An endogenous chemical present in the brain, a neurotransmitter is responsible for transmitting messages or nerve impulses from one brain cell to another. Also known as chemical messengers, neurotransmitters have the ability to have an effect on various physical as well as psychological functions - such as sleep, mood, appetite, heart rate and even fear. It is worth mentioning here that several billion neurotransmitters work incessantly to ensure that our brain always functions seamlessly. These neurotransmitters are also responsible for administering all our bodily functions - breathing, heartbeat and even the levels of our learning and concentration.
It is essential for neurons or nerve cells to communicate with each other so that they can transmit signals and send messages all through the body. Although the neurons are in constant touch with one another, they are not simply connected. Each neuron has a tiny opening at its end known as synapse and they communicate with other neurons through signals that are emitted from these small spaces. It is only when the signals pass through these small spaces the neurons are able to correspond with one another. Neurons communicate with one another by means of a process called neurotransmission.
In majority of instances, a part of the neuron called the axon terminal releases a neurotransmitter when the potential action reaches the synapse - a structure from where a neuron or nerve cell can transmit signals to one another.
It is only after an electrical signal reaches the terminal of a neuron that the latter activates the release of tiny sacs known as vesicles, which contain the neurotransmitters. In turn, these vesicles spill out the neurotransmitters into the synapse from where these neurotransmitters move through the gaps to the nerve cells in their neighbourhood. All neurons have receptors which bind to the neurotransmitters and can activate the necessary changes in the cells.
Once the neurotransmitters are released, they move across the gap of synapse and get in touch with the receptor of the neighbouring neuron. The receiving neurotransmitter either excites or slows down the neurons subject to the nature of the neurotransmitter.
Every so often the neurotransmitters are able to bind to the receptors of the neighbouring neuron and result in an electrical signal that is sent out down the cell. In this case, the action of neurotransmitters is called excitatory. In some other instances, a neurotransmitter has the ability to obstruct the passage of the signals, thereby putting a stop to the chemical message. In such cases, the action of the neurotransmitter is called inhibitory.
Many may wonder about the fate of a neurotransmitter after its job is done. In fact, once the activity of a neurotransmitter is stopped by means of different mechanisms. For instance, a neurotransmitter can be deactivated or degraded by the action of enzymes. Then again, a neurotransmitter may also go past the receptor of the neighbouring neuron. Another way by which a neuron is stopped from carrying on signals is by a process called reuptake. In this case the axon terminal of the neuron which earlier released the neurotransmitter takes it back.
Neurotransmitters have a vital role in our everyday life as they regulate all our bodily and psychological functions. While scientists have already identified over 100 different types of neurotransmitters, they are yet to ascertain the number of these chemical messengers.
All neurotransmitters are classifies according to their respective functions. The major functions on the basis of which these chemical messengers are classified include inhibitory neurotransmitters, excitatory neurotransmitters, and modulatory transmitters.
As the name suggests, neurotransmitters in this category have inhibit the neurons. In other words, these neurotransmitters reduce the chances of a neuron triggering its action potential. Gamma-aminobutyric acid (GABA) and serotonin are two brain chemical messengers classified as inhibitory neurotransmitters.
The effect of neurotransmitters in this category is excitatory on the neurons. In other words, these neurotransmitters enhance the possibilities of the neurons to trigger their action potential. There are several excitatory neurotransmitters and major among them are norepinephrine and epinephrine.
It is worth mentioning here that some neurotransmitters can have both inhibitory as well as excitatory effects on neurons. This depends on the nature of the receptors present. Such neurotransmitters include dopamine and acetylcholine.
Often known as neuromodulators, neurotransmitters in this category have the potential to influence a number of neurons simultaneously. In fact, modular neurotransmitters also possess the ability to maneuver the action of other neurotransmitters. Axon terminals of neurons release synaptic neurotransmitters that have a quick effect on the other receptor neurons, whereas neuromodulators or modulatory neurotransmitters are slow-acting and disseminate over a large area.
Neurotransmitters can be categorized and classified in various different ways. In some cases, the neurotransmitters are simply categorized as amino acids, monoacids and peptides.
In addition, these chemical messengers can be classified into any of the following six types:
Two major neurotransmitters that can be classified as amino acids are gamma-aminobutyric acid (GABA) and glutamate.
Gamma-aminobutyric acid (GABA): This neurotransmitter works as the foremost inhibitory chemical messenger of our body. This chemical messenger contributes to motor control, vision in addition to playing an important role in regulating anxiety. In fact, benzodiazepines that are prescribed to aid in anxiety treatment work be augmenting the GABA neurotransmitter's efficiency. GABA helps to enhance the relaxation feeling and helps us to remain calm.
Glutamate: There is an abundance of this chemical messenger in our nervous system. In the nervous system, glutamate has several cognitive functions like learning and memory. However, presence of too many glutamate in the nervous system can often be dangerous, as they may lead to cellular death. The upsurge in glutamate lead to excitotoxicity is related to a number of ailments as well as brain injuries such as epileptic seizures, Alzheimer’s disease and even stroke.
The two major neurotransmitters classified as peptides are oxytocin and endorphins.
Oxytocin: This is both a neurotransmitter as well as a hormone. The hypothalamus produces this chemical messenger, which has a vital role in bonding, social recognition as well as sexual reproduction. Oxytocin can also be produces synthetically such as pitocin, which is frequently used to help in labor as well as delivery. Oxytocin and its synthetic version pitocin both work to contract the uterus during labor.
