Structure and function of a synapse — synaptic transmission mechanism

Question

Describe the structure of a chemical synapse. Explain how a nerve impulse is transmitted across the synaptic cleft.

(NEET 2022, similar pattern)

Solution — Step by Step

A synapse is the junction between two neurons (or a neuron and an effector). A chemical synapse has three parts:

Pre-synaptic membrane: The axon terminal of the transmitting neuron. Contains synaptic vesicles filled with neurotransmitters (e.g., acetylcholine, noradrenaline) and numerous mitochondria for ATP production.
Synaptic cleft: A narrow gap (~20 nm wide) between the two neurons. The neurotransmitter must cross this gap.
Post-synaptic membrane: The membrane of the receiving neuron (usually a dendrite or cell body). Contains receptor proteins specific to the neurotransmitter.

A nerve impulse (action potential) arrives at the axon terminal of the pre-synaptic neuron.
The depolarisation opens voltage-gated $\text{Ca}^{2+}$ channels in the pre-synaptic membrane.
$\text{Ca}^{2+}$ ions rush into the axon terminal.
$\text{Ca}^{2+}$ triggers the synaptic vesicles to fuse with the pre-synaptic membrane (exocytosis).
Neurotransmitter molecules are released into the synaptic cleft.
Neurotransmitters diffuse across the cleft and bind to specific receptors on the post-synaptic membrane.
This binding opens ion channels (e.g., $\text{Na}^+$ channels), generating a new electrical signal (either excitatory or inhibitory) in the post-synaptic neuron.
The neurotransmitter is quickly removed from the cleft — either by enzymatic degradation (e.g., acetylcholinesterase breaks down acetylcholine) or by reuptake into the pre-synaptic terminal.

Synaptic transmission is one-way because:

Neurotransmitter vesicles are present only in the pre-synaptic terminal.
Receptors for the neurotransmitter are present only on the post-synaptic membrane.

The post-synaptic side cannot release neurotransmitter, and the pre-synaptic side lacks the receptors. This ensures signals travel in one direction — from axon terminal to dendrite.

Why This Works

Chemical synapses convert an electrical signal (action potential) into a chemical signal (neurotransmitter) and back to an electrical signal. This chemical intermediary step provides several advantages: it allows signal modulation (amplification or inhibition), integration of multiple inputs, and unidirectional transmission.

The synaptic delay (~0.5 ms) is the time needed for neurotransmitter release, diffusion, and receptor binding. While slower than electrical synapses (which use gap junctions), chemical synapses offer far greater flexibility in signal processing.

NEET frequently asks about specific neurotransmitters. Key ones to know: Acetylcholine (neuromuscular junction, parasympathetic), Noradrenaline (sympathetic nervous system), Dopamine (reward pathways), GABA (main inhibitory neurotransmitter in the brain).

Common Mistake

Students often forget the role of $\text{Ca}^{2+}$ in synaptic transmission. It is calcium entry (not the action potential itself) that triggers vesicle fusion and neurotransmitter release. If $\text{Ca}^{2+}$ channels are blocked, the action potential will still arrive at the terminal, but no neurotransmitter will be released — and transmission stops.

Another error: writing that the neurotransmitter “stays in the cleft” after transmission. Rapid removal of the neurotransmitter is essential. If it lingers, the post-synaptic neuron keeps firing uncontrollably. This is actually how nerve agents and organophosphate pesticides work — they inhibit acetylcholinesterase, causing continuous stimulation.

Question

Solution — Step by Step

Why This Works

Common Mistake

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