In the beginning of this course, we talked about neurons and the action potential. You might remember that there are more than 100 billion neurons in your brain, which in turn are connected by more than 1000 trillion synaptic connections (the ends of a neuron's dendrites).

The synapses release a chemical (a neurotransmitter) which works as a biochemical messenger that carries  information from one neuron to another. The kind of neurotransmitter defines how the neuron will behave, causing it to pass some information on, while blocking other types of information.

Let's look at some of the most common neurotransmitters and their roles:

1. Acetylcholine

This neurotransmitter is active in our parasympathetic system - the system that is active when we sleep, rest and eat - hence its nickname "rest & digest". Acetylcholine is also active in the connection to the hippocampus, thus playing a role in memory and learning.  Some medicine used to treat depression and schizophrenia blocks acetylcholine, which may explain why one common side effect of this type of treatment is memory loss. 

2. Norepinephrine

This neurotransmitter mediates the effect of the sympathetic nervous system, also known as the fight or flight system. Norepinephrine causes our hearts to beat faster, inhibits our digestion and increases our blood pressure. It is also involved in the amygdala when we react with fear or aggression. 

3. Dopamine

This neurotransmitter can be found in the mesolimbic system, also known as our reward system. The reason for this is that almost all types of addictive drugs activate this system. However, it's false to say that dopamine mediates a rewarding sensation. What dopamine does is play an important role in establishing a connection between the stimuli (e.g. taking a drug, drinking, etc.) and the reward. This is why scientists have detected an increase in dopamine among addicts just by exposing them to an image of their drug of choice.

4. Serotonin

Serotonin appears almost everywhere in the brain and is thus associated with all kinds of functions. In the hypothalamus, it is involved in regulating body temperature and sexual behavior. In the spinal cord, serotonin has a pain relieving effect. A well-known hypothesis is that people who are depressed have a lower level of serotonin transmission. There are, however, no conclusive findings of this. What we do know is that pharmaceuticals that increase serotonin activity (SSRI) have antidepressive effects.