How do neurones communicate with one another? How are impulses sent across the brain? Neurones are the basic units of the nervous system, integral to many functions, and they number in the billions. Neurones are essential in how information from the outside world is received, integrated, processed, and passed on to other neurones or non-neuronal structures (known as effectors). They communicate through nerve impulses, which are enabled by neurotransmitters. So, what are neurotransmitters?
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Jetzt kostenlos anmeldenHow do neurones communicate with one another? How are impulses sent across the brain? Neurones are the basic units of the nervous system, integral to many functions, and they number in the billions. Neurones are essential in how information from the outside world is received, integrated, processed, and passed on to other neurones or non-neuronal structures (known as effectors). They communicate through nerve impulses, which are enabled by neurotransmitters. So, what are neurotransmitters?
Fig. 1 - Neurotransmitters are chemical messengers in the nervous system.
Neurotransmitters aid the communication between neurones.
Neurotransmitters are chemical messengers that send signals from neurones to other neurones or receiving structures.
Receiving structures include glands, muscles, and organs, formally known as effector organs. They are tissues or organs that respond to the signal de neurones and the nervous system as a whole is sending them, like for example excreting hormones, contracting, etc.
There is a synaptic cleft between neurones, a little space where neurotransmitters pass from one neurone to another. The synaptic cleft is where many important processes occur and where neurotransmitters 'work'.
A neurotransmitter's function is to aid communication between neurones, or between neurones and effector organs. They transmit the information that allows animals to understand their environment and emit a response to (changes in) it.
We can summarise neurotransmitter release in the following way: neurotransmitters are the chemical messengers released by presynaptic neurones (the cell that sends the signals) into the synaptic cleft, to bind to specific receptors on postsynaptic neurones or non-neural cells (the cell that receives the signals).
Presynaptic neurones are the neurones that send the signal, i.e. that releases de neurotransmitters. Neurotransmitters are released at the end of the presynaptic neurone's axon (axon terminal).
Postsynaptic neurones are the neurones that receive the signal, i.e. that generate a response (an action potential) to the neurotransmitters binding to their receptors. Signal reception usually happens at the dendrites. A postsynaptic neuron can be presynaptic too if it releases neurotransmitters after receiving and processing the neurotransmitters from another neuron.
The synaptic cleft is the very small space between the pre- and postsynaptic neurones.
Fig. 2. Synapse diagram.
An action potential is a change in the electrical charge in a cell. Only certain cells, like neurones, can produce an action potential. Usually, the internal side of the membrane has a negative charge compared to the outside of a cell's membrane. However, when an action potential is initiated, the inside of the cell turns more positive. Not all changes in membrane potential transform into action potentials, but when they do this change in charge initiates other processes in the cell. In neurones, positive charge change usually travels down the axon to the axon terminal.
Membrane potential refers to the difference in electrical charge across the membrane of a neurone between its internal environment and external environment.
Neurotransmitters can have an excitatory or inhibitory effect, which we will discuss soon! A neurotransmitter's function and effect on the receiving cell (postsynaptic neurone) varies depending on the neurotransmitter released and the receptor the receiving cell has to receive the neurotransmitter.
Fig. 3: (a) The synaptic cleft is the space between the axon terminal or terminal button of one neurone and the dendrite of another neurone. (b) In this pseudo-coloured image from a scanning electron microscope, a terminal button (green) has been opened to reveal the synaptic vesicles (orange and blue) inside.
Neurotransmitters trigger or inhibit an impulse in the next neurone. After release, neurotransmitters are removed from the synaptic cleft through several processes, such as reuptake and the breakdown of the neurotransmitter.
There are a lot of keywords here that can be confusing. An excellent way to remember them is to prioritise learning the main ones. The prefixes (pre- and post-) indicate the direction of where the neurotransmitter is going.
For instance, a neurotransmitter diffuses from the PREsynaptic neurone, across the synaptic cleft, to the POSTsynaptic neurone.
Neurotransmitters need to be eliminated after a short time in the synaptic cleft because if not the neuronal signal would not stop and neurones or effector cells would not be able to go back to their resting state. In other words, it is essential for terminating synaptic transmission.
Neurotransmitters can be removed from the synaptic cleft through three methods:
After each signal, the synaptic cleft is cleared of neurotransmitters to enable the processing of upcoming signals independently. However, if a new neurotransmitter release occurs before the previous one is eliminated, the two signals work together.
Depending on the reaction of the postsynaptic cell, neurotransmitters can be classified as excitatory or inhibitory. They can also be divided depending on their chemical structure:
Monoamine neurotransmitters are molecules formed by an amino group (-NH2) bound to an aromatic ring by a two-carbon chain (-CH2-CH2). All monoamines are derived from the aromatic amino acids phenylalanine, tyrosine, or tryptophan.
As we said before, the receptor has a role in how a neurotransmitter affects the postsynaptic neurone. In the same way, each type of neurotransmitter will impact the postsynaptic neurone differently.
Research into the topic of neurotransmitters highlights how scientists are still discovering more and more neurotransmitter types every day, so we cannot say we have found all the different types of neurotransmitters.
This table summarises the differences between excitatory and inhibitory neurotransmitters.
Neurotransmitter type | Definition | Examples |
Excitatory neurotransmitters | These neurotransmitters increase the chances of a resulting action potential occurring in the postsynaptic neurone or receiving cell. | Glutamate, epinephrine, norepinephrine |
Inhibitory neurotransmitters | These neurotransmitters decrease the chances of a resulting action potential occurring in the postsynaptic neurone or receiving cell. | GABA, glycine and serotonin |
It essentially boils down to whether or not the neurotransmitter will cause an action potential in the postsynaptic neurone.
