Question
Describe the structure of a chemical synapse and explain the mechanism of synaptic transmission. How does an impulse cross from one neuron to the next?
This is a direct NEET 2022 question and a consistent 1-marker in CBSE Class 11 board exams. Understanding the mechanism — not just the diagram — is what separates 160+ scorers from the rest.
Solution — Step by Step
A synapse is the junction between two neurons. The neuron sending the signal is the pre-synaptic neuron; the one receiving is the post-synaptic neuron. Between them lies the synaptic cleft — a fluid-filled gap of about 20–40 nm.
The pre-synaptic terminal has membrane-bound vesicles called synaptic vesicles, each packed with chemical messengers called neurotransmitters (e.g., acetylcholine, dopamine).
When an action potential travels down the axon and reaches the axon terminal (knob), it triggers a voltage change in the pre-synaptic membrane. This voltage change opens voltage-gated Ca²⁺ channels.
Calcium ions (Ca²⁺) rush into the pre-synaptic terminal from the extracellular fluid. This is the trigger — without Ca²⁺ influx, neurotransmitters are NOT released.
The influx of Ca²⁺ causes synaptic vesicles to fuse with the pre-synaptic membrane and release neurotransmitters into the synaptic cleft by exocytosis. The neurotransmitters diffuse across the cleft — this takes about 0.5 ms (the synaptic delay).
This diffusion is why impulse conduction across a synapse is slower than conduction along an axon.
Neurotransmitter molecules bind to specific receptor proteins on the post-synaptic membrane. These receptors are ligand-gated ion channels — binding opens them and allows ions to flow in or out of the post-synaptic neuron.
If the net effect is depolarisation, an excitatory post-synaptic potential (EPSP) is generated. If it’s hyperpolarisation, an inhibitory post-synaptic potential (IPSP) is generated.
After the signal is transmitted, neurotransmitters must be cleared from the cleft — otherwise the post-synaptic neuron would keep firing. They are either:
- Enzymatically degraded (e.g., acetylcholine is broken down by acetylcholinesterase)
- Reabsorbed by the pre-synaptic terminal (reuptake)
This resets the synapse for the next impulse.
Why This Works
The synapse acts as a one-way valve for nerve signals. Because neurotransmitters are only stored in the pre-synaptic vesicles and receptors only exist on the post-synaptic membrane, impulses can only travel in one direction — pre to post. This directionality is essential for organised nervous system function.
The chemical nature of synaptic transmission (rather than direct electrical coupling) also allows for signal modulation. A single pre-synaptic neuron can influence many post-synaptic neurons, and multiple inputs can be summed — this is the basis of integration in the nervous system.
- Synaptic cleft width: 20–40 nm
- Synaptic delay: ~0.5 ms
- Trigger for exocytosis: Ca²⁺ influx
- Neurotransmitter at neuromuscular junction: Acetylcholine
Alternative Method — Electrical Synapse
Not all synapses are chemical. In electrical synapses, the pre- and post-synaptic membranes are connected through gap junctions — protein channels that directly link the cytoplasm of both neurons.
Here, ions flow directly from one neuron to the next — no neurotransmitter needed. Transmission is nearly instantaneous (no synaptic delay) and bidirectional. These are found in the heart muscle and in certain brain circuits where speed matters more than modulation. For NEET, know that electrical synapses exist but chemical synapses are the focus.
Common Mistake
Students often write that “the nerve impulse crosses the synapse.” It doesn’t. The electrical impulse stops at the pre-synaptic terminal. What crosses the cleft is the chemical neurotransmitter. A new electrical impulse is generated fresh in the post-synaptic neuron. Writing “impulse crosses” in NEET/CBSE answers costs marks — write “signal is transmitted via neurotransmitters” instead.
For NEET MCQs, the most tested fact is the role of Ca²⁺ in triggering exocytosis. If an option mentions Na⁺ or K⁺ as the trigger for neurotransmitter release — it’s wrong. Calcium is the key.