Every time you move a finger, recognize a familiar face, remember a conversation, feel pain, or take a breath, cells in your body are handling electrical signals. These signals are extraordinarily small. They are generated by the movement of charged particles across cell membranes, travel along nerve cells, cross tiny gaps between neurons, and ultimately influence muscles, glands, organs, and other neurons.
The electricity of the nervous system is not the same as the current flowing through a household wire. Neurons do not contain miniature batteries sending electricity down copper-like cables. Instead, they maintain carefully controlled differences in electrical charge across their membranes. Changes in those differences create signals that can travel through neural circuits and carry information.
Understanding these signals helps explain how the brain communicates with the rest of the body, how sensations reach conscious awareness, why reflexes can happen almost instantly, and how patterns of neural activity contribute to thoughts, emotions, learning, and behavior.
Where the nervous system’s electricity comes from
The basic electrical machinery of the nervous system begins with the cell membrane.
A typical neuron is surrounded by a thin membrane made largely of lipids. This membrane separates the watery interior of the cell from the fluid outside it. Dissolved in these fluids are electrically charged atoms and molecules called ions. Important ions for neural signaling include sodium, potassium, calcium, and chloride.
Because the concentrations of these ions differ between the inside and outside of a neuron, and because the membrane is selectively permeable to them, the neuron can maintain a voltage difference across its membrane. The inside of a resting neuron is generally electrically negative relative to the outside. This membrane potential is commonly around −70 millivolts in many neurons, although the precise value varies among cells and conditions.
That voltage is tiny compared with the roughly 120 volts supplied by a typical U.S. household outlet. Yet it is biologically powerful because the membrane is extremely thin and because neurons have molecular channels capable of responding to changes in voltage or to chemical signals.
The unequal distribution of ions is maintained largely through proteins embedded in the membrane. Among the most important is the sodium-potassium pump, which uses cellular energy to move sodium ions outward and potassium ions inward. Other channels allow particular ions to cross the membrane under specific circumstances.
The result is a system that resembles a microscopic electrical circuit, but one built from membranes, ions, proteins, and chemical energy rather than wires and metal conductors.
What a neuron looks like
Neurons come in many shapes and sizes, but many have three broad structural regions: dendrites, a cell body, and an axon.
Dendrites are branching extensions that receive signals from other cells. The cell body, or soma, contains the nucleus and much of the machinery needed to keep the neuron alive. The axon is a specialized projection that carries electrical signals away from the cell body toward other cells.
The axon can be extremely short or, in some human neurons, more than three feet long. Long axons make it possible for signals originating in the spinal cord to reach muscles in the feet, for example.
Many axons are surrounded by myelin, a fatty insulating material produced by specialized glial cells. In the brain and spinal cord, myelin is produced by oligodendrocytes; in peripheral nerves, it is produced by Schwann cells.
Myelin does not simply make an axon stronger. It changes how electrical signals travel. Gaps between myelin segments, called nodes of Ranvier, expose small sections of the axon membrane. Electrical activity can effectively leap from node to node in a process called saltatory conduction. This greatly increases the speed of signal transmission while reducing the amount of membrane that has to actively exchange ions during each signal.
How a neuron turns a small change into a signal
Neurons constantly receive information from other neurons and from sensory receptors. Many of these inputs produce relatively small changes in membrane voltage.
These changes are called graded potentials because their size can vary. A small input may produce a small voltage change, while a stronger or more numerous set of inputs can produce a larger one.
Some inputs make a neuron more likely to generate an electrical impulse. Others make it less likely. A neuron therefore behaves somewhat like an integrator, continuously combining many incoming signals.
A particularly important region is the beginning of the axon, often called the axon initial segment. If the combined electrical influence reaches a critical level, voltage-sensitive ion channels open and an action potential begins.
This is the fundamental electrical event used for long-distance signaling in many neurons.
The action potential: a pulse of neural electricity
An action potential is a rapid, temporary change in a neuron’s membrane potential.
At rest, the inside of the neuron is relatively negative. When the membrane at the appropriate location reaches threshold, voltage-gated sodium channels open. Sodium ions rapidly move into the neuron, making the membrane potential less negative and then briefly positive.
This change triggers additional sodium channels to open, producing a rapid rise in voltage.
The sodium channels then become inactivated, while voltage-gated potassium channels open. Potassium moves out of the neuron, helping restore the membrane toward its resting state.
For a short time, the membrane can become more negative than its resting potential. This is called hyperpolarization. The membrane then returns to its usual resting condition as ion channels return to their resting states and cellular mechanisms restore the appropriate ionic conditions.
