Neurotransmitters: Definition, Function, & Types

By Justine Saavedra, MS
Reviewed by Tchiki Davis, M.A., Ph.D.

Discover the function and importance of neurotransmitters. Explore the function of neurotransmitters in sleep, mood, happiness, and more.

Neurotransmitters: Definition, Function, & Types

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Your brain is made up of millions of individual cells that perform different functions. Some cells are responsible for functions like memory while others perform maintenance activities such as clearing debris. You can think of each of the cells as an individual worker — but how do these workers, or cells, communicate about what to do? Part of the answer lies in the chemical messengers found in your brain and body.

These chemical messengers are called neurotransmitters and they play a vital role in almost every aspect of thinking and cognition. Neurotransmitters can also affect functions in other parts of the body such as temperature and heart rate regulation.

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What Are Neurotransmitters? (A Definition)

Simply put, neurotransmitters are chemicals found in your brain and body that help tell it what to do. These chemicals are produced naturally by your body and they are responsible for many aspects of your brain’s functioning. The first neurotransmitter was discovered in the early 1900s; over 100 more have been identified since then (Hyman, 2005).

Neurotransmitters work by binding to specific receptors on cells which causes an electrical response. The electrical response started by the neurotransmitter will turn the function of the cell on or off, depending on what the signal was. When a molecule is “excitatory” it turns cells on and if it is “inhibitory” it turns cells off. By turning cells on or off, neurotransmitters play a huge role in almost every aspect of human functioning. They can bind to a wide variety of cells including nerve, muscle, or endocrine cells.

Why Are Neurotransmitters Important?

Neurotransmitters are similar to water and blood in that they are essential to basic human functioning. When something is off balance in the neurotransmitter system, it can have a variety of effects on the human body. Listed below are some common functions of neurotransmitters. As you read through the list, you will see that these chemicals play a role in basically every aspect of your day-to-day life.

Important neurotransmitter functions:

Regulating heart rhythm
Body temperature regulation
Mood regulation
Forming memories
Feeling emotions
Digesting food
Sleep (Hyman, 2005)

Video: Neurotransmitters - What Are Neurotransmitters And What Do They Do In The Body?

Function of Neurotransmitters

Neurotransmitters are part of a complex feedback system that includes electrical signals, hormones, and other information inputs. This system is what drives human emotion, behavior, thoughts, and other bodily functions, both conscious and unconscious. Each of the neurotransmitters in your body plays a unique role and can have a variety of functions throughout the body. For example, norepinephrine is a critical component of the “fight or flight” response while Gaba is the chemical that helps calm you down. Specific functions of a few of the most well-known neurotransmitters will be discussed in more detail below.

Neurotransmitters vs Hormones

Neurotransmitters and hormones are both distinct chemical substances produced in the human body. While both play a critical role in human health and behavior, there are some key differences as well. To start, neurotransmitters are produced mostly in the central nervous system while hormones are produced by endocrine glands. Both hormones and neurotransmitters act as messengers throughout the body by delivering instructions to various bodily systems but they do this in different ways. Hormones are secreted in the blood while neurotransmitters are secreted in the synapse, the space between the neuron cells in your brain. In addition, neurotransmitters tend to have fast, short effects while hormones tend to have longer-lasting effects (McCorry, 2007).

Video: Cell Communication: Hormones and Neurotransmitters

The Role of Neurotransmitters in the Autonomic Nervous System

The autonomic nervous system is responsible for the body’s automatic, or unconscious, functions and can be divided into two main parts — the sympathetic and parasympathetic nervous systems. All of the effects of the autonomic nervous system can be attributed to three neurotransmitters — acetylcholine, norepinephrine, and epinephrine. These chemicals are what drive all of the automatic, unconscious nervous system functions and reactions in your body. These neurotransmitters are responsible for contracting and relaxing muscles, altering cardiac activity, regulating glandular secretions, and other involuntary actions (McCorry, 2007).

The Role of Neurotransmitters in the Sympathetic Nervous System

The sympathetic nervous system activates your “fight or flight” response. This involuntary response occurs when something in your environment scares or threatens you. Your body reacts to the stressor by preparing you to defend yourself if need be. When your body goes into fight or flight mode, it will try to protect you in various ways including increased heart rate, dilating the pupils of your eyes, and inhibited digestion. This quick, immediate response prepares your body for strenuous physical activity.

