Neurotransmitters: Definition, Function, & Types

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.

You can test your neurotransmitters with this at-home kit.

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

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.​

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). 

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)

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:
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• 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. 

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. But if you’re curious, you can test your neurotransmitters with this take-home kit.

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).

If you’d like to keep learning more, here are a few books that you might be interested in.

• Your Brain Electric: Everything you need to know about optimising neurotransmitters including serotonin, dopamine and noradrenaline
• Secrets to Controlling your Weight, Cravings and Mood: Understand the biochemistry of neurotransmitters and how they determine our weight and mood
• Happy Brain: Boost Your Dopamine, Serotonin, Oxytocin & Other Neurotransmitters Naturally, Improve Your Focus and Brain Functions (38+ Tips, Train, Power, Function, Science, Endorphins)

• 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.