The central nervous system (CNS) consists of two organs: the brain and the spinal cord. Together, they serve as the body’s processing center, receiving sensory information from every part of your body, interpreting it, and sending out instructions that control movement, organ function, thought, and emotion. Every other nerve in your body belongs to the peripheral nervous system, which acts as the communication network carrying signals to and from these two central organs.
The Brain: Three Major Divisions
The brain is the larger and more complex of the two CNS organs. It has three main structural divisions: the cerebrum, the cerebellum, and the brainstem. Each handles distinct tasks, though they constantly communicate with one another.
Cerebrum
The cerebrum is the largest part of the brain, filling most of the skull. Its outer layer of gray matter, called the cerebral cortex, is covered in ridges and folds that dramatically increase its surface area. Beneath that outer layer sits white matter, which contains the nerve fibers connecting different brain regions. The cerebrum is split into two hemispheres, and each hemisphere controls the opposite side of the body. The two halves stay in constant communication through a thick band of nerve fibers called the corpus callosum.
Virtually everything you think of as “thinking” happens in the cerebrum. It initiates and coordinates movement, processes vision, hearing, and touch, and handles speech, reasoning, problem-solving, emotions, and learning. It also regulates body temperature.
Cerebellum
The cerebellum sits at the back of the head, below the cerebrum and above the brainstem. It’s roughly the size of a fist. Like the cerebrum, it has two hemispheres and an outer layer of neurons. Its primary job is coordinating voluntary muscle movements and maintaining posture, balance, and equilibrium. When you reach for a glass of water without thinking about it, your cerebellum is fine-tuning those movements in real time.
Brainstem
The brainstem connects the cerebrum to the spinal cord and is made up of three parts: the midbrain, the pons, and the medulla. It controls many of the automatic functions you never have to think about. The medulla, for example, produces reflexive actions like sneezing, coughing, swallowing, and vomiting. The midbrain serves as a critical relay point between the upper brain and the lower brain and spinal cord. Damage to the brainstem is particularly dangerous because it governs life-sustaining processes like breathing and heart rate.
The Spinal Cord: Structure and Regions
The spinal cord is a cylindrical column of nervous tissue that runs from the base of the brainstem down through the vertebral column. It is made of both white matter (which carries signals up and down the cord) and gray matter (which processes information locally). The cord is divided into four main regions: cervical, thoracic, lumbar, and sacral. In total, 31 pairs of spinal nerves branch off from these regions, with 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve pair.
The spinal cord does more than just relay messages between the brain and body. It performs its own initial processing of incoming sensory information. Different zones within the cord handle different types of signals. The dorsal horn (toward the back) receives and processes sensory input like touch and pain. The ventral horn (toward the front) contains motor neurons that send commands to skeletal muscles, controlling voluntary movement. A middle zone called the lateral horn houses neurons that manage autonomic functions, regulating organs like the heart, lungs, and digestive tract without any conscious effort on your part.
This local processing is why spinal reflexes work so fast. When you touch something hot, the spinal cord can trigger a withdrawal reflex before the pain signal even reaches your brain.
Cells That Build the CNS
The CNS contains two broad categories of cells: neurons, which transmit electrical signals, and glial cells, which support and protect those neurons. Glial cells actually outnumber neurons, and each type plays a specific role.
Astrocytes are star-shaped cells that maintain the chemical environment neurons need to function. They regulate neurotransmitter levels around synapses, control the concentration of important ions like potassium, and provide metabolic support. They can also sense neural activity and release molecules that influence how neurons fire, making them active participants in brain signaling rather than passive support structures.
Oligodendrocytes produce myelin, a fatty substance that wraps around the long extensions of neurons like insulation around a wire. This myelin sheath allows electrical signals to travel much faster. Microglia are the CNS’s immune cells, identifying threats and clearing away dead cells or toxic substances. They also play a role in brain development by pruning unnecessary connections between neurons. Ependymal cells line the brain’s internal cavities and the central canal of the spinal cord, where they help produce cerebrospinal fluid.
How the CNS Protects Itself
Because the brain and spinal cord are irreplaceable, the body wraps them in multiple layers of protection. The most obvious layer is bone: the skull encases the brain, and the vertebral column surrounds the spinal cord. Beneath the bone sit three membrane layers called the meninges.
The outermost layer, the dura mater, is a thick, tough membrane made of two layers of connective tissue. One side attaches directly to the skull, while the other connects to the middle membrane. The dura also folds inward to form partitions that separate different sections of the brain, adding structural stability. The middle layer, the arachnoid mater, is thinner and has a spiderweb-like appearance due to delicate connective tissue strands that stretch down to the innermost layer. Between the arachnoid and the innermost layer is a fluid-filled space containing cerebrospinal fluid, which cushions the brain and spinal cord against impact. 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 brain tissue and helps maintain the structural stiffness of the spinal cord.
The Blood-Brain Barrier
The CNS has one more layer of defense that no other organ system has: the blood-brain barrier. This is not a single membrane but a system of tightly sealed blood vessel walls that control what enters the brain from the bloodstream. In most of the body, blood vessel walls have small gaps that allow molecules to pass through freely. Brain capillaries are different. Their cells are locked together by continuous tight junctions, leaving virtually no gaps between them and blocking most substances from slipping through.
These specialized capillaries are supported by a basement membrane that provides structural reinforcement and by astrocytes, whose foot-like projections wrap around the blood vessels and release chemical signals essential for maintaining the barrier. The result is a highly selective filter that lets in oxygen, glucose, and other essentials while keeping out most toxins, pathogens, and large molecules. This is why many medications that work elsewhere in the body cannot easily reach the brain.