The nervous system works by passing electrical and chemical signals between specialized cells, allowing you to sense the world, think, move, and react in fractions of a second. At its core, the system relies on neurons that generate rapid voltage changes and pass messages to one another across tiny gaps called synapses. But neurons do not work alone: they are supported and actively modulated by a second class of cells, glia, which make up more than half the cells in the brain. The interplay between these cell types, the signals they produce, and the circuits they form gives rise to everything from a knee-jerk reflex to a memory of your childhood home.
Neurons and Glia, the Two Pillars
The nervous system is built from two broad cell categories. Neurons are the signal generators. They produce electrical impulses and release chemical messengers. Glia, once dismissed as mere structural packing material, turn out to be active participants in nearly every aspect of brain function. They make myelin (the insulating sheath around nerve fibers), help form and eliminate synapses, regulate blood flow, and maintain the chemical balance neurons need to fire properly.1PubMed Central. Glial Contributions to Neural Function and Disease
Glial cells account for more than half the total cell count in the mammalian brain.2PubMed Central. The Structure and Function of Glial Networks: Beyond the Neuronal Connections Among them, astrocytes are especially interesting. They wrap around synapses and respond to neuronal activity, adjusting how strongly signals pass between neurons. Research now treats astrocytes as functional elements of the synapse itself, actively involved in processing, transferring, and storing information. That finding challenges the older view that brain function comes exclusively from neuronal networks and suggests that the real operating unit is a neuron-glia network.3PubMed Central. Glial cells in neuronal network function
How an Electrical Signal Starts
A neuron at rest has a slight negative charge inside compared to outside, maintained by the selective permeability of its membrane to different ions. When a stimulus pushes the neuron past a threshold, voltage-gated sodium channels snap open and sodium ions rush in, briefly reversing that charge. This rapid flip is the action potential, the fundamental electrical signal of the nervous system. Voltage-gated sodium channels initiate action potentials in nerve cells, muscle fibers, and other excitable tissues. The sodium current underlying this process was first described mathematically by Hodgkin and Huxley in 1952, and their quantitative model for how action potentials are generated has held up for decades.4PubMed Central. Voltage-gated sodium channels at 60: structure, function and pathophysiology
These same sodium channels are also essential at the sensory end of the system. They are critical for the initial detection of sensory stimuli, for generating the action potential that carries the signal, and for triggering neurotransmitter release at sensory nerve terminals.5PubMed. The Role of Voltage-Gated Sodium Channels in Pain Signaling In other words, the same basic channel type does triple duty: it detects, it transmits, and it communicates.
Speeding Things Up With Myelin
An action potential traveling along a bare nerve fiber moves relatively slowly because the electrical signal has to regenerate continuously along the membrane. Myelin, a fatty sheath wrapped around axons by glial cells, changes the game. It insulates stretches of the axon (called internodes) and leaves small exposed gaps (nodes of Ranvier) where ion channels are concentrated. The signal effectively jumps from node to node, a process called saltatory conduction, which dramatically increases speed.
Recent work has refined our understanding of what “jumping” actually means at the microscale. Researchers found that the action potential at each downstream node begins earlier than the slower electrical wave still spreading through the upstream internode. The signal at the nodes appears displaced backward in time relative to the internodal wave, creating what the authors call “temporal saltation”: the nodal signals are not simply leapfrogging through space, they are leapfrogging through time as well.6PubMed Central. Saltatory Conduction along Myelinated Axons Involves a Periaxonal Nanocircuit The voltage within each internode gradually weakens and slows toward the middle, but the rapid charging of the upstream end of each myelin segment feeds forward to charge the downstream end, priming the next node to fire.7Cell. Nanoscale Periaxonal Resistance, Myelin Circuitry, and Saltatory Conduction in Myelinated Mammalian Axons
The practical payoff is enormous. Myelinated fibers carry signals many times faster than unmyelinated ones of the same diameter, which is why diseases that damage myelin, like multiple sclerosis, can disrupt movement, vision, and sensation so profoundly.
