The nervous system is an enormous network of nerve cells (neurons). These cells receive, process, and transmit information between each other through electrical/chemical signals via a specialized structure called synapse and which is where the magic happens.

To send a signal, neurons must release chemical messengers called neurotransmitters in the synapse. Neurotransmitters attach to their specific receptor in the opposing neuron resulting in an inhibitory or an excitatory signal.

Neurotransmitters play a major role in shaping everyday life and our functions, in fact, these chemical messengers dictate how we feel, react and act, thus, making them the main target of many different drugs. 

Each neurotransmitter has its own specific receptors, in fact, the same receptors can be occupied by more than one neurotransmitter.

There are many neurotransmitters, in fact, their exact numbers are still unknown, but more than 100 chemical messengers have been uniquely identified. Here are the most "important" ones to know about

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Dopamine

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Dopamine (3,4-dihydroxyphenethylamine) is a neurotransmitter manufactured in nerve cell bodies located within the ventral tegmental area which is basically a group of neurons located close to the midline on the floor of the midbrain. 

Our reward system is generally considered to be made up of the main dopamine pathways of the brain (primarily the mesolimbic pathway) and other structures like the ventral tegmental area and nucleus accumbens. Dopamine is normally released with pleasant/ fulfilling activities like eating and sexual intercourse.

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Glutamate

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Glutamate is the principal excitatory neurotransmitter in the brain and is involved in most aspects of normal brain function including cognition, memory, and learning. It's the most abundant (plentiful) neurotransmitter in the brain.

Glutamate is a non-essential amino acid that plays a major role, in facilitating muscle movements: When glutamate is released it facilitates the opening of sodium channels within the post-synaptic membrane allowing sodium ions to access the membrane and causing depolarization, thus, it makes it easier for the cell to reach its depolarisation threshold and generate an action potential. 

Excess glutamate might damage nerve cells, and how this information is being transmitted. A study shows that Amyotrophic lateral sclerosis (ALS) disorder is linked to excessive glutamate.

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GABA

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GABA short for Gamma-aminobutyric acid is the main inhibitory neurotransmitter found in the nervous systems of widely divergent species including humans. It's synthesized in the brain from glutamate and an active form of vitamin B6 as a cofactor.

GABA acts by binding to GABA(a) or GABA(b) receptors to cause an opening of ion channels and allowing a flow of negatively charged chloride ions into the cell or positively charged potassium ions out of the cell which causes a hyperpolarization of the cell, thus, Its principal role is reducing neuronal excitability throughout the nervous system: GABA prevents neurons from over-reacting. A study showed that patients who suffer from epilepsy (a disease caused by neurons over-firing) have decreased GABA levels.

GABA plays also a major role in motor control and muscle tone, in fact, it's used to treat the trembling encountered in patients with Huntington’s disease.

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Acetylcholine (ACh)

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Acetylcholine's name is derived from its chemical structure (an ester of acetic acid and choline). ACh has two types of receptors Nicotinic acetylcholine receptors which are responsible for the addictive effect of nicotine and Muscarinic acetylcholine receptors

As a neurotransmitter, ACh is responsible for the learning process, memory formation and the enhancement of alertness. while, outside the nervous system, it's the final product of the cholinergic system which has different functions including muscle movement, respiration, digestion, regulating heart rate...

References :   

https://www.ncbi.nlm.nih.gov/pubmed/10736372

https://www.ncbi.nlm.nih.gov/pubmed/25245510

https://www.ncbi.nlm.nih.gov/pubmed/2144428

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