The action potential itself is remarkably brief, often lasting only a few milliseconds.
Why neurons use an all-or-none signal
An action potential is often described as an all-or-none event. Once a neuron’s membrane reaches the necessary threshold and an action potential begins, the resulting pulse has a broadly similar size each time.
A stronger stimulus does not normally produce a dramatically larger action potential. Instead, stimulus intensity can be represented through other features of neural activity, particularly how frequently action potentials occur and which populations of neurons become active.
This distinction is important. A neuron does not usually encode “very strong pain” by producing a giant action potential. Instead, stronger or more persistent stimulation can alter the pattern and frequency of activity across neural populations.
The all-or-none nature of action potentials also makes them reliable for long-distance communication. Once initiated, an action potential is regenerated as it moves along the axon rather than simply fading away like a small electrical disturbance.
How an electrical signal travels down an axon
An action potential at one section of an axon changes the electrical conditions in nearby membrane. That change influences voltage-sensitive channels farther along the axon, causing the action potential to propagate.
Behind the advancing signal, the membrane enters a refractory period during which it is temporarily unable, or less able, to generate another action potential. This helps ensure that the signal normally travels forward rather than immediately reversing direction.
In an unmyelinated axon, the action potential is regenerated continuously along the membrane. In a myelinated axon, it is regenerated primarily at the nodes of Ranvier, allowing rapid saltatory conduction.
The speed of conduction depends on factors including axon diameter and myelination. Large, heavily myelinated axons can conduct signals much faster than small, unmyelinated ones.
This difference helps explain why some neural messages are transmitted extremely rapidly while others move much more slowly.
Electrical signals are only half the story
It is tempting to imagine the nervous system as a network in which electrical signals simply pass directly from one neuron to another. Most of the time, that is not what happens.
Neurons are usually separated by microscopic junctions called synapses. At many synapses, the electrical signal arriving at the end of one neuron triggers the release of chemical messengers called neurotransmitters.
These are called chemical synapses.
When an action potential reaches the axon terminal, it causes voltage-gated calcium channels to open. Calcium ions enter the terminal and help trigger the fusion of neurotransmitter-filled vesicles with the cell membrane. The neurotransmitter is released into the tiny space between cells, known as the synaptic cleft.
The molecules then diffuse across the cleft and bind to receptors on the receiving cell.
This electrical-to-chemical-to-electrical sequence is one of the central mechanisms of nervous-system communication.
How neurotransmitters change the receiving neuron
Neurotransmitters do not all have the same effect. What a neurotransmitter does depends heavily on the receptor it activates and the type of cell receiving the signal.
A neurotransmitter can produce an excitatory effect, making a neuron more likely to fire an action potential. Another receptor activated by the same or a different neurotransmitter can produce an inhibitory effect, making firing less likely.
Some receptors act rapidly by directly controlling ion channels. Others activate signaling pathways inside the cell and can produce slower, longer-lasting changes.
The nervous system therefore does not operate through a simple collection of “on” and “off” switches. Its cells continually influence one another through combinations of excitatory, inhibitory, and modulatory signals.
After a neurotransmitter has acted, it must be removed or otherwise inactivated. Depending on the neurotransmitter and synapse, this can occur through reuptake into cells, enzymatic breakdown, or diffusion away from the synapse.
The brain is an enormous network of interacting signals
A human brain contains roughly 86 billion neurons, along with large populations of glial cells. Individual neurons can form connections with many other cells, creating an extraordinarily complex network.
A single neuron can receive inputs from hundreds or thousands of other neurons. Its activity depends on how those inputs arrive in space and time.
Inputs occurring simultaneously at different locations can combine, a process called spatial summation. Inputs arriving repeatedly over a short period can also accumulate, known as temporal summation.
The result is a constantly changing pattern of activity. Individual action potentials matter, but much of the nervous system’s information processing depends on patterns across populations of neurons rather than on any single electrical pulse.
This is one reason the brain cannot be understood simply as a collection of individual “thought neurons.” Perception, memory, movement, emotion, and other functions emerge from interactions among large, distributed neural circuits.
How electrical signals let you sense the world
Sensory systems begin with specialized cells that detect physical or chemical changes.
Photoreceptors in the eyes respond to light. Mechanoreceptors in the skin respond to mechanical deformation such as pressure, vibration, or stretch. Hair cells in the inner ear respond to mechanical movements associated with sound and balance. Chemoreceptors contribute to senses such as taste and smell.