Crossing the Gap at Chemical Synapses
Neurons do not physically touch each other at most of their communication points. Instead, a tiny gap called the synaptic cleft separates the sending neuron from the receiving one. When an action potential arrives at the axon terminal of the sending neuron, it opens voltage-gated calcium channels. Calcium ions flood in and trigger the release of chemical messengers called neurotransmitters from small storage bubbles (vesicles) docked at the membrane.8PubMed Central. Ca(2+) influx and neurotransmitter release at ribbon synapses
This calcium-triggered release is astonishingly fast. Within a few hundred microseconds of calcium entry, a sensor protein called synaptotagmin grabs the calcium ions and mechanically activates the machinery that fuses vesicles with the membrane, dumping neurotransmitter into the cleft.9PubMed Central. Calcium control of neurotransmitter release The neurotransmitter drifts across the cleft, binds to receptors on the receiving neuron, and either excites it (pushing it toward firing) or inhibits it (making it less likely to fire). After the message is delivered, the neurotransmitter is cleared away, mainly by specialized uptake pumps that suck it back into the sending neuron or surrounding cells, or by enzymes that break it down.10PubMed Central. Role of transmitter uptake mechanisms in synaptic neurotransmission This cleanup is what limits how long each signal lasts and prevents the receiving neuron from being overstimulated.
Electrical Synapses and Gap Junctions
Not all synaptic communication involves chemicals. At electrical synapses, two neurons are directly connected by protein channels called gap junctions, which allow ions and small molecules to pass straight from one cell to the next.11PubMed Central. The electrical synapse: Molecular complexities at the gap and beyond Electrical synapses are faster than chemical ones because there is no delay for vesicle fusion and neurotransmitter diffusion. They are especially useful in circuits that need precise timing, like those coordinating rhythmic movements or synchronizing groups of neurons.
For a long time, electrical synapses were considered the simpler, less interesting cousin of chemical transmission. That view has shifted. Researchers now recognize that gap junctions have their own forms of plasticity and can be modulated by the same kinds of signals that adjust chemical synapses, making them more flexible than the old “hard-wired connection” metaphor suggested.12PubMed Central. Gap junction-mediated electrical transmission: regulatory mechanisms and plasticity
How a Neuron Decides Whether to Fire
A typical neuron in the brain receives thousands of inputs, some excitatory and some inhibitory, arriving at different times and at different locations on its surface. The neuron has to combine all these signals and “decide” whether to fire an action potential of its own. This process is called integration, and it takes two main forms. Spatial summation occurs when signals arriving at different locations on the neuron add together. Temporal summation occurs when signals arrive at the same location in rapid succession, each one building on the previous one before it fades.13PubMed. Basic principles of synaptic physiology illustrated by a computer model
The balance between excitation and inhibition is what keeps the nervous system stable. Too much excitation and neurons fire uncontrollably, as happens during an epileptic seizure. Too much inhibition and signals can’t get through, which is essentially what general anesthesia accomplishes. Healthy neural circuits maintain a dynamic equilibrium where inhibition sculpts excitation into precise, meaningful patterns.
Reflexes, the Fastest Circuits
Reflexes are the nervous system’s speed champions because they bypass the brain entirely. The simplest reflex arc involves just two neurons: a sensory neuron that detects a stimulus and a motor neuron that drives a muscle response. The classic example is the monosynaptic stretch reflex, better known as the knee-jerk reflex. A tap on the tendon below your kneecap stretches specialized sensors in the quadriceps muscle called muscle spindles. These sensors activate sensory neurons whose cell bodies sit in the dorsal root ganglia, just outside the spinal cord. Those sensory neurons make a single direct synapse onto motor neurons in the spinal cord, which fire and contract the quadriceps, kicking your lower leg forward.
