The central nervous system consists of two structures: the brain and the spinal cord. Together, they act as the body’s command center, collecting sensory information from every part of your body, processing it, and sending instructions back out. Everything else, every nerve branching through your limbs and organs, belongs to the peripheral nervous system. But the central nervous system is where all decisions get made.
The Brain’s Three Main Divisions
Your brain is the larger and more complex of the two central nervous system structures. It divides into three major parts, each handling a different category of work.
The cerebrum is the largest portion, taking up most of the skull. It’s split into left and right hemispheres and handles everything you think of as “thinking”: speech, memory, reasoning, judgment, personality, and voluntary movement. It also interprets input from your five senses. The cerebellum sits at the back of the brain and manages balance, posture, coordination, and fine motor skills. You can thank your cerebellum every time you catch a ball or walk in a straight line without concentrating on it. The brainstem connects the brain to the spinal cord and controls automatic functions you never have to think about: heart rate, breathing, swallowing, and sleep-wake cycles.
The Spinal Cord
The spinal cord is a long, thin bundle of nervous tissue running from the base of the brainstem down through the vertebral column. It serves as the primary highway between the brain and the rest of the body. When your brain decides to move your hand, the signal travels down the spinal cord and out through peripheral nerves to reach the right muscles. Sensory information, like the feeling of a hot stove, travels the opposite direction.
The cord is organized into 31 segments, each giving rise to a pair of spinal nerves that branch out to specific body regions. These break down into 8 cervical segments (serving the neck and arms), 12 thoracic (the trunk), 5 lumbar (the lower back and legs), 5 sacral (the pelvis), and 1 coccygeal segment at the very base. This segmental organization is why spinal cord injuries at different levels affect different parts of the body.
Gray Matter and White Matter
If you could slice the brain or spinal cord open, you’d see two distinct types of tissue. Gray matter has a grayish-pink color and contains the cell bodies of neurons, along with the short branching structures (dendrites) that receive signals from other cells. This is where the actual processing happens. White matter, by contrast, is made up of long nerve fibers coated in a fatty substance called myelin, which speeds up signal transmission. White matter functions like the wiring that connects processing centers to each other.
Here’s what’s interesting: the arrangement of these two tissues flips between the brain and the spinal cord. In the brain, gray matter sits mostly on the outside (the cerebral cortex, the thin outer layer responsible for higher thinking) with white matter underneath. In the spinal cord, it’s reversed. Gray matter forms a butterfly-shaped core on the inside, surrounded by white matter on the outside. Deep within the brain, clusters of gray matter called nuclei also handle specialized processing tasks like movement coordination and emotional responses.
Neurons and Supporting Cells
The central nervous system is built from two broad categories of cells. Neurons are the ones that carry electrical signals. A typical neuron has a cell body, branching dendrites that pick up incoming signals, and a long axon that transmits signals outward to other neurons or to muscles and glands. Neurons come in several structural types: multipolar neurons with many dendrites, bipolar neurons with one dendrite and one axon, and unipolar neurons with a single extension that branches in two directions.
The second category, glial cells, vastly outnumber neurons. They don’t carry signals themselves, but they’re essential for keeping the system running. Astrocytes provide structural support and help regulate the chemical environment around neurons. Oligodendrocytes produce the myelin coating that insulates nerve fibers in the brain and spinal cord. Microglia act as the central nervous system’s immune cells, clearing away debris and responding to injury or infection. Unlike neurons, glial cells can divide and reproduce throughout your life.
Protective Layers: The Meninges
Both the brain and spinal cord are wrapped in three membrane layers called the meninges. These layers cushion, anchor, and protect the delicate nervous tissue underneath.
- Dura mater (Latin for “hard mother”) is the outermost layer, sitting directly beneath the skull and vertebral bones. It’s thick and tough, made of two layers of connective tissue. It contains a drainage system called the dural venous sinuses that allows blood to leave the brain.
- Arachnoid mater is the middle layer, named for its spiderweb-like appearance. It’s thin, contains no blood vessels or nerves of its own, and has delicate connective tissue projections that bridge down to the innermost layer.
- Pia mater is the innermost layer, clinging tightly to the surface of the brain and spinal cord like shrink wrap. It’s rich with blood vessels that supply the brain tissue directly and helps contain cerebrospinal fluid. In the spinal cord, it also helps maintain the cord’s structural stiffness.
Cerebrospinal Fluid
Floating between the meninges and filling cavities within the brain is cerebrospinal fluid, a clear liquid that acts as both a shock absorber and a waste-removal system. Adults carry roughly 150 milliliters of it at any given time, about two-thirds of a cup. But the body produces 400 to 600 milliliters per day, meaning the entire supply gets recycled multiple times every 24 hours. This constant turnover keeps the chemical environment around the brain and spinal cord stable and flushes out metabolic waste products.
The Blood-Brain Barrier
The central nervous system has an additional layer of protection that most organs don’t: the blood-brain barrier. This is a tightly sealed lining of specialized cells in the brain’s blood vessels that controls exactly which substances can pass from the bloodstream into brain tissue. The cells forming this barrier are connected by extremely tight junctions that prevent molecules from slipping through the gaps between them. There are also no significant transport channels through the cells themselves, making the barrier highly selective.
This matters because the brain is extraordinarily sensitive to chemical changes. Many molecules circulating in the blood can alter how neurons fire, and even small fluctuations could trigger uncontrolled brain activity. The barrier keeps the brain’s internal environment stable. Surrounding the blood vessels, astrocytes extend foot-like projections that help maintain and develop the barrier, while a structural basement membrane provides additional support.
How It All Works Together
The central nervous system’s core job is a three-step loop: take in sensory data, process it, and generate a response. Sensory neurons throughout your body collect information (temperature, pressure, light, sound, chemical signals) and relay it through spinal nerves up to the brain. The brain integrates this information, compares it against memory and context, and decides on a response. It then sends electrical signals back down through the spinal cord and out through peripheral nerves to trigger a motor action, whether that’s pulling your hand away from something sharp or adjusting your posture as you shift in a chair.
This loop runs continuously and largely without conscious effort. Your brainstem keeps your heart beating and lungs expanding. Your cerebellum fine-tunes every movement so you don’t overshoot when reaching for a glass of water. Your cerebrum lets you read this sentence, understand it, and remember it later. All of it originates in two pounds of brain tissue and a cord of nerves no thicker than your thumb.