These receptors convert information about the outside or inside of the body into changes in electrical activity. This process is called sensory transduction.
The resulting signals enter neural pathways that carry information toward the central nervous system.
The brain then processes these signals in stages and across interconnected regions. Seeing an object, for example, is not simply a matter of electrical signals arriving in the brain. The nervous system must analyze features such as contrast, color, shape, motion, location, and relationships among objects.
What you consciously experience as a perception is associated with the coordinated activity of extensive neural networks.
Why touching something hot can make you pull away
One of the clearest demonstrations of neural electrical signaling is a reflex.
Suppose you accidentally touch a very hot surface. Temperature- and pain-sensitive sensory neurons in your skin become activated. Their signals travel along peripheral nerves toward the spinal cord.
Some of that information is rapidly routed through spinal circuits to motor neurons. The motor neurons then activate muscles that withdraw the affected limb.
This can happen before the brain has fully processed the sensation as conscious pain.
The brain still receives information about what happened, allowing you to become aware of the painful stimulus and respond in more deliberate ways. But the initial withdrawal can be organized through circuitry in the spinal cord.
This arrangement is useful because it provides a rapid protective response without requiring every step of the response to wait for conscious processing.
How electrical signals produce movement
Voluntary movement begins with activity in the nervous system but ultimately depends on muscle cells.
When the brain decides to move, patterns of neural activity travel through motor pathways. Motor neurons in the spinal cord and brainstem send signals through peripheral nerves to skeletal muscles.
At the neuromuscular junction, the connection between a motor neuron and a skeletal muscle fiber, the neurotransmitter acetylcholine is released.
Acetylcholine binds to receptors on the muscle cell membrane and produces an electrical change. If the signal reaches the necessary threshold, an action potential travels along the muscle fiber.
That electrical event triggers the release of calcium inside the muscle cell. Calcium interacts with proteins involved in contraction, allowing the muscle’s contractile machinery to generate force.
The process illustrates a chain extending across several levels: electrical activity in the brain influences electrical activity in motor neurons, which triggers chemical signaling at the neuromuscular junction, which produces electrical activity in muscle, which ultimately produces mechanical movement.
How the nervous system controls movement precisely
Moving a limb is not simply a matter of telling a muscle to contract.
Most useful movements require precise coordination among many muscles. Some muscles generate force while others stabilize joints or counter unwanted movements.
The brain and spinal cord continuously adjust motor commands based on sensory feedback. Information from muscles, tendons, joints, skin, the eyes, and the balance organs of the inner ear helps the nervous system determine where the body is and how it is moving.
The cerebellum plays an important role in coordinating movement and adjusting motor commands. Other brain regions contribute to planning, initiating, selecting, and controlling actions.
This feedback makes movement dynamic rather than purely preprogrammed. Even a simple action such as reaching for a cup involves ongoing adjustments based on the cup’s position, the body’s posture, the amount of force being used, and sensory information arriving during the movement.
How electrical signals contribute to thinking
Thinking does not occur as one identifiable electrical pulse traveling through a single brain region.
Instead, thoughts depend on coordinated activity among networks of neurons. Different brain systems contribute to attention, language, working memory, decision-making, planning, perception, emotion, and many other functions.
When you hold a phone number in mind for a few seconds, for example, the relevant information is associated with changing patterns of activity across neural networks. When you recognize a familiar person, sensory processing interacts with stored information and associations distributed across the brain.
Electrical activity is therefore fundamental to cognition, but the relationship is not as simple as one type of electrical waveform corresponding to one particular thought.
The brain’s signals operate across many timescales. Individual action potentials occur within milliseconds, while coordinated neural processes involved in learning, decision-making, and behavior can unfold over seconds, minutes, or much longer periods.
How memories change the brain
Learning involves changes in the connections and properties of neurons.
When particular groups of neurons repeatedly become active together, the strength and behavior of their synaptic connections can change. This general ability of the nervous system to alter its connections and function is called neural plasticity.
One important form is long-term potentiation, in which certain patterns of activity can produce a lasting increase in the strength of particular synapses. Long-term depression can produce lasting decreases in synaptic strength under other conditions.
Plasticity is more complicated than simply making synapses “stronger” whenever something is learned. Changes can involve the number and properties of receptors, the structure of synapses, the growth or withdrawal of connections, and alterations in gene expression and cellular signaling.
Memory therefore reflects physical and functional changes in neural circuits. Electrical activity helps initiate and shape those changes, while the resulting changes influence how future electrical activity flows through the network.
How emotions involve electrical signaling
Emotions also depend on coordinated activity across neural circuits.
