Your nervous system is a communication network that carries electrical signals between your brain and every other part of your body. It takes in information from your surroundings, processes it, and triggers responses, all within milliseconds. The system relies on roughly 86 billion neurons in the brain alone, plus an equal number of support cells that keep everything running.
The Two Main Divisions
The nervous system splits into two parts that work together constantly. The central nervous system (CNS) is your brain and spinal cord. It’s the processing center where signals get interpreted and decisions get made. Your brain reads incoming signals and coordinates how you think, move, and feel.
The peripheral nervous system (PNS) is everything else: a branching network of nerves that extends from your spinal cord out to your organs, limbs, fingers, and toes. It acts as the relay system, carrying sensory information inward and motor commands outward. Your eyes, ears, tongue, nose, and nerves throughout your skin all collect data about your environment and send it to the brain through this network.
Three Types of Neurons
Not all neurons do the same job. Sensory neurons gather information from your senses (what you see, hear, touch, taste, and smell) and send it toward the brain. Motor neurons carry instructions from your brain and spinal cord out to your muscles. Interneurons sit between the two, acting as connectors. They regulate your movement in response to sensory input and play a central role in thinking, learning, and memory.
A simple example ties all three together: when you touch a hot surface, sensory neurons detect the heat and fire a signal toward your spinal cord. Interneurons in the spinal cord process that signal and immediately activate motor neurons that pull your hand away. This happens before the pain signal even reaches your brain.
How Neurons Send Signals
At rest, a neuron holds a small electrical charge across its outer membrane, typically between -80 and -40 millivolts. This is its resting potential, maintained by a careful balance of charged particles (ions) inside and outside the cell. When a neuron receives enough stimulation, the balance tips. Channels in the membrane open, positively charged ions rush in, and the voltage spikes. This rapid voltage shift is called an action potential, and it lasts about one millisecond.
The action potential travels down the length of the neuron like a wave. How fast it moves depends on the type of nerve fiber. Small, uninsulated fibers that carry pain signals transmit at roughly 0.5 to 2 meters per second, about walking speed. The fastest fibers, which are larger and wrapped in a fatty insulating layer called myelin, send signals at 80 to 120 meters per second. That’s up to 268 miles per hour.
What Happens at the Gap Between Neurons
Neurons don’t physically touch each other. Between them is a tiny gap called a synapse. When an electrical signal reaches the end of one neuron, it can’t simply jump across. Instead, the arriving signal causes calcium to flow into the nerve terminal. That calcium triggers tiny packets of chemical messengers (neurotransmitters) to fuse with the cell membrane and release their contents into the gap.
Those neurotransmitter molecules drift across the synapse and lock onto receptors on the next neuron, like keys fitting into locks. This either excites the receiving neuron, pushing it closer to firing its own signal, or inhibits it, making it less likely to fire. A single packet of neurotransmitter can open about 1,000 receptor channels on the other side. During a typical muscle command, a single nerve signal opens roughly 100,000 channels in the receiving muscle cell.
Once the message is delivered, the neurotransmitter is quickly cleared from the gap. It gets broken down by enzymes, reabsorbed by the sending neuron, or drifts away. This cleanup ensures signals stay crisp rather than blurring together.
Key Chemical Messengers
Your nervous system uses dozens of neurotransmitters, but four do most of the heavy lifting. Glutamate is the most abundant in the brain and the primary excitatory messenger. It drives thinking, learning, and memory by making neurons more likely to fire. GABA is its counterpart: the most common inhibitory messenger, which calms brain activity and helps regulate anxiety, sleep, concentration, and seizure risk.
Dopamine fuels your brain’s reward system. It’s involved in pleasure, motivation, focus, and learning from experience. Serotonin, an inhibitory messenger, helps regulate mood, sleep, appetite, anxiety, and pain perception. Imbalances in any of these can contribute to a wide range of neurological and psychiatric conditions.
The Autonomic System: Running Things Automatically
Much of what your nervous system does happens without conscious input. The autonomic nervous system manages your organs, glands, and internal processes through two complementary branches.
The sympathetic branch handles your “fight or flight” response. When you face a threat or stressor, it increases your heart rate, dilates your pupils to let in more light, opens your airways, and diverts blood away from digestion toward your muscles. It prepares your body for action.
The parasympathetic branch does the opposite, often called “rest and digest.” It slows your heart rate and reduces pumping force, constricts your pupils, increases saliva production, speeds digestion, and signals your pancreas to release insulin so cells can absorb sugar for energy. It also relaxes the muscles controlling your bladder and bowels. These two branches constantly push and pull against each other, fine-tuning your body’s state moment to moment based on what’s happening around you.
Support Cells That Keep It All Running
Neurons get the spotlight, but the 86 billion non-neuronal cells in the brain are just as essential. Three types of glial cells stand out.
Astrocytes are star-shaped cells that maintain the working environment around neurons. They control neurotransmitter levels near synapses, manage the concentration of important ions like potassium, and provide metabolic fuel. They can also sense neurotransmitter activity and release molecules that directly influence how neurons fire, making them active participants in brain signaling rather than passive bystanders.
Oligodendrocytes produce myelin, the fatty insulation wrapped around long nerve fibers in the brain and spinal cord. This is what gives white matter its color, and it’s the reason some signals travel at hundreds of miles per hour instead of a slow crawl. Schwann cells do the same job in the peripheral nervous system.
Microglia are the brain’s immune cells. They patrol for injury and disease, clearing away dead cells and toxic material. They also contribute to “synaptic pruning,” physically consuming synapses tagged as unnecessary. This cleanup process is especially active during childhood and adolescence and plays an important role in shaping efficient brain circuits.
How the Nervous System Adapts
Your nervous system isn’t a fixed circuit. It rewires itself based on experience through a process called synaptic plasticity. When two connected neurons fire together repeatedly, the synapse between them grows stronger, making future communication easier. This strengthening is called long-term potentiation and is widely considered the cellular basis of learning and memory. The process depends on precise timing: when a sending neuron fires just before the receiving neuron, a surge of calcium enters the receiving cell and triggers molecular changes that boost the connection.
The reverse also happens. When connections are used infrequently or their timing is off, synapses weaken through long-term depression. This pruning of underused pathways is just as important as strengthening active ones. Together, these two mechanisms allow your brain to prioritize relevant information, form new memories, recover from injuries, and adapt to new skills throughout your life.