The central nervous system (CNS) consists of two main structures: the brain and the spinal cord. Together, they serve as the body’s command center, processing every sensation you feel, every thought you have, and every movement you make. Everything else, including the nerves that branch out to your arms, legs, and organs, belongs to the peripheral nervous system. Understanding the parts of the CNS starts with the brain’s major divisions and works down through the spinal cord, along with the protective layers and specialized cells that keep everything running.
The Cerebrum: The Largest Part of the Brain
The cerebrum is the massive, wrinkled structure that makes up roughly 85% of the brain’s total weight. It’s divided into two halves, called hemispheres, and its outer surface is a thin layer of gray matter known as the cerebral cortex. This is where your most complex thinking happens: language, creativity, decision-making, and conscious awareness all originate here.
Each hemisphere is divided into four lobes, each with distinct responsibilities:
- Frontal lobe: The largest lobe, sitting behind your forehead. It handles planning, reasoning, emotional regulation, personality, and voluntary movement. It also contains the brain’s primary speech production area.
- Parietal lobe: Located at the top-middle of the brain. It processes touch, temperature, pressure, and pain, and helps you understand spatial relationships, like where your body is relative to objects around you.
- Temporal lobe: Found on the sides of the brain, near your ears. It’s critical for hearing, language comprehension, and memory. Deep inside the temporal lobe sits the hippocampus, a structure essential for forming new memories and learning.
- Occipital lobe: At the very back of the brain. This is the primary visual processing center, where raw information from your eyes gets interpreted into the shapes, colors, distances, and faces you recognize.
Deep Brain Structures
Buried beneath the cerebral cortex are several smaller structures that play outsized roles in daily life. The thalamus acts as a relay station, routing incoming sensory signals to the correct part of the cortex for processing. Almost every sensation you experience, except smell, passes through the thalamus before you become aware of it.
Just below it, the hypothalamus regulates drives you rarely think about consciously: hunger, thirst, body temperature, and mood. It also triggers the release of hormones that control growth, metabolism, and stress responses. Nearby, the amygdala is the brain’s emotional alarm system. It attaches emotional weight to your memories, which is why frightening or deeply meaningful experiences tend to stick with you more vividly than neutral ones. The amygdala is especially active in processing fear, anxiety, and anger.
The Cerebellum
Tucked beneath the back of the cerebrum, the cerebellum looks like a smaller, tightly folded version of the brain. It contains more neurons than the rest of the brain combined, and its primary job is coordination. Every time you catch a ball, walk in a straight line, or type without looking at the keyboard, your cerebellum is fine-tuning those movements in real time. It also plays a role in balance and motor learning, which is why damage to this area often causes unsteady, clumsy movement rather than paralysis.
The Brainstem
The brainstem connects the brain to the spinal cord and controls the automatic functions that keep you alive without any conscious effort. It has three sections, stacked from top to bottom:
- Midbrain: The uppermost section, involved in motor control, particularly eye movements, and in processing visual and auditory information.
- Pons: The middle section, which coordinates facial movements, facial sensation, hearing, and balance.
- Medulla oblongata: The lowest section, sitting right where the brain transitions into the spinal cord. It regulates breathing, heart rate, blood pressure, and swallowing. Because these functions are essential for survival, injuries to the medulla are often life-threatening.
The Spinal Cord
The spinal cord is a long, thin bundle of nervous tissue that runs from the base of the brainstem down through the protective canal inside your vertebral column. It extends about 18 inches in adults and serves two critical purposes: carrying signals between the brain and the rest of the body, and coordinating certain reflexes on its own without waiting for brain input (like pulling your hand off a hot surface).
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 are grouped into five regions: 8 cervical (neck), 12 thoracic (mid-back), 5 lumbar (lower back), 5 sacral (pelvis), and 1 coccygeal (tailbone). The level at which a spinal cord injury occurs determines which parts of the body lose sensation or movement, which is why doctors pay close attention to the exact segment affected.
One interesting structural difference between the brain and spinal cord involves the arrangement of gray matter and white matter. In the brain, gray matter (where the cell bodies of neurons are concentrated) forms the outer layer, with white matter underneath. In the spinal cord, that pattern is reversed: gray matter sits in the center, surrounded by white matter on the outside.
Protective Layers and Fluid
The brain and spinal cord are too vital and too delicate to sit unprotected inside bone. Three membranes called the meninges wrap around them in layers, providing cushioning and structural support. When these membranes become infected or inflamed, the result is meningitis.
Between the meningeal layers, cerebrospinal fluid (CSF) circulates continuously. The body maintains about 150 milliliters of this clear fluid at any given time. CSF cushions the brain against impacts, essentially allowing it to float so that sudden head movements don’t slam brain tissue against the skull. Beyond physical protection, CSF also helps maintain the chemical environment neurons need to function properly and plays a role in clearing waste products from the brain.
An additional layer of defense comes from the blood-brain barrier, a wall of tightly packed cells lining the blood vessels inside the brain. These cells are locked together so closely that bacteria, viruses, and most toxins in the bloodstream cannot slip through into brain tissue. This barrier is highly selective, letting in essential nutrients like oxygen and glucose while blocking nearly everything else. It’s one reason why treating brain infections or brain tumors can be so challenging: many medications also struggle to cross this barrier.
The Cells That Make It Work
The CNS contains two broad categories of cells: neurons, which transmit electrical signals, and glial cells, which support and protect neurons. Glial cells actually outnumber neurons and come in several specialized types.
Astrocytes are star-shaped cells that maintain the environment around neurons. They regulate the levels of chemical messengers at the junctions between nerve cells, control concentrations of important ions, and provide metabolic fuel. They were once thought to be passive scaffolding, but they actively influence how neurons communicate.
Oligodendrocytes produce myelin, a fatty insulating material that wraps around the long extensions of neurons called axons. This insulation allows electrical signals to travel much faster, which is why nerve impulses can cross the length of your spinal cord in milliseconds. When myelin breaks down, as it does in multiple sclerosis, signals slow or stop entirely.
Microglia serve as the brain’s immune cells. They patrol for signs of injury or infection, clear away dead cells and debris, and even prune unnecessary connections between neurons during development. Ependymal cells line the fluid-filled spaces inside the brain and spinal cord and are involved in producing cerebrospinal fluid, keeping that protective cushion constantly refreshed.