The human nervous system functions depend on the neurotransmitters, which are molecules specialized in the transmission of neuronal information that our organism produces from the nutrients we obtain when eating.
In this article we are going to explain what a neurotransmitter is and which are the main types, including dopamine, glutamic acid or GABA; these are excellent examples of how different the functions of this type of molecule can be.
Neurotransmitters are endogenous molecules, this means created by the organism, and they are in charge of transmitting information from one neuron to another, to a gland (which synthesizes hormones) or to a muscle cell (myocyte). They are also called "chemical messengers."
Neurons are cells of the nervous system that are in charge of the transmission of information as electrochemical impulses. Neurotransmission takes place in the synapse, the space between a presynaptic cell (which has to be a neuron) and a postsynaptic cell -the one that the neurotransmitter receives.
The neurotransmitters are released in the synaptic vesicles, receptacles that are located in the axon terminal button of the presynaptic neuron. Axons are neuronal extensions that transmit the electrochemical impulses, and the terminal buttons are their extremes. Opposed to this, the segments called "dendrites" are the ones that receive the transmission.
The primary function of neurotransmitters is to stimulate or inhibit the postsynaptic cell activity: depending on the information (i.e., the neurotransmitters) it receives, its functioning will be enhanced of interfered. Every neurotransmitter has the potential to bind only to specific types of postsynaptic receptors.
Most of the neurotransmitters are synthesized by the human organism from simple and very common precursors in our body, such as the amino acids we obtain from our diet.
Although nowadays, we know about more than 100 types of neurotransmitters with different functions, in this article we are only going to mention some of the most significant ones: dopamine, adrenaline, noradrenaline, serotonin, acetylcholine, glutamic acid, glycine, GABA (or Gamma-aminobutyric acid), histamine, and endogenous opioid peptide.
One of the most popular neurotransmitters is dopamine, which is involved in brain networks related to motivation and reward-driven behavior. In this sense, many people associate dopamine with pleasure, although it would be better to say that its activity depends on salience or the degree of surprise caused by specific stimuli.
This neurotransmitter is also essential for movement: lesions in dopaminergic pathways, which are produced for example in Parkinson's disease (PD), cause motor-type symptoms such as resting tremors, muscle stiffness, slowness of movement, and difficulty walking or even keeping balance.
Dopamine is a catecholamine, like epinephrine and norepinephrine, which we are going to talk about next. These types of neurotransmitters have the precursor in common from which they are synthesized: tyrosine, a non-essential amino acid (because the body produces it from phenylalanine, an essential amino acid we get from our diet).
Epinephrine or adrenaline is considered a neurotransmitter when it works in the nervous system, but it is probably more relevant as a hormone -this is, the effects it has when it is secreted in the bloodstream by adrenal glands.
Epinephrine depends on the fight-or-flight response of our organism, which is activated in front of situations that we perceive as a threat to our physical or psychological integrity. It depends on the sympathetic nervous system, it is closely related to stress, and it implies physiological changes such as the increase of breathing and heart rate, subordination and the contraction of blood vessels.
Norepinephrine can also be conceptualized as a neurotransmitter or as a hormone depending on whether it works inside or outside of the nervous system. But, what is the difference between epinephrine and norepinephrine? In this case, the function of the neurotransmitter is more relevant than the one of the hormone.
Both adrenaline and noradrenaline are synthesized from dopamine. Tyrosine, the amino acid that is used as the precursor for all the catecholamines, is transformed into DOPA due to the effects of the tyrosine hydroxylase enzyme. When decarboxylated, DOPA becomes dopamine; if it oxidizes it becomes noradrenaline, and finally, adrenaline is obtained from the methylation of noradrenaline.
Serotonin is also called "5-hydroxytryptamine" (5-HT). In this case, the amino acid used as a precursor is not tyrosine (and in consequence serotonin doesn't belong to the group of catecholamines) but the tryptophan, which can be obtained from food such as eggs, milk, whole grains or chocolate, among others.
Serotonin is in charge of the regulation of other neurotransmitters' activities. Nowadays, it is known that it is involved in several processes such as the decrease of the levels of anxiety and physiological stress, the feeling of increased tiredness and appetite, the improvement of the mood or cell division.
Acetylcholine comes from the glucose we obtain from food. Among the functions of the organism in which this neurotransmitter participates, we can highlight the stimulation (the contraction) of muscle cells in general and the brain, the production of saliva, urination, erection or the decrease of the heart rate frequency.
Glutamic acid is the primary excitatory neurotransmitters of the human brain. It is widespread throughout this organ but, despite the importance of its excitatory effects, the neurons that use it as a neurotransmitter are relatively few; they usually use it for other purposes, such as transamination or protein synthesis.
The GABA neurotransmitter or gamma-aminobutyric acid is the most important neurotransmitter for the neuronal inhibition in the central nervous system, and particularly in the brain. The muscle tone depends on the GABA -so the deficits of this neurotransmitter are related to hypertonia and rigidity.
The eighth neurotransmitter that we have included in this list is glycine, which also has inhibitory effects in the central nervous system but, contrary to GABA, its activity is more important in the spine than in the brain. Glycine is also very important for the synthesis of collagen, present in the skin and bones.
When it acts as a neurotransmitter in the nervous system, histamine is in charge of the regulation of tiredness and alertness, as well as the secretion of hormones by the hypothalamic system.
Histamine is especially known by its role in the immunity system responses. Among other body processes, this type of neurotransmitters is associated with inflammation and itching.
There are a lot of opioid peptides that the brain produces and that are considered neurotransmitters due to the way they fulfill their functions.
Within this type of neurotransmitters, encephalins, endorphins, and dynorphins stand out. Their functions have to do with the regulation of sensations of pain and hunger, body temperature or reproduction, among other aspects.