Building that seemingly simple circuit during development is anything but simple. Multiple families of transcription factors control the step-by-step specification of the different sensory neuron subtypes in the dorsal root ganglia. The formation of muscle spindles themselves requires a signaling molecule called neuregulin 1, provided by the sensory neurons, acting through receptors on specialized muscle fibers. After the basic circuit is wired, signals from the peripheral muscles fine-tune the connections, and neurotrophin 3 released from muscle spindles adjusts the strength of sensory-motor connections within the spinal cord after birth.14PubMed. Development of the monosynaptic stretch reflex circuit
Most reflexes are more complex than the two-neuron stretch reflex. Pulling your hand away from a hot stove, for example, involves at least three neurons: a sensory neuron, one or more interneurons in the spinal cord that relay and distribute the signal, and a motor neuron. Adding interneurons allows the circuit to coordinate more muscles and to simultaneously inhibit the opposing muscles, so your arm flexes smoothly rather than fighting itself.
Where Nerve Meets Muscle
The final step in voluntary movement happens at the neuromuscular junction, the specialized synapse between a motor neuron and a skeletal muscle fiber. The neuromuscular junction is designed to reliably convert an action potential in the motor neuron into a contraction of the muscle fiber.15PubMed. Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting The neurotransmitter here is acetylcholine, which is released from vesicles at the nerve terminal, crosses the synaptic cleft, and binds to receptors concentrated in folds on the muscle surface.16PubMed. Molecular architecture of the neuromuscular junction
This junction has almost no margin for casual failure under normal conditions, but sustained activity can expose its limits. Research in rat muscles has shown that even brief activity performed a few seconds before a contraction can markedly reduce the force output when neuromuscular transmission is compromised, because the nerve terminal cannot replenish its acetylcholine stores fast enough.17PubMed. Role of recovery of acetylcholine release in compromised neuromuscular junction function That finding is directly relevant to conditions like myasthenia gravis, where an autoimmune attack on acetylcholine receptors makes every contraction depend on an already-strained supply of neurotransmitter. The weakness and fatigue these patients experience may partly reflect how long the nerve terminal needs to restock between bursts of activity.
How Connections Change Over Time
The nervous system is not a fixed circuit board. Synapses strengthen or weaken depending on how they are used, a property called synaptic plasticity, and it is the cellular basis of learning and memory. The best-studied forms are long-term potentiation (LTP) and long-term depression (LTD). When two connected neurons fire together repeatedly, the synapse between them tends to get stronger; when they fire out of sync, it tends to weaken.
The early phase of strengthening works by shuttling more receptors into the receiving side of the synapse, so each burst of neurotransmitter produces a bigger response. Weakening does the opposite: receptors are pulled out. These changes are triggered by a specific type of receptor, the NMDA receptor, that acts like a coincidence detector, only opening when both the sending and receiving neurons are active at roughly the same time.18PubMed Central. NMDA receptor-dependent long-term potentiation and long-term depression (LTP/LTD) Later, if the potentiation is strong enough and accompanied by the right neuromodulatory signals (like dopamine), the synapse physically grows, adding new structural elements. This late phase requires the synthesis of new proteins and takes about an hour to develop.19PubMed Central. Glutamatergic synapses are structurally and biochemically complex because of multiple plasticity processes That timeline maps neatly onto the common experience that something learned casually fades quickly, while something learned with emotional engagement or repeated practice sticks around.
Sensory Transduction, Turning the World Into Electricity
Before any of the signal-processing machinery described above can do its work, the nervous system has to convert physical reality into electrical language. That conversion happens at sensory receptors, specialized cells or cellular structures that turn mechanical pressure, temperature, light, or chemical concentrations into changes in membrane voltage called receptor potentials. Some receptors use ion channels that respond directly to physical force or temperature. Others rely on more elaborate signaling chains in which a receptor protein on the cell surface activates internal messenger molecules, which in turn open or close ion channels further along.20PubMed Central. Design principles of sensory receptors
The elegance of this arrangement is that every sensory modality, no matter how different the stimulus, ultimately speaks the same language: voltage changes and action potentials. Your brain does not “see” light or “hear” sound. It interprets patterns of electrical activity arriving along specific neural pathways. What makes the signal meaningful is not its form but its origin and its destination.
