The Acetylcholine Is the specific neurotransmitter in the systems of the somatic nervous system and the ganglionic synapses of the autonomic nervous system .
It is a chemical that allows the functioning of a large number of neurons and, at the same time, allows the performance of various brain activities.
It was the first isolated neurotransmitter, conceptualized and characterized, so according to many scientists is the"oldest"substance of the brain.
Acetylcholine was pharmacologically described by Henry Hallet Delt in 1914 and was later confirmed by Otto Loewi as a neurotransmitter.
The main activity of acetylcholine lies in the cholinergic system, the system responsible for producing and synthesizing acetylcholine.
With regard to its most important effects, it emphasizes muscle contraction, movement, digestive and neuroendocrine processes, and the activation of cognitive processes such as attention and arousal.
How does acetylcholine work?
As we have seen, in the mammalian brain information between neurons is transmitted through a chemical called the neurotransmitter.
This substance is released at the synapse in response to a specific stimulus and upon release transmit certain information to the next neuron.
The segregated neurotransmitter acts in specialized and highly selective receptor sites, thus, as different types of neurotransmitters exist, each acts on certain systems.
Thus, a cholinergic neuron may produce acetylcholine (but not other types of neurotransmitters), as well, a cholinergic neuron may produce specific receptors for acetylcholine but not for other neurotransmitters.
Thus, the exchange of information performed by acetylcholine takes place in neurons and systems determined and called cholinergic.
For acetylcholine to act requires an emitting neuron that produces this substance and a receptor neuron that produces a cholinergic receptor that is capable of transporting the acetylcholine when it is released from the first neuron.
How is acetylcholine synthesized?
Acetylcholine is synthesized from choline, an essential nutrient that generates the organism.
Choline accumulates in cholinergic neurons through a reaction with acyl CoA and under the enzymatic influence of choline acetyltransferase.
These three elements are found in the specific regions of the brain where acetylcholine will be produced, which is why acetylcholine makes a neurotransmitter belonging to a specific system, the cholinergic system.
When in a neuron we find these three substances that we just mentioned, we know that it consists of a cholinergic neuron and that this will produce acetylcholine by the interaction of choline and the associated enzymatic elements.
The synthesis of acetylcholine is performed inside the neuron, specifically in the nucleus of the cell.
Once synthesized, acetylcholine leaves the nucleus of the neuron and travels through the axon and dendrites, that is, the parts of the neuron that are in charge of communication and association with other neurons.
Release of acetylcholine
So far we have seen what it is, how it works and how acetylcholine is produced in the human brain.
Thus, we already know that the function of this substance is to associate and communicate specific (cholinergic) neurons with other specific (cholinergic) neurons.
To perform this process, the acetylcholine that is inside the neuron, must be released to travel to the receiving neuron.
In order for the acetylcholine to be released, the presence of a stimulus is required to motivate its exit from the neuron.
Thus, if there is no action potential performed by another neuron, acetylcholine will not be able to emerge.
And is that for the acetylcholine to be released, an action potential must reach the nerve terminal in which the neurotransmitter is.
When this happens, the same action potential generates a membrane potential, a fact that motivates the activation of the calcium channels.
Due to the electrochemical gradient, an influx of calcium ions is generated which allows the membrane barriers to open and acetylcholine can be released.
As we see, the release of acetylcholine responds to chemical mechanisms of the brain in which many substances and different molecular actions participate.
Acetylcholine Receptors
Once released, the acetylcholine stays in no man's land, that is, it is outside the neurons and is in the intersynaptic space.
Thus, in order for the synapse to be performed and acetylcholine can fulfill its mission of communicating with the consecutive neuron, the presence of substances known as receptors is required.
Receptors are chemicals whose main function is to transduce the signals emitted by the neurotransmitter.
As we have seen previously, this process is performed selectively, so not all receptors respond to acetylcholine.
For example, the receptors of another neurotransmitter such as Serotonin , Will not pick up the signals of the acetylcholine, so that for it to work it must couple to a series of specific receptors.
Generally, the receptors that respond to acetylcholine are those called cholinergic receptors.
We can find 4 main types of cholinergic receptors: muscarinic agonist receptors, nicotinic agonist receptors, muscarinic receptor antagonists and nicotinic receptor antagonists.
