Your nervous system is a communication network that carries electrical and chemical signals between your brain and every other part of your body. It controls everything from pulling your hand away from a hot stove to digesting your lunch, and it does most of this work in milliseconds. Understanding how it operates comes down to a few key ideas: how it’s organized, how individual nerve cells fire, how signals jump between cells, and how the whole system adapts over time.
The Two Main Divisions
The nervous system splits into two broad parts. The central nervous system is your brain and spinal cord, the command center where information is processed and decisions are made. The peripheral nervous system is everything else: the vast web of nerves that branch out from your brain and spinal cord to reach your skin, muscles, organs, and glands. The peripheral system has one job in two directions. It carries sensory information inward (what you’re touching, seeing, hearing) and delivers motor commands outward (telling a muscle to contract or a gland to release a hormone).
Within the peripheral nervous system, two subsystems handle different kinds of tasks. The somatic nervous system manages things you consciously control, like moving your legs or focusing your eyes on a page. The autonomic nervous system runs processes your brain handles automatically, without you thinking about them: heart rate, blood pressure, breathing, and digestion. Most peripheral nerves enter or exit through the spinal cord, but twelve pairs of cranial nerves connect directly to the brain, handling specialized jobs like smell, vision, and facial movement.
How a Single Nerve Cell Fires
Neurons, the cells that carry signals, communicate using a combination of electricity and chemistry. At rest, a neuron holds a slight negative charge inside compared to outside, roughly between negative 40 and negative 90 millivolts. This resting state exists because the cell keeps more sodium ions outside and more potassium ions inside, creating an imbalance that functions like a loaded spring.
When a signal from another cell pushes that charge closer to zero (a process called depolarization), it can trigger an action potential, the neuron’s version of “firing.” Here’s the sequence:
- Sodium rushes in. Channels along the nerve fiber snap open, letting positively charged sodium ions flood into the cell. This flips the local charge from negative to positive and triggers the next set of channels down the line to open, so the signal races along the fiber like a chain of dominoes.
- Potassium flows out. Almost immediately, the sodium channels slam shut and potassium channels open. Because potassium is more concentrated inside the cell, it pours out, dragging the charge back toward its resting negative state.
- Brief overshoot. The potassium channels stay open a beat too long, making the cell temporarily more negative than usual. Then a molecular pump restores the original sodium and potassium balance, and the neuron is ready to fire again.
This entire cycle takes just a few milliseconds. The signal travels faster in nerve fibers that are insulated by a fatty coating called myelin, which is produced by specialized support cells. Myelin lets the electrical impulse skip along the fiber rather than crawl through every segment, dramatically increasing speed.
How Signals Jump Between Cells
Neurons don’t physically touch each other. There’s a tiny fluid-filled gap between them, less than 40 nanometers wide, called the synaptic junction. When an electrical signal reaches the end of a neuron, it triggers small packets of chemical messengers (neurotransmitters) to spill into that gap. Those molecules drift across, land on specific receptors on the next cell, and either encourage or discourage that cell from firing its own signal.
Different neurotransmitters have different jobs. Glutamate is the brain’s most abundant excitatory messenger, meaning it pushes the next cell closer to firing. It plays a central role in thinking, learning, and memory. GABA is the most common inhibitory messenger, doing the opposite: it calms neural activity and helps regulate anxiety, sleep, and concentration. Acetylcholine does double duty. In the brain it supports memory and learning. In the peripheral nervous system it triggers muscle contractions and helps regulate heart rate, blood pressure, and digestion.
Each neurotransmitter fits only its matching receptor, like a key in a lock. Once it binds and delivers its message, the neurotransmitter is either broken down or recycled. This precise lock-and-key system is what lets the brain run billions of signals simultaneously without them blurring together.
The Autonomic System: Rest vs. Alert
The autonomic nervous system has two competing branches that keep your body balanced. The sympathetic branch is your alert mode. It speeds up your heart rate, dilates your pupils, opens your airways, and diverts blood toward your muscles. It’s the system behind the “fight or flight” response. The parasympathetic branch is your rest mode, and it does roughly the opposite at every organ.
When the parasympathetic branch is in charge, your heart rate and pumping force drop, your pupils constrict, your airways tighten to conserve energy, and your digestive tract ramps up activity. It tells your pancreas to release insulin so your cells can use sugar from food. It stimulates saliva and mucus production. It even manages aspects of sexual arousal. You don’t choose to activate either branch. Your brain reads the situation and adjusts the balance automatically, hundreds of times a day.
Reflexes: The Fastest Response
Some signals never make it to the brain before your body responds. When you touch something sharp, pain receptors in your skin fire an impulse along a sensory neuron to your spinal cord. There, a relay neuron passes the signal directly to a motor neuron, which commands the muscles in your hand to contract and pull away. This three-cell shortcut, called a reflex arc, is why you jerk your hand back before you consciously feel pain. Your brain gets the memo afterward.
Support Cells That Keep It Running
Neurons get most of the attention, but they depend on a large population of support cells called glia. These cells don’t carry signals the way neurons do, but the nervous system couldn’t function without them.
Astrocytes maintain the chemical environment around neurons. They regulate water and ion levels, help form the blood-brain barrier, and limit tissue damage during injury by blocking inflammation from spreading. Oligodendrocytes produce the myelin insulation that speeds up electrical signals along nerve fibers, and they continue generating new myelin throughout your life. Microglia act as the nervous system’s immune cells. They constantly survey brain tissue, and when they detect injury, infection, or disease, they shift into an active state: migrating to the damage site, clearing debris, and modulating the connections between neurons. During development, microglia also prune excess synapses, helping to sculpt efficient neural circuits.
The Blood-Brain Barrier
Your brain has its own security system. The cells lining blood vessels in the brain are packed so tightly together that almost nothing can slip between them. Small, fat-soluble molecules (like oxygen and certain hormones) can pass through on their own. But large or water-soluble molecules, including many nutrients, need dedicated transport proteins to escort them across. Pathogens like bacteria and viruses, along with most toxins, are blocked entirely. This selective barrier protects the brain’s delicate chemical environment from the fluctuations happening in the rest of your bloodstream.
How the System Rewires Itself
Your nervous system is not fixed wiring. It physically changes based on how you use it, a property called neuroplasticity. The core mechanism works at the level of individual connections between neurons. When one neuron repeatedly helps fire another, the connection between them gets stronger and more efficient. This principle was proposed decades ago, and the biological process behind it, called long-term potentiation, has been studied for over fifty years.
Long-term potentiation requires two things to happen at the same time: the sending neuron must release glutamate, and the receiving neuron must already be somewhat activated. Special receptors at the synapse act as coincidence detectors, only triggering the strengthening process when both conditions are met simultaneously. This ensures that only meaningful, repeated patterns get reinforced rather than random noise. The same mechanism works in reverse: connections that rarely fire together gradually weaken. This balance of strengthening and weakening is the physical basis of learning, memory formation, and skill development. It’s why practicing a piano piece makes it easier and why unused knowledge fades.