Neurotransmitters are chemical messengers that facilitate communication between neurons in the brain and elsewhere in the body.
Neurons (also called nerve cells) communicate with each other by transmitting information through electrical and chemical signals.
These signals travel from one neuron to another via synapses, specialized sites where one neuron’s axon terminus meets another neuron’s dendrite or cell body.
There are two main types of neurotransmission: synaptic transmission and paracrine transmission. In synaptic transmission, neurotransmitters are released at the synapse and affect a nearby cell; this is the type of transmission discussed in this article. Paracrine transmission occurs when neurotransmitters are released at one site but affect cells at a different site.
Neurotransmitter receptors are located within the plasma membrane of the receiving neuron. When a certain neurotransmitter molecule from the preceding neuron enters the receiving cell’s plasma membrane, it binds to one of these receptors, which triggers a series of molecular events inside the cell.
Neurotransmitter release
Once a neurotransmitter has diffused away from the presynaptic neuron, it is off the radar.
What happens next depends on what kind of receptor is nearby on the receiving neuron. If the right kind of receptor is nearby, then things can get exciting!
Receptors are located on the plasma membrane of the receiving neuron. When a neurotransmitter molecule binds to a receptor, it sends a signal into the cell through pathways that control things like cell function or movement.
Certain receptors can trigger either a short- or long-term response in the receiving cell. Short-term responses can include things like turning down cell volume or moving cells closer together, while long-term responses can involve growing new connections or dying.
These responses are part of how our brains develop and reorganize in response to use.
Synaptic vesicles
When a neurotransmitter molecule docks with the receptor, it can cause a change in the receptor itself, in the cell membrane it resides in, or in the cell itself.
Neurotransmitter molecules are stored in tiny sacks called vesicles. When a nerve cell receives a signal to transmit a message, these vesicles fill up with neurotransmitters and then fuse with the nerve cell’s plasma membrane, releasing the neurotransmitters into the gap between cells.
This process is called synaptic transmission, and it is how one neuron communicates with the next. The neurotransmitters then bind to receptors located on the next neuron’s plasma membrane, which starts that neuron functioning.
In addition to transmitting signals down neurons, neurotransmitters can also act on other types of cells found in the brain called glial cells. Glial cells support and protect neurons, but they also can receive signals from neurotransmitters.
Action potential
When a neuron fires an action potential, it does so by means of an electrical charge that travels down the cell body and axon.
This charge is called a voltage spike or voltage decline. When the voltage spike reaches the end of the axon, it triggers a chemical called glutamate to be released into the synapse.
Glutamate is one of over 100 kinds of chemical messengers in the brain, called neurotransmitters. Neurotransmitters are manufactured in nerve cells and some other cell types in the brain.
Neurotransmitters attach to receptors on the surface of neighboring nerve cells (or other cell types) and modify their behavior. There are over 100 different types of receptors in the brain, each one specific to a particular neurotransmitter.
When a neurotransmitter molecule binds to its receptor on the next neuron, it can have one of several effects: It can inhibit or stimulate activity in the next neuron, it can regulate activity in parts of the brain other than the direct next neuron, or it can have no effect at all.
Neurons and neurotransmitters
The word “neuron” is short for “neural” and “unit.” A neuron is the primary component of the nervous system, which also includes glial cells.
Glial cells support and protect neurons, regulate body temperature, and provide structure and chemical support for the brain. These non-nucleated cells outnumber neurons by at least ten to one.
Neurons are specialized cells that transmit information to other neurons via chemical and electrical signals. These signals are transmitted down a long fiber called an axon, and terminate at structures called synapses.
At the synapse, an electrochemical signal from the presynaptic neuron is transferred to a dendrite of a postsynaptic neuron, generating another electrical or chemical signal. This process is how one nerve cell communicates with another.
Plasticity
A fifth mechanism of synaptic change is through the process of plasticity. Plasticity refers to the ability of the brain to change, reorganize, and rewire itself.
This process occurs at the molecular, cellular, and systems levels. Molecular plasticity refers to changes in neurotransmitter concentrations and composition, cellular plasticity refers to changes in the number and type of cells in a region, and systems plasticity refers to changes in how regions communicate with each other.
Overall, plasticity represents a dynamic balance between stable long-term representations and adaptability. For example, neurons may have strong long-term connections that maintain their function over time but also have the ability to adapt to new environments or stimuli by changing their output.
Plasticity can be pathological or non-pathological. Pathological plasticity occurs when neurons lose some of their stability thus leading to altered function or dysfunction. Non-pathological plasticity is when these changes are adaptive and serve a functional purpose.
Neurotransmitter receptors
A neurotransmitter molecule can bind to one of several receptor types. The receptors are located in the plasma membrane of the receiving neuron.
Some receptors are internal, located either inside the cell or within a structure called the synaptic cleft. These are called ganglionic or neuromodulatory receptors.
Receptors can be G-protein coupled, ligand-gated, ionotropic, or nuclear. Each type has a different way of interacting with the neurotransmitter that binds to it.
Internal neuromodulatory receptors can affect internal processes in the cell, like regulating growth or controlling the distribution of chemicals within the cell. This is called modulation.
Ionotropic receptors are capable of activating changes directly in the cell’s inner environment, or intracellular fluid.
Different types of neurotransmitters and their functions
Neurotransmitters are chemicals that neurons use to communicate with each other and other cell types. There are many different neurotransmitters, and each one has a specific function.
Some neurotransmitters are excitatory, meaning they cause the receiving neuron to become excited and fire an impulse. Other neurotransmitters are inhibitory, meaning they cause the receiving neuron to become less excited and fire an inhibitory impulse.
Neurotransmitter receptors can be located inside or outside of the plasma membrane of the receiving neuron. When a neurotransmitter molecule binds to its receptor, it either causes a change in voltage inside the cell or it triggers a chemical reaction outside of the cell.
Disruption of any of these processes can lead to mental health disorders and symptoms. For example, people with depression may have low levels of certain excitatory neurotransmitters but high levels of inhibitory neurotransmitters. Thus, treating symptoms with medications that increase excitatory neurotransmitters may help.
Synaptic plasticity
A related concept is synaptic plasticity, or the ability of a synapse to change in strength and composition. This happens through a process called long-term potentiation (LTP) and long-term depression (LTD).
LTP and LTD are regulated by the actions of neurochemical receptors at the synapse. These receptors are normally activated or inhibited by neurotransmitters in the synapse fluid.
For example, serotonin receptors on the postsynaptic neuron can activate proteins in the cell membrane that increase the connection’s sensitivity to incoming signals. This makes it easier for the cell to transmit a signal into the nervous system.
Serotonin can also inhibit other cells’ sensitivity to signals, making it harder for them to receive signals. This is why mood disorders like depression are linked to decreased serotonin activity in the brain.
