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.