Neurons are specialized cells that receive, process, and transmit information throughout the nervous system. They allow the brain to interpret sensory information, coordinate movement, form memories, regulate internal organs, and support thinking and emotion. Their defining feature is their ability to communicate through electrical signals within a cell and chemical or electrical signals between cells.
The human nervous system contains billions of neurons connected into complex networks. Each neuron has a structure suited to its role, from detecting a touch on the skin to transmitting commands from the brain to a muscle. Understanding how neurons are built, how they generate signals, and how they communicate helps explain how the nervous system works.
What is a neuron?
A neuron is a nerve cell specialized for communication. Unlike cells that primarily store energy, produce structural materials, or transport substances, neurons are adapted to carry information over distances and pass it to other cells.
Neurons are found in the brain, spinal cord, and peripheral nerves. They work alongside glial cells, which support, protect, nourish, and regulate the environment around them. Together, these cells make the nervous system capable of processing information and coordinating responses throughout the body.
A neuron can receive signals from other neurons or from sensory receptors, integrate those signals, and produce an electrical impulse if the conditions are right. That impulse travels along the neuron and can trigger communication with another neuron, a muscle cell, or a gland cell.
Neurons do not work independently. Their functions depend on connections with other cells, forming circuits that process information. A single neuron may receive input from many other neurons and influence multiple targets, allowing networks to perform tasks that individual cells cannot accomplish alone.
The structure of a neuron
Most neurons share several basic structural features: a cell body, dendrites, an axon, and specialized endings that communicate with other cells. However, their shapes vary considerably depending on where they are located and what they do.
Cell body
The cell body, also called the soma, contains the nucleus and most of the cell’s essential machinery. The nucleus holds the cell’s DNA, while structures in the surrounding cytoplasm produce proteins, generate energy, and maintain cellular functions.
Neurons require substantial energy to maintain their electrical properties, transport materials, and repair cellular components. The cell body helps coordinate these processes and supports the long extensions that carry signals throughout the nervous system.
The soma also receives and integrates incoming electrical signals, particularly through its membrane and the nearby dendrites. In many neurons, the decision to generate an action potential depends on the combined effects of these inputs near the beginning of the axon.
Dendrites
Dendrites are branching extensions that receive much of the information arriving from other neurons. Their tree-like structure provides a large surface area for connections, called synapses.
When another neuron communicates with a dendrite, it can change the electrical state of the receiving cell. These changes may make the neuron more or less likely to generate an action potential, depending on the type of input.
Dendrites are not simply passive wires. Many contain specialized structures and molecular machinery that modify incoming signals. Small protrusions called dendritic spines, common on many neurons in the brain, provide sites for synaptic connections and can change as connections strengthen or weaken during learning.
Axon
The axon is a long cellular extension that carries electrical signals away from the cell body toward other cells. Some axons are extremely short, while others extend over substantial distances. For example, certain motor neurons send axons from the spinal cord to muscles in the limbs.
Most neurons have one axon, although it may branch extensively to communicate with multiple targets. The axon begins near a specialized region called the axon initial segment, where action potentials commonly originate when incoming signals bring the membrane to the necessary threshold.
An axon’s ability to transmit signals over distance is essential to nervous system function. Without it, the brain could not rapidly coordinate muscles, communicate with sensory receptors, or regulate distant organs.
Myelin sheath
Many axons are wrapped in myelin, a fatty, insulating material that helps electrical signals travel more quickly and efficiently.
In the central nervous system, myelin is produced by cells called oligodendrocytes. In the peripheral nervous system, it is produced by Schwann cells. A single oligodendrocyte can myelinate portions of multiple axons, whereas each myelinating Schwann cell typically covers one segment of a single axon.
Myelin does not form one continuous covering. Small gaps between myelinated segments, called nodes of Ranvier, contain concentrations of ion channels that help regenerate the electrical signal as it travels along the axon. This arrangement allows action potentials to appear to jump from node to node, a process known as saltatory conduction.
Not all axons are myelinated. Some conduct signals more slowly, but their speed may be sufficient for their particular functions.
Axon terminals
At the end of an axon, branches often form specialized endings called axon terminals. These terminals communicate with target cells at synapses.
In a typical chemical synapse, an arriving electrical impulse triggers the release of chemical messengers called neurotransmitters. These molecules cross a narrow gap and bind to receptors on the receiving cell, changing its activity.
Some terminals connect with other neurons, while others communicate with muscle fibers or gland cells. The type of target and the signaling molecules involved help determine the response.
