A neuron is a specialized cell that allows the nervous system to receive information, process it, and send signals to other cells. Neurons make it possible to sense a touch, move a muscle, recognize a face, remember an experience, and regulate functions such as breathing and heart rate.
The human nervous system contains enormous numbers of neurons, organized into networks that communicate through electrical and chemical signals. Although neurons come in many shapes and sizes, most use the same basic strategy: they receive signals through their branches, integrate those signals in the cell body, and, when the conditions are right, send an electrical impulse down a long extension called an axon. At the axon’s end, chemical messengers called neurotransmitters can carry the signal to another cell.
Understanding how this works requires looking at both the structure of a neuron and the changes in electricity and chemistry that occur across its membrane.
The basic parts of a neuron
Most neurons have three major structural regions: dendrites, a cell body, and an axon.
Dendrites are branching extensions that receive signals from other neurons or from sensory cells. A neuron can have many dendrites, giving it a large surface area for incoming information. Signals arriving at different dendrites can have different effects on whether the neuron ultimately sends its own signal.
The cell body, also called the soma, contains the nucleus and most of the structures needed to keep the cell alive. It also integrates incoming signals. Near the point where the axon begins is a specialized region called the axon hillock and initial segment. This area plays an important role in deciding whether the neuron will generate an action potential.
The axon is a usually elongated projection that carries electrical signals away from the cell body. Some axons are extremely short, while others extend a considerable distance through the body. At its far end, an axon typically branches into axon terminals, which communicate with other neurons, muscles, or glands.
Many axons are wrapped in myelin, a fatty insulating material produced by specialized glial cells. Myelin helps electrical signals travel rapidly along the axon. Small gaps in the myelin, called nodes of Ranvier, allow the action potential to be regenerated at intervals along the axon. This arrangement allows the signal to appear to jump from node to node, a process known as saltatory conduction.
Neurons are not the only cells in nervous tissue. Glial cells, or glia, support neurons in many ways, including providing metabolic support, maintaining the chemical environment around neurons, producing myelin, and participating in immune defense. Neurons depend on this surrounding cellular environment to function properly.
How a neuron uses electricity
A neuron does not send electricity through a wire in the same way a household circuit does. Instead, its electrical signaling comes from the movement of charged particles, called ions, across its cell membrane.
The membrane separates the fluid inside the neuron from the fluid outside it. These fluids contain different concentrations of ions, including sodium, potassium, and chloride. Proteins embedded in the membrane control which ions can cross.
At rest, the inside of a neuron is electrically negative relative to the outside. This difference is called the resting membrane potential. It is maintained by a combination of ion concentration differences, selective ion permeability, and active transport mechanisms such as the sodium-potassium pump.
The neuron constantly uses energy to maintain these ion gradients. The gradients are essential because they provide the conditions that allow rapid changes in membrane voltage when ion channels open.
How neurons receive and integrate signals
When another neuron communicates with a neuron, it usually does so at a specialized junction called a synapse. The receiving neuron may experience a small change in its membrane voltage when neurotransmitters bind to receptors on its surface.
Some incoming signals make the receiving neuron more likely to produce an action potential. These are generally called excitatory signals. Others make an action potential less likely and are called inhibitory signals.
A single neuron can receive signals from many other cells at once. The effects of these inputs are combined in the receiving neuron. Their timing and location matter: signals arriving close together in time can add up, and signals arriving at different parts of the neuron can influence the cell differently.
If the combined effect at the axon’s initial segment brings the membrane voltage to a critical threshold, the neuron can generate an action potential.
This integration is one of the most important features of neural signaling. A neuron is not simply an on-off relay. It continuously combines many incoming influences and produces an output according to the properties of its membrane, its connections, and the state of its surrounding network.
What an action potential is
An action potential is a brief, rapid change in a neuron’s membrane voltage that travels along the axon.
It begins when voltage-sensitive ion channels open in response to the membrane reaching threshold. Sodium channels typically open first, allowing sodium ions to enter the neuron. This causes the membrane voltage to become less negative and then temporarily positive relative to the resting state.
Sodium channels then become inactivated, while potassium channels open. Potassium ions move out of the cell, helping bring the membrane voltage back toward its resting level. The membrane may briefly become more negative than its resting state before returning to normal.
The sequence is tightly controlled by the behavior of ion channels.
An action potential is often described as an all-or-none event. Once threshold is reached, the action potential has a broadly consistent size rather than becoming larger in proportion to how strong the original stimulus was. Stronger or more persistent inputs can instead affect how frequently a neuron generates action potentials and how many neurons become active.
This distinction is important: information in the nervous system is encoded not only by whether neurons fire, but also by patterns and timing of their activity across networks.
How an action potential travels down the axon
An action potential at one part of the axon changes the local electrical conditions of the membrane. That change influences nearby sections of the axon, causing them to reach threshold and generate their own action potentials.
In this way, the signal propagates along the axon.
The action potential does not simply travel as the original group of ions moving from one end of the axon to the other. Instead, each successive section of membrane undergoes the characteristic sequence of ion-channel changes.
