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Welcome to 2 minute neuroscience, where I simplistically explain neuroscience topics
in 2 minutes or less.
In this installment I will discuss synaptic transmission.
Most communication between neurons occurs at a specialized structure called a synapse.
A synapse is an area where two neurons come close enough to one another that they are
able to pass chemical signals from one cell to another.
The neurons are not actually connected, but are separated by a microscopically small space
called the synaptic cleft.
The cleft is less than 40 nm wide; by comparison a human hair is about 75,000 nanometers.
The neuron where the signal is initiated is called the presynaptic neuron, while the neuron
that receives the signal is called the postsynaptic neuron.
In the presynaptic neuron, there are chemical signals called neurotransmitters that are
packaged into small sacs called vesicles.
Each vesicle can contain thousands of neurotransmitter molecules.
When the presynaptic neuron is excited by an electrical signal called an action potential,
this causes the vesicles to fuse with the presynaptic membrane and release their contents
into the synaptic cleft.
Once they are in the synaptic cleft, neurotransmitters interact with receptors on the postsynaptic
membrane.
They bind to these receptors and can cause an action to occur in the postsynaptic cell
as a result.
This action may involve increasing the likelihood that the postsynaptic cell will become activated
and fire an action potential, or decreasing it.
Eventually, the neurotransmitter molecules must be cleared from the synaptic cleft.
Some of them will simply drift away in a process called diffusion.
In some cases, the neurotransmitter is taken back up into the presynaptic neuron in a process
called reuptake.
Once back inside the presynaptic neuron, the neurotransmitter can be recycled and reused.
In other cases, enzymes break down the neurotransmitter within the synaptic cleft.
Then the component parts of the neurotransmitter can be sent back into the presynaptic neuron
to make more neurotransmitter.