The nervous system has two main parts: the central nervous system (the brain and spinal cord) and the peripheral nervous system (every nerve that branches out from there to the rest of your body). Together, these divisions contain roughly 86 billion neurons in the brain alone, plus hundreds of millions more spread throughout your body. Here’s how each part is organized and what it does.
The Central Nervous System
The central nervous system, or CNS, is your body’s processing center. It consists of just two structures: the brain and the spinal cord. Both are wrapped in three protective layers of membrane called meninges and enclosed in bone (the skull and the vertebral column, respectively). Every decision your body makes, whether conscious or automatic, originates here.
The spinal cord acts as the main highway between the brain and the rest of the body. It’s organized into four regions, each sending out pairs of nerves to specific areas:
- Cervical (8 nerve pairs): starting at the neck, these run mostly to your face, head, neck