Neurotransmitters: what they are and different types

The neurotransmitters are physiological substances which allow the transmission of nerve impulses between two anatomically separated regions. The neurotransmitters are physiological substances which allow the transmission of nerve impulses between two anatomically separate regions and put into connection by synapses, or nerve fibers, or the nerves and muscle fibers present in the endplates. Within the nervous system, neurotransmitters play an essential role in the transmission of excitatory or inhibitory impulses.

What are neurotransmitters

The neurotransmitters are substances released by neurons at the synaptic level and carry on its function of a neuron or effector organ. They are synthesized in the neuron and are found in the synaptic termination; they are released in sufficient quantity to exert the excitatory action on a postsynaptic neuron they are therefore produced by the transmitting (presynaptic) cell and placed in the space that divides it from the receiving (postsynaptic) cell of the nervous system. They adhere to the membrane of the recipient cell and transmit its information. Subsequently, they detach from the membrane and are destroyed or reabsorbed by the transmitting cell.

The neurotransmitters are produced using the amino acids within the presynaptic cell via the endoplasmic reticulum and Golgi apparatus and are stored in the vesicles that roam into the cytosol of the nerve cell. As the nerve impulse occurs, the vesicles merge with the cell membrane, freeing the neurotransmitters in the synaptic cleft.

The neurotransmitters are detected by specific receptors, ion channels, seats on the postsynaptic cell membrane. The interaction between neurotransmitters and the receptor/ion channel triggers an excitatory or inhibitory response in the postsynaptic neuron. 

The chemical signal carried by the neurotransmitters is translated into an electrical signal, and therefore, after having performed its function, the neurotransmitters are removed from the receptors. This process is called reuptake and sees their reabsorption by the presynaptic cell, which will destroy them in the cytosol or reintegrate them into the vesicles. Without reuptake, neurotransmitters could continue to stimulate or depress the postsynaptic neuron.

Types of neurotransmitters

In relation to the type of response produced, neurotransmitters can be exciters, inhibitors, or suppressors. Therefore they can respectively promote the creation of a nerve impulse in the receiving neuron or inhibit the impulse itself.

Furthermore, based on size, neurotransmitters can be distinguished into neuropeptides and small molecules. Neuropeptides comprise 3 to 36 amino acids, while in the group of small molecules; there are single amino acids, such as glutamate and neurotransmitters such as acetylcholine, serotonin, and histamine. The two groups of neurotransmitters also have different synthesis and release modes.

 Basically, there are two groups of synaptic neurotransmitters, the one consisting of fast-acting low molecular weight transmitters and the group of larger neuropeptides with slower action.

The first group is composed of neurotransmitters responsible for most of the responses released by the nervous system, such as the transmission of sensory signals to the brain and motor commands to the muscles. The low molecular weight neurotransmitters are synthesized in the cytosol of the presynaptic termination and, subsequently, by active transport, they are absorbed inside the numerous vesicles present in the synaptic terminal. When a signal reaches the synaptic terminal, a few vesicles at a time-release their neurotransmitter into the synaptic cleft, this process usually takes place over a millisecond.

Neuropeptides, on the other hand, are implicated in more prolonged effects, such as long-term changes in the number of receptors and the prolonged closure or opening of some ion channels. Neuropeptides are synthesized as parts of large protein molecules from the ribosomes of the neuronal soma.

These proteins are transported within the endoplasmic reticulum and, therefore, inside the Golgi apparatus, where the protein from which the neuropeptide will originate is enzymatically split into smaller fragments. Some of them constitute the neuropeptide or its precursor, and subsequently, the Golgi apparatus packs the neuropeptide into small vesicles that are generated from it. Due to the axonal flow, the vesicles are transported to the ends of the nerve endings, ready to be released in the nerve terminal when an action potential arrives.

Among the low molecular weight neurotransmitters, we find acetylcholine, biogenic amines (dopamine, adrenaline, and norepinephrine), histamine, amino acids (GABA, glycine, and glutamate) and ATP.

Neuropeptides include opioids, neurohypophyseal hormones, tachykinins, insulin, somatostatin, and gastrins.

Drugs and neurotransmitters

Drugs and other substances can interfere with the functioning of neurotransmitters. Many stimulants and anti-depressants alter the transmission of the neurotransmitters dopamine, norepinephrine (or norepinephrine), and epinephrine (adrenaline), called in the complex catecholamines. For example, cocaine blocks dopamine recapture, allowing it to stay longer in the inter-synaptic space. In particular, cocaine alters the dopaminergic circuits of the nucleus accumbens, a region of the brain that is involved in motivational drive and emotional strengthening. 

Reserpine was first used as an anti-hypertensive agent and later as an antipsychotic in the treatment of schizophrenia, causing depletion of neurotransmitters by rupture of the synaptic vesicles and degradation by monoamine oxidases (MAO-A and MAO-B). Finally, AMPT prevents the conversion of tyrosine into L-DOPA, and deprenyl inhibits the action of monoamine oxidase B, increasing the level of dopamine between the synapses.

Low molecular weight neurotransmitters 

There are nine low molecular weight substances that are recognized as neurotransmitters. Eight of these are amines, and seven of them are amino acids or derivatives of the latter. The synthesis of these neurotransmitters is catalyzed by enzymes present in the cytosol.

Acetylcholine (ACH) is the neurotransmitter used by spinal cord motor neurons, and consequently, it is present at the level of all the neuromuscular junctions of vertebrates. In the autonomic nervous system, it is the neurotransmitter of all preganglionic and postganglionic parasympathetic neurons. ACH is also present at many brain synapses, especially the basal nucleus.

The acetylcholine molecule was the first neurotransmitter to be identified. It is responsible for nerve transmission both in the central nervous system and in the peripheral nervous system. It is freed from the terminals of motor neurons, from preganglionic neurons, from postganglionic neurons of the parasympathetic and in various areas of the central nervous system, where it plays an essential role in cognitive processes (Alzheimer).

There are two categories of ACH

Receptors: - nicotinic receptors, ionotropic type

- Muscarinic receptors: metabotropic type.

 The muscarinic actions induced by the ACH generate generalized vasodilation and hypersecretion of the sweat glands, which are innervated by cholinergic fibers of the sympathetic nervous system.

Nicotinic actions occur on the ganglia of the sympathetic and parasympathetic systems, the neuromuscular plaque of the voluntary muscles, and the nerve endings of the splanchnic nerves that surround the secretory cells of the adrenal medulla.

Author: Vicki Lezama