Last week, we explained how a neuron can pick up information, and how this information is transformed into an electric current. Today, we will continue to see how the information travels, this time from one neuron to another, through a structure called the synapse.

The Synaptic structure

Let’s recap what we know from last week. A neuron has received information, and this information was transformed into an action potential thanks to the inflow and outflow of ions. This action potential travels down the neuron to the end, towards the synapse. The synapse is a structure that links two neurons. The neurons who is sending the information, or the presynaptic neuron, is very close to the neuron that will receive the information, or the postsynaptic neurons. this proximity is what allows the information to go from one neuron to another. However, the information will not be transmitted as an electric current; instead, it will become a chemical signal. In the presynaptic neuron, we can find small pockets, called vesicles, that contains molecules called neurotransmitters. We all know some neurotransmitters by name: dopamine and serotonin are examples. The role of a neurotransmitter is to transmit the information to the postsynaptic neuron. To better understand how it works, we will go through two examples: first the excitation of the neuron, then the inhibition of a neuron [source / source / source].

The Excitation of a neuron

In this example, the presynaptic neuron received an information telling him to « activate » the postsynaptic neuron. This is called excitation. The action potential will thus travel down to the synapse, and activate the release of neurotransmitters. It does so by again causing an influx of Ca2+ ions. There are many neurotransmitters that can excite a neuron, but the most common one in the brain is called glutamate. Now, once the vesicles have been opened, a lot of glutamate are found in the junction between the pre- and postsynaptic neuron, ready for action. At the postsynaptic neuron, we can find receptor specific for glutamate. There are two main one, AMPA receptor, and NMDA receptor. The glutamate will bind both, however AMPA receptor will activate first, causing an influx of positive ions inside the postsynaptic neuron. NMDA receptors will then activate, due to both glutamate and the influx of ions inside the cell, causing even more positive ions to enter the cell. In the end, the influx of ions have changed the membrane potential, depolarizing the neuron and creating a new action potential. This action potential will then travel down the neuron to the synapse, starting the cycle again [source / source / source].

The Inhibition of a neuron

Neuronal inhibition works similarly to excitation, but the results are different. Just as the name suggests, the presynaptic neuron will want to inhibit the postsynaptic neuron, preventing it from creating an action potential. To do so, we use another neurotransmitter, GABA. Once the presynaptic neuron has released GABA, it will bind to its receptor, aptly named the GABA receptor, and cause an influx of ions. This time however, it is an influx of negative ions, mainly Cl-. This causes the membrane potential to drastically decrease, causing hyperpolarization. It is then very hard for the postsynaptic neuron to create an action potential [source / source / source].

Learning and memory: How to be more efficient

Synapses also have the crucial role of causing learning and memory in humans. In terms of neuronal function, learning simply means that the neuron will perform its task faster and stronger than the last time it was used. There are many ways to do so, but the main one is to increase the number of receptors at the postsynaptic neuron, particularly AMPA receptors. With more AMPA receptors, the neuron will be depolarized faster, and the action potential will be created faster, making the overall information more rapidly conveyed. This particular form of learning and memory is called long-term potentiation (or LTP), and is the basis for all memory in the body. Another form of memory, called long-term depression (or LTD) actively reduces the amount of AMPA receptors, rendering the neuron slower [source / source].

Now we understand how the information goes from one neuron to another. But now what? Obviously, the information will not go from one neuron to the next indefinitely, there has to be a goal. Next week, we will talk about how the information is used, notably by talking about the neuromuscular junction, a special synapse converting electrical current into movement.