A synapse is the point where two neurons can communicate with each other. The two neurons involved are called the pre-synaptic and the post-synaptic neuron. The pre-synaptic neuron is located before the synapse and sends signals to the post-synaptic neuron. These signals are located at a certain point between the two neurons. This space is called the synaptic cleft.

Because neurons cannot talk and listen to each other, a different type of signal transmission takes place. Chemical processes take place in the synapse, which means that the cells communicate by passing on certain substances. These substances are called neurotransmitters and are passed onto the next cell. This is done in a several steps.

1. In the cell body and axon of the presynaptic cell, neurotransmitters are produced.
2. The neurotransmitters are transferred to the synapse at the end of the axon.
3. An action potential that reaches the synapse causes calcium to enter the pre-synaptic cell. The calcium releases neurotransmitters into the synaptic cleft.
4. The neurotransmitters in the synaptic cleft bind to receptors of the post-synaptic cell, changing its charge.
5. The neurotransmitters release from the post-synaptic receptors and become inactive.
6. What remains of the neurotransmitters dissolves in the fluids of the brain, or is being recycled.
7. The post-synaptic cell sends a substance back to the pre-synaptic cell. This causes the pre-synaptic cell to stop releasing the neurotransmitters.

The dendrites of the post-synaptic cell can be either excitatory or inhibitory. When it comes to an excitatory neuron, stimulation from the synaptic cleft increases the likelihood that the post-synaptic neuron will also fire, thus transmitting the signal even further. This is caused by the fact that the excitatory membrane creates a positive charge in the cell. This charge is called an EPSP (excitatory post-synaptic potential). In an inhibitory cell, just the opposite happens. A negative charge, an IPSP (inhibitory post-synaptic potential), is created and the chance that the post-synaptic neuron will fire is reduced.

Author: Myrthe Princen (translated by Pauline van Gils)

Image: Marcel Loeffen