The Role of Neurotransmitters in the Parasympathetic Nervous System

While the sympathetic nervous system is known for the “fight or flight” response, the parasympathetic nervous system is known for the opposite “rest and digest” response. This system signals to your body that it is safe to rest and relax. “The parasympathetic system predominates during quiet, resting conditions. The overall effect of the parasympathetic system under these conditions is to conserve and store energy and to regulate basic body functions such as digestion and urination” (McCorry, 2007).

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Types of Neurotransmitters: Gaba

Gaba is a neurotransmitter that is mainly described as “inhibitory” which means that it limits or represses the activity of the cell it binds to. The balance between inhibitory and excitatory feedback drives central nervous system functions. Since Gaba is largely responsible for inhibition in the brain, it plays a role in a number of various functions. When Gaba and its receptors are not functioning properly it can cause a wide range of problems.

Disorders associated with Gaba:

Epilepsy
Anxiety disorder
Schizophrenia
Depression
Premenstrual dysphoric disorder (PMDD)
Alcohol use disorder (Ochoa-de la Paz et al., 2021)

Types of Neurotransmitters: Glutamate

While Gaba is considered the driver of inhibitory responses, glutamate is considered the driver of excitatory responses, meaning a cell is activated or turned on. Glutamate can be found abundantly throughout the body and is responsible for functions such as memory, cognition, and brain development (Fontana, 2015). Glutamate is especially important for neuroplasticity which is the ability of your brain to adapt and change over time. Because of this, glutamate is an essential component of learning and memory. Dysfunctional glutamate receptors are associated with pathologies such as Alzheimer’s disease or stroke.

Types of Neurotransmitters: Norepinephrine

Norepinephrine can have a wide variety of functions in the body by influencing the brain and other types of cells. This chemical is both short and long-acting, meaning it has both fast and long-term effects on the body. Norepinephrine is a key part of multiple processes including cellular energy metabolism, regulating cognition, and neuroplasticity (O’Donnell et al., 2012). Your survival fight-or-flight response is driven by norepinephrine. This response helps increase your heart rate, slow down digestion, and focus your mind to prepare you for strenuous physical activity.

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Types of Neurotransmitters: Dopamine

While all neurotransmitters are important, dopamine is one of the standouts. This neurotransmitter is produced in both the central nervous system and other parts of the body. It can have both excitatory and inhibitory effects, meaning it is involved in a wide variety of functions.

Functions associated with dopamine:

Body movement
Motor control
Memory
Motivation
Arousal
Reward
Sleep regulation
Attention
Hormone regulation
Influences cardiovascular, renal, and digestive systems (Klein et al., 2018)

Dopamine is a very influential chemical when it comes to human well-being. It is sometimes known as the “feel-good” neurotransmitter and substances such as drugs and alcohol can briefly create an abundance of dopamine in the brain, giving people a euphoric feeling. When the person comes down, their dopamine dips below baseline levels, which can make them feel sad and unmotivated. Dopamine is an interesting molecule in that it can cause problems when there is both not enough and too much of it present in the brain. Low dopamine levels are associated with problems with attention and high dopamine levels are associated with problems like planning and switching tasks. When dopamine is not functioning properly, it can produce negative outcomes including Parkinson’s disease, Schizophrenia, Huntington’s disease, Attention Deficit and Hyperactivity Disorder, and Addiction.

Types of Neurotransmitters: Serotonin

Serotonin is arguably one of the most well-known neurotransmitters and has long been thought to be a major driver of depression, though that has since been disproven. It was discovered in the early 1900s and has been studied extensively since then. As a result, serotonin has been found to play a role in several bodily functions. It plays a role in various body systems including the gastrointestinal, cardiovascular, circulatory, and nervous systems (Jonnakuty & Gragnoli, 2008).

Functions associated with serotonin:

Memory
Mood
Emotions
Wakefulness
Sleep
Appetite
Temperature regulation

Serotonin is ubiquitous throughout the body, not just in the brain. Interestingly, it is estimated that 95% of the systemic serotonin in your body is produced in your gut with the other 5% produced in the central nervous system (Jonnakuty & Gragnoli, 2008). It is a central neurotransmitter meaning that it plays a huge role in human cognition and has been implicated in a variety of psychiatric disorders. Serotonin’s role in depression will be discussed in more detail below.

Neurotransmitters and Depression

For at least 50 years, the prevailing theory in medicine was that depression was caused by lowered serotonin levels and lowered serotonin receptor activity. This theory gained traction after drugs called selective serotonin reuptake inhibitors (SSRIs) seemed to help alleviate symptoms of depression. Since these drugs increase the concentration of serotonin in the brain and relieve depression symptoms, a theory emerged stating that lower levels of serotonin cause depression.