After firing, a section of membrane enters a refractory period during which it cannot immediately generate another normal action potential, or requires a stronger stimulus to do so. This helps ensure that the signal moves forward rather than repeatedly traveling backward along the same recently activated membrane.
Myelinated axons conduct signals differently. Because myelin electrically insulates much of the axon, significant membrane current exchange occurs mainly at the nodes of Ranvier. The action potential is regenerated at these nodes, allowing rapid transmission along the length of the axon.
How a neuron communicates with another cell
When an action potential reaches an axon terminal, it triggers a sequence of events that can release neurotransmitters.
At many chemical synapses, the arriving action potential opens voltage-sensitive calcium channels. Calcium ions enter the axon terminal and trigger synaptic vesicles—small membrane-bound packets containing neurotransmitter—to fuse with the cell membrane.
The neurotransmitter is then released into the narrow synaptic cleft between the cells. It crosses this space and binds to receptors on the receiving cell.
The receptors determine what happens next. Depending on the neurotransmitter, receptor type, and cell involved, the result may be excitatory, inhibitory, or capable of producing other changes in the receiving cell.
Neurotransmitters do not simply have one universal effect. The same chemical messenger can produce different effects when it acts on different receptor types.
Afterward, neurotransmitters are cleared from the synapse through processes such as reuptake into cells, enzymatic breakdown, or diffusion away from the synaptic region. This limits the duration of the signal and allows the synapse to be used again.
Electrical and chemical signaling work together
Neural communication is often described as electrical signaling within a neuron and chemical signaling between neurons. That is a useful general description, but it is not an absolute division.
The action potential is an electrical event produced by ion movement across the membrane. At a typical chemical synapse, that electrical event causes chemical release. The neurotransmitter then changes the electrical or biochemical state of the next cell.
There are also electrical synapses, in which cells are connected by specialized channels called gap junctions. Ions and electrical current can pass directly between the cells, allowing very rapid communication and, in some neural circuits, synchronization of activity.
The nervous system therefore uses several forms of signaling rather than relying on one simple mechanism.
Different neurons perform different jobs
Not all neurons process information in the same way.
Sensory neurons detect changes in the body’s internal or external environment. Depending on the system, they can respond to stimuli such as pressure, temperature, light, sound, or chemical changes.
Motor neurons transmit signals that ultimately control muscles or other effectors. Their activity helps produce voluntary movement as well as many involuntary actions.
Interneurons connect neurons within the nervous system. They perform much of the processing that transforms incoming information into appropriate patterns of activity.
Neurons also differ in their shapes, electrical properties, neurotransmitters, connections, and gene expression. A neuron in a sensory pathway can therefore operate quite differently from one involved in controlling movement or supporting memory.
How neurons create complex functions
A single neuron has limited computational ability compared with the nervous system as a whole. The extraordinary capabilities of the brain arise from networks containing large numbers of interconnected cells.
Each neuron can receive many inputs, combine them, and influence many other cells. Connections between neurons can also change. This ability to modify the strength or effectiveness of connections is known as synaptic plasticity.
Synaptic plasticity is important for learning, memory, development, and adaptation. Neural circuits can change in response to experience, altering how strongly particular patterns of activity influence later responses.
The brain’s information processing therefore depends not simply on individual neurons firing, but on which neurons are connected, how strongly they influence one another, and when they become active.
What happens when a neuron is damaged?
Neurons are highly specialized cells and can be vulnerable to disruptions in their energy supply, ion balance, chemical environment, and structural integrity. Because neurons depend heavily on continuous energy production to maintain their ion gradients, interruptions in the supply of oxygen or nutrients can cause serious dysfunction.
Damage can occur to the cell body, dendrites, axon, synapses, or myelin. The consequences depend on which neurons and pathways are affected.
Some parts of the nervous system have limited capacity for repair, while peripheral nerves can have a greater ability to regenerate under appropriate conditions. The central nervous system—the brain and spinal cord—generally presents greater obstacles to effective axon regeneration.
Disorders affecting neurons or their supporting cells can interfere with signaling in many ways. Problems may involve the loss of neurons, abnormal electrical activity, damaged myelin, altered neurotransmitter systems, or disrupted synaptic connections.
The basic sequence of neural signaling
A neuron can therefore be understood as part of a repeating communication process:
- Inputs arrive, usually at dendrites and the cell body through synapses or sensory receptors.
- The inputs are integrated, with excitatory and inhibitory influences combining across the neuron.
- Threshold may be reached at the axon’s initial segment.
- An action potential is generated and travels along the axon.
- The axon terminal responds when the action potential arrives.
- Neurotransmitter may be released into a synapse.
- The next cell responds, potentially producing another action potential or another type of cellular change.
This process occurs repeatedly across interconnected neural circuits. The result is a communication system capable of detecting changes, coordinating movement, regulating organs, and supporting perception, thought, learning, and memory.
A neuron is therefore more than an electrical wire or a simple switch. It is a living cell that maintains complex ion gradients, responds to thousands of potential inputs, generates precisely timed electrical signals, and communicates chemically and electrically with other cells. The coordinated activity of vast networks of these cells is what allows the nervous system to turn signals into behavior and experience.