How neurons generate and transmit electrical signals
Neurons communicate using changes in electrical voltage across their cell membranes. These changes arise from the movement of electrically charged particles called ions, including sodium, potassium, calcium, and chloride.
The cell membrane separates fluid inside the neuron from fluid outside it. Ion channels and transport proteins regulate how these charged particles move across the membrane, creating and maintaining the electrical conditions necessary for signaling.
Resting membrane potential
When a neuron is not generating an action potential, its membrane usually maintains a voltage difference called the resting membrane potential. The inside of the cell is generally more negative than the outside.
This voltage results from unequal ion concentrations across the membrane, selective membrane permeability, and the activity of transport proteins such as the sodium-potassium pump. The pump helps maintain the concentration gradients by moving sodium out of the cell and potassium into it.
The resting state does not mean the neuron is inactive. It is a prepared electrical condition that allows the cell to respond rapidly when it receives sufficient stimulation.
Action potentials
An action potential is a rapid, temporary change in membrane voltage that travels along an axon. It is the main electrical signal used by many neurons to transmit information over long distances.
When incoming signals depolarize the membrane near the axon initial segment enough to reach a threshold, voltage-gated sodium channels open. Sodium enters the cell, causing a rapid rise in membrane voltage. Shortly afterward, sodium channels become inactivated and voltage-gated potassium channels allow potassium to leave the cell, helping restore the membrane toward its resting state.
The membrane then passes through a recovery period before it can generate another action potential normally. This process helps action potentials propagate in one direction along the axon under typical conditions.
Action potentials follow an all-or-none principle: once the threshold is reached, a full action potential occurs. A stronger stimulus does not ordinarily produce a proportionally larger action potential. Instead, stimulus strength is often represented by changes in the frequency and timing of action potentials and by the activity of populations of neurons.
How neurons process incoming signals
Not every signal received by a neuron produces an action potential. Many incoming signals create smaller, graded changes in membrane voltage.
Excitatory inputs make an action potential more likely, while inhibitory inputs make one less likely. Their effects depend on the receptors and ion channels involved, as well as where and when the signals arrive.
A neuron combines these inputs through a process called synaptic integration. Inputs arriving close together in time can add together, as can inputs arriving at different locations on the cell. Inhibition can counteract excitation or influence how signals are integrated.
If the combined effect at the axon initial segment reaches the required threshold, the neuron generates an action potential. Otherwise, it may remain below threshold. This integration allows neurons to respond selectively to patterns of activity rather than simply passing every incoming signal onward.
How neurons communicate at synapses
A synapse is a specialized junction through which one neuron communicates with another cell. Synapses allow nervous system circuits to transmit, modify, and coordinate information.
Most synapses in the human nervous system are chemical synapses, although electrical synapses also play important roles.
Chemical synapses
At a chemical synapse, the sending cell releases neurotransmitters into the synaptic cleft, the small space between the communicating cells.
The process begins when an action potential reaches the axon terminal and opens voltage-gated calcium channels. Calcium enters the terminal and triggers synaptic vesicles, which store neurotransmitters, to release their contents.
The neurotransmitters cross the cleft and bind to receptors on the receiving cell. Depending on the receptor, this interaction may open ion channels directly or activate biochemical pathways that alter cell activity.
The response can be excitatory, inhibitory, or modulatory. It may occur rapidly or develop more slowly, depending on the neurotransmitter, receptor, and signaling mechanism.
Afterward, neurotransmitters are cleared from the synaptic cleft through mechanisms such as reuptake into cells, enzymatic breakdown, or diffusion away from the synapse. Clearing the signal helps regulate when communication ends and prepares the synapse for subsequent activity.
Electrical synapses
Electrical synapses allow ions to pass directly between neighboring cells through specialized channels called gap junctions. This direct connection enables electrical changes to spread from one cell to another with very little delay.
Electrical synapses can help groups of neurons coordinate their activity. They are particularly useful in circuits where rapid synchronization matters, although they generally offer less flexibility in signal modification than chemical synapses.
The nervous system uses both types of synapses because they serve different purposes. Chemical synapses provide diverse ways to regulate signaling, while electrical synapses support fast, direct communication between connected cells.
Neurotransmitters and their effects
Neurotransmitters are chemical messengers that influence the activity of receiving cells. Their effects depend on the receptors they activate, not simply on the identity of the chemical itself.
Several neurotransmitters are especially important in nervous system function:
- Glutamate is the main excitatory neurotransmitter in much of the brain and plays a major role in learning and memory.
