The central nervous system (CNS) has two main components: the brain and the spinal cord. Together, they serve as the body’s command center, processing sensory information, generating thoughts and emotions, and coordinating every voluntary and involuntary action. These two organs are supported by specialized cells, protective membranes, and a circulating fluid that keeps them nourished and cushioned from harm.
The Brain: Three Major Divisions
The brain is the larger and more complex of the two CNS components. It contains roughly 86 billion neurons, though recent analyses suggest the true number could fall anywhere between 61 and 99 billion depending on the individual. The brain divides into three broad regions: the cerebrum, the cerebellum, and the brainstem. Each handles distinct tasks, but they work together constantly.
Cerebrum
The cerebrum is the largest part of the brain and the structure most people picture when they think of one. It splits into left and right hemispheres, each covered by the cerebral cortex, a folded outer layer of tissue where most higher-level processing happens. The cortex is organized into four lobes. The frontal lobe handles planning, judgment, problem-solving, and voluntary movement. The parietal lobe processes touch and spatial awareness. The temporal lobe manages hearing, language comprehension, and memory formation. The occipital lobe, at the back, is devoted to vision.
Deeper inside the cerebrum sit several important clusters of cells. The thalamus acts as a relay station, routing incoming sensory signals to the correct part of the cortex. The hypothalamus links the nervous system to the hormone system through the pituitary gland, and it regulates heart rate, blood pressure, appetite, thirst, body temperature, and sleep-wake cycles. A group of structures called the basal ganglia helps fine-tune movement, while the limbic system, which includes the hippocampus and amygdala, governs emotion, motivation, and the formation of new memories.
Cerebellum
Tucked beneath the back of the cerebrum, the cerebellum coordinates voluntary muscle movements, posture, and balance. It doesn’t initiate movement on its own but refines the signals the cerebrum sends so that actions come out smooth and accurate. Without it, reaching for a cup of coffee would be jerky and imprecise. Newer research also points to a role in thought, emotions, and social behavior.
Brainstem
The brainstem connects the cerebrum and cerebellum to the spinal cord. It contains three sections stacked vertically. The midbrain, at the top, helps process hearing and movement and calculates responses to environmental changes. The pons, in the middle, gives rise to several cranial nerves that control chewing, blinking, tear production, facial expression, and balance. The medulla, at the bottom, is essential to survival: it regulates heart rhythm, breathing, blood flow, and oxygen levels, and it produces reflexes like coughing, sneezing, swallowing, and vomiting.
The Spinal Cord
The spinal cord is a long, thin column of nervous tissue that runs from the base of the brainstem down to roughly the first or second lumbar vertebra in the lower back. It serves as the main information highway between the brain and the rest of the body, carrying motor commands downward and sensory signals upward.
The cord is divided into four regions, each giving rise to a set of spinal nerves that branch out to specific parts of the body. The cervical region (neck area) produces 8 nerve pairs and contains an enlarged section, spanning segments C5 through T1, dedicated to the arms and hands. Below that, the thoracic region produces 12 nerve pairs serving the trunk. The lumbar region produces 5 nerve pairs, and the sacral region produces another 5, with an enlargement spanning L2 through S3 that supplies the legs. A single coccygeal nerve pair rounds out the total of 31 pairs.
Grey Matter and White Matter
All CNS tissue falls into one of two types: grey matter and white matter. Grey matter gets its color from densely packed neuron cell bodies, the parts of nerve cells that contain the nucleus and do the actual computing. White matter is made up of long nerve fibers (axons) coated in a fatty insulation called myelin, which gives it a pale appearance and speeds up signal transmission.
Their arrangement flips between the brain and spinal cord. In the brain, grey matter forms the outer cortex while white matter fills the interior. In the spinal cord, the pattern reverses: grey matter sits in the center, forming a butterfly- or horn-shaped core, while white matter surrounds it on the outside. Scattered clusters of grey matter also exist deep within the brain, forming nuclei like the thalamus and basal ganglia.
Cells That Support Neurons
Neurons get most of the attention, but the CNS also depends on a class of non-neuronal cells called glia. These cells maintain the chemical environment neurons need to fire properly, modulate how fast signals travel, help clean up chemical messengers after they’ve done their job, and guide brain development.
Three types do most of the work. Astrocytes are the most abundant and maintain the right chemical balance around neurons so signals transmit cleanly. Oligodendrocytes wrap myelin insulation around axons in the CNS, dramatically increasing the speed of electrical impulses. Microglia are smaller immune cells that act as the brain’s cleanup crew, removing debris from injured or dying cells, much like immune cells in the rest of the body.
How CNS Signals Travel
Neurons communicate through two basic mechanisms. At chemical synapses, a sending neuron releases signaling molecules into a tiny gap between cells, and the receiving neuron picks them up, translating the chemical message back into an electrical impulse. This process introduces a small delay but allows the signal to be amplified or modified along the way.
At electrical synapses, two neurons are connected by direct channels called gap junctions. Electrical current flows passively through these pores almost instantaneously, with no chemical middleman. Electrical synapses can also transmit in both directions, unlike most chemical synapses, which send signals one way. The brain uses both types depending on how fast and how precisely a particular circuit needs to operate.
Protective Layers and Fluid
Because CNS tissue cannot regenerate easily, the body wraps it in multiple layers of protection. Three membranes called meninges encase both the brain and spinal cord. The outermost layer, the dura, is thick and tough. Beneath it sits the arachnoid, a web-like middle layer. The innermost layer, the pia, clings directly to the surface of the brain and spinal cord.
Between the arachnoid and pia layers flows cerebrospinal fluid (CSF). Adults carry about 150 milliliters of it at any given time, but the body produces 400 to 600 milliliters per day, constantly cycling out old fluid and replacing it with fresh. CSF cushions the brain and spinal cord against physical impact, delivers nutrients, and carries away metabolic waste. It circulates through a series of internal chambers called ventricles before flowing out into the space surrounding the brain and cord.
The Blood-Brain Barrier
The CNS has one more layer of defense that no other organ system shares. The blood-brain barrier is formed by the cells lining the brain’s blood vessels, which are sealed together by junctions roughly 50 to 100 times tighter than those in blood vessels elsewhere in the body. This means most substances dissolved in the blood simply cannot leak through into brain tissue.
Astrocytes play a supporting role here as well. Their extensions wrap around the outer surface of these blood vessels in a fine mesh, helping maintain the barrier’s tight seal. The result is a highly selective filter that lets oxygen and essential nutrients pass through while blocking toxins, pathogens, and fluctuations in blood chemistry that could disrupt neural function.