Types of Neurotransmitters: Characteristics and Functions

Neurotransmitters are essential biomolecules for the transmission of information in higher vertebrates. Without them, the nervous system couldn't function properly.

Written and verified by el biólogo Samuel Antonio Sánchez Amador.

Last update: 22 March, 2021

Adult human beings have approximately 100 billion neurons in their brains. These form the control center of movement, sleep, hunger, and almost all the vital functions of the body, but how are these signals produced by the body transported? We find the answer in neurotransmitters.

A neurotransmitter is a chemical made by nerve cells, and which is used to communicate with other cells. It’s part of a complex network of cellular communication called the chemical synapse. If you want to know everything about these essential biomolecules, read on.

Step one: understanding the synapse

If a signal is emitted from the brain that encodes how you need to perform a movement, how does it reach the tip of the foot? Simple: through communication between neurons. Here the synapse comes into play, a functional and specialized approach, in which an excited neuron transmits an electrical impulse to another neuron or to the relevant effector cell.

The presynaptic cell (presynaptic neuron) is excited, and so the nerve impulse reaches the axon (the body of the neuron) and travels through it until it reaches the end of the cell body. According to the Atlas of Plant and Animal Histology, some axons can measure up to several meters, and so the distance traveled can be considerable.

Once at the “tip” of the axon, the neuron encounters two different scenarios: a tight bond between neurons or an impassable physical space. At this time, the two types of synapses take place: electrical and chemical. Read on as we briefly tell you about this.

1. Electrical synapse

As indicated in the general physiology document of the University of Cantabria (UC), in the electrical synapse, the presynaptic and postsynaptic neurons have a GAP or communicating junction. Therefore, the ionic current flows from one cell to another, without the need for a neurotransmitter to mediate the process.

These channels have a high conductance, and so the current flows from neuron to neuron, hyperpolarizing or depolarizing the postsynaptic. It’s a very simple type of synapse, which occurs mainly in less complex vertebrates and in a few parts of the human body.

The electrical synapse is all or nothing, and, since both neurons are in permanent contact, there are no intermediate points.

2. Chemical synapse

Actually, this is the type of synapse that interests us and the one that occurs between the vast majority of neurons in the human body. This time, there is a physical space between the presynaptic neuron and the postsynaptic neuron, which is known as a synaptic cleft. To establish communication, the action of neurotransmitters is necessary.

For the synaptic potential to occur despite this physical space, we must have the following elements:

1. Presynaptic element: This is the axon termination of the presynaptic neuron. Synaptic vesicles are stored in this termination, which contain the neurotransmitters that we’ll explore below. Without exaggeration, there may be 10,000 to 50,000 neurotransmitters locked in a single vesicle.
2. Synaptic cleft: This is the space between both neurons or between the neuron and effector cell. This can range from 20 nm to 50 nm in length.
3. Postsynaptic element: The dendrites of the postsynaptic neuron (extensions of the cell body) have membrane receptors, which are activated when they come into contact with the neurotransmitter.

Once neurotransmitters are received at the postsynaptic cell membrane, a series of membrane channels open, forcing ions into or out of the cell. This causes a change in the membrane potential, which is translated into an excitatory or inhibitory electrical signal.

The chemical synapse is modular. The amount of neurotransmitters or membrane receptors can increase or decrease signal strength.

Second step: what are neurotransmitters?

This definition is from the National Cancer Institute of the United States of America: they are chemical substances made by nerve cells and used to communicate with other cells, including other nerve cells and muscle cells. As we have seen, they are essential components of the chemical synapse.

For a neurotransmitter to be considered as such, it must meet the following requirements:

1. The substance must be present inside the neurons. As redundant as it sounds, a substance cannot be secreted by a neuron if it hasn’t previously been contained within it.
2. The enzymes that allow the synthesis of the neurotransmitter must be present in areas close to it. In other words, the presence of enzymes and metabolic intermediates within the neuron show that the neurotransmitter has been created there.
3. The effect of the neurotransmitter must be reproduced if this substance is applied from the outside. If a test is carried out with the same elements outside the body environment, the reaction should be the same.

Third step: the types of neurotransmitters

Once we have described what a neuronal synapse is and what a neurotransmitter consists of, we’re ready to show you, at least briefly, the most important and well-known neurotransmitters.

1. Acetylcholine

According to a publication on the Medigraphic portal, acetylcholine was the first neurotransmitter described in both the peripheral nervous system (PNS) and the central nervous system (CNS) of mammals.

