THE JUNCTION

A gap between cells

A synapse is the junction where a neuron passes a signal to another cell. At a chemical synapse the two cells do not touch. They are separated by a narrow gap, and the message crosses it as molecules.

The synapse is the brain's smallest switch.

THE PICTURE

Three parts

A presynaptic terminal sends, a cleft separates, and a postsynaptic cell receives.

Sender, gap, receiver.

THE PARTS

What's in each region

Presynaptic terminal Holds vesicles, small bubbles filled with neurotransmitter

Cleft A fluid-filled gap about 20 to 40 nanometers wide

Postsynaptic membrane Studded with receptors that bind the neurotransmitter

Helpers Astrocytes wrap many synapses and help clear the transmitter

A synapse is a tiny, precisely organized machine.

SPEED

Small distances, fast crossing

📏

Short trip

Molecules only need to diffuse tens of nanometers

⏱️

Short delay

The whole handoff takes roughly half a millisecond to a millisecond

➡️

One direction

Transmitter is released on one side and received on the other

Diffusion is fast enough because the gap is so small.

TWO KINDS

Chemical and electrical synapses

Most synapses in the human brain are chemical. Some neurons also link through electrical synapses, in which tiny channels join the cells directly. This course focuses on chemical synapses, because that is where neurotransmitters act.

Where there is a transmitter, there is a chemical synapse.

UNIT 3 STUDY COMPLETE

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Ready for the Fab Challenge?

You've learned the parts of a chemical synapse and why the handoff is so fast.