Endorphins: These are inhibitory neurotransmitters and help to hold back the chemical signals that cause pain. At the same time, they help to promote a euphoric feeling. While our body produces endorphins whenever we endure pain, their production can also be activated by various other activities like aerobic workouts. For instance, when a person experiences a "runner’s high" (a euphoric feeling together with less anxiety and a reduced aptitude to experience pain), which is an ideal case of enjoyable feeling that is caused due to endorphins production.
There are a number of transmitters that are classified as monoamines, such as dopamine, epinephrine, norepinephrine, histamine, and serotonin.
Dopamine: This neurotransmitter is classified as monoamines and has a vital role in coordinating the various movements of our body. In addition, dopamine also contributes to motivation, reward and additions. Various addictive medications enhance the levels of dopamine in the brain. When the brain loses neurons that produce dopamine it may often lead to the degenerative disease known as Parkinson’s disease that causes impairments in motor movements and tremors.
Epinephrine: This monoamine chemical messenger in the brain is a neurotransmitter as well as a hormone. Normally, epinephrine (also called adrenaline) is a stress hormone that is secreted by the body's adrenal system. Nevertheless, in the brain, this chemical works as a neurotransmitter.
Norepinephrine: This neurotransmitter has a vital role in keeping the brain always alert and is also engaged in the body's flight or fight response. Its main work is to aid in mobilizing the body as well as the brain to initiate necessary actions when one is under stress or in danger. When an individual is sleeping, the levels of norepinephrine is said to be at the lowest. On the other hand, its level is highest when one is under duress.
Histamine: This monoamine chemical messenger serves the purpose of a neurotransmitter in our brain and the spinal cord. Histamine is a part of our immune system and has a function in allergic reactions. It is produced to combat pathogens responsible for allergic reactions.
Serotonin: This neurotransmitter has a number of crucial roles. It regulates as well as monitors sleep, mood, appetite, anxiety and an individual's sexuality. Selective serotonin reuptake inhibitors, more commonly known as SSRIs, are actually one variety of antidepressant drugs that are usually prescribed for treating conditions like anxiety, depression, panic attacks and panic disorder. SSRIs also help to maintain a healthy balance of serotonin levels in the system by means of obstructing serotonin reuptake in the brain. This, in turn, can aid in improving mood and also reduce anxiety.
There are two types of neurotransmitters under this category - adenosine and adenosine triphosphate (ATP).
Adenosine: In our brain, this neurotransmitter works like a neuromodulator and helps to hold back arousing and promote sound sleep.
Adenosine triphosphate (ATP): In our central as well as peripheral nervous systems, this purine serves as a neurotransmitter. It plays vital roles in the sensory transduction, autonomic control, and also helps in the communication will glial cells (cells surrounding the neurons and providing them insulation and support). Findings of different studies suggest that adenosine triphosphate may also be involved in some way in different neurological problems such as trauma, pain and also neurodegenerative disorders.
There are two major types of gasotransmitters, namely nitric oxide and carbon monoxide and each have different roles to play.
Carbon monoxide: It is an odourless, colorless gas that is toxic and can even prove to be fatal when one is exposed to very high levels of this gas. However, our body also produces this substance naturally and it does the work of a neurotransmitter helping to modulate the inflammatory responses of the body.
Nitric oxide: This substance works as a neurotransmitter and affects smooth muscles helping them to relax. This, in turn, helps to dilate the blood vessels and ensure the flow of blood to specific parts of the body.
Acetylcholine (ACh): This chemical messenger is the sole neurotransmitter in its group. Acetylcholine (ACh) is found in the central as well as the peripheral nervous systems. This chemical messenger is the main neurotransmitter related to the motor neurons. This neurotransmitter plays a vital role in forming memories, concentration and logical and verbal reasoning. It has also been found that acetylcholine helps in producing new and vigorous synapses (a connection between two nerve cells, comprising a minute space through which nerve impulses pass by dispersing neurotransmitters) all over the brain by means of a process known as synaptogenesis. In fact, acetylcholine is derived from the chemical called choline, which is often present in nuts, seafood and eggs. ACh also plays a major role in our body's movements. When a neuron releases acetylcholine into a neuromuscular intersection - a synaptic junction between a nerve cell and a muscle fiber, it leads to a succession of chemical and mechanical reactions, which eventually causes the muscles to contract. Absence or deficiency of this neurotransmitter in the neuromuscular junction results in the muscles to relax as the chemical and mechanical reactions come to a halt.
As the body has numerous processes, sometimes things can go out of kilter. Therefore, it is not at all surprising that the immense and complex system of our nervous system may also be vulnerable to various problems. Some of the things that may sometimes go awry include the failure of the neurons to produce sufficient number of specific neurotransmitters or releasing an excessive of specific neurotransmitters into the system. When excessive neurotransmitters are released by the neurons it is possible that enzymes may deactivate them. Some other problems may include fast absorption of neurotransmitters by the receptors. Moreover, if drugs and diseases affect the neurotransmitters they may have a negative effect on our body. Deficit of certain types of neurotransmitters in the system may sometimes result in development of diseases like epilepsy, Alzheimer’s disease and Parkinson's disease.
Medical practitioners are aware of the function of neurotransmitters in promoting our mental health conditions. This is the main reason why health professionals often prescribe medications that influence these chemical messengers' actions in our body, especially for treating various types of psychiatric conditions. It has been found that conditions like schizophrenia and addictions are associated with the neurotransmitter known as dopamine. Similarly, serotonin plays a major part in different mood disorders such as OCD and depression. Physicians often prescribe medications like selective serotonin reuptake inhibitors (SSRIs) for treating the different symptoms related to anxiety and depression. Sometimes these medications are used alone. However, they may also be used together with other curative treatments, such as cognitive-behavioural therapy and others.