Brain systems involved in evaluating situations, generating bodily responses, assigning significance to experiences, retrieving memories, and regulating behavior interact continuously. Structures including the amygdala, hypothalamus, hippocampus, prefrontal cortex, and other regions participate in different aspects of emotional processing.
Emotional states also involve the body’s physiology. The autonomic nervous system can change heart rate, breathing, digestion, sweating, pupil size, and other functions.
When you feel frightened, for example, the experience is not produced by one electrical signal labeled “fear.” It involves interactions among sensory processing, memory, attention, bodily regulation, hormonal systems, and brain networks involved in evaluating and responding to the situation.
The same broad principle applies to positive emotions, stress, motivation, and other affective experiences.
Why pain is more than a signal from an injured body part
Pain provides an especially useful example of how the nervous system transforms information rather than merely transmitting it.
Specialized sensory neurons called nociceptors can detect potentially damaging mechanical, thermal, or chemical conditions. Their activity enters the spinal cord and travels through ascending pathways to the brain.
But pain is not simply a direct readout of tissue damage.
The nervous system can increase or decrease the influence of incoming signals through spinal and brain mechanisms. Attention, previous experience, expectations, emotional state, and context can affect how pain is experienced.
This does not mean pain is imaginary or that the nervous system is “making it up.” Pain is a real biological experience produced by the nervous system. Its complexity reflects the fact that the brain evaluates sensory information in the context of the organism’s overall state and circumstances.
The role of glial cells
Neurons receive much of the attention in discussions of electrical signaling, but they cannot function properly without glial cells.
Astrocytes help maintain the chemical environment around neurons, provide metabolic support, participate in signaling, and contribute to the regulation of synapses. Oligodendrocytes and Schwann cells produce myelin around axons. Microglia serve important immune and surveillance functions in the nervous system.
Glial cells can influence neuronal activity by regulating ions, neurotransmitters, metabolic resources, and the local environment around synapses.
The nervous system is therefore not simply a collection of electrically active neurons. It is an integrated cellular system in which neurons and glia continually interact.
Why the heart also has electrical activity
The brain is not the only organ in the body that uses electrical signaling.
The heart has its own specialized electrical conduction system. Certain cardiac cells can spontaneously generate rhythmic electrical activity. These signals spread through the heart and coordinate the contraction of its chambers.
The sinoatrial node normally acts as the heart’s primary pacemaker. Electrical activity then travels through specialized pathways, including the atrioventricular node and the conduction system that distributes the signal through the ventricles.
This electrical activity can be measured at the skin with an electrocardiogram, or ECG.
The electrical signals of the heart and the electrical signaling of neurons use related principles involving membrane potentials and ion channels, but their organization and functions are different.
How scientists measure the nervous system’s electrical activity
Because neural electrical signals are so small, measuring them requires specialized equipment.
An electroencephalogram, or EEG, records electrical activity from electrodes placed on the scalp. EEG does not record individual neurons firing one by one. Instead, it primarily reflects synchronized electrical activity from large populations of neurons, particularly activity associated with cortical regions near the electrodes.
Researchers can also record electrical activity directly from individual neurons or small groups of neurons using electrodes placed close to or inside cells. Such techniques can reveal action potentials and other electrical events with much greater spatial precision.
Electromyography, or EMG, measures electrical activity associated with skeletal muscles. It can provide information about how nerves and muscles are functioning.
Electrocardiography measures the heart’s electrical activity.
These techniques demonstrate an important point: biological electricity is measurable, but the meaning of a signal depends on where it was recorded, how it was generated, and what physiological process produced it.
Why brain scans are not simply pictures of thoughts
Modern neuroscience also uses techniques such as functional magnetic resonance imaging, or fMRI, to study brain activity. Unlike an electrode recording, fMRI does not directly measure action potentials.
Instead, it detects changes in blood oxygenation and blood flow associated with neural activity. Because active brain tissue changes its metabolic demands, these physiological changes can provide an indirect measure of where activity is occurring.
This distinction matters. A colorful brain image does not show individual thoughts as if they were objects inside the skull. It represents measurements related to physiological changes associated with neural activity.
Similarly, EEG patterns can reveal information about brain states without providing a simple translation from a waveform to a specific thought or memory.
What happens when electrical signaling goes wrong
Because nervous-system function depends on precise electrical and chemical processes, disruptions can have profound effects.
In epilepsy, abnormal patterns of synchronized neuronal activity can produce seizures. Different forms of epilepsy involve different brain networks and mechanisms, and seizures can range from brief changes in awareness to dramatic convulsions.