This neurotransmitter participates in the regulation of various functions, such as cortical activation phenomena, the transition from sleep to wakefulness, and various memory and association processes.

Thus, acetylcholine is fairly well distributed in the central nervous system, particularly in memory, reward, and other circuits. At the metabolic level, various functions can be attributed to it, among which we find the following:

1. Vasodilation and decreased heart rate, as far as the cardiovascular system is concerned.
2. Increased motility, glandular secretion and peristaltic movements at the intestinal level. This can lead to nausea, vomiting, and diarrhea.
3. In the respiratory system it causes bronchoconstriction.
4. Increases the secretion of the sweat glands of the skin, thus producing more sweat on the epidermal surface of the individual.

2. Dopamine

As indicated by the News Medical portal, dopamine is produced in dopaminergic neurons in the ventral tegmental area (VTA) of the midbrain. This neurotransmitter has a lot of influence in the brain, as it plays essential roles in cognition, personality, motivation, and the feeling of reward and humor, among many others.

In addition to facts related to human psychology, it also has clear functionalities at an anatomical level. It has locomotor, muscular and cardiac activity regulation functions, for example.

3. Noradrenaline

Norepinephrine is a catecholamine with multiple physiological and homeostatic functions, which is why it acts both as a hormone and as a neurotransmitter. According to studies, this biomolecule has been associated with motivation, alertness and wakefulness, level of consciousness, perception of sensory stimuli, and many other aspects.

As a stress hormone, norepinephrine activates certain parts of the brain and, together with epinephrine, works to put us in a state of alert or fight and flight. This results in an increase in heart rate, the release of glucose to give the muscles energy, and an increase in respiratory rate and blood flow.

In the responses to danger, effectiveness and immediacy are sought in the short term, so many underlying physiological processes are underestimated.

4. Serotonin

According to the Association of Occupational Health and Prevention Specialists (AEPSAL), serotonin is a neurotransmitter that is synthesized, among many other places, in the brain. Its function has been associated with emotions and mood, but it also fulfills many others, including the following:

1. Regulates appetite, as it encourages the feeling of satiety in the individual
2. To a certain extent it also acts on the individual’s sexual appetite
3. Regulates body temperature
4. It controls motor activity, perception and cognition
5. It participates in the mechanisms that govern anxiety, fear and phobias, along with other neurotransmitters
6. Regulates the secretion of certain hormones
7. It plays an important role in the formation and maintenance of the body’s bone structure.
8. It is involved in the functioning of the vascular system.
9. It promotes cell division.

5. GABA (γ-aminobutyric acid)

GABA is a neurotransmitter well distributed by neurons in the cerebral cortex. GABA’s role is to inhibit or reduce neuronal activity, as well as playing an important role in behavior, cognition, and the body’s response to stress.

6. Glycine

Glycine is one of the amino acids that are part of the proteins of living beings, so it is not a biomolecule isolated from protein metabolism like the rest of those mentioned. According to the Metabolic Guide, glycine has a double function:

1. As an inhibitory neurotransmitter: it acts on specific receptors present in the brain stem and spinal cord. For this reason, glycine is considered a central nervous system (CNS) inhibitor.
2. As an excitotoxic neurotransmitter: it acts by negatively modulating a receptor in the cerebral cortex, which plays a role in the development of the nervous system, brain plasticity, and also in degenerative processes. Therefore, excitotoxicity promotes the appearance of illnesses and diseases.

7. Glutamate

This is another of the 20 biological protein-forming amino acids. It’s the excitatory neurotransmitter par excellence in humans, as it participates in brain development, learning, memory, and synaptic plasticity.

It’s interesting to know that, in addition to this, it’s one of the most active amino acids from a metabolic point of view in the entire human body, as it’s used as a “wild card” for the exchange of energy between tissues. Thus, its function is both nervous and metabolic.

Summary

As you may have seen, the world of neurotransmitters is, to say the least, difficult to cover sufficiently in such a short article. Some of them are biomolecules with associated hormonal functions, while others are the most basic protein-forming subunits (amino acids).

If we want you to understand one thing after reading these lines, it’s that life as we know it today in the lives of the most complex vertebrates wouldn’t be possible without neurotransmitters. Without them, the synaptic clefts wouldn’t be able to communicate and, therefore, the neurons wouldn’t be able to transmit electrical impulses effectively.