The central nervous system (CNS) is made up of two structures: the brain and the spinal cord. Together, they serve as the body’s command center, receiving sensory information from the environment, processing it, and sending out instructions that control movement, organ function, and virtually every other bodily process. Everything outside the brain and spinal cord belongs to the peripheral nervous system, which acts as the wiring that carries signals between the CNS and the rest of the body.
The Brain: Three Major Divisions
The brain is the larger and more complex of the two CNS structures. It sits inside the skull’s cranial cavity and contains roughly 20 billion neurons in the outer layer alone, each forming an average of 7,000 connections with other neurons. That adds up to about 0.15 quadrillion total connections in just the cerebral cortex. The brain breaks down into three broad regions, each handling different jobs.
The cerebrum is the largest part, making up the bulk of what you picture when you think of a brain. It handles movement, body temperature, speech, reasoning, emotions, learning, and all five senses. Beneath the cerebrum sits the brainstem, which connects the brain to the spinal cord. The brainstem controls survival-level functions: heart rhythm, breathing, blood pressure, and oxygen levels. It also manages reflexes like coughing, swallowing, and sneezing. The brainstem itself has three subregions (the midbrain, pons, and medulla), each with specialized roles ranging from hearing and balance to facial expression and chewing.
At the back of the head, tucked below the cerebrum and behind the brainstem, sits the cerebellum. It’s roughly the size of a fist and coordinates voluntary muscle movements, posture, and balance. Without the cerebellum, even simple actions like walking or reaching for a cup would be clumsy and imprecise.
The Spinal Cord: Structure and Segments
The spinal cord is a long, narrow column of nervous tissue running from the base of the brainstem down through the vertebral column. It serves as the main communication highway between the brain and the body. When your brain creates a message, it sends that signal down the spinal cord, which passes it along to peripheral nerves that reach muscles, organs, and glands. Signals also travel in the opposite direction, carrying sensory data from your skin, joints, and internal organs back up to the brain.
The spinal cord is divided into 31 segments, each defined by a pair of nerve roots that branch out from it. These segments are grouped into five regions: 8 cervical (neck), 12 thoracic (mid-back), 5 lumbar (lower back), 5 sacral (pelvis), and 1 coccygeal (tailbone). At each segment, a dorsal root carries incoming sensory signals and a ventral root carries outgoing motor signals. These two roots merge to form a single spinal nerve, which then connects to the peripheral nervous system. This is the point where the CNS hands off to the PNS.
Grey Matter and White Matter
Both the brain and spinal cord are built from two types of tissue: grey matter and white matter. Grey matter contains the cell bodies of neurons, along with their short branching extensions, supporting cells, and tiny blood vessels. This is where processing happens. White matter is made up of long nerve fibers coated in myelin, a fatty insulating layer that helps electrical signals travel quickly and reliably. White matter carries signals between different areas of the CNS.
Interestingly, the arrangement of these two tissues is reversed between the brain and spinal cord. In the brain, grey matter forms the outer surface (the wrinkled cortex you see in pictures), while white matter is buried underneath. In the spinal cord, the layout flips: grey matter forms a butterfly-shaped core in the center, surrounded by an outer layer of white matter. Despite the reversed arrangement, the principle is the same. Grey matter processes information locally, and white matter transmits it over longer distances.
Cells That Build the CNS
Two broad categories of cells make up the central nervous system: neurons and glial cells. Neurons are the signaling cells. They generate and transmit electrical impulses, forming the circuits that underlie everything from reflexes to abstract thought. Glial cells were long thought to outnumber neurons by 10 to 1, but more recent counting methods suggest the ratio in humans is closer to 1 to 1, though it varies considerably from one brain region to another.
Glial cells play supporting roles that are essential to CNS function. Some produce the myelin coating on white matter fibers. Others regulate the chemical environment around neurons, clear waste, or help direct blood flow to active brain areas. Without glial cells, neurons couldn’t survive or signal effectively.
How the CNS Protects Itself
Because the brain and spinal cord are irreplaceable (CNS neurons generally cannot regenerate after injury, unlike peripheral nerves), the body wraps them in multiple layers of protection. The most obvious is bone: the skull encases the brain, and the vertebral column encases the spinal cord.
Beneath the bone sit three membranes called the meninges. The outermost layer, the dura mater, is tough and sits closest to the bone. The middle layer, the arachnoid mater, is more delicate. The innermost layer, the pia mater, clings directly to the surface of the brain and spinal cord. Between the arachnoid and pia layers, cerebrospinal fluid circulates, cushioning the CNS against impacts and carrying away metabolic waste.
The CNS also has a chemical barrier. The blood-brain barrier is formed by the tightly sealed cells lining the brain’s capillaries, supported by surrounding cells that regulate what crosses from the bloodstream into brain tissue. This barrier allows essential nutrients and oxygen through while blocking most bacteria, toxins, and large molecules. It is one reason brain infections are relatively rare, and also why delivering medications to the brain can be difficult.
How the CNS Differs From the PNS
The simplest way to distinguish the two systems: the CNS is everything inside the skull and vertebral column, and the peripheral nervous system is everything outside. That is a slight oversimplification (a few peripheral elements exist within those cavities), but it captures the core distinction. Structurally, clusters of neuron cell bodies in the CNS look different under a microscope than the equivalent structures in the PNS, which form visible clumps called ganglia along peripheral nerves.
The most important practical difference is regeneration. Peripheral nerves can often regrow after being damaged, which is why sensation and movement may return after certain injuries. CNS neurons, as a general rule, do not regenerate. This is why spinal cord injuries and many forms of brain damage result in permanent loss of function.