Functions of acetylcholine
Acetylcholine has many functions both physically and psychologically or brain.
In this way, this neurotransmitter is responsible for performing basic activities such as movement or digestion and, at the same time, participates in more complex brain processes such as cognition or memory.
Here we review the main functions of this important neurotransmitter.
1- Motor functions
It is probably the most important activity of acetylcholine.
This neurotransmitter is responsible for producing muscle contraction, controlling the resting potential of the intestinal muscle, increasing the production of spikes and modulating the blood pressure .
It acts mildly as a vasodilator in the blood vessels and contains a certain relaxing factor.
2- Neuroendocrine functions
Another key function of acetylcholine is to increase the secretion of vasopressin by stimulation of the posterior lobe of the hypophysis .
The Vasopressin Is a peptide hormone that controls the reabsorption of water molecules, so its production is vital for neuroendocrine functioning and development.
Also, acetylcholine decreases prolactin secretion in the posterior pituitary.
3- Parasympathetic functions
Acetylcholine plays an important role in the ingestion of food and in the functioning of the digestive system.
This neurotransmitter is responsible for increasing the blood flow of the gastrointestinal tract, increases gastrointestinal muscle tone, increases gastrointestinal endocrine secretions and decreases heart rate.
4- Sensory functions
Cholinergic neurons are part of the great ascending system, so they also participate in sensory processes.
This system starts in the brainstem And innervates large areas of the cerebral cortex where acetylcholine is found.
The main sensory functions that have been associated with this neurotransmitter lie in the maintenance of consciousness, the transmission of visual information and the perception of pain.
5- Cognitive functions
It has been shown how acetylcholine plays a critical role in the formation of memories, the ability to concentrate, and the development of attention and logical reasoning.
This neurotransmitter provides protective benefits and may limit the occurrence of cognitive impairment.
In fact, it has been shown as the main substance Alzheimer disease Is acetylcholine.
Related diseases
As we have seen, acetylcholine participates in various brain functions, so that the deficiency of these substances can be reflected in the deterioration of some of the activities discussed above.
Clinically, acetylcholine has been associated with two major diseases, Alzheimer's disease and Parkinson's disease .
Alzheimer's
As regards Alzheimer's disease, in 1976 the levels of the enzyme choline acetyltransferase were found to be up to 90% lower than normal in different brain regions of patients with this disease.
As we have seen, this enzyme is vital for the production of acetylcholine, so it was postulated that Alzheimer's disease could be caused by the deficiency of this brain substance.
At present, this factor is the main clue that points to the cause of Alzheimer's and covers much of the scientific attention and research that is carried out both on the disease and on the making of possible treatments.
Parkinson's
As regards Parkinson's, the association between the cause of the disease and acetylcholine is less clear.
Parkinson's is a disease that mainly affects movement, which is why acetylcholine could play an important role in its genesis.
However, the cause of the disease is unknown today and in addition, another neurotransmitter such as dopamine seems to play a more important role and most drugs for this pathology focus on the function of this neurotransmitter.
However, the close relationship between dopamine and acetylcholine suggests that the latter is also an important neurotransmitter in the disease.
What is a neurotransmitter?
Neurotransmitters are biomolecules that transmit information from one neuron to another neuron in a row.
The brain is full of neurons that allow brain activity, however, they must be able to communicate with each other to be able to perform its functions .
In this way, the neurotransmitters are the key substances of the brain that allow its activity and functionality.
The transmission of information between one neuron and another is done through the synapse, that is, through the transport of information between an emitting neuron and a receiving neuron (or cell).
Therefore, the synapse is performed by the neurotransmitters, since it is these substances that allow the exchange of information.
How does a neurotransmitter work?
When the synapse occurs, a neurotransmitter is released by the vesicles at the tip of the presynaptic neuron (the one that emits the information).
In this way, neurotransmitters are located inside the neuron and when they want to communicate with one another they are released.
Once released, the neurotransmitter crosses the synaptic space and acts by changing the action potential in the next neuron, that is, it modifies the electric shock waves of the neuron with which it wants to communicate.
Therefore, by the wave that releases the neurotransmitter when it is outside the neuron, it is achieved to excite or inhibit (depending on the type of neurotransmitter) the next neuron.
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