In this tutorial, you will learn all about neurotransmitters. We begin with an introduction to neurotransmitters and an explanation of how they work. Then, we consider the different ways in which neurotransmitters are classified. Lastly, we provide common examples and discuss their functions in the human body.
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What Are Neurotransmitters?
Neurotransmitters are chemical messengers within your body. They transmit cellular signals from neurons (i.e., nerve cells) to various target cells in muscles, glands, or other nerves. Your nervous system uses neurotransmitters to help control a wide range of bodily functions, including (but not limited to) those listed below.
- Heartbeat and blood pressure
- Muscle movement/coordination
- Hormone regulation
How Do Neurotransmitters Work?
A synapse is the site of communication between a neuron and its target cell. The term presynaptic cell refers to the neuron sending the signal, while the term postsynaptic cell refers to the target cell receiving the signal. Presynaptic cells communicate with postsynaptic cells by releasing neurotransmitters across a synapse through a process known as synaptic transmission. This process can be divided into four major steps.
- Step 1: Neurotransmitters are synthesized within the neuron and stored in synaptic vesicles, which are located in the axon terminal of the presynaptic neuron.
- Step 2: An electrical signal (known as an action potential) travels down the presynaptic neuron and reaches the axon terminal. This triggers a cascade of events. Voltage-gated calcium channels open, and the resulting influx of calcium ions causes the synaptic vesicles to fuse with the presynaptic membrane. As a result, neurotransmitters are released into the synaptic cleft (i.e., the small gap between presynaptic and postsynaptic cells).
- Step 3: Neurotransmitters diffuse across the synaptic cleft and interact with receptor proteins on the postsynaptic membrane. The binding of neurotransmitters with their corresponding receptors can have an excitatory, inhibitory, or modulatory effect on the postsynaptic cell.
- Step 4: Neurotransmitters are inactivated or removed from the synaptic cleft to prevent continuous stimulation of the postsynaptic cell. Some neurotransmitters diffuse out of the synaptic cleft and are absorbed by glial cells, while others are reabsorbed by the presynaptic neuron. It is also possible for neurotransmitters to be broken down by enzymes.
Classification of Neurotransmitters
There are several different ways to categorize neurotransmitters. For example, they are commonly classified according to the effect they have on a postsynaptic cell.
- Excitatory neurotransmitters depolarize the postsynaptic membrane (i.e., make it more positive) and increase the likelihood that an action potential will fire. They do this by binding to and activating receptors on a target cell, which, in turn, cause sodium ion channels on the membrane to open and the cell to depolarize.
- Inhibitory neurotransmitters hyperpolarize the postsynaptic membrane (i.e., make it more negative) and decrease the likelihood that an action potential will fire. They do this by binding to and activating receptors on a target cell, which, in turn, cause potassium (or chlorine) ion channels on the membrane to open and the cell to hyperpolarize.
It is important to note that some neurotransmitters are capable of producing both excitatory and inhibitory effects in target cells. For example, the neurotransmitter acetylcholine produces excitatory effects in skeletal muscle (causing muscle to contract) but inhibitory effects in the heart (slowing heart rate). In these cases, the effect depends on the target cell and its receptors.
Neurotransmitters can also be classified based on their chemical structure. The list below contains some of the major groups (along with examples).
- Amino Acids: include glycine, glutamate, and GABA.
- Monoamines: include serotonin, dopamine, epinephrine, norepinephrine, and histamine.
- Peptides: include endorphins, oxytocin, somatostatin, and substance P.
- Purines: include adenosine and ATP.
- Others: include acetylcholine (ACh) and nitric oxide.
Common Neurotransmitters (and Their Functions)
Acetylcholine, the first neurotransmitter to be discovered, has numerous functions throughout the body. It activates skeletal muscles in the somatic nervous system, and it is capable of exciting or inhibiting internal organs in the autonomic nervous system. In the brain, acetylcholine helps to regulate cognitive processes such as memory and attention.
Dopamine is often nicknamed the “feel-good chemical” for its role in the brain’s reward system. It is released during pleasurable situations (e.g., when you are eating your favorite food or engaging in sexual activity). Dopamine also improves mood, increases motivation, enhances attention, and regulates motor control. Abnormal levels of dopamine in the brain can potentially lead to conditions such as Parkinson’s disease, schizophrenia, and attention-deficit/hyperactivity disorder (ADHD).
Serotonin is generally classified as an inhibitory neurotransmitter. The majority of the body’s total serotonin (roughly 90%) is located in the gastrointestinal tract, where it helps regulate digestion. In addition, platelets release serotonin, which plays an important role in the healing of wounds. In the brain, serotonin helps to stabilize mood and decrease anxiety. As a result, low levels can potentially lead to anxiety and mood disorders. Serotonin also helps to regulate sleep, appetite, and sexual function.
Glutamate is the most abundant excitatory neurotransmitter in the nervous system. It plays an important role in the sleep-wake cycle, learning/memory, and pain signaling. It can also be used as an energy source for brain cells when glucose levels are low. Furthermore, an enzyme (glutamate decarboxylase) uses glutamate to synthesize GABA, another key neurotransmitter in the brain.
Gamma-aminobutyric acid (GABA), on the other hand, is the most common inhibitory neurotransmitter in the nervous system. It can help reduce anxiety, relieve stress, and improve quality of sleep. In fact, many different sedatives and tranquilizers work by enhancing the effects of GABA.