An Overview of Neurotransmitters
What are Neurotransmitters?
The human brain is the most complex structure in the body. It has around 80 billion neurons or nerve cells. That’s more than ten times as many neurons as there are people currently living on Earth. These people interact, whisper, make public announcements, and talk to each other through various forms of communication. The same thing happens for neurons through the use of neurotransmitters.
Neurotransmitters are chemical messengers that carry signals from one neuron, referred to as the sender, to the receiver cell.
That receiver cell can be a muscle cell, gland, or another neuron. The nervous system is made up of a massive network of nerves that convey and receive signals from neurons and their target cells all over the body. It’s involved in everything that you do, feel, and think. It also receives constant feedback from all your body parts to keep your body functioning optimally. Neurotransmitters make it possible for the messages to be delivered.
How does a neurotransmitter work? How are they produced?
A neuron or nerve cell is composed of three connected parts: cell body, axon, and axon terminal.
The cell body is essential for manufacturing neurotransmitters and preserving the overall function of the neuron. The axon carries the signal (electrical) along the neuron to the axon terminal. Lastly, the axon terminal is where the electrical signal is transformed into a chemical signal. This is through neurotransmitters that convey the message from the sender neuron to the receiver cell. In the axon terminal, neurotransmitters are found and stored in sacs called synaptic vesicles. A vesicle can hold thousands of neurotransmitter molecules.
As the signal reaches the axon terminal, the electrical charge of the signal enables the vesicles to fuse with the neuron membrane (cell membrane). As a result, the neurotransmitters are released into a small space or gap called the synaptic junction. This junction is between the sender neuron and the receiver cell. The neurotransmitters then land on the receiver cell (which can be another neuron, gland, or muscle cell) and bind to specific receptors.
After binding, the neurotransmitters trigger an action or a change in the receiver cell, which can be a muscle contraction, the release of hormones from a gland, or an electrical signal in another neuron. The receiver cell may get excited (excitatory action) and pass the message to another cell. Conversely, the receiver cell may be inhibited (inhibitory action) and prevent the message from being conveyed. And at times, the action may be both excitatory and inhibitory based on the location. Please see the table below to identify which neurotransmitters are excitatory, inhibitory or both. After delivering the message, the neurotransmitter may dissolve on its own, be reabsorbed and utilised again by the sender cell or broken down by enzymes at the synaptic junction.
Production of neurotransmitters includes intake of nutrient-rich foods that contain precursors to neurotransmitters.
Precursors are the raw materials or building blocks needed to produce neurotransmitters. The greater the amount of neurotransmitter precursor you consume, the more neurotransmitters are produced, making an imbalance less likely to happen. This is straightforward thinking. It sounds simple, doesn’t it? Unfortunately, with neuroscience and medicine, it never is. It’s complicated by the fact that foods are made up of several nutrients, and how those nutrients interact with each other will also affect the production of neurotransmitters. The health of your gastrointestinal tract also plays a huge role in the absorption of precursors. To simplify the explanation regarding their production, here are some examples:
1. Do you want to be more alert and attentive? Eat protein.
Digestion in your intestines breaks down protein into amino acids. A healthy digestive system will absorb these amino acids. One of these amino acids, tyrosine, increases the production of the neurotransmitters norepinephrine, epinephrine, and dopamine. These neurotransmitters can increase alertness and boost energy. Nobody in their right mind eats 100% pure tyrosine and expects superhuman alertness, but consuming foods high in protein can boost alertness and respond better to stressful situations. Food rich in protein includes meat, eggs, poultry, and fish.
2. Do you want to relax? Eat carbohydrates.
Eating carbohydrate-rich foods will promote the release of insulin into your bloodstream. Insulin decreases the release of amino acids into your blood except for one amino acid, tryptophan. If there’s something that you need to know about tryptophan is that it’s a loner and doesn’t want to mingle with the other amino acids. It passes into your brain alone, without the noisy company of others. It enters your brain and is transformed into serotonin, a neurotransmitter that reduces pain, decreases appetite, induces sleep, and produces a sense of calm. Research studies have documented that people who want to lose weight and go on extreme diets become depressed because their serotonin levels have dropped due to low carbohydrate intake. So, if you want to relax, load up a little on carbohydrates.
3. Do you want to focus more and improve your memory and concentration? Eat more eggs.
Choline is neither a mineral nor a vitamin, but it tags along with the vitamin Bs because of their similarities. It’s found in high cholesterol foods like liver and eggs. If you don’t have enough choline in your diet, you may suffer from difficulty concentrating and memory impairment. Choline is a precursor to the neurotransmitter acetylcholine, which has an essential role in learning and memory.
Note: The examples given here are simplified for ease of understanding. Numerous factors can affect the production, storage, and release of neurotransmitters. Neurotransmitters also interact differently with other chemicals in your body.
Some common neurotransmitters
There are more than 100 known neurotransmitters in the human brain. They’re most commonly classified based on their action or structure. The table below lists some of the common ones based on their structure:
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Neurotransmitter Imbalance: When messengers don't function as they should
There has to be a balance between their production, release, and after delivery state, for neurotransmitters to work. An imbalance may occur as a result of one or more of the following factors:
- There’s something wrong with the receiver neuron, muscle cell or gland. The receptors on the receiver cell may not take up sufficient numbers of neurotransmitters because of inflammation and damage at the synaptic junction. This happens in a medical condition called myasthenia gravis. The receiving cell is a muscle cell. Myasthenia gravis is a chronic autoimmune disease in which antibodies attack the receptors on the receiving cell (muscle cell), making them less available for binding to the neurotransmitter acetylcholine resulting in an imbalance.
