Neurotransmitters play an important role in neural communication. They are chemical messengers that carry messages between nerve cells (neurons) and other cells in your body, influencing everything from mood to involuntary movements. This process is generally referred to as neurotransmission or synaptic transmission.
Specifically, excitatory neurotransmitters have excitatory effects on the neuron. This means they increase the likelihood that the neuron will fire a signal called an action potential in the receiving neuron.
Neurotransmitters can act in predictable ways, but they can be affected by drugs, disease, and interaction with other chemical messengers.
To send messages throughout the body, neurons need to transmit signals to communicate with one another. But there is no physical connection with each other, just a minuscule gap. This junction between two nerve cells is called a synapse.
To communicate with the next cell, a neuron sends a signal across the synapse by diffusion of a neurotransmitter.
Neurotransmitters affect neurons in one of three ways: they can be excitatory, inhibitory, or modulatory. An excitatory transmitter generates a signal called an action potential in the receiving neuron. An inhibitory transmitter prevents it. Neuromodulators regulate groups of neurons.
- Excitatory neurotransmitters have excitatory effects on the neuron. This means they increase the likelihood that the neuron will fire an action potential.
- Inhibitory neurotransmitters have inhibitory effects on the neuron. This means they decrease the likelihood that the neuron will fire an action.
- Modulatory neurotransmitters can affect a number of neurons at the same time and influence the effects of other chemical messengers.
Some neurotransmitters, such as dopamine, depending on the receptors present, create both excitatory and inhibitory effects.
The most common and clearly understood types of excitatory neurotransmitters include:
Acetylcholine
This is an excitatory neurotransmitter that is found throughout the nervous system. One of its many functions is muscle stimulation, including those of the gastrointestinal system and the autonomic nervous system.
Are you familiar with cosmetic Botox injections? They’re used to eliminate wrinkles by temporarily paralyzing certain muscles. This procedure uses botulinum toxin to freeze the muscles in place by preventing neurons in the area from releasing acetylcholine.
Epinephrine
Also called adrenaline, epinephrine is an excitatory neurotransmitter produced by the adrenal glands. It is released into the bloodstream to prepare your body for dangerous situations by increasing your heart rate, blood pressure, and glucose production.
Are you familiar with the fight-or-flight response? Adrenaline helps your nervous and endocrine systems prepare for extreme situations in which you might be making a fight-or-flight decision.
Glutamate
This is the most common neurotransmitter in the central nervous system. It is an excitatory neurotransmitter and usually ensures balance with the effects of gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter.
Histamine
This is an excitatory neurotransmitter primarily involved in inflammatory responses, vasodilation, and the regulation of your immune response to foreign bodies such as allergens.
Dopamine
Dopamine has effects that are both excitatory and inhibitory. It is associated with reward mechanisms in the brain.
Drugs such as cocaine, heroin, and alcohol can temporarily increase its levels in the blood. This increase can lead to nerve cells firing abnormally that can result in intoxication along with consciousness and focus issues.
A typical secretion of dopamine in your bloodstream can contribute to motivation.
Also called noradrenaline, norepinephrine is the primary neurotransmitter in the sympathetic nervous system where it works to control heart rate, blood pressure, liver function, and other functions.
Gamma-aminobutyric acid
Also known as GABA, gamma-aminobutyric acid is an inhibitory neurotransmitter that acts as a brake to the excitatory neurotransmitters. GABA has wide distribution in the brain and has a major role in reducing neuronal excitability throughout the nervous system.
Serotonin
Serotonin is an inhibitory neurotransmitter that is involved in emotion and mood, balancing excessive excitatory neurotransmitter effects in your brain. Serotonin also regulates processes, such as sleep cycle, carbohydrate cravings, food digestion, and pain control.
There are billions of neurotransmitter molecules working constantly to keep your brain functioning and managing everything from your breathing to your heartbeat to your ability to concentrate.
Understanding the way that nerve cells communicate, as well as how increases and decreases in neurotransmitters affect our physical and mental well-being, helps researchers and doctors find ways to make us happier and healthier.
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As briefly described in the preceding chapter, neurotransmitters are chemical signals released from presynaptic nerve terminals into the synaptic cleft. The subsequent binding of neurotransmitters to specific receptors on postsynaptic neurons (or other classes of target cells) transiently changes the electrical properties of the target cells, leading to an enormous variety of postsynaptic effects (see Chapters 7 and 8).
The notion that electrical information can be transferred from one neuron to the next by means of chemical signaling was the subject of intense debate through the first half of the twentieth century. A key experiment that supported this idea was performed in 1926 by German physiologist Otto Loewi. Acting on an idea that allegedly came to him in the middle of the night, Loewi proved that electrical stimulation of the vagus nerve slows the heartbeat by releasing a chemical signal. He isolated and perfused the hearts of two frogs, monitoring the rates at which they were beating (Figure 6.1). The gist of his experiment was to collect the perfusate flowing through the stimulated heart and transfer it to the second heart. Even though the second heart had not been stimulated, its beat also slowed, showing that the vagus nerve regulates the heart rate by releasing a chemical that accumulates in the perfusate. Originally referred to as “vagus substance,” the agent was later shown to be acetylcholine (ACh), which over the years has become the most thoroughly studied neurotransmitter. ACh acts not only in the heart but at a variety of postsynaptic targets in the central and peripheral nervous systems, preeminently at the neuromuscular junction of striated muscles and in the visceral motor system (see Chapters 5 and 21).
Over the years, a number of formal criteria have emerged that definitively identify a substance as a neurotransmitter (Box A). Nonetheless, identifying the neurotransmitters active at any particular synapse remains a difficult undertaking, and for many synapses (particularly in the brain), the nature of the neurotransmitter is not well established. Substances that have not met all the criteria outlined in Box A are referred to as “putative” neurotransmitters.
Criteria That Define a Neurotransmitter.
The distinctive characteristics of neurotransmitters, compared to other signaling molecules, are made clearer by comparison with the actions of the hormones secreted by the endocrine system. Hormones typically influence target cells far removed from the hormone-secreting cell (see Chapter 8). This “action at a distance” is achieved by the release of hormones into the bloodstream. In contrast, the distance over which neurotransmitters act is miniscule. At many synapses, transmitters bind only to receptors on the postsynaptic cell that directly underlies the presynaptic terminal (Figure 6.2A); in such cases, the transmitter acts over distances less than a micrometer. Even when neurotransmitters diffuse locally to alter the electrical properties of multiple postsynaptic (and sometimes presynaptic) cells in the vicinity (Figure 6.2B), they act only over distances of tens to hundreds of micrometers. While the elongated axonal processes of neurons allow neurotransmitters to be released as much as a meter away from the neuronal cell body, these transmitters still act only near the presynaptic site of release (Figure 6.2C).
While the distinction between neurotransmitters and hormones is generally clear-cut, a substance can act as a neurotransmitter in one region of the brain while serving as a hormone elsewhere. For example, vasopressin and oxytocin, two peptide hormones that are released into the circulation from the posterior pituitary, also function as neurotransmitters at a number of central synapses. A number of other peptides also serve as both hormones and neurotransmitters.