Every thought, movement, sensation, and many automatic functions of the body depend on neurons communicating with one another. These specialized cells carry information through the nervous system using a combination of electrical changes within a neuron and chemical signals passed between neurons.
The basic process is remarkably consistent. A neuron receives information, integrates it, and, when the incoming signals reach a sufficient level, generates an electrical impulse called an action potential. That impulse travels along the neuron to its ending, where it triggers the release of chemical messengers called neurotransmitters. The neurotransmitters cross a tiny gap and influence the next cell.
This electrical-to-chemical-to-electrical sequence allows networks containing billions of neurons to process information rapidly and coordinate the body.
The basic structure of a neuron
Although neurons come in many shapes and sizes, most have three functional regions: dendrites, the cell body, and an axon.
Dendrites are branching structures that commonly receive signals from other neurons or from sensory receptors. The cell body, or soma, contains the nucleus and maintains the cell’s metabolic machinery. The axon is a long projection that carries electrical signals away from the cell body toward other neurons, muscles, or glands.
The point where the axon begins, near the cell body, is especially important for deciding whether a neuron will produce an action potential. Signals arriving at different parts of the neuron can be either excitatory, making an action potential more likely, or inhibitory, making one less likely. The neuron effectively integrates these influences.
Not every neuron follows exactly the same structural arrangement, but the principle is similar: information is received, processed, and transmitted.
How a neuron creates an electrical signal
A neuron’s electrical behavior comes from the movement of charged particles, particularly sodium and potassium ions, across its cell membrane.
At rest, the inside of a neuron is electrically negative relative to the outside. This resting membrane potential is maintained by differences in ion concentrations and by proteins in the membrane that selectively allow certain ions to cross. The sodium-potassium pump also helps maintain the underlying concentration gradients by moving sodium and potassium ions across the membrane.
When a neuron receives sufficiently strong excitatory input, its membrane becomes less negative. If the membrane at the neuron’s trigger region reaches a critical threshold, voltage-sensitive sodium channels open rapidly. Sodium ions then flow into the cell, causing a rapid change in membrane voltage.
This sudden electrical event is the action potential.
The action potential is an all-or-none event: once threshold is reached, it occurs rather than becoming a weaker or stronger version of itself. The intensity of information is therefore not normally represented by making individual action potentials larger. Instead, information can be encoded in patterns such as how frequently action potentials occur and which neurons are active.
Soon after sodium channels open, they become temporarily inactive, while voltage-sensitive potassium channels allow potassium to flow out of the neuron. This helps restore the membrane toward its resting state. A brief period of reduced excitability follows, called the refractory period, which helps limit how rapidly another action potential can occur and helps keep the signal moving forward.
How the action potential travels along the axon
An action potential at one point on an axon changes the electrical conditions in the neighboring membrane. That change brings the next section of the axon toward threshold, causing an action potential there. The process continues along the axon.
The signal does not physically travel down the axon as a single object. Rather, each section of membrane triggers the next section in sequence.
Many axons are surrounded by myelin, an insulating layer produced by glial cells. In the brain and spinal cord, myelin is produced by oligodendrocytes; in peripheral nerves, it is produced by Schwann cells. Myelin is interrupted at small gaps called nodes of Ranvier.
In a myelinated axon, action potentials are regenerated mainly at these nodes. The electrical change effectively moves rapidly between nodes, a process known as saltatory conduction. This allows signals to travel much faster than they would along an axon without myelin.
Axon diameter also affects conduction speed: larger axons generally conduct signals more rapidly because electrical current encounters less internal resistance.
What happens when the signal reaches the end of the neuron
An axon often branches near its end into numerous axon terminals. Most neurons do not make direct electrical contact with the next neuron. Instead, they communicate across specialized junctions called synapses.
When an action potential reaches an axon terminal, it changes the terminal’s membrane voltage. This opens voltage-gated calcium channels, allowing calcium ions to enter.
The increase in calcium triggers small membrane-bound packages called synaptic vesicles to fuse with the cell membrane. The vesicles release neurotransmitter molecules into the narrow space between cells, called the synaptic cleft.
The neurotransmitter then reaches receptors on the receiving cell.
How neurotransmitters affect the next cell
A neurotransmitter does not automatically “turn on” the next neuron. Its effect depends largely on which receptor it activates.
Some receptors produce changes that make the receiving neuron more likely to generate an action potential. These are excitatory effects. Others make an action potential less likely and are inhibitory effects.
A single neuron can receive thousands of inputs, and those inputs can arrive at different locations and at different times. The neuron combines these signals. Excitatory and inhibitory effects can partially or completely offset one another, while repeated inputs arriving close together can combine.