- Too much or too little production or release of one or more neurotransmitters can lead to an imbalance. An example would be the excess levels of the neurotransmitter serotonin and its possible connection to autism spectrum disorders (ASD).
- Rapid neurotransmitter reabsorption may be seen among people with depression. A low level of the neurotransmitter serotonin is present. However, it’s believed to be quickly reabsorbed into the sender cell, making it less available for binding to the receiver cell and exerting its action.
- Some people with Alzheimer’s disease have low levels of the neurotransmitter acetylcholine. An enzyme called acetylcholinesterase breaks down the few acetylcholine left and prevents it from binding to the receptors on the receiver cell. To make matters worse, neurons that receive the acetylcholine are lost or damaged in people with Alzheimer’s disease.
The factors mentioned above are just some of several things that can go wrong and lead to neurotransmitter imbalance. More often than not, it’s not just one pathologic process going on. Multiple simultaneous processes happen at different levels and areas, all contributing to neurotransmitter imbalance. Lucky for us, there are some useful medications that can affect the action of neurotransmitters and help in the treatment of diseases that include an imbalance in neurotransmitters.
How do medications help restore the balance in neurotransmitters?
Neuroscientists, neuropharmacologists, and other experts have developed different types of medicines that can affect chemical transmission and alter the effects of neurotransmitters. Several drugs, particularly those that address and treat brain diseases, work in various ways to influence neurotransmitters. Below are some examples:
- Drugs that block the enzyme acetylcholinesterase. Acetylcholinesterase is an enzyme that breaks down the neurotransmitter acetylcholine. Drugs that block acetylcholinesterase allow more acetylcholine to reach the receptors of the receiver. Rivastigmine, galantamine, and donepezil are examples of anticholinesterase inhibitors used to improve cognitive function and memory in people with Alzheimer’s disease and other neurodegenerative disorders.
- Drugs that block the reabsorption into the sender neuron. Selective serotonin reuptake inhibitors (SSRIs) are a type of antidepressant drugs that relieve the symptoms of anxiety, posttraumatic stress disorder, obsessive-compulsive disorder, phobias, and depression. They work by blocking and inhibiting the reabsorption of the neurotransmitter serotonin into the sender. This leads to a build-up of serotonin levels in the synaptic junction, making It more likely that serotonin will bind to the receptors of the receiver. Examples of SSRIs include sertraline (Zoloft) and fluoxetine (Prozac).
- Drugs that block the release of a neurotransmitter. Lithium partially works in treating mania (emotional high) among people with bipolar disorder by blocking the release of the neurotransmitter norepinephrine.
- Drugs that increase the response to a neurotransmitter. Benzodiazepines reduce the excitability of nerve signals in the brain and are primarily given to people who have anxiety, panic disorder, insomnia, and selected types of epilepsy. They heighten the brain’s response to the neurotransmitter gamma-aminobutyric acid (GABA) and produce a calming effect on people. Benzodiazepines are usually only given for a few weeks because they can have side effects leading to more anxiety or a change in mood and behaviour. Nitrazepam, lormetazepam, and loprazolam are some of the most potent benzodiazepines available now.
- Drugs that block a neurotransmitter from binding to the receptors at the receiver cell. Antipsychotic drugs are commonly used to treat symptoms linked to psychosis, paranoia, delusions, and hallucinations. These are primarily seen among people who have schizophrenia. People with schizophrenia have an excess of the neurotransmitter dopamine. These drugs bind to dopamine receptors on the receiver and prevent dopamine from binding. They’re also used for people diagnosed with bipolar disorder, dementia, and depression. Chlorpromazine, benztropine, and clozapine are some of the antipsychotic drugs used to treat schizophrenia.
Message Delivered?
The explanations laid out in this article on neurotransmitters have been simplified for ease of understanding. When it comes to neurotransmitters in the realm of neuroscience and diseases, the in-depth explanations are far too complicated and confusing for the layman. Neurotransmitters are involved in every function in our bodies. We can’t survive without them. Any imbalance in their levels may lead to a particular health problem. Some medications help restore the balance. But they’re only part of a bigger solution. Lastly, don’t just focus on taking good care of the messengers. Remember the senders and receivers as well. They deserve your care and attention too.
We have another article on neurotransmitters coming soon that you might enjoy reading:
Neurotransmitters Part 2: Does happiness all boil down to chemical messengers?
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References
Ropper, A., Samuels, M., Klein, J., & Prasad, S. (2019). Adams and Victor’s Principles of Neurology 11th Edition (11th ed.). McGraw Hill / Medical.
Hall, J. E., & Hall, M. E. (2020). Guyton and Hall Textbook of Medical Physiology E-Book (Guyton Physiology) (14th ed.). Elsevier.
Katzung, B., & Trevor, A. (2020). Basic and Clinical Pharmacology 15e (15th ed.). McGraw Hill / Medical.
Rodwell, V., Bender, D., Botham, K., Kennelly, P., & Weil, A. P. (2018). Harper’s Illustrated Biochemistry Thirty-First Edition (31st ed.). McGraw Hill / Medical.
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