In order for the body to be able to react to a stimulus, it must be passed on in several ways in order to trigger a reaction in the target organ. The transmission takes place from one neuron to the neighboring neuron, via the connection points of the neurons – so-called synapses. This transfer takes place via the neurotransmitters.
neurotransmitters (from the Latin transmittere = to bring) are messenger substances that play an important role in the transmission of a stimulus from one neuron (nerve cell) to another neuron. Within a synapse, they are usually released from the presynaptic membrane via exocytosis into the synaptic cleft, where they diffuse to the postsynaptic membrane. Neurotransmitters are therefore responsible for transmitting stimuli from one cell to the other cell.
Neurotransmitters as part of the synapse
Synapses are where the excitation transmission takes place – this is where electrical stimuli (action potentials) are transmitted from neurons to the downstream muscle, nerve and gland cells. are there neurotransmitters of great importance.
Here is a brief synapse overview. If you want to learn more about it, take a look at the Synapse article!
Synapses are roughly made up of three parts:
- The presynaptic membrane: It contains neurotransmitters, which are packed in vesicles. This neurotransmitters serve as messengers for the transmission of excitation.
- The synaptic cleft: This is the space between the pre- and postsynaptic membrane and consists of extracellular matrix.
- The postsynaptic membrane: Here there are receptors that receive information through dendrites.
Neurotransmitter – function
As mentioned earlier, neurotransmitters transmit signals within a synapse from the presynaptic to the postsynaptic membrane. To do this, electrical signals are converted into chemical signals.
This is how it works:
First, electrical impulses, so-called action potentials, the presynaptic membrane and lead to the opening of calcium channels. The neurotransmitters, which are contained in vesicles in the presynaptic membrane, are then released into the synaptic cleft. This process is called exocytosis.
Next, a vesicle diffuses to the receptors on the postsynaptic membrane. Binding of the neurotransmitter to a receptor leads to changes in the structure of the receptor proteins.
Channels open that allow ions to flow in or out. The influx or efflux of ions can cause either an excitatory (excitatory) postsynaptic potential (EPSP) or an inhibitory (inhibitory) postsynaptic potential (IPSP). Depending on the potential is strengthened or inhibited.
Whether a reinforced (EPSP) or inhibited (IPSP) excitation is transmitted depends on the postsynaptic receptors and ion channels. You can find out more about this below.
At a excitatory synapse an excitatory postsynaptic potential (EPSP). Here, the tension on the postsynaptic neuron becomes more positive. One speaks of one depolarization.
This is how it works: the corresponding neurotransmitters bind to receptors in the postsynapse and open sodium ion channels. Na+ ions flow into the cell and cause the voltage in the nerve cell to rise. She becomes depolarized.
At a inhibitory synapse an inhibitory postsynaptic potential (IPSP). It comes to one hyperpolarization. This describes a drop in the resting potential.
More specifically, potassium and chloride channels open upon binding of the appropriate neurotransmitters to postsynaptic receptors. Positively charged K+ ions flow out of the cell and negatively charged Cl- ions flow into the cell. The voltage becomes more negative – the nerve cell becomes hyperpolarized.
After binding to the receptor of the postsynaptic membrane, the neurotransmitters are broken down into inactive components by various enzymes.
The enzyme cholinesterase in the synaptic cleft breaks down the neurotransmitter acetylcholine. Acetylcholine is split into acetate (acetic acid) and choline and diffuses back to the presynaptic membrane. The stimulus is passed on as long as acetylcholine is still present in the synaptic cleft.
These components diffuse back to the presynaptic membrane and are reassembled using energy and stored in vesicles – ready for a new electrical signal.
ionotropic receptors
Whether a neurotransmitter is excitatory or inhibitory depends on the properties of its receptor at the postsynapse. It will ionotropic and metabotropic differentiated receptors.
ionotropes receptors (or ligand-gated ion channels) are receptors on which ligands bind and lead to the opening of the ion channel. They are membrane receptors and at the same time they represent an ion channel. The opening of the ion channel causes the inflow of ions that membrane potential change.
ligands are substances (e.g. neurotransmitters) that bind to the ion channels according to the key-lock principle and activate them. Activation can be reversed by detaching the molecule. You can think of the ligand-gated ion channel as a pore in the postsynaptic membrane. Ions flow out or in here.
That membrane potential is the voltage or potential difference between two liquid spaces that are separated from each other. More precisely, the intercellular and the extracellular space – i.e. between the inner cell space and the space outside the cell. Nerve cells have a resting potential when they are not excited. The effects of stimulus and the resulting influx of ions lead to voltage changes that can lead to an action potential.
