The human nervous system has two major parts: the central nervous system (CNS), which includes the brain and spinal cord, and the peripheral nervous system (PNS), which includes every nerve that branches out from the CNS to reach the rest of your body. Together, these two divisions control everything from conscious thought to automatic processes like heartbeat and digestion.
The Central Nervous System: Brain and Spinal Cord
The CNS is the command center. It processes incoming information, stores memories, generates thoughts, and sends out instructions to muscles and organs. It consists of just two structures, but they’re extraordinarily complex.
The brain divides into three main regions: the cerebrum, the brainstem, and the cerebellum. The cerebrum is the largest portion, responsible for thinking, planning, language, and voluntary movement. The cerebellum sits at the back and coordinates balance and fine motor control. The brainstem connects the brain to the spinal cord and handles essential survival functions like breathing, heart rate, and sleep cycles.
The spinal cord extends from the base of the brainstem down through the vertebral column. It serves as the main highway between the brain and the body, carrying signals in both directions. It also handles some responses on its own: when you pull your hand away from a hot stove before you consciously feel the pain, that’s a spinal reflex that never needed to reach the brain. The cord is organized into 31 segments, each giving rise to a pair of spinal nerves. These segments break down into 8 cervical (neck), 12 thoracic (mid-back), 5 lumbar (lower back), 5 sacral (pelvis), and 1 coccygeal (tailbone) nerve pair.
How the CNS Protects Itself
Because the brain and spinal cord are so critical, they have layered defenses. Three membranes called meninges wrap around both structures. The skull and vertebral column provide a hard outer shell. Between the meninges, cerebrospinal fluid (CSF) surrounds and cushions the brain and spinal cord, washes out waste products, and delivers nutrients.
The brain also has a filtering system called the blood-brain barrier. The cells lining blood vessels in the brain are locked together with tight junctions, preventing most substances in the bloodstream from freely entering brain tissue. Star-shaped support cells called astrocytes wrap around these blood vessels and act as an additional filter. They intercept glucose from the blood, process it, and pass it along to neurons in a form they can use. This barrier keeps out toxins and pathogens while still allowing essential fuel through.
The Peripheral Nervous System
The PNS is everything outside the brain and spinal cord: the nerves running through your limbs, torso, and face. It splits into two functional divisions, each handling a different category of activity.
The somatic nervous system controls conscious, voluntary actions. It connects the CNS to your skin and skeletal muscles, allowing you to feel a tap on your shoulder and then turn your head in response. When you decide to walk across a room, it’s your somatic system carrying those commands from the brain to your leg muscles.
The autonomic nervous system handles everything you don’t have to think about. It connects the CNS to internal organs like the heart, stomach, intestines, and glands. It runs continuously and automatically, keeping your organs functioning without conscious effort.
Sympathetic vs. Parasympathetic Nervous Systems
The autonomic nervous system further divides into two branches that work in opposition, like a gas pedal and a brake.
The sympathetic nervous system drives the “fight or flight” response. When you perceive danger, it raises your heart rate, dilates your pupils, opens your airways, and redirects blood flow toward your muscles. It prepares your body for action.
The parasympathetic nervous system does the opposite: it promotes “rest and digest” functions. It slows your heart rate and reduces the pumping force of your heart. It constricts your pupils, increases saliva production, ramps up digestion, and signals the pancreas to release insulin so your cells can use sugar from the food you’re processing. It also manages functions like tear production, bladder control, and aspects of sexual arousal. During calm, restful moments, your parasympathetic system is dominant.
These two branches don’t take turns in a simple on/off pattern. Both are always active to some degree, and your body’s state at any given moment reflects the balance between them.
Cranial and Spinal Nerves
Peripheral nerves reach the CNS through two routes. Twelve pairs of cranial nerves connect directly to the brain, mostly serving the head and neck. These include the nerve responsible for smell, several controlling eye movement, the nerve carrying signals from the face, and the nerve that moves your tongue. The vagus nerve, the tenth cranial nerve, is a notable exception to the head-and-neck pattern. It travels all the way down into the chest and abdomen, regulating heart rate, digestion, blood pressure, breathing, and mood. It’s the primary nerve of the parasympathetic system.
The 31 pairs of spinal nerves exit the spinal cord and branch outward to serve specific regions of the body. Each spinal nerve carries both sensory fibers (bringing information in) and motor fibers (sending commands out), making them “mixed” nerves. Each one maps to a specific strip of skin called a dermatome, which is why doctors can test sensation in different areas to pinpoint where a spinal nerve might be compressed or damaged.
The Enteric Nervous System
Sometimes called the “second brain,” the enteric nervous system is a mesh of 200 to 600 million neurons embedded in the walls of your gastrointestinal tract. It’s technically part of the autonomic nervous system, but it can operate with a surprising degree of independence. It controls the muscular contractions that push food through your digestive tract, regulates stomach acid secretion, adjusts local blood flow, triggers the release of gut hormones, and interacts with the immune cells in your gut lining. While it communicates with the brain through the vagus nerve and sympathetic connections, it can coordinate basic digestive functions even when those links are disrupted.
The Cells That Make It All Work
Every part of the nervous system is built from the same basic cell types. Neurons are the signaling cells, and they come in three functional categories. Sensory neurons detect information from your environment, like the heat of a surface against your fingertips, and relay it inward. Motor neurons carry commands outward from the spinal cord and brain to muscles, glands, and organs. Interneurons sit between the two, connecting sensory and motor neurons into circuits. They form the basis of complex processing, from simple spinal reflexes to the vast networks of the brain.
Neurons get most of the attention, but they’re outnumbered by glial cells, the support staff of the nervous system. Glial cells don’t carry electrical signals themselves. Instead, they nourish neurons, insulate nerve fibers to speed up signal transmission, clear away waste, and help maintain the chemical environment neurons need to function. Unlike neurons, glial cells can divide and replace themselves throughout life.