The neurotransmitter will cause an action potential by affecting and influencing the ion flow across cell membranes of the neurones to cause an excitatory or inhibitory effect.
There's a vast array of neurotransmitters, and they can have different effects on their target cell. Let's explore the various examples of neurotransmitters.
Serotonin and dopamine are also examples of neuromodulators. Neuromodulators are molecules that affect neuronal signalling.
When we consider the effects of neurotransmitters such as serotonin, GABA, and epinephrine on the body, we can say that neurotransmitters significantly impact behaviours.
Serotonin may make a person more calm and relaxed, whilst epinephrine (adrenaline), an essential factor in the fight-or-flight response, can have the opposite effect on behaviour. A person will feel more alert and anxious and experience feelings of fear with epinephrine.
Similarly, dopamine significantly affects behaviours, primarily when these behaviours result in dopamine release.
The ventral tegmental area (VTA) is one of the major areas associated with dopamine within the brain, as it has a lot of dopaminergic neurones. It is connected to the substantial nigra, another hotspot of the dopaminergic areas in the brain.
The VTA is very heavily associated with feelings of reward and motivation. As a result, it is closely linked to drug abuse and addiction, as drugs impact the release and reuptake of dopamine and artificially extend these feelings.
The drug cocaine has an inhibitory effect on dopamine transporters within these two dopaminergic areas: it inhibits dopamine reuptake in the VTA. Thus, dopamine in these dopaminergic areas remains in the synapses for longer, extending the rewarding feelings. This is what causes that infamous feeling of euphoria when people take cocaine. Cocaine essentially prolongs the effects of dopamine in the brain's reward pathways.
Addiction becomes a problem when it takes more quantities of cocaine to produce the same effect.
When things are going right with neurotransmitters, you can navigate your daily life without much hindrance. Your brain can operate and react accordingly to different situations.
However, certain disorders can occur when there is an imbalance in neurotransmitters.
Medications for these disorders often affect the neurotransmitter associated with them.
For instance, schizophrenic patients often take antipsychotic medications that influence their dopamine levels. Typical antipsychotics block dopamine receptors in the brain, preventing dopamine uptake. Atypical antipsychotics affect dopamine and other neurotransmitters, such as serotonin.
Those with depression often use drugs that increase serotonin levels by preventing serotonin reuptake. Thus, serotonin remains in the synapses for longer.
We often refer to drugs affecting neurotransmitters as agonists or antagonists.
Agonists work by aiding the uptake of neurotransmitters at the receptors on the postsynaptic neurone/receiving cell. They bind to the synaptic receptors and facilitate the effects of the neurotransmitter. Antagonists work by binding to synaptic receptors as well. However, they reduce the effects of a neurotransmitter.
Do not confuse this with excitatory and inhibitory effects. Agonists will aid the effect of the excitatory and inhibitory neurotransmitters. It applies to antagonists, too, as antagonist drugs will reduce the excitatory or inhibitory effect.
Neurotransmitters and hormones are both molecules generated in the body that serve for communication between tissues, organs and organ systems. However, even if the function is similar, hormones and neurotransmitters are very different. They vary in their:
Let's summarise the differences in the following table:
Differences | Neurotransmitters | Hormones |
Chemical properties | Small molecules | Large molecules |
Effect location | Synapses | Endocrine glands, target organs |
Effect speed | Rapid, milliseconds to seconds | Slower, seconds to minutes |
Duration of effect | Brief, seconds to minutes | Longer, minutes to hours |
Target cells | Neurons, muscle cells | Cells throughout the body |
Function | Regulate synaptic transmission, communication from neurons | Regulate various physiological processes, such as growth and development, metabolism, and reproduction |
Neurotransmitters are chemical messengers that send signals from neurones to other neurones or receiving structures (effector organs). Examples of effector organs include organs, glands, and muscles.
Neurotransmitters affect behaviour in different ways as they have different effects depending on the neurotransmitter. Certain neurotransmitters will make a person behave calmly and more relaxed, whilst others cause feelings of anxiety to behave more erratically. For instance, dopamine gives you a sense of motivation and is a significant part of the reward pathways in the brain. If you do something that activates the reward system in the brain, it will provide you with a 'good feeling', making you more inclined to repeat the behaviour.
Key neurotransmitters of interest in psychology include serotonin, dopamine, glutamate, and GABA, although research into neurotransmitters is extensive and encompasses many different types of neurotransmitters.
Neurotransmitters are the chemical messengers released by presynaptic neurones (the cell that sends the signals) into the synaptic cleft, to bind to specific receptors on postsynaptic neurones or non-neural cells (the cell that receives the signals).
Typically, this occurs when an action potential is generated and transmitted to the synapse in the presynaptic neurone, which triggers the release of neurotransmitters from the axon terminal (the ends of the axons). Neurotransmitters then bind to the receptors and either inhibit or excite the postsynaptic neurone, creating an action potential in the postsynaptic neurone.
Psychologists may use a variety of means to remember the different neurotransmitters. Acronyms., for instance, help shorten complicated names, as seen in GABA (gamma-aminobutyric acid).
What is a neurotransmitter?
It is a chemical messenger that allows for communication between one neurone and another.
Why do neurones need neurotransmitters to communicate?
The synaptic cleft means that neurones do not connect, so neurotransmitters bridge this gap.
What is the synaptic cleft?
It is the small gap between neurones.
What are synaptic vesicles?
They are the small, bubble-like structures that hold neurotransmitters in the presynaptic neurone, and release them into the synaptic cleft.
What is the presynaptic neuron?
The neuron proceeding the synaptic cleft from which the action potential originates.
What is the postsynaptic neuron?
The neuron that receives the neurotransmitter.
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