The Autonomic System, Running Things in the Background
Much of what your nervous system does never reaches conscious awareness. The autonomic nervous system controls heart rate, digestion, breathing rate, pupil dilation, and dozens of other functions through two opposing branches. The sympathetic branch gears you up for action (the “fight or flight” response), while the parasympathetic branch promotes rest and recovery. Most organs receive input from both branches, and proper physiological regulation depends on their coordinated activity.21PubMed. Identification of CNS neurons with polysynaptic connections to both the sympathetic and parasympathetic innervation of the submandibular gland
The popular framing of “sympathetic versus parasympathetic” as a toggle switch oversimplifies what actually happens. Both systems are always active to some degree, and their balance shifts continuously. Your heart is not just sped up by adrenaline or slowed down by the vagus nerve; it is simultaneously held at a setpoint by a tug-of-war between the two. That dual control is what lets the body make fine adjustments rather than lurching between extremes.
How Drugs and Toxins Exploit the System
Because synaptic transmission relies on such precise molecular machinery, it is exquisitely vulnerable to interference. Neurotoxins, whether from venomous animals, bacteria, or the pharmacy shelf, generally work by either blocking or overstimulating some step in the signaling chain. They may inhibit neurotransmitter release, flood the synapse with excess transmitter, or bind to receptors on the receiving cell and either activate or block them.22PubMed Central. Neurotoxins Acting at Synaptic Sites: A Brief Review on Mechanisms and Clinical Applications
Botulinum toxin, for instance, prevents acetylcholine release at the neuromuscular junction, causing paralysis. Nerve agents and some pesticides block the enzyme that breaks down acetylcholine, so the neurotransmitter accumulates and muscles go into uncontrollable spasm. Caffeine, alcohol, antidepressants, and anesthetics all act on various points in synaptic transmission. Understanding the signaling chain makes it obvious why a single molecule targeting one specific protein can have such outsized effects on behavior, consciousness, or survival.
An Ancient System With Deep Roots
The basic plan of nervous system signaling is very old. Comparing the nervous systems of distantly related animal groups, like jellyfish and humans, reveals a surprising degree of conservation. Many of the genes governing how neurons are born, how they develop, and how they function are shared across these lineages, which split more than 500 million years ago. Researchers have concluded that the last common ancestor of these groups was already an animal with a well-established nervous system, in which neurons arose from epithelial cells through a cell-determination mechanism that was inherited from an even deeper ancestor.23PubMed Central. Evolution of eumetazoan nervous systems: insights from cnidarians
During development in modern animals, this ancient toolkit still shapes the wiring. Growing axons navigate toward their targets guided by at least four mechanisms acting simultaneously: they can be attracted or repelled by molecules they physically contact, and they can be attracted or repelled by diffusible chemical gradients in their environment.24PubMed. The molecular biology of axon guidance The idea that axons follow chemical trails was first proposed by Santiago Ramón y Cajal over a century ago, and modern molecular biology has largely confirmed it while adding layers of complexity he could not have anticipated.25Current Biology. Neural Development: Chemoattractants for navigating axons
Reading Nerve Signals From Outside the Body
The fact that the nervous system runs on electricity has made it possible to eavesdrop on it with electrodes. Brain-computer interfaces record neural signals from the cortex and translate them into commands for a computer screen, a robotic limb, or a communication device. The technology ranges from scalp electrodes that pick up the summed electrical activity of millions of neurons (what you see in a standard EEG) to tiny needle-like probes inserted into the brain tissue itself, which can record the firing of individual neurons.26PubMed. Brain-computer interfaces: an overview of the hardware to record neural signals from the cortex
The tradeoff is straightforward: recordings from the scalp are safe and easy but blurry, while intracortical electrodes give sharp, detailed signals but require surgery and face long-term challenges like tissue scarring around the implant. A middle ground is epicortical arrays placed on the brain surface, which capture cleaner signals than scalp electrodes without penetrating the tissue. The field is still working out which approach gives the best balance of signal quality, safety, and durability for real-world use in people with paralysis or locked-in syndrome.