- GABA (gamma-aminobutyric acid) is the principal inhibitory neurotransmitter in the adult brain, helping regulate neuronal activity and prevent excessive excitation.
- Acetylcholine contributes to muscle contraction, attention, learning, and several automatic bodily functions.
- Dopamine helps regulate movement, motivation, reinforcement learning, and aspects of attention.
- Serotonin influences mood, sleep, appetite, and other processes.
- Norepinephrine contributes to alertness, attention, and responses to changing demands.
- Glycine is an important inhibitory neurotransmitter, particularly in the spinal cord and brainstem.
These chemicals do not operate in isolation. Their effects depend on the circuits in which they act, the receptors present on target cells, and the pattern of neural activity.
The main types of neurons
Neurons can be classified according to their function, shape, or the connections they make. These classification systems describe different aspects of the same cells rather than mutually exclusive categories.
Sensory neurons
Sensory neurons, also called afferent neurons, carry information from sensory receptors toward the central nervous system.
They help detect conditions inside and outside the body. Specialized receptors respond to stimuli such as pressure, temperature, tissue damage, light, sound, and chemical substances.
For example, sensory neurons in the skin transmit information about touch and potentially damaging heat. Other sensory neurons convey information about the position and movement of muscles and joints, helping the brain and spinal cord coordinate posture and movement.
Many sensory neurons have a distinctive structure in which a single extension branches into peripheral and central processes. This arrangement allows signals from sensory endings to travel toward the spinal cord or brain.
Motor neurons
Motor neurons, also called efferent neurons, transmit commands from the central nervous system to muscles or, through particular autonomic pathways, influence glands and internal organs.
Somatic motor neurons control skeletal muscles involved in voluntary movement and many reflexes. Their axons typically connect with muscle fibers at specialized junctions called neuromuscular junctions, where acetylcholine triggers electrical activity in the muscle.
Autonomic motor pathways regulate functions such as heart rate, digestion, and glandular secretion. These pathways generally use a chain of neurons between the central nervous system and the target organ rather than a single neuron extending directly from the central nervous system to the target.
Motor neurons are essential for turning neural processing into physical action, from moving a finger to maintaining balance or adjusting breathing-related muscle activity.
Interneurons
Interneurons are neurons that connect and process information within neural circuits. They are especially abundant in the brain and spinal cord, where they help integrate sensory input, coordinate movement, and support complex functions such as learning and decision-making.
Some interneurons are excitatory, increasing the likelihood that their targets will become active. Others are inhibitory, reducing the activity of connected neurons. This balance allows circuits to select relevant signals, coordinate timing, and prevent activity from spreading uncontrollably.
In a withdrawal reflex, for example, sensory neurons can activate spinal interneurons that help recruit motor neurons to pull a limb away from a harmful stimulus. The spinal cord can organize this rapid response before the brain has fully processed the event.
Interneurons also form the complex networks underlying higher cognitive functions. Their connections help regulate which signals are amplified, suppressed, or combined.
Neuron shapes and structural diversity
Neurons also differ in shape, reflecting the kinds of connections they form and the distances over which they communicate.
Multipolar neurons have one axon and multiple dendrites. This is the most common structural arrangement among neurons in the brain and spinal cord. Many motor neurons and interneurons are multipolar, allowing them to receive information from numerous sources.
Bipolar neurons have one main dendritic process and one axon extending from opposite sides of the cell body. They occur in specialized sensory systems, including parts of the retina and the olfactory system.
Pseudounipolar neurons have a single process that branches into two major pathways, one extending toward peripheral sensory endings and the other toward the central nervous system. Many sensory neurons that carry information from the skin, joints, and muscles have this arrangement.
Some neurons have highly specialized shapes. Purkinje cells in the cerebellum, for example, possess elaborate branching dendrites that receive extensive input and help coordinate the processing needed for precise movement.
These structural differences affect how neurons receive, integrate, and distribute signals. A neuron’s shape is therefore closely related to its role in a neural circuit.
How neurons work together in neural circuits
A neural circuit is a group of interconnected neurons that processes information to produce a particular effect or contribute to a broader function. Circuits can be relatively simple, such as those involved in certain reflexes, or extremely complex, involving many regions of the brain.
Sensory information often passes through several stages of processing. Signals from receptors may travel through sensory neurons to the spinal cord or brainstem and then reach brain regions that interpret the information. Along the way, circuits can adjust signal strength, combine inputs, and generate appropriate responses.
