
NEET 2017
Receptor sites for neurotransmitters are present on:
A · Membranes of synaptic vesicles
B · Pre-synaptic membrane
C · Tips of axons
D · Post-synaptic membrane ✓
Solution: When the impulse reaches the axon terminal, synaptic vesicles fuse with the membrane and release neurotransmitters into the synaptic cleft. These released neurotransmitters bind to their specific receptors located on the post-synaptic membrane, which opens ion channels and generates a new potential. So the receptor sites are on the post-synaptic membrane.
Solved Neural Control and Coordination NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What are neurotransmitters in synaptic transmission?
Neurotransmitters are the chemicals stored inside synaptic vesicles at the axon terminal. When an impulse arrives, they are released into the synaptic cleft and carry the signal to the post-synaptic neuron by binding to receptors.
What triggers the release of neurotransmitters?
The arrival of an action potential (nerve impulse) at the axon terminal. It stimulates the synaptic vesicles to move to the membrane and fuse with it, releasing neurotransmitters into the synaptic cleft.
What is the new potential generated at the post-synaptic neuron?
After neurotransmitters bind to receptors and ion channels open, a new potential develops in the post-synaptic neuron. NCERT says it may be either excitatory or inhibitory, depending on which ions enter.
Is a chemical synapse the common type in humans?
Yes. NCERT states electrical synapses are rare in our system, so most transmission in the human neural system happens through chemical synapses using neurotransmitters.
Why does a chemical synapse cause a synaptic delay?
Because the signal must be converted from electrical to chemical and back: vesicle fusion, neurotransmitter release, diffusion across the cleft, and receptor binding all take a small amount of time, unlike the direct current flow in an electrical synapse.
