The central nervous system (CNS) is the part of your nervous system that includes your brain and spinal cord. It serves as the body’s command center, receiving sensory information from the world around you, processing it, and sending out instructions that control movement, thought, and virtually every other function. Everything else in the nervous system, the nerves branching out to your limbs, organs, and skin, belongs to the peripheral nervous system, which acts as the CNS’s communication network with the rest of the body.
How the Brain Is Organized
The brain is the largest and most complex part of the CNS. It contains roughly 86 billion neurons, the specialized cells that transmit electrical signals, along with a nearly equal number of support cells. For decades, scientists believed the brain held about 100 billion neurons and ten times as many support cells, but modern counting methods have revised both numbers downward.
The brain has three major divisions, each handling different jobs:
- Cerebrum: The largest part, divided into left and right hemispheres. Its outer surface is folded into ridges and grooves that pack an enormous amount of tissue into a limited space. Different regions of the cerebrum handle distinct tasks. The frontal lobes contain a motor cortex that plans and executes voluntary movement, like reaching for a cup or kicking a ball. The parietal lobes process touch, temperature, and taste. The occipital lobes at the back handle vision, linking what your eyes see with images stored in memory. The temporal lobes at the sides receive sound from the ears and help integrate memories with sensory experiences.
- Cerebellum: Tucked beneath the cerebrum at the back of the skull, the cerebellum coordinates movement and manages learned physical skills. Playing piano, catching a ball, or keeping your balance while walking all depend on it.
- Brainstem: The lowest part of the brain, connecting directly to the spinal cord. It controls reflex actions, eye movements, and many automatic functions you never consciously think about, like breathing and heart rate. A structure called the thalamus, sitting deep inside the brain above the brainstem, acts as a relay station, sorting sensory information traveling between the spinal cord and the cerebrum.
Deep clusters of nerve cells called the basal ganglia surround the thalamus and play a key role in initiating and integrating movements. Damage to these clusters is involved in conditions like Parkinson’s disease, which is why movement becomes difficult when they malfunction.
What the Spinal Cord Does
The spinal cord is a tube of nerve tissue about the diameter of your index finger that runs from the base of the brainstem down through your spine, ending in a tapered point in your lower back. It has 31 pairs of nerves branching off from it, connecting the brain to the peripheral nervous system and, through it, to the rest of the body. Its three regions correspond to the three sections of the spine: cervical (neck), thoracic (upper back), and lumbar (lower back).
The spinal cord has two main jobs. First, it relays motor commands from the brain to the body and sends sensory information from the body back up to the brain. Second, it handles certain reflexes entirely on its own, without waiting for input from the brain. When a doctor taps just below your kneecap and your leg kicks forward, that response is managed by the spinal cord. This shortcut allows your body to react to danger faster than the brain could process a conscious decision.
How Signals Travel Through the CNS
Neurons communicate using a combination of electrical impulses and chemical messengers. When a neuron fires, an electrical signal races along its length. In neurons wrapped with myelin, a fatty insulating layer, signals travel remarkably fast, up to 120 meters per second (about 270 miles per hour). Neurons without this insulation conduct signals much more slowly, around 2 meters per second. That speed difference explains why some sensations, like a sharp pinprick, register almost instantly, while others, like a dull ache, take longer to reach your awareness.
When the electrical signal reaches the end of a neuron, it triggers the release of chemical messengers that cross the tiny gap to the next neuron. The two most important of these in the CNS work as opposites. One type excites the receiving neuron, making it more likely to fire. The other inhibits it, making it less likely to fire. The constant push and pull between excitation and inhibition across billions of connections creates the complex patterns of brain activity behind everything from a passing thought to coordinated movement.
Support Cells That Keep Neurons Working
Neurons get most of the attention, but the CNS also depends on several types of non-neuronal cells (collectively called glial cells) that maintain the environment neurons need to function.
- Astrocytes regulate the chemical environment around neurons, ensuring the right balance of ions and nutrients for proper signaling.
- Oligodendrocytes produce myelin, the insulating wrapping that speeds up electrical signals. When myelin breaks down, as it does in multiple sclerosis, signal transmission slows or fails.
- Microglia act as the CNS’s immune cells, clearing away damaged cells and debris from injury or normal wear and tear.
How the CNS Protects Itself
The brain and spinal cord are soft, delicate tissue, so the body surrounds them with multiple layers of protection. Bone comes first: the skull encases the brain, and the vertebral column shields the spinal cord. Beneath the bone, three membrane layers called meninges wrap the entire CNS. The outermost layer (dura mater) is thick and tough, providing a physical barrier against injury. The middle layer (arachnoid mater) contains a fluid-filled space, and the innermost layer (pia mater) clings directly to the surface of the brain and spinal cord tissue.
Cerebrospinal fluid fills the space between these membranes and the cavities inside the brain, cushioning the CNS against sudden impacts. It also helps carry away waste products. Beyond these physical barriers, a network of tightly packed cells lining the brain’s blood vessels, known as the blood-brain barrier, filters what enters the CNS from the bloodstream. This barrier lets essential nutrients through while blocking most bacteria, toxins, and large molecules. It’s also why many medications have difficulty reaching the brain.
How the CNS Forms Before Birth
The entire central nervous system develops from a single structure called the neural tube, which begins forming during the third week of pregnancy. A flat sheet of cells along the embryo’s back folds inward, starting in the middle and zipping closed in both directions, much like closing a jacket from the center outward. By the end of the fourth week, the tube is fully sealed. The top end goes on to become the brain, while the lower portion becomes the spinal cord.
This process is remarkably sensitive to disruption. Neural tube defects can occur between days 21 and 28 after conception, often before a person even knows they’re pregnant. Conditions like spina bifida (where the spinal portion doesn’t close completely) and anencephaly (where the brain portion fails to develop) result from errors during this brief window. Adequate folate intake before and during early pregnancy significantly reduces the risk of these defects.
What Happens When the CNS Is Damaged
Unlike many other tissues in the body, the CNS has very limited ability to repair itself. A broken bone can heal. A cut on your skin can close. But neurons in the brain and spinal cord generally do not regenerate after injury. This is why spinal cord injuries often cause permanent paralysis, and why strokes can leave lasting deficits in speech, movement, or memory depending on which brain region lost its blood supply.
The consequences of CNS damage depend entirely on location. Injury to the cervical spinal cord can affect all four limbs, while damage lower down may only affect the legs. Brain injuries vary just as widely: damage to the motor cortex impairs movement, damage to the occipital lobes affects vision, and damage to areas of the temporal lobe can disrupt language comprehension. This specificity is one reason brain imaging after a stroke or injury is so important. Knowing exactly where the damage occurred tells clinicians what functions are likely affected and what recovery may look like.