Neuroscience Crash Course #3

Understanding Neurotransmission

We ended off last time (link here to part 2) discussing the action potential and its purpose within neurons. That is, to bring a positive charge from the cell body to the terminals in order to trigger the release of neurotransmitters.

The axon terminals are the sight of neurotransmitter release. They are small hubs located at the end of axons where neurotransmitters are both synthesized and stored while they await release.

Diagram of a stereotypical neuron

When the action potential reaches the terminal, the positive charge it brings opens voltage-gated calcium channels. These are similar to the voltage-gated sodium and potassium channels involved in the propagation of the action potential, except they're only permeable to calcium ions (Ca2+). 

Calcium ions are very highly concentrated outside of neurons while there is almost none inside most of the time. As a result, when these voltage-gated channels open calcium floods into the terminal. This is significant because calcium ions are required for neurotransmitter release to happen.

How do calcium ions trigger neurotransmitter release?

Once neurotransmitters are synthesized they are packaged into vesicles, small bubbles composed of a plasma membrane similar to the neuron itself. They act as storage vessels for neurotransmitters, and bind with the membrane itself to release its contents out of the neuron.