Movement also depends on coordinated circuits. The motor cortex contributes to planning and initiating voluntary actions, the basal ganglia help select and regulate actions, and the cerebellum contributes to timing, coordination, and error correction. Spinal circuits then help organize the activation of individual muscles.
Neural circuits also support functions that are not directly tied to immediate movement or sensation. Networks involving the hippocampus contribute to forming new memories, while distributed cortical and subcortical networks support attention, language, planning, and emotional processing.
The same neuron can participate in different aspects of processing depending on its connections and patterns of activity. The function of a neural circuit therefore emerges from the interactions among its cells rather than from any single neuron acting alone.
How neurons change during learning and memory
Neurons are not fixed communication units. Their connections and response properties can change with experience, a capacity known as neuroplasticity.
One important form of plasticity is a change in synaptic strength. Some synapses become more effective at influencing their target neurons, while others become less effective. These changes can occur through alterations in neurotransmitter release, receptor number or activity, and the structure of synaptic connections.
Long-term potentiation is a persistent increase in synaptic strength that has been studied extensively as a mechanism involved in learning and memory. Long-term depression, by contrast, is a lasting decrease in synaptic strength. Both processes help neural networks adjust how they respond to patterns of input.
Learning can also involve changes in dendritic spines, the formation or elimination of synaptic connections, and changes in the organization of larger neural circuits. Memory is not stored in one isolated neuron; it depends on patterns of activity and lasting changes distributed across networks.
Plasticity also occurs outside learning. Neural circuits can adapt after injury, during development, and as sensory or behavioral demands change. However, the nervous system’s capacity for repair varies, and changes in connectivity do not always restore lost functions completely.
Neurons and glial cells: A coordinated system
Although neurons carry and process much of the nervous system’s information, they depend on glial cells to function properly. Glia were once described mainly as supporting cells, but they also actively regulate the neural environment and influence communication.
Astrocytes help maintain the chemical environment around neurons, regulate certain aspects of synaptic signaling, and support the relationship between neural activity and blood supply. They also help manage extracellular potassium and neurotransmitters released during neural activity.
Oligodendrocytes and Schwann cells produce myelin in the central and peripheral nervous systems, respectively. By insulating axons, they help maintain reliable and efficient electrical conduction.
Microglia act as immune-related cells in the central nervous system. They monitor their surroundings, respond to injury or infection, and help remove cellular debris. During development, they also contribute to the refinement of neural connections.
Other glial cells provide structural and metabolic support. Together, neurons and glia maintain the conditions necessary for reliable signaling, adaptive circuit function, and the long-term health of nervous tissue.
What happens when neurons are damaged?
Neuronal damage can disrupt communication within a circuit, leading to changes in movement, sensation, cognition, or automatic bodily functions. The effects depend on which neurons are affected, how extensive the damage is, and whether other parts of the nervous system can compensate.
Some neurons can recover from limited injury, but mature neurons in the central nervous system generally have a limited ability to regenerate long axons. Peripheral nerve axons can sometimes regrow when the cell body survives and the surrounding structures support regeneration, although recovery may be incomplete.
Several neurological disorders involve neuronal dysfunction, damage, or loss. Parkinson’s disease is associated with the degeneration of dopamine-producing neurons in a region of the midbrain called the substantia nigra, disrupting circuits involved in movement. Alzheimer’s disease involves progressive changes in brain function and the loss of neurons and synapses, particularly affecting memory and other cognitive abilities as the disease advances. Multiple sclerosis primarily damages myelin and related structures in the central nervous system, interfering with the transmission of signals along affected axons.
Neurons can also be harmed by inadequate blood flow, physical trauma, infections, toxic substances, and prolonged or excessive electrical activity. Because neural circuits depend on precise communication, even damage that affects a relatively small population of cells can have substantial effects when those cells occupy an important position in a network.
The consequences of neuronal damage also depend on the nervous system’s ability to reorganize. Surviving circuits may sometimes compensate by changing their connections or patterns of activity, but this adaptation has limits and cannot always replace the functions of lost cells.
Why neurons are essential to the human body
Neurons link sensation, internal regulation, movement, and cognition through a common system of electrical and chemical communication. Their specialized structures allow them to receive information, integrate competing signals, and transmit messages to other cells. Their synapses allow those messages to be modified, while plasticity enables circuits to change with experience.
From a sensory neuron detecting pressure to a motor neuron activating a muscle, each cell contributes to a broader network. The coordinated activity of these networks makes it possible to perceive the environment, respond to danger, learn new skills, remember experiences, and maintain the body’s internal balance.
