The nervous system has two main organs: the brain and the spinal cord. Together, these form the central nervous system, which serves as the body’s command center. Beyond these two organs, a vast network of nerves branches out to every part of your body, forming the peripheral nervous system. Understanding how these parts work together explains how you think, move, feel, and keep your heart beating without a conscious thought.
The Brain
The brain is the most complex organ in the human body. It handles sensation, movement, emotions, communication, thinking, and memory. It contains somewhere between 67 and 86 billion neurons, plus tens of billions of supporting cells that help those neurons function. All of this sits inside your skull, protected by bone and a series of cushioning layers.
The brain has three major divisions, each responsible for different types of work.
The cerebrum is the largest part, making up the bulk of what you see when you picture a brain. It’s split into left and right hemispheres and handles everything that feels “conscious”: interpreting your five senses, forming speech, storing memories, guiding your behavior and personality, and carrying out reasoning and judgment. When you recognize a friend’s face, recall a phone number, or decide what to eat for dinner, your cerebrum is doing the work.
The cerebellum is a smaller, half-circle structure tucked behind the brainstem. Its job is coordination. It fine-tunes your balance, posture, and motor skills so your movements are smooth rather than jerky. Walking in a straight line, catching a ball, or typing on a keyboard all depend on the cerebellum working in the background.
The brainstem sits at the base of the brain and connects directly to the spinal cord. It controls the automatic functions you never have to think about: heart rate, breathing, sleep and wake cycles, and swallowing. Damage to the brainstem is especially dangerous because these functions are essential for survival.
The Spinal Cord
The spinal cord is a long, narrow column of nervous tissue that runs from the base of the brain down through the vertebral column. It extends roughly 45 cm in men and 43 cm in women, ending in the lower back. Despite its relatively small size, it serves as the primary highway between the brain and the rest of the body.
The spinal cord has two core jobs. First, it relays motor commands from the brain to your muscles and organs, allowing you to move your limbs or adjust your posture. Second, it carries sensory information from your skin, joints, and internal organs back up to the brain, letting you feel touch, temperature, and pain. It also handles some responses on its own: when you pull your hand away from a hot stove before you even feel the burn, that’s a spinal reflex that doesn’t need the brain’s input.
The cord is organized into 31 segments, each giving rise to a pair of spinal nerves (one on each side). These 31 pairs of spinal nerves exit through small gaps between the vertebrae and branch out to serve specific regions of the body, from the top of your neck down to your toes.
How the Brain and Spinal Cord Are Protected
Both the brain and spinal cord are fragile, so the body wraps them in multiple layers of protection. The outermost layer is bone: the skull surrounds the brain, and the stacked vertebrae of the spine encase the spinal cord. Beneath the bone, three membrane layers called meninges provide additional shielding. The outermost membrane (dura mater) sits closest to the bone. The middle layer (arachnoid mater) is separated from the innermost layer (pia mater) by a fluid-filled space. That fluid, called cerebrospinal fluid, acts as a shock absorber, cushioning the brain and spinal cord against sudden jolts or impacts.
The Peripheral Nervous System
Everything outside the brain and spinal cord belongs to the peripheral nervous system. This includes all the nerves that travel from your spinal cord and brain out to your face, limbs, chest, and abdomen. Each nerve is a bundle of nerve cells with long extensions called axons, twisted together like cables.
Twelve pairs of cranial nerves connect directly to the brain rather than the spinal cord. These handle specialized tasks tied to the head and neck. The olfactory nerve carries your sense of smell. The optic nerve transmits visual information from your eyes. The facial nerve controls your facial expressions and carries taste signals from the front two-thirds of your tongue. The vestibulocochlear nerve is responsible for hearing and balance. Perhaps the most far-reaching is the vagus nerve, which travels all the way down into the chest and abdomen. It accounts for roughly 75% of the parasympathetic nervous system and influences heart rate, digestion, breathing, and even speech.
The remaining peripheral nerves exit through the spinal cord. These 31 pairs of spinal nerves divide into smaller and smaller branches as they spread outward, eventually reaching every patch of skin, every muscle fiber, and every internal organ. This branching network is what lets you feel a tap on your shoulder and, a fraction of a second later, turn your head to see who’s there.
The Autonomic Nervous System
Within the peripheral nervous system, a specialized division called the autonomic nervous system controls processes you don’t consciously manage: heart rate, blood pressure, breathing, digestion, and sexual arousal. It has two opposing branches that work like a gas pedal and a brake.
The sympathetic branch triggers the “fight or flight” response. When you’re startled or under stress, it raises your heart rate and blood pressure, releases stored energy, and slows digestion so your body can focus on the immediate threat. The parasympathetic branch does the opposite, promoting “rest and digest” functions. It slows the heart, lowers blood pressure, and restores normal digestion.
Your gut also has its own semi-independent network called the enteric nervous system, sometimes nicknamed the “second brain.” It contains two layers of nerve clusters embedded in the walls of the digestive tract. One layer controls the muscular contractions that push food along. The other regulates how water and nutrients are absorbed across the intestinal wall. While it can operate on its own through local reflexes, it constantly communicates with the sympathetic and parasympathetic branches to coordinate digestion with whatever else is happening in your body.
How It All Works Together
The brain and spinal cord don’t function in isolation. Every second, sensory nerves in the periphery collect information (a change in temperature, a sound, the stretch of a full stomach) and send it up through spinal nerves or cranial nerves to the central nervous system. The brain processes that information, makes decisions, and sends commands back down through the same nerve pathways to trigger a response, whether that’s pulling on a jacket, turning toward a noise, or releasing digestive enzymes.
This loop happens continuously and largely without your awareness. The autonomic system adjusts your heart rate beat by beat. Your cerebellum recalculates your balance with every step. Your brainstem keeps you breathing while you sleep. The two main organs, the brain and spinal cord, sit at the center of all of it, while the peripheral nerves serve as the wiring that connects them to every corner of your body.