Our body needs hormones and neurotransmitters to function properly.
Neurotransmitters allow communication between neurons and other cells, crossing the synaptic space and through nerve impulses. Hormones, on the other hand, are secreted by the endocrine glands, intervening in the regulation of many basic body functions.
Although they share structural and functional characteristics, there are also aspects that differentiate them. In this article we explain what they are, how they act and what the main characteristics of hormones and neurotransmitters are , as well as the most important differences between them .
Hormones: definition, characteristics and classification
Hormones are chemical substances that act as messengers and activate different processes for our body to function properly. They are produced in the endocrine or secretory glands (such as the hypothalamus, pituitary or thyroid, for example) and are released into the extracellular space, diffusing through the blood vessels into the blood.
The main characteristics of these chemical messengers are that they intervene in the metabolism and other functions (immune system, sexual reproduction, etc.); they act in tissues of the organism that can be at long distances from the point of origin from where the hormone is released; the effect they cause depends on the amount of hormones that are present, being directly proportional to their concentration.
Almost all hormones can be classified, chemically, in three large groups : peptide hormones, composed of amino acids, polypeptides or oligopeptides, such as insulin or vasopressin; the hormones derived from amino acids, which are used by these to synthesize themselves, such as adrenaline; and the lipid hormones, of the eicosanoid or steroid group, such as cortisol and testosterone.
Hormones can have stimulant effects, promoting activity in a tissue (e.g. prolactin); inhibitory effects, decreasing activity (e.g. somatostatin, which inhibits growth hormone release); antagonistic effects, when two hormones have opposite effects on each other (e.g. insulin and glucagon); and synergistic effects, when two hormones together produce a stronger effect than separately (e.g. growth hormone and thyroid hormones).
Neurotransmitters: definition, classification and characteristics
Neurotransmitters are chemical substances that our body uses to send information from one neuron to another , through the synaptic space between them. These signals travel to and from the central nervous system, with the aim of coordinating our body, regulating muscle activity, body secretions and the activity of different organs.
Chemical messengers that act as neurotransmitters share some basic characteristics: they are stored in synaptic vesicles, are released when calcium ions (Ca2+) are introduced into the terminal axon in response to the action potential, and produce their effect by binding to receptors on the post-synaptic cell membrane.
The main function of neurotransmitters is to inhibit or stimulate the activity of post-synaptic cells , depending on the type of receptor on which they exert their effect, triggering changes in the permeability of the neuronal membrane and in its enzymatic activity, with the mediation of other neuromodulators (such as cAMP and cGMP).
There are different types of neurotransmitters that can be classified as follows:
- Amines : neurotransmitters derived from different amino acids. In this group we can include dopamine or serotonin.
- Amino acids : are the precursor substances of amines (e.g. glutamate or glycine).
- Purines : substances like ATP or adenosine, can act as chemical messengers as well.
- Peptides : distributed throughout the brain, the most well known are the opioid peptides (e.g. encephalines and endorphins), responsible for modulating pain, among other functions.
- Gases : the most representative, nitric oxide, which produces vasodilatory effects.
- Esters : in this group, the most representative neurotransmitter, acetylcholine, which intervenes in the regulation of sleep or muscle activity, among many other functions.
Differences between hormones and neurotransmitters
Hormones and neurotransmitters share a basic characteristic and that is that they both act as chemical messengers intervening in the regulation of different body functions. However, there are important differences between a hormone and a neurotransmitter . We will now see which ones.
One of the differences between hormones and neurotransmitters is that the former are released by the endocrine glands into the bloodstream; in contrast, neurotransmitters are released in the synaptic space that exists between neurons. This leads us to point out another basic difference, and that is that the effect produced by hormones is generally much more prolonged than that of neurotransmitters.
Another feature that differentiates these two types of chemical messengers is that the neurotransmitter, when released, only communicates with the nearest neuron , through the synaptic space; however, the hormones communicate with other cells that may be at a great distance, as they travel through the bloodstream. The difference would also be that the neurotransmitters act specifically in the nervous system, whereas the hormones could act anywhere in the body.
Sometimes the distinction between hormone and neurotransmitter is not entirely clear . Some neurotransmitters also act as hormones, such as catecholamines (adrenaline, noradrenaline and dopamine). These can be produced by the adrenal glands and released into the bloodstream, exerting a hormonal effect; at the same time, they are released into the nerve endings, acting as neurotransmitters. In these cases, they are also called neurohormones.
According to the French doctor, Roger Guillemin, a neurotransmitter would be nothing more than a paracrine secretion hormone (a type of cellular communication by chemical secretion), although due to their specific characteristics, they are usually considered to be another type of messenger other than the hormone.
However, at present there are still authors who consider that a hormone is any substance that is released by a cell to act on another cell , whether near or far, and regardless of its origin or location, as well as the route used for its transport (blood circulation, extracellular fluid or synaptic space). The definitions of hormone and neurotransmitter therefore remain open to interpretation.
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