It can be said that all neurons have a way of communicating with each other called synapses.

At synapses, neurons communicate with each other by means of neurotransmitters , which are molecules in charge of sending signals from one neuron to the next. Other particles called neuromodulators also intervene in the communication between nerve cells

Thanks to neurotransmitters and neuromodulators, the neurons in our brain are capable of generating the torrents of information that we call “mental processes” , but these molecules are also found in the periphery of the nervous system, in the synaptic terminals of the motor neurons (neurons of the central nervous system that project their axons to a muscle or gland), where they stimulate the muscle fibres to contract them.

Differences between neurotransmitter and neuromodulator

Two or more neuroactive substances may be in the same nerve endings and one may function as a neurotransmitter and another as a neuromodulator.

Hence their difference: neurotransmitters do or do not create action potentials (electrical impulses that are produced in the cell membrane), activate post-synaptic receptors (receptors of post-synaptic cells or neurons) and open ionic channels (proteins of the neuronal membranes that contain pores that when opened, allow the passage of charged particles such as ions) while neuromodulators do not create action potentials but regulate the activity of ionic channels.

In addition, neuromodulators modulate the efficacy of post-synaptic cell membrane potentials produced at ion channel-associated receptors. This occurs through the activation of G-proteins (particles that carry information from a receptor to the effector proteins). A neurotransmitter opens a channel, whereas a neuromodulator affects one or two tens of G-proteins , which produce cAMP molecules, opening many ion channels at once.

There is a possible relationship of rapid changes of the nervous system and neurotransmitters and slow changes with neuromodulators. Similarly, the latency (i.e. changes in post-synaptic membrane potential due to the effect of a neurotransmitter) of neurotransmitters is 0.5-1 milliseconds, whereas that of neuromodulators is several seconds. In addition, the “life expectancy” of neurotransmitters is 10-100 ms. and that of neuromodulators is minutes to hours.

As for the differences between neurotransmitters and neuromodulators according to their shape, that of neurotransmitters is similar to that of small vesicles of 50 mm in diameter, but that of neuromodulators is that of large vesicles of 120 mm in diameter.

Types of receptors

Neuroactive substances can bind to two types of receptors, which are

Ionotropic receptors

These are receivers that open ionic channels . In most, neurotransmitters are found.

Metabotropic receptors

G-protein bound receptors . In metabotropic receptors, neuromodulators are usually attached.

There are also other types of receptors, which are the autoreceptors or presynaptic receptors that participate in the synthesis of the substance released in the terminal. If there is an excess of release of the neuroactive substance, it binds to the autoreceptors and produces an inhibition of the synthesis avoiding the exhaustion of the system.

Neurotransmitter classes

Neurotransmitters are classified into groups: acetylcholine, biogenic amines, transmitting amino acids and neuropeptides.

1. Acetylcholine

Acetylcholine (ACh) is the neurotransmitter of the neuromuscular junction , it is synthesized in the septal and nasal Meynert nuclei (nuclei of the forebrain), can be in both the central nervous system (where the brain and spinal cord are located) and the peripheral nervous system (the rest) and causes diseases such as myasthenia gravis (a neuromuscular disease due to weakness of skeletal muscles) and muscular dystonia (a disorder characterized by involuntary twisting movements).

2. Biogenic amines

The biogenic amines are serotonin and catecholamines (adrenaline, noradrenaline and dopamine) and act mainly by metabotropic receptors.

  • Serotonin is synthesized from raphe nuclei (in the brain stem); noradrenaline in the locus coeruleus (in the brain stem) and dopamine in the substantia nigra and ventral tegmental area (from where projections are sent to various regions of the forebrain).
  • Dopamine (DA) is related to pleasure and mood. A deficit of it in the substantia nigra (portion of the midbrain and fundamental element in the basal ganglia) produces Parkinson’s and the excess produces schizophrenia.
  • Noradrenaline is synthesized from dopamine, is related to fight and flight mechanisms and a deficit causes ADHD and depression.
  • Adrenaline is synthesized from noradrenaline in the adrenal capsules or adrenal medulla, activates the sympathetic nervous system (system responsible for the innervation of smooth muscles, heart muscle and glands), participates in fight and flight reactions, increases heart rate and contracts blood vessels, produces emotional activation and is related to stress disorders and general adaptation syndrome (syndrome that consists of subjecting the body to stress).
  • The biogenic amines play important roles in the regulation of affective states and mental activity.

3. Transmitting amino acids

The most important excitatory transmitting amino acids are glutamate and aspartate and the inhibitors are GABA (gamma immunobutyric acid) and glycine. These neurotransmitters are distributed throughout the brain and participate in almost all the synapses of the CNS, where they bind to ionotopic receptors.

4. Neuropeptides

Neuropeptides are formed by amino acids and act mainly as neuromodulators in the CNS . The mechanisms of chemical synaptic transmission can be affected by psychoactive substances whose effect on the brain is to modify the efficiency with which chemical nerve communication takes place, and this is why some of these substances are used as therapeutic tools in the treatment of psychopathological disorders and neurodegenerative diseases.