The central nervous system consists of two structures: the brain and the spinal cord. Together, they receive sensory information from the body and the outside world, process it, and send out instructions that control movement, organ function, thought, and emotion. Everything else in the nervous system, every nerve branching through your limbs and torso, belongs to the peripheral nervous system and serves as a communication line running to and from this central hub.
The Three Major Parts of the Brain
The brain divides into three main regions: the cerebrum, the cerebellum, and the brainstem. Each handles a different category of work, though they constantly exchange information with one another.
The cerebrum is the largest region, making up the bulk of what you see when you picture a brain. It controls motor and sensory processing, conscious and unconscious behavior, feelings, intelligence, and memory. Its outer surface, the cerebral cortex, is the folded, wrinkled layer responsible for higher-level thinking. Deeper structures within the cerebrum handle things like habit formation, emotional memory, and relaying sensory data to the cortex.
The cerebellum sits at the back of the skull, beneath the cerebrum. Its primary job is coordination. It receives sensory information from both the brain and spinal cord, then uses it to fine-tune the precision and accuracy of voluntary movements. Without it, reaching for a glass of water would be clumsy and imprecise. The cerebellum also contributes to attention, language processing, and certain emotional responses like fear memory.
The brainstem connects the brain to the spinal cord and houses the control centers for functions you never have to think about: breathing, heart rate, body temperature, digestion, swallowing, coughing, sneezing, and sleep-wake cycles. It is made up of three subregions stacked on top of one another: the midbrain, the pons, and the medulla.
How the Spinal Cord Is Organized
The spinal cord is a long, cylindrical bundle of nervous tissue that extends from the brainstem down through the vertebral column. In cross-section, it has a simple layout: a butterfly-shaped core of gray matter surrounded by white matter.
The gray matter contains the cell bodies of neurons and is divided into regions called horns. The back (dorsal) horns receive incoming sensory information from the body, like touch, pressure, and pain signals. The front (ventral) horns contain motor neurons whose fibers exit the cord and travel to muscles, telling them when to contract. A lateral horn, present mainly in the middle portion of the cord, controls involuntary functions like blood vessel diameter and organ activity.
The white matter surrounds the gray matter and is organized into columns of nerve fibers that act as highways between the brain and the rest of the body. Columns along the back of the cord carry sensory information upward to the brain. Columns along the sides carry movement commands downward from the cerebral cortex to the spinal motor neurons. Columns along the front carry a mix of ascending pain and temperature signals and descending motor instructions. The general principle is straightforward: sensory pathways run along the back of the cord, motor pathways run along the front.
Neurons and Support Cells
The human brain contains roughly 86 billion neurons, though individual counts in studies have ranged from about 62 billion to 95 billion. The brain holds a roughly equal number of non-neuronal cells. An average adult male brain weighs about 1,336 grams; an average adult female brain weighs about 1,198 grams. Brain weight decreases with age, losing a few grams per year in both sexes.
Neurons do the signaling work, but they depend on three types of support cells called glial cells. Astrocytes, star-shaped cells found throughout the brain and spinal cord, maintain the chemical environment neurons need to send signals properly. Oligodendrocytes wrap certain nerve fibers in a fatty insulation called myelin, which dramatically increases the speed at which electrical signals travel. Microglial cells act as the CNS immune system, scavenging debris from damaged or dying cells and responding to injury or infection.
How the CNS Processes Information
The central nervous system works as an integration center. Sensory signals from the skin, muscles, joints, and organs travel inward through peripheral nerves and enter the spinal cord or brainstem. From there, a relay station deep in the brain called the thalamus routes the information to the appropriate processing areas in the cerebral cortex.
Once the cortex interprets the sensory input, it generates a motor plan. The cerebellum plays a key role here: it predicts the sensory consequences of a planned movement, then compares that prediction against the actual feedback coming in during the movement. If there is a mismatch, it adjusts the motor output in real time. This prediction-and-correction loop is why you can learn to catch a ball more accurately with practice. The refined motor commands then travel down through the spinal cord’s white matter columns to activate the appropriate muscles.
Three Layers of Protective Membranes
Both the brain and spinal cord are wrapped in three membranes called the meninges. From outermost to innermost, they are the dura mater, the arachnoid mater, and the pia mater.
The dura mater is a thick, dense, fibrous layer that sits just inside the skull and vertebral column. It is quite inelastic and serves as a tough outer shield. The arachnoid mater is a thinner, web-like membrane beneath it. It does not follow the brain’s folds and grooves but bridges over them. Between the arachnoid and the innermost layer lies the subarachnoid space, which is filled with cerebrospinal fluid. The pia mater is the deepest layer, a delicate membrane that clings directly to every contour of the brain and spinal cord surface.
Cerebrospinal Fluid
The brain and spinal cord essentially float in cerebrospinal fluid (CSF), a clear liquid that cushions them against impact, delivers nutrients, and carries away waste. Your body produces about 500 milliliters of CSF per day, but only about 125 to 150 milliliters exists in the system at any given moment because it is constantly absorbed and recycled.
CSF is produced inside a series of cavities within the brain called ventricles. It flows from the two lateral ventricles into a third ventricle, then through a narrow channel into a fourth ventricle near the brainstem. From there, most of it exits into the subarachnoid space surrounding the brain and spinal cord. A smaller amount flows down through a tiny central canal running the length of the spinal cord. Eventually, the fluid passes over the surface of the brain and is reabsorbed into the bloodstream.
The Blood-Brain Barrier
Unlike most organs, the brain does not allow free exchange between blood and tissue. A specialized filtration system called the blood-brain barrier tightly controls what enters the CNS from the bloodstream. It is built from three components: the endothelial cells lining the brain’s tiny blood vessels, support cells called pericytes embedded in the vessel wall, and the end-feet of astrocytes wrapping around the outside.
The endothelial cells in brain capillaries are different from those elsewhere in the body. They have no small pores, and their connections to each other, called tight junctions, are far more restrictive. These tight junctions block water-soluble molecules from slipping between cells. As a general rule, only fat-soluble, positively charged molecules smaller than about 400 to 600 daltons can cross on their own. This keeps most blood-borne toxins, pathogens, and large proteins out of the brain while still allowing essential nutrients like oxygen and glucose to pass through using dedicated transport systems. The barrier also actively pumps certain unwanted substances back out into the blood, adding another layer of defense.