The central nervous system (CNS) consists of two structures: the brain and the spinal cord. Together, they contain roughly 86 billion neurons and an equal number of supporting cells, all enclosed within bone and wrapped in protective membranes. Every thought, movement, sensation, and automatic body function originates from or passes through one of these two structures.
The Brain’s Three Major Divisions
The brain divides into three main parts: the cerebrum, the brainstem, and the cerebellum.
The cerebrum is the largest portion, filling most of the skull. Its outer surface, the cerebral cortex, is a layer of gray matter covered in ridges and folds that dramatically increase its surface area. Beneath the cortex sits white matter, made up of long nerve fibers coated in a fatty insulation called myelin. Gray matter contains the actual cell bodies of neurons (where processing happens), while white matter carries signals between regions, functioning like the brain’s internal wiring.
The cerebral cortex splits into left and right hemispheres, and each hemisphere has four lobes:
- Frontal lobes: control thinking, planning, problem-solving, short-term memory, and voluntary movement
- Parietal lobes: process sensory information like touch, taste, texture, and temperature
- Temporal lobes: handle smell, taste, and sound processing, and play a role in storing memories
- Occipital lobes: process visual information from your eyes and match it against stored images so you can recognize what you see
The cerebellum sits at the back of the head, below the occipital and temporal lobes. Roughly the size of a fist, it has its own two hemispheres and is sometimes called the “little brain.” It fine-tunes movement, balance, and coordination, making the difference between a smooth reach for a coffee cup and a clumsy swipe past it.
The Brainstem
The brainstem connects the cerebrum to the spinal cord and manages many of the functions you never have to think about: breathing, heart rate, blood pressure, and swallowing. It has three sections stacked on top of each other.
The midbrain, at the top, processes visual and auditory information and helps control eye movements. The pons, in the middle, coordinates facial and eye movements, facial sensation, hearing, and balance. Its name comes from the Latin word for “bridge” because it links the midbrain to the lowest section, the medulla oblongata. The medulla sits where the brain meets the spinal cord and regulates the most basic life-sustaining functions, including breathing rhythm and heart rate.
Deep Brain Structures
Tucked between the cerebrum and the brainstem is a region called the diencephalon, which contains several critical relay and control centers. The two most important are the thalamus and the hypothalamus.
The thalamus acts as the brain’s switchboard. Nearly all sensory information heading to the cerebral cortex (with the exception of smell) passes through the thalamus first, where it gets sorted and routed to the correct lobe for processing. The hypothalamus, sitting just below it, is the brain’s master regulator for internal balance. It controls body temperature, hunger, thirst, sleep cycles, and hormonal output by directing the pituitary gland. It also governs the autonomic nervous system, the network that runs unconscious processes like digestion and stress responses.
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 extends about 18 inches in most adults, ending around the level of the first or second lumbar vertebra, well above the bottom of the spine.
The cord is organized into 31 segments, each giving rise to a pair of spinal nerves that branch out to specific regions of the body. These segments break down into five groups: 8 cervical (neck), 12 thoracic (mid-back), 5 lumbar (lower back), 5 sacral (pelvis), and 1 coccygeal (tailbone). The cervical segments control the arms, shoulders, and diaphragm. Thoracic segments serve the trunk and parts of the abdomen. Lumbar and sacral segments handle the legs, bladder, and bowel.
The spinal cord does more than relay messages between the brain and body. It also processes certain reflexes on its own. When you touch a hot surface, sensory neurons send a signal into the cord, which triggers motor neurons to yank your hand away before the pain signal even reaches your brain.
Gray and White Matter Distribution
The brain and spinal cord share the same two tissue types, but their arrangement flips. In the brain, gray matter forms the outer cortex while white matter fills the interior. In the spinal cord, the pattern reverses: gray matter sits in the center (forming a butterfly-shaped core visible on cross-section) and white matter surrounds it on the outside. This central gray matter contains the neuron cell bodies that process local reflexes, while the outer white matter carries long-distance signals up to the brain and back down.
The Protective Layers
Both the brain and spinal cord are wrapped in three membrane layers called the meninges, which sit between the nervous tissue and the surrounding bone.
The outermost layer, the dura mater, is a thick, tough membrane made of two layers of connective tissue. It lines the inside of the skull and vertebral column and contains a drainage system that allows blood to leave the brain and cerebrospinal fluid to re-enter circulation. Beneath it lies the arachnoid mater, a thin, web-like membrane (its name literally means “spider”) that contains no blood vessels or nerves of its own. The innermost layer, the pia mater, clings tightly to the surface of the brain and spinal cord like shrink wrap. It carries many of the blood vessels that supply nervous tissue and helps maintain the structural stiffness of the spinal cord.
Cerebrospinal Fluid and the Ventricles
Inside the brain sit four interconnected, fluid-filled chambers called ventricles. There are two lateral ventricles (one in each cerebral hemisphere), a third ventricle nestled between the two halves of the thalamus, and a fourth ventricle located near the brainstem and cerebellum.
These ventricles produce cerebrospinal fluid, a clear liquid that fills the chambers and flows into the space between the arachnoid and pia mater, bathing the entire surface of the brain and spinal cord. This fluid serves three purposes: it cushions the CNS against impact, delivers nutrients, and carries away waste products. After circulating, the fluid drains back into the bloodstream through small projections in the arachnoid layer. At any given moment, roughly 150 milliliters of cerebrospinal fluid surrounds the central nervous system, continuously produced and reabsorbed throughout the day.