A chemical synapse must remove neurotransmitter after release. Otherwise postsynaptic receptors would remain activated and separate signaling events would blur together.

Several mechanisms terminate transmitter action.

Reuptake

Transport proteins can move neurotransmitter from the extracellular space back into the presynaptic terminal or into nearby glial cells, non-neuronal nervous-system cells that can support and regulate the neuronal environment.

The recovered molecules may be recycled directly or broken down and resynthesized.

Enzymatic degradation

Some neurotransmitters are destroyed by extracellular enzymes, biological catalysts that accelerate specific chemical reactions. At the neuromuscular junction, for example, acetylcholinesterase catalyzes hydrolysis of acetylcholine in the synaptic cleft.

Diffusion away

Neurotransmitter can also diffuse out of the narrow synaptic region, lowering its local concentration and reducing receptor occupancy.

Why clearance kinetics matter

Release determines when transmitter concentration rises; clearance determines how quickly it falls.

vesicle release → transmitter concentration rises
clearance       → transmitter concentration falls

A slower clearance process prolongs receptor activation even if presynaptic release has already stopped. Drugs that inhibit degradation or reuptake can therefore strengthen or prolong signaling without increasing the number of action potentials arriving at the terminal.

Termination is distributed across the synapse

Different transmitters rely on different combinations of uptake, degradation and diffusion. Postsynaptic receptors can also become temporarily less responsive during sustained exposure, and presynaptic release machinery resets after Ca$^{2+}$ falls.

Chemical synaptic duration is therefore not determined by one universal off-switch. It emerges from the balance between release, receptor kinetics and transmitter clearance.