The central nervous system (CNS) is made up of two organs: the brain and the spinal cord. Together, they act as the body’s command center, processing every sensation you feel, every thought you have, and every movement you make. Everything else, the nerves branching out to your limbs, organs, and skin, belongs to the peripheral nervous system. The boundary between the two is where nerve roots exit the skull and spinal column.
The Brain’s Three Major Divisions
Your brain has three main parts, each handling different jobs.
The cerebrum is the largest, stretching across the top of the head down to about ear level. Its outer surface, called the cerebral cortex or “gray matter,” is where the most complex thinking happens: language, problem-solving, memory, and voluntary movement. The cerebrum is divided into left and right hemispheres, each further divided into lobes that specialize in different functions like vision, hearing, and spatial reasoning.
The cerebellum sits underneath the cerebrum, behind your ears toward the back of your head. It’s smaller, but it’s packed with neurons. Its main job is coordination. When you ride a bike, catch a ball, or type without looking at the keyboard, your cerebellum is running those automatic movement programs so you don’t have to consciously think through every step.
The brainstem is the smallest of the three and sits at the base of the brain, extending downward toward the neck. It connects the brain to the spinal cord and controls the functions you can’t afford to think about: heart rate, breathing, blood pressure, and sleep cycles. A region within the brainstem called the medulla keeps your heart beating and your lungs working even while you’re unconscious or asleep.
The Spinal Cord
The spinal cord runs from the base of the brainstem down through the vertebral column. Despite making up only about 2% of the entire CNS, it’s essential. It serves as the main highway between the brain and the rest of the body, carrying signals in both directions: sensory information traveling up and movement commands traveling down.
The cord is organized into segments that correspond to different regions of the body: cervical (neck), thoracic (mid-back), lumbar (lower back), and sacral (base of the spine). Each segment sends out pairs of nerve roots that exit between vertebrae and branch into the peripheral nervous system. The spinal cord also handles some tasks on its own. Reflexes, like pulling your hand away from a hot surface, are processed right at the spinal cord level before the pain signal even reaches your brain.
Neurons and Glial Cells
At the cellular level, the CNS is built from two broad categories of cells: neurons and glial cells.
Neurons are the signaling cells. Your brain alone contains roughly 100 billion of them. Each neuron has a cell body, branching extensions called dendrites that receive incoming signals, and a long fiber called an axon that sends signals outward to other neurons, muscles, or glands. Neurons don’t physically touch each other. Instead, they communicate across tiny gaps using chemical messengers.
Glial cells were long assumed to vastly outnumber neurons, sometimes cited at a 10-to-1 ratio. More recent counts tell a different story. The human brain contains about 80 billion neurons and roughly 60 billion glial cells, making the ratio close to 1-to-1 overall, though it varies by brain region. In the cerebral cortex, glial cells outnumber neurons by about 1.5 to 1.
Glial cells don’t transmit electrical signals the way neurons do, but they’re far from passive. They provide structural support, supply nutrients to neurons, insulate nerve fibers, clear away debris, and help regulate the chemical environment around synapses. Without them, neurons couldn’t function properly.
Gray Matter and White Matter
If you were to slice through the brain or spinal cord, you’d see two distinct tissue types: gray matter and white matter.
Gray matter has a grayish-pink color and is made up of neuron cell bodies, dendrites, and short axon terminals. This is where the actual processing happens: receiving information, making decisions, and generating responses. In the brain, gray matter forms the outer cortex. In the spinal cord, the arrangement flips: gray matter sits in the center, forming a butterfly-shaped core.
White matter gets its pale color from myelin, a fatty coating that wraps around longer axons like insulation on a wire. Myelin dramatically speeds up signal transmission, allowing messages to travel faster across longer distances. White matter forms the deep interior of the brain and the outer portion of the spinal cord, creating the communication cables that connect different brain regions to each other and to the spinal cord.
Protective Layers Around the CNS
The brain and spinal cord are soft, delicate tissue. They’re protected by several layers of defense, starting with bone (the skull and vertebral column) and continuing with three membrane layers called the meninges.
The outermost membrane is the dura mater, a thick, tough layer that sits directly against the inside of the skull and vertebral column. Beneath it is the arachnoid mater, a thinner, web-like layer. The innermost layer is the pia mater, which clings tightly to the surface of the brain and spinal cord like shrink wrap. In the spinal cord, the pia mater also helps maintain the cord’s stiffness.
Between the arachnoid and pia layers is a fluid-filled gap called the subarachnoid space. This space contains cerebrospinal fluid (CSF), a clear liquid that cushions the brain and spinal cord against impacts. An adult carries about 150 milliliters of CSF at any given time, split between the brain’s internal chambers, the space around the brain, and the space around the spinal cord. Your body produces 400 to 600 milliliters of fresh CSF per day, turning over the entire supply about three times daily. This constant renewal helps flush out waste products and maintain a stable chemical environment.
The Blood-Brain Barrier
The CNS has one more layer of protection that’s invisible to the eye. The blood-brain barrier is a filtering system built into the walls of the brain’s tiniest blood vessels. In most of the body, capillary walls have small gaps that let molecules pass through easily. In the brain, the cells lining these capillaries are wedged together so tightly that they form what scientists call “tight junctions,” leaving almost no space between cells.
This arrangement is highly selective. Small molecules, fat-soluble substances, and certain gases like oxygen and carbon dioxide pass through freely. Larger molecules that the brain needs, like glucose for energy, get in through specialized transporter proteins that act like dedicated doors, opening only for specific substances. Toxins, bacteria, and most drugs in the bloodstream are blocked. This selectivity is why treating brain diseases can be particularly challenging: many medications simply can’t cross the barrier.