The central nervous system (CNS) is made up of two organs: the brain and the spinal cord. Together, they collect sensory information from your body, process it, and send out responses that control everything from breathing to complex thought. Every movement you make, every sensation you feel, and every organ function in your body runs through this two-part command center.
The Brain: Three Regions, One System
The brain divides into three basic units: the forebrain, the midbrain, and the hindbrain. Each handles different jobs, but they work as an integrated system.
The forebrain is the largest and most developed part of the human brain. It consists primarily of the cerebrum, the wrinkled outer structure you picture when you think of a brain. This is where your thoughts, personality, language, and voluntary decisions originate. Tucked beneath the cerebrum are deeper structures that relay sensory information and regulate hormones, hunger, sleep, and emotional responses.
The midbrain sits at the uppermost part of the brainstem. It controls certain reflex actions and plays a role in eye movements and other voluntary movements. The hindbrain includes the lower brainstem and a dense, textured structure called the cerebellum. The hindbrain controls vital functions like heart rate and breathing. The cerebellum coordinates movement, especially learned physical skills like walking, typing, or catching a ball.
The Spinal Cord: The Body’s Information Highway
The spinal cord is a long column of nerve tissue that runs from the base of the brainstem down through your vertebral column. It has 31 segments, each defined by a pair of nerves that branch out to specific parts of your body. These segments break down into 8 cervical (neck), 12 thoracic (mid-back), 5 lumbar (lower back), 5 sacral (pelvic), and 1 coccygeal (tailbone) nerve pair.
When your brain makes a decision, such as telling your hand to pull away from a hot surface, it sends an electrical signal down through the spinal cord to the appropriate muscles. Sensory information travels the reverse path: nerves in your skin or organs send signals up the spinal cord to the brain for processing. Some reflexes, like jerking your knee when a doctor taps it, are handled at the spinal cord level without waiting for the brain to weigh in.
Gray Matter and White Matter
If you sliced into the brain or spinal cord, you’d see two distinct tissue types. Gray matter sits on the surface of the brain and contains the cell bodies of neurons, the cells that generate and process signals. This is where the actual computing happens. White matter lies in the deeper layers beneath it and is made of long nerve fibers (axons) that connect different regions. Many of these fibers are wrapped in a fatty coating called myelin, which gives white matter its pale color and speeds up signal transmission.
In the spinal cord, this arrangement flips. Gray matter forms a butterfly-shaped core in the center, while white matter surrounds it on the outside. The white matter tracts in the spinal cord are essentially bundled cables carrying signals between the brain and the rest of the body.
Neurons and Supporting Cells
Two broad categories of cells make up the CNS: neurons and glial cells. Neurons are the signaling cells. They generate electrical impulses and pass information to each other across tiny gaps called synapses. When a neuron fires, it sends a brief electrical pulse (lasting about one millisecond) down its axon. At the synapse, that electrical signal converts into a chemical one: the neuron releases molecules called neurotransmitters into a gap roughly 20 to 40 nanometers wide. These molecules land on the next neuron and either encourage it to fire or suppress it from firing, converting the chemical signal back into an electrical one. This rapid electrical-to-chemical-to-electrical chain is how every thought, sensation, and command travels through your CNS.
Glial cells outnumber neurons in many brain regions and handle the support work. There are three major types. Astrocytes help regulate the chemical environment around synapses, supply energy to neurons, and synchronize neural networks. Oligodendrocytes wrap myelin around axons, which dramatically increases the speed of electrical signals. Microglia act as the brain’s immune system, clearing out dead cells and debris and responding to infection or injury. The ratio of glial cells to neurons isn’t uniform across the brain. It varies by region and actually increases with brain size, meaning larger-brained mammals tend to have more support cells per neuron.
How the CNS Protects Itself
The brain and spinal cord are soft, delicate tissue. They sit inside bone (the skull and vertebral column), but that’s only the first layer of protection. Beneath the bone are three membranes called the meninges, which wrap around the entire CNS. Between two of these membrane layers is a space filled with cerebrospinal fluid (CSF).
Your body produces 400 to 600 milliliters of CSF every day, though only about 150 milliliters circulates at any given time in an adult. This fluid acts as a shock absorber, cushioning the brain and spinal cord against sudden impacts. It also delivers nutrients, removes waste products, and helps maintain stable pressure inside the skull.
Beyond the physical barriers, the CNS has a chemical gatekeeper: the blood-brain barrier. The blood vessels inside your brain are lined with endothelial cells packed so tightly together that almost nothing can slip through the gaps. This lipid-based membrane selectively filters what enters brain tissue from the bloodstream, blocking toxins and pathogens while letting in oxygen, glucose, and other essentials. This is why many medications that work elsewhere in the body can’t easily reach the brain.
How the Two Parts Work Together
The brain and spinal cord aren’t independent systems that happen to be connected. They function as a single processing unit with a clear division of labor. The brain handles interpretation, decision-making, memory, emotion, and complex coordination. The spinal cord serves as both a relay cable and a local processing center for reflexes. Sensory neurons throughout your body feed information into the spinal cord, which routes it up to the brain. The brain integrates that data with everything else it knows, then sends motor commands back down the spinal cord to your muscles and glands.
This loop, from sensation to processing to response, is the core function of the CNS. It runs continuously whether you’re solving a math problem, digesting lunch, or sleeping. Damage to either component disrupts the loop. A spinal cord injury can cut off communication between the brain and everything below the injury site, even though both the brain and the lower body remain functional on their own. A brain injury can leave the spinal cord intact but impair the processing that gives those signals meaning.