The nervous system is your body’s communication and control network. It detects what’s happening inside and outside your body, processes that information, and sends commands that drive everything from moving your hand to digesting your lunch. It operates at speeds of 10 to 120 meters per second, depending on the type of nerve fiber, making it the fastest signaling system in the body.
Two Main Divisions
The nervous system has two major parts. The central nervous system consists of the brain and spinal cord. The peripheral nervous system is everything else: the vast web of nerves that extends outward into your limbs, organs, and skin. Think of it like an upside-down tree. Your brain is the root system, your spinal cord is the trunk, and the peripheral nerves are the branches reaching into every corner of your body.
Your brain is a powerful processor, but it knows nothing about the outside world on its own. It depends entirely on peripheral nerves to relay information from your eyes, ears, skin, and internal organs. Those same peripheral nerves then carry the brain’s commands back out to your muscles and glands. Without this two-way relay, neither half of the system could do its job.
How Nerve Cells Communicate
Nerve cells, or neurons, send messages through a combination of electrical and chemical signals. When a neuron is stimulated enough to reach a critical threshold, it fires an electrical pulse called an action potential. That pulse races down the length of the cell until it reaches a tiny gap, roughly 20 to 40 nanometers wide, between one neuron and the next.
At that gap, the electrical signal converts into a chemical one. The first neuron releases chemical messengers called neurotransmitters into the space. Those molecules drift across and latch onto receptors on the receiving neuron, where they trigger a new flow of charged particles that converts the message back into an electrical signal. This electrical-to-chemical-to-electrical relay happens in milliseconds, billions of times per day, across trillions of connections.
The speed of these signals varies dramatically. The largest motor nerve fibers, which control skeletal muscles, conduct impulses at 80 to 120 meters per second. Smaller fibers carrying less urgent information may transmit at less than one meter per second. That range explains why you can yank your hand from a hot stove almost instantly, yet a dull ache from a stubbed toe takes a beat to fully register.
Sensing the World and Producing Movement
One of the nervous system’s most fundamental jobs is the loop between sensation and action. Sensory receptors throughout your body, in your skin, eyes, ears, joints, and internal organs, constantly feed information to the brain. The brain integrates all of this input, generates possible responses, and selects the best one before sending a command to the relevant muscles. Researchers describe this as a process of parallel sensory input converging into a single, selected motor output. In practical terms, it means your brain is evaluating multiple options simultaneously, then committing to one coordinated movement.
This loop runs continuously, even during something as simple as standing upright. Your muscles, inner ear, and the pressure sensors in your feet all send streams of data that your brain uses to make constant micro-adjustments to keep you balanced.
Automatic Body Functions
You don’t have to think about keeping your heart beating, your lungs breathing, or your blood pressure steady. The autonomic nervous system, a branch of the peripheral nervous system, handles these processes without conscious effort. It has two opposing arms that work as a balancing act.
The sympathetic nervous system activates your body during stress or danger. It’s responsible for the “fight or flight” response: faster heart rate, dilated pupils, redirected blood flow to your muscles, and a surge of energy. The parasympathetic nervous system does the opposite. It governs “rest and digest” functions, slowing your heart rate, stimulating digestion, and promoting recovery. These two systems constantly push and pull against each other to keep your body calibrated for whatever situation you’re in.
Temperature Regulation
Maintaining a stable body temperature is one of the nervous system’s most important homeostatic roles. Temperature sensors in your skin detect the outside environment, while additional sensors inside your abdomen and brain monitor your core temperature. All of this information flows to a region at the front of the hypothalamus called the preoptic area, which acts as the body’s thermostat.
When cooling signals arrive, the preoptic area releases its usual brake on heat-generating circuits deeper in the brain. That triggers a cascade: your blood vessels near the skin constrict to conserve heat, your heart rate increases, and you may start shivering. When warming signals dominate, the opposite happens. The preoptic area strengthens its inhibition of those same circuits, blood vessels near the skin dilate to release heat, and shivering stops. During an infection, inflammatory molecules hijack this same system to raise the set point, producing a fever.
Memory and Higher Thinking
Beyond reflexes and organ control, the nervous system is the basis for thought, learning, and memory. New memories are initially encoded in a structure called the hippocampus, deep within the brain. Over time, those memories are gradually stabilized and transferred to the outer layer of the brain, the cortex, for long-term storage. This process, called consolidation, is why a fresh memory can feel fragile but an older one becomes more durable.
The same signaling mechanisms that neurons use for basic communication also underlie learning. When connections between neurons are repeatedly activated, those connections strengthen. This flexibility, known as synaptic plasticity, is the physical basis of how your brain rewires itself in response to new experiences.
The “Second Brain” in Your Gut
Your digestive tract contains its own extensive nervous system called the enteric nervous system. It’s embedded in the walls of your gastrointestinal tract and contains enough neurons and supporting cells to regulate digestion largely on its own, without moment-to-moment input from the brain. Researchers sometimes call it the “brain in the gut” because it independently coordinates the muscular contractions that move food along, controls the secretion of digestive enzymes, and manages the timing of these processes with remarkable precision.
This autonomy is actually a practical advantage. Because the enteric nervous system handles digestion locally, your brain is freed from having to consciously manage every stage of breaking down a meal. The two systems still communicate through what’s known as the gut-brain axis, a bidirectional link that integrates digestive function with the rest of the body. The “butterflies in your stomach” feeling during stress is a familiar example of this connection in action.
The Support Cells Behind the Scenes
Neurons get most of the attention, but the nervous system also relies heavily on glial cells, which perform critical support roles. For a long time, scientists believed glial cells outnumbered neurons ten to one, but more recent counts suggest the ratio in parts of the human brain is closer to one to one, varying significantly by region.
There are several types. Oligodendrocytes produce myelin, a fatty insulating sheath that wraps around nerve fibers and allows electrical signals to travel much faster. Without myelin, the rapid conduction speeds that make coordinated movement possible would collapse. Astrocytes maintain the chemical environment neurons need to function. They regulate water and ion balance, help form the blood-brain barrier, and clear away damaging reactive molecules that would otherwise harm neurons. They also participate directly in signaling at the junctions between neurons.
Microglia serve as the nervous system’s immune cells. They constantly survey brain tissue, extending fine projections that sense injury or infection. They also play a surprising role in shaping neural circuits by pruning unnecessary connections during development and modulating existing ones throughout adulthood. Together, these support cells are not just passive scaffolding. They actively maintain the conditions that allow neurons to do their work.