Excitatory receptor channels
The main excitatory (stimulating) ligands are the neurotransmitters glutamate and acetylcholine. The ion channels or receptor channels are named after the activating ligand.
Accordingly, their receptors are called ionotropic acetylcholine receptors and ionotropic glutamate receptors.
Synapses at which excitatory receptor channels are also referred to as excitatory synapses. They generate a postsynaptic potential at the postsynapse that is more positive than the resting potential.
This is called the excitatory postsynaptic potential, for short EPSP.
Inhibitory receptor channels
An important inhibitory (inhibiting) ligand is the neurotransmitter GABA (γ-aminobutyric acid). The receptor channels are called GABA receptors and are found in the central nervous system.
Synapses at which inhibitory receptor channels are also referred to as inhibitory synapses. The resting potential at the postsynapse becomes more negative and one speaks of an inhibitory postsynaptic potential, in short IPSP.
metabotropic receptors
metabotrops receptors Like ionotropic receptors, they are transmembrane proteins. Binding of a ligand to a metabotropic receptor triggers a signal cascade inside the cell that serves to transmit information. This process of signal transmission is called signal transduction.
A G protein is usually activated after a neurotransmitter binds to a metabotropic receptor. Check out the article on G-protein coupled receptors if you want to learn more!
The fact that a signal is transmitted to metabotropic receptors over several steps results in longer reaction times. Unlike the ionotropic receptors, the metabotropic receptors have no influence on the amount of ions and therefore also not on the membrane potential.
Classification of neurotransmitters
Based on their chemical characteristics, neurotransmitters can be classified into monoamines, peptides and amino acids.
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Monoamines: are z. B. adrenaline, dopamine, histamine and melatonin.
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Neuropeptides: are z. B. oxytocin, somatostatin and vasopressin.
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Amino acids: are z. B. γ-aminobutyric acid (GABA) and glutamic acid (glutamate).
Important neurotransmitters
Now you have already heard about some neurotransmitters and their peculiarities. You can learn more about their functions here.
neurotransmitter GABA
GABA (gamma(γ)-aminobutyric acid) is an inhibitory neurotransmitter found in the central nervous system and regulates blood sugar levels in the pancreas. The release of the hormone glucagon, which causes the blood sugar level to rise, is inhibited. In addition, GABA is also involved in the regulation of sleep through its inhibitory function.
As an inhibitory neurotransmitter, the effect of GABA opposes the effect of glutamate, so they act in opposite directions.
neurotransmitter glutamate
Glutamate is the antagonist of the GABA neurotransmitter. It also occurs in the central nervous system and has an excitatory effect – i.e. arousing. Among other things, glutamate is used for muscle building, coordination and the secretion of hormones from the pituitary gland (such as ACTH).
neurotransmitter acetylcholine
Acetylcholine is a neurotransmitter found in the central and peripheral nervous systems.
In the central nervous system, it is involved in cognitive processes and in the transmission of excitation from one nerve cell to the next nerve cell. In addition, it is involved in mediating muscle contraction and other important bodily functions. It is also important in the autonomic nervous system, especially in the sympathetic and parasympathetic nervous systems.
Information about two processes is passed on in the sympathetic and parasympathetic nervous system. First from pre- on the postganglionic neuron and then from the postganglionic neuron to the target organ. Acetylcholine is the neurotransmitter in both the sympathetic and parasympathetic nervous systems preganglionic neurons. In the parasympathetic he also takes over postganglionic excitation transmission.
Check out the articles on acetylcholine, central nervous system, peripheral nervous system, parasympathetic, and sympathetic! You can find out more about these terms there.
neurotransmitter dopamine
Along with serotonin, dopamine is one of the happy hormones because it plays an important role in the internal reward system.
After doing its work, dopamine diffuses back to the original nerve cell. This ends the effects of dopamine. If an experience is rated as positive, dopamine is released. This increases the drive and motivation and the experience is saved as pleasant.
Drugs affect dopamine levels because they inhibit reuptake at the original neuron. In the long term, however, the receptors to which dopamine binds become blunt and, over time, require more dopamine for arousal.
In addition, dopamine is important in the brain for controlling movement. For example, diseases such as Parkinson’s, in which the dopamine-controlled neurons degenerate, lead to symptoms such as shaking (tremor) and muscle stiffness (rigor).
Neurotransmitters – The Most Important
- are neurotransmitters messengersfabricswhich play an important role in the transmission of excitation in synapses….