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When a person thinks, speaks, eats, walks, or just sits comfortably with all bodily systems functioning normally, the billions of cells that make up the brain and the rest of the nervous system are hard at work.

The brain is an information-processing organ, and the brain cells that relay and handle all of this information are called neurons. With their special ability to produce and share electrical signals and a capacity to link up in complex webs, neurons are a basic component of the nervous system. The brain contains more than 100 billion of them.

It is through the development, activation, and modification of these neuronal networks that the brain is able to make sense of information about the world, to adapt and learn, and to direct behavior. But neurons are not alone in the brain or elsewhere in the nervous system: Brain cells called glia play various supporting roles to keep the system running smoothly.

What Is a Neuron?

A neuron or nerve cell is the brain's fundamental building block for the transmission of information. The cell body (or soma) is a neuron’s center, from which different types of extensions project outward toward other neurons. The branch-like dendrites of a neuron receive incoming signals from thousands of other neurons. Outgoing signals are then transmitted along a single extension called the axon, which can span long distances (as far as meters) to reach yet more neurons or other types of cell.

Between the axon of a transmitting neuron and the dendrite of a receiving neuron is a gap called the synapse—the site at which signals are passed between the brain cells.

The nervous system contains hundreds of different types of neurons that have physical forms specialized for their functions.

  • Sensory neurons transmit signals from the sense organs about touch, sights, sounds, tastes, and smells.
  • Motor neurons carry signals from the brain to the body’s muscles in order to control movements.
  • Interneurons convey signals between sensory and motor neurons and among themselves.

How Neurons Connect

Neurons communicate using electrical signals, or action potentials. In order for a neuron to transmit its own signal, it must first be sufficiently activated by incoming signals from other neurons via its dendrites. Then, an action potential is generated in its axon and carried along the length of the axon.

To travel from the axon of one neuron to the dendrite of another neuron, a signal must cross the gap between neurons (the synapse), where it is translated from an electrical signal to a chemical signal. The action potential causes the release of molecules called neurotransmitters, which pass from one end of the synapse to the other. Once these molecules cross the synapse, the chemical signal is translated back to an electrical signal that continues toward the next neuron.

The ultimate effect of a signal from one neuron on the next neuron depends on the function of the synapse, which is based on the type of neurotransmitter released. Excitatory synapses send signals that encourage the creation of an action potential by the next neuron; inhibitory synapses work against it. Each neuron receives signals from both excitatory and inhibitory synapses.

The interconnections between neurons allow for the formation of highly complex information-processing networks. Over time, these connections can grow stronger or weaker depending on the neurons’ patterns of activation. Changes in structure and function take place at the level of the synapses between neurons. This capacity for change is called synaptic plasticity and it is essential to the brain’s ability to learn.

Neurotransmitters

Neurotransmitters, chemical molecules that pass between neurons, are a key part of the system that allows neurons to transmit information. Some types of molecules (excitatory neurotransmitters) promote the activation of a neuron, while others (inhibitory neurotransmitters) reduce activation.

Key neurotransmitters include the following:

  • Acetylcholine is important for the control of muscles and the secretion of hormones, as well as for cognitive function.
  • Norepinephrine (noradrenaline) is key to the function of the sympathetic nervous system and to the “fight-or-flight” response.
  • Dopamine helps to regulate reward behavior and mood, as well as in the control of body movements.
  • Serotonin plays a role in the initiation of sleep and in appetite, mood, temperature control, and other functions.
  • Glutamate is the most common excitatory neurotransmitter in the nervous system; it is involved in a wide variety of functions.
  • GABA (gamma-aminobutyric acid) is the most widespread inhibitory neurotransmitter; it is also involved in a vast array of functions.

Some neurological and psychiatric disorders have been linked to problems with the activity of one or more neurotransmitters. For example, a loss of dopamine activity is a part of Parkinson’s disease, and Alzheimer’s disease is associated with dysfunction involving acetylcholine.

Many medications act to increase or reduce the activity of neurotransmitters. Selective serotonin reuptake inhibitors (SSRIs), commonly prescribed for depression and other psychiatric disorders, increase levels of serotonin. Anticonvulsants used to treat epilepsy work on GABA. And drugs used to treat Parkinson’s disease, such as L-DOPA, promote dopamine function.

Other Brain Cells (Glia)

Although neurons are considered the basic units of the brain and nervous system, they do not do all the work. Glia are a non-neuronal category of cells that do not transmit electrical signals but still form an important part of the nervous system. Glial cells provide structural and other kinds of support to neurons and help to ensure that neurons can function properly.

There are multiple types of glial cells with distinct purposes. Major kinds of glia include:

  • Microglia: provide the central nervous system’s immune defense against potential disease threats. They also dispose of leftovers from cell injury and death.
  • Astrocytes: named for their star-like shape, support the function of neurons in part by helping to maintain an appropriate environment outside of the neurons. They can also directly affect neurotransmitter activity at the synapses between neurons.
  • Oligodendrocytes: provide structural support to the axons of neurons through a process called myelination. They produce a substance called myelin, which makes up an insulating sheath around axons that allow them to carry electrical signals more efficiently.

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