However, this theory has recently proven to be inaccurate. A 2022 meta-analysis of 17 separate studies on the relationship between depression and serotonin found that there was no relation between serotonin levels and depression symptoms. This paper found that “the main areas of serotonin research provide no consistent evidence of there being an association between serotonin and depression, and no support for the hypothesis that depression is caused by lowered serotonin activity or concentrations” (Moncrieff et al., 2022). This means that depression is caused by a variety of factors, both biological and environmental, not just being deficient in a single neurotransmitter.

Video: Interactions of Hormones and Neurotransmitters and Mood

Neurotransmitters and Happiness

Happiness can be a difficult construct to define since it looks different for each person and is a result of a combination of biological and environmental factors. Two of the most important biological factors for happiness are the neurotransmitters serotonin and dopamine. While no single neurotransmitter is solely responsible for happiness, these two chemicals play a key role in mood and emotions. In general, it is thought that dopamine regulates positive mood while serotonin regulates negative mood (Mitchell & Phillips, 2007).

Neurotransmitters and Sleep

Sleep is a complex human behavior that requires input from many bodily systems to work properly and this could not happen without neurotransmitters. Several neurotransmitters play a role in sleep including Gaba and serotonin. Gaba helps induce sleep because it inhibits the functions of excitatory molecules such as serotonin. If the Gaba in your body is not functioning properly, or at “normal” levels, you may experience issues with sleep. Gaba is an essential piece of achieving REM sleep, a critical part of optimum human functioning (Siegel, 2004).

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Final Thoughts on Neurotransmitters

Neurotransmitters are one of the essential chemical messengers in the human body. They can be found in the brain and throughout all of the other body systems. There are over 100 distinct neurotransmitters, each with a unique function. Neurotransmitters are responsible for some of the most basic human functions such as heart rate regulation and sleep, to the most complex ones including thought and emotion. When a neurotransmitter is not functioning properly it can cause a variety of issues ranging from mild to severe. In summary, neurotransmitters are essential for communication between neurons, regulation of mood and emotions, motor control, memory and learning, and maintaining homeostasis. Without proper neurotransmitter function, the nervous system and overall brain function would be severely impaired.

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References

Fontana, A. C. (2015). Current approaches to enhance glutamate transporter function and expression. Journal of Neurochemistry, 134(6), 982–1007. https://doi.org/10.1111/jnc.13200
Hyman, S. E. (2005). Neurotransmitters. Current Biology, 15(5). https://doi.org/10.1016/j.cub.2005.02.037
Jonnakuty, C., & Gragnoli, C. (2008). What do we know about serotonin? Journal of Cellular Physiology, 217(2), 301–306. https://doi.org/10.1002/jcp.21533
Klein, M. O., Battagello, D. S., Cardoso, A. R., Hauser, D. N., Bittencourt, J. C., & Correa, R. G. (2018). Dopamine: Functions, signaling, and association with neurological diseases. Cellular and Molecular Neurobiology, 39(1), 31–59. https://doi.org/10.1007/s10571-018-0632-3
McCorry, L. K. (2007). Physiology of the autonomic nervous system. American Journal of Pharmaceutical Education, 71(4), 78. https://doi.org/10.5688/aj710478
Mitchell, R. L. C., & Phillips, L. H. (2007). The psychological, neurochemical and functional neuroanatomical mediators of the effects of positive and negative mood on executive functions. Neuropsychologia, 45(4), 617–629. https://doi.org/10.1016/j.neuropsychologia.2006.06.030
Moncrieff, J., Cooper, R. E., Stockmann, T., Amendola, S., Hengartner, M. P., & Horowitz, M. A. (2022). The serotonin theory of depression: A Systematic Umbrella Review of the evidence. Molecular Psychiatry. https://doi.org/10.1038/s41380-022-01661-0
Ochoa-de la Paz, L. D., Gulias-Cañizo, R., D´Abril Ruíz-Leyja, E., Sánchez-Castillo, H., & Parodí, J. (2021). The role of GABA neurotransmitter in the Human Central Nervous System, physiology, and pathophysiology. Revista Mexicana de Neurociencia, 22(2). https://doi.org/10.24875/rmn.20000050
O’Donnell, J., Zeppenfeld, D., McConnell, E., Pena, S., & Nedergaard, M. (2012). Norepinephrine: A neuromodulator that boosts the function of multiple cell types to optimize CNS performance. Neurochemical Research, 37(11), 2496–2512. https://doi.org/10.1007/s11064-012-0818-x
Siegel, J. M. (2004). The neurotransmitters of sleep. J Clin Psychiatry, 65(Suppl 16), 4-7.

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