The nervous system is your body’s communication network, responsible for everything from sensing heat on your fingertips to forming memories to keeping your heart beating. It consists of two major divisions: the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which includes every nerve structure outside those two organs. Together, they collect information from your environment and body, process it, and coordinate a response, all within fractions of a second.
The Two Main Divisions
The central nervous system is the command center. Your brain handles everything from conscious thought and emotion to regulating breathing and temperature, while the spinal cord serves as the main highway connecting the brain to the rest of the body. Signals travel up the spinal cord to deliver sensory information and back down to trigger movement and organ function.
The peripheral nervous system is the wiring that reaches every other part of you. It branches out from the brain and spinal cord into nerves that extend to your skin, muscles, organs, and glands. When you stub your toe, peripheral nerves carry that pain signal to your spinal cord and brain. When your brain decides to pull your foot away, peripheral nerves carry that command back to the muscles in your leg.
How Nerve Cells Work
The basic unit of the nervous system is the neuron, a specialized cell built to transmit electrical and chemical signals. The human brain alone contains roughly 67 to 86 billion neurons, supported by an additional 40 to 50 billion glial cells that insulate, nourish, and protect them.
Three types of neurons handle different jobs. Sensory neurons fire when you encounter something in your environment, like touching a hot pan, and send that information inward toward the brain and spinal cord. Motor neurons work in the opposite direction, carrying signals outward to muscles and glands so you can move, sweat, or digest food. Interneurons sit between the two, connecting sensory and motor neurons and forming the complex circuits that allow for things like reflexes, decision-making, and learning.
Electrical Signals and the Synapse
Neurons communicate through brief electrical impulses called action potentials. When a neuron is stimulated strongly enough, channels in its membrane open and allow positively charged particles to rush in, creating a rapid spike of electrical activity that lasts about one millisecond. That spike races down the length of the neuron toward the next cell in the chain.
Speed varies enormously depending on the nerve fiber. The fastest signals, carried by large insulated fibers, travel at up to 200 meters per second (roughly 450 miles per hour). The slowest, in thin uninsulated fibers, crawl at less than 0.1 meters per second. The difference comes down to a fatty coating called myelin that wraps around many nerve fibers. Myelin acts like insulation on an electrical wire, allowing the signal to leap from one gap to the next rather than traveling as a slow continuous wave. This leaping pattern can increase speed more than tenfold.
When an electrical signal reaches the end of a neuron, it can’t jump directly to the next cell. Instead, the neuron releases chemical messengers called neurotransmitters into a tiny gap (the synapse) between the two cells. These chemicals cross the gap and trigger or suppress a new electrical signal in the receiving neuron.
Key Chemical Messengers
Different neurotransmitters have different effects throughout the nervous system. Glutamate is the brain’s primary excitatory messenger, meaning it makes the receiving neuron more likely to fire. It plays a central role in learning and memory by strengthening connections between neurons that are repeatedly activated together. GABA works in the opposite direction, accounting for roughly 40% of the brain’s inhibitory signaling. It calms neural activity and helps prevent circuits from becoming overactive.
Dopamine is involved in learning, movement control, reward, and motivation. It is the chemical behind the satisfying feeling you get when you accomplish a goal or eat something you enjoy. Serotonin influences mood, sleep, and digestion. Most psychiatric medications that treat depression or anxiety target serotonin pathways, but this chemical also helps regulate bowel motility, bladder control, and cardiovascular function.
The Autonomic Nervous System
Much of what the nervous system does happens without your conscious input. The autonomic nervous system controls heart rate, digestion, breathing, and dozens of other processes automatically. It has two opposing branches that work like a gas pedal and a brake.
The sympathetic branch activates your “fight or flight” response. When you perceive danger, it increases heart rate and blood pressure, releases stored energy into the bloodstream, and slows digestion. These changes redirect resources toward your muscles and brain so you can react quickly. The parasympathetic branch does the opposite, driving “rest and digest” functions. It slows the heart, promotes salivation, and restores digestive activity. In a healthy body, the two branches constantly balance each other, shifting emphasis based on what the moment demands.
Your Gut Has Its Own Nervous System
The enteric nervous system is a vast network of neurons embedded in the walls of your gastrointestinal tract. Sometimes called the “second brain,” it monitors and integrates gut functions largely on its own, controlling things like the muscular contractions that move food through your intestines and the secretion of digestive enzymes.
The enteric nervous system doesn’t work in isolation, though. It communicates constantly with the brain through what researchers call the gut-brain axis. The vagus nerve, the longest nerve running from the brain to the abdomen, serves as the primary communication line. Studies in mice have shown that severing the vagus nerve eliminates many of the behavioral and neurochemical effects that gut bacteria have on the brain, confirming the vagus nerve as the major pathway linking intestinal activity to mood, cognition, and immune function.
How the Nervous System Protects Itself
The brain and spinal cord are physically shielded by bone (the skull and vertebral column), three layers of protective tissue called the meninges, and a cushion of cerebrospinal fluid that absorbs impact. But physical protection is only part of the story.
A specialized barrier between the bloodstream and the brain, known as the blood-brain barrier, filters what enters the brain’s environment. It blocks pathogens, toxins, and many drugs circulating in the blood while still allowing essential nutrients, oxygen, and water through. It also maintains stable levels of hormones and other chemicals in the brain, preventing the kind of fluctuations that could disrupt normal signaling.
The Nervous System Keeps Changing
For decades, scientists believed the adult brain was essentially fixed. That view has been overturned by more than 40 years of research into neuroplasticity, the nervous system’s ability to reorganize its structure and function throughout life. This reorganization takes several forms: existing connections between neurons can strengthen or weaken based on use, entirely new connections can form, and in at least two brain regions (the area near the brain’s fluid-filled cavities and a structure involved in memory), new neurons are generated even in adulthood.
One well-studied form of plasticity is long-term potentiation, a lasting strengthening of the signal between two neurons after they fire together repeatedly. This is thought to be the cellular basis of learning and memory. On the flip side, stress can cause neurons to lose connections, but the brain begins replacing those connections as soon as the stress is removed. This capacity for remodeling is what allows people to recover function after a stroke, adapt to new skills, and form memories at any age.