In multiple sclerosis, the immune system damages myelin in the central nervous system. Loss of normal myelin can interfere with the rapid and reliable transmission of electrical signals along affected axons.
In peripheral neuropathies, damage to peripheral nerves can interfere with sensory or motor signaling, potentially causing symptoms such as numbness, weakness, pain, or altered sensation.
A stroke can deprive brain tissue of oxygen and nutrients because blood flow is interrupted or because a blood vessel ruptures. Neurons in affected regions can rapidly lose their ability to maintain the ion gradients required for normal electrical signaling.
These conditions illustrate how dependent neural function is on the integrity of cellular membranes, ion concentrations, blood supply, myelin, synapses, and the supporting environment.
Why ion channels are so important
Ion channels are among the most important molecular components of electrical signaling.
These proteins form selective pathways through cell membranes. Some open in response to changes in voltage. Others respond to neurotransmitters, mechanical forces, temperature, or intracellular chemical signals.
Voltage-gated sodium channels are essential for the rapid rising phase of many neuronal action potentials. Voltage-gated potassium channels help restore the membrane potential afterward. Calcium channels are crucial for processes including neurotransmitter release.
Because different tissues express different combinations of ion channels, their electrical behavior can vary substantially. This helps explain how neurons can be specialized for different functions and why heart cells, skeletal muscle cells, and other excitable cells have distinct electrical properties.
Ion channels are also important targets of many medications and toxins because altering them can change the excitability of cells.
The nervous system uses electricity and chemistry together
It is sometimes useful to distinguish “electrical signaling” from “chemical signaling,” but the two are deeply intertwined.
Within a neuron, changes in membrane voltage are central to long-distance signaling. At many synapses, that electrical activity triggers chemical neurotransmitter release. The neurotransmitter then changes the electrical state of another cell.
Chemical signaling can also produce longer-lasting changes in cells, altering gene expression, metabolism, receptor availability, and synaptic strength.
The nervous system therefore works through an integrated language of electrical pulses, ion movements, neurotransmitters, receptors, intracellular signaling pathways, and changing connections.
Why these signals are incredibly energy-intensive
Maintaining electrical gradients costs energy.
The sodium and potassium distributions across neuronal membranes are continually disturbed by ion movement during neural activity. Cellular mechanisms, particularly the sodium-potassium pump, must work to maintain those gradients.
The brain therefore consumes a substantial amount of energy even though it makes up only a small fraction of the body’s mass. Much of that energy supports the maintenance and restoration of ion gradients, along with other demanding processes such as neurotransmitter cycling, synaptic activity, cellular maintenance, and information processing.
This energetic cost is one reason the brain depends so heavily on a continuous supply of oxygen and glucose delivered through its blood vessels.
How fast are neural signals?
Neural communication operates across a wide range of speeds.
Some sensory and motor signals travel through fast, heavily myelinated fibers at well over 100 meters per second. Other signals, particularly those carried by small unmyelinated fibers, travel much more slowly.
Synaptic transmission introduces additional delays, often on the order of milliseconds at individual chemical synapses.
These delays may sound insignificant, but neural circuits can contain many interconnected stages. The brain has evolved ways to coordinate these signals despite the fact that neural processing is not instantaneous.
Your experience of the world therefore involves information arriving and being processed over different timescales, with the brain integrating those signals into coherent perception and behavior.
Why electrical signals do not explain the brain by themselves
Knowing how action potentials work is essential to neuroscience, but an action potential is only one level of explanation.
To understand a movement, scientists may need to consider ion channels, individual neurons, synapses, circuits, muscles, sensory feedback, and behavior. To understand memory, they may also need to examine changes in synaptic connections, gene expression, network activity, and interactions among brain regions.
The electrical signals provide one of the fundamental mechanisms by which nervous-system cells communicate. But the remarkable capabilities of the brain arise from the organization of billions of cells into dynamic, interacting systems.
A thought is not simply an electrical spark. A feeling is not a single chemical. A movement is not one command traveling down one wire. They are the results of patterns of activity distributed across biological networks whose components are constantly influencing one another.
At the smallest scale, the process begins with ions moving across a membrane only a few nanometers thick. Those tiny movements can trigger an action potential, which can travel down an axon, release neurotransmitters at a synapse, alter another neuron’s activity, influence a circuit, activate a muscle, change a heartbeat, alter a perception, or contribute to a thought. The extraordinary scale of what the nervous system accomplishes emerges from the organization and interaction of these microscopic electrical events.
