The central nervous system, or CNS, is the body’s main command center. It consists of two structures: the brain and the spinal cord. Together, they receive sensory information from the rest of the body, process it, and send out instructions that control movement, emotions, memory, organ function, and nearly every other bodily process. Despite making up only about 2 percent of your body weight, the brain alone consumes 20 percent of your body’s oxygen supply, reflecting just how metabolically demanding this system is.
The Brain and Spinal Cord
The brain handles sensation, movement, emotional responses, communication, cognition, and memory. It’s where conscious thought happens and where unconscious processes like regulating your heart rate and breathing are coordinated. Different regions specialize in different tasks, but they work together through vast networks of interconnected cells.
The spinal cord is a long column of nervous tissue that runs from the base of the brain down through the spine. In adults, it averages about 41 centimeters in length and typically ends between the first and second lumbar vertebrae, roughly at the level of your lower back. Its primary job is acting as a two-way highway: it relays motor commands from the brain down to the body, and it carries sensory information from the body back up to the brain. The spinal cord also manages certain rapid responses on its own, like the reflex that jerks your hand away from a hot surface before your brain even registers the pain.
How the CNS Communicates
All communication in the CNS runs on a combination of electrical and chemical signals. Neurons, the primary signaling cells, generate electrical impulses that travel along their length. When an impulse reaches the end of a neuron, it triggers the release of chemical messengers called neurotransmitters into the tiny gap between that neuron and the next one.
Here’s how it works in sequence: an electrical impulse arrives at the end of a neuron, causing calcium to flood into the cell. That calcium surge triggers tiny packets of neurotransmitters to fuse with the cell membrane and release their contents into the gap. Those chemical messengers drift across and bind to receptors on the next neuron, which can then fire its own electrical impulse. Afterward, the cell recycles the packaging for reuse. This entire process takes milliseconds and repeats billions of times throughout the CNS every second.
Neurons and Support Cells
Neurons get most of the attention, but they’re actually outnumbered by support cells called glia. These glial cells don’t just fill space. They actively shape how the CNS works.
Astrocytes are the most abundant type. They regulate blood flow to active brain areas, maintain the chemical environment neurons need to function, help build and prune the connections between neurons, and clean up excess neurotransmitters that could otherwise overstimulate surrounding cells.
Oligodendrocytes produce myelin, a fatty insulation that wraps around the long fibers (axons) extending from neurons. This insulation dramatically speeds up signal transmission by forcing electrical impulses to jump between gaps in the coating rather than traveling slowly along the entire length of the fiber. A single oligodendrocyte can insulate portions of many different axons at once. These cells also supply axons with fuel, delivering nutrients like lactate and cholesterol to keep signals running.
Microglia serve as the brain’s immune cells. In their resting state, they extend thin branching arms that constantly survey their surroundings. When they detect injury or infection, they migrate to the damage site, release immune signaling molecules, and engulf dead cells and debris. They also play a role in normal brain development and maintenance by pruning unnecessary connections between neurons.
Gray Matter and White Matter
If you’ve ever seen a cross-section image of the brain or spinal cord, you’ll notice two distinct tissue types: gray matter and white matter. The difference comes down to what’s concentrated in each area.
Gray matter gets its color from dense clusters of neuron cell bodies. In the brain, it forms the outer layer (the cortex) and sits in deeper clusters called nuclei. This is where the actual processing happens: interpreting sensory input, planning movement, forming memories, experiencing emotions. In the spinal cord, gray matter sits in the interior, forming a butterfly or horn-shaped core.
White matter, by contrast, is made up of myelinated axons, the long insulated fibers that carry signals between regions. The myelin coating gives it its pale appearance. White matter surrounds the gray matter core in the spinal cord and fills much of the brain’s interior, forming the communication cables that connect processing centers to each other and to the rest of the body.
How the CNS Is Protected
Because the CNS is so critical, the body wraps it in multiple layers of protection. The first line of defense 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 tough, durable sheet closest to the bone. The middle layer, the arachnoid mater, is a web-like membrane. The innermost layer, the pia mater, clings directly to the surface of the brain and spinal cord. Between the arachnoid and pia layers is a space filled with cerebrospinal fluid, a clear liquid that cushions the brain and spinal cord against impacts, much like a shock absorber.
The CNS also has a chemical barrier. The blood-brain barrier is a tightly sealed lining in the blood vessels of the brain that prevents most substances circulating in the blood from freely entering brain tissue. This keeps out toxins, pathogens, and many drugs, though it also makes treating brain diseases more challenging since medications have difficulty crossing it.
How the CNS Connects to the Rest of the Body
The CNS doesn’t work alone. It depends on the peripheral nervous system (PNS), the network of nerves branching out from the brain and spinal cord to every part of the body. Sensory nerves in the PNS detect changes in your environment and internal organs, then send that information to the CNS as electrical signals traveling along incoming (afferent) pathways. The CNS processes this input, decides on a response, and sends commands back out through outgoing (efferent) pathways to muscles, glands, and organs.
This loop is constant and largely automatic. Sensory neurons in your skin, joints, and organs are always feeding data to the CNS about temperature, pressure, pain, body position, blood chemistry, and more. The CNS integrates all of this to maintain homeostasis, keeping your internal conditions stable even as the world around you changes.
Conditions That Affect the CNS
Because the CNS controls so many functions, damage or disease here tends to have widespread effects. Conditions that affect the CNS range from injuries like concussions and spinal cord trauma to degenerative diseases and autoimmune disorders.
Multiple sclerosis (MS) is one of the better-known CNS conditions. In MS, the immune system attacks the myelin insulation on nerve fibers in the brain and spinal cord, disrupting signal transmission. An estimated 2.8 million people worldwide live with MS, with prevalence varying sharply by region. Europe and the Americas have the highest rates (roughly 143 and 117 per 100,000 people, respectively), while South East Asia and the Western Pacific have the lowest. Symptoms depend on which nerve fibers are damaged and can include vision problems, numbness, muscle weakness, and difficulty with coordination.
Other common CNS conditions include Parkinson’s disease and Alzheimer’s disease, both of which involve progressive loss of neurons in specific brain regions. Stroke, caused by interrupted blood flow to the brain, kills neurons within minutes of onset. Epilepsy involves abnormal electrical activity in the brain that causes seizures. Spinal cord injuries can partially or completely sever the communication lines between the brain and the body below the injury site, resulting in loss of sensation or paralysis.