If the resulting electrical change at the neuron’s trigger region reaches threshold, the next neuron generates its own action potential. The signal has therefore crossed the synapse and been converted back into an electrical event.
Neurotransmitter signaling is terminated in several ways. A neurotransmitter may be broken down by an enzyme, taken back into the releasing neuron or nearby cells through reuptake, or diffuse away from the synapse. These mechanisms prevent a signal from continuing indefinitely.
Electrical synapses work differently
Not all neuronal communication uses neurotransmitters. At electrical synapses, neighboring cells are connected by specialized channels called gap junctions. Ions can pass directly from one cell to another, allowing electrical activity to spread with very little delay.
Electrical synapses are useful when groups of cells need highly synchronized activity. Chemical synapses are more common in the nervous system and offer much greater flexibility because neurotransmitters and their receptors can amplify, inhibit, modify, or otherwise regulate signaling.
How signals move through the nervous system
Neurons do not operate as isolated wires. They form interconnected circuits.
The central nervous system, consisting of the brain and spinal cord, processes and coordinates information. The peripheral nervous system connects the central nervous system with sensory receptors, muscles, organs, and other tissues.
For example, touching a hot surface activates sensory receptors in the skin. Sensory neurons carry information into the nervous system, where circuits in the spinal cord and brain process it. Motor neurons can then activate muscles to withdraw the hand. The brain can also process the sensory information, allowing the experience to be consciously perceived.
Not every response requires conscious awareness. The spinal cord can organize certain rapid protective responses through reflex circuits, allowing an appropriate motor response to begin before the brain has fully processed the sensation.
Communication also works in the other direction. Signals originating in the brain and spinal cord travel through motor pathways to control skeletal muscles, while other pathways regulate internal organs, blood vessels, glands, and other involuntary functions.
How the nervous system carries different kinds of information
The nervous system does not use a single electrical signal to represent every kind of information. Meaning depends on which neurons are active, when they are active, how frequently they fire, and how their activity is organized within a circuit.
Sensory neurons, for example, can convey information about touch, temperature, body position, sound, light, and other forms of stimulation. The nervous system interprets these signals according to the receptors and neural pathways involved.
The location of activity matters as well. Information from different parts of the body generally enters and travels through distinct neural pathways, helping the brain determine where a stimulus originated.
Timing is also important. A rapid sequence of action potentials can convey different information from a slower sequence, and groups of neurons can encode information through coordinated patterns of activity.
Why neurotransmitters do not tell the whole story
It is common to describe neurotransmitters as if each one had a single fixed function—such as an “excitatory” neurotransmitter or an “inhibitory” neurotransmitter. In reality, their effects depend on the receptors present on the receiving cell and on the cellular context.
For example, the same neurotransmitter can produce different effects when it binds to different receptor types. Some receptors directly open or close ion channels, producing relatively rapid effects. Others activate intracellular signaling pathways that alter the cell’s behavior more indirectly and can produce slower, longer-lasting changes.
This receptor-dependent signaling gives neural circuits considerable flexibility. The same chemical messenger can therefore participate in very different functions in different parts of the nervous system.
Where glial cells fit in
Neurons receive much of the attention because they carry and process electrical signals, but they depend heavily on glial cells, or glia.
Glia perform several supporting and regulatory functions. They help maintain the chemical environment around neurons, provide metabolic support, participate in immune defense within the nervous system, and produce myelin. Some glial cells also influence synaptic activity and help maintain the environment required for neurons to function normally.
The nervous system is therefore not simply a network of neurons. Its signaling depends on interactions among neurons, glia, blood supply, and the surrounding chemical environment.
From one signal to a functioning nervous system
A single action potential is relatively simple: a rapid, temporary change in membrane voltage that travels along an axon. The extraordinary capabilities of the nervous system arise from what happens when enormous numbers of neurons communicate through interconnected circuits.
At each stage, information can be transformed. Sensory receptors convert physical or chemical stimuli into neural signals. Neurons integrate excitatory and inhibitory inputs. Action potentials carry signals over distances. Synapses transfer information between cells and can strengthen, weaken, or otherwise modify its effect. Networks of neurons then combine these signals to produce perception, movement, memory, thought, and regulation of the body’s internal functions.
The essential sequence is straightforward: neurons receive signals, integrate them, generate action potentials when appropriate, carry those impulses along their axons, and communicate with other cells at synapses. The complexity of the nervous system comes not from a fundamentally different signaling mechanism, but from the vast number of neurons and the intricate ways in which their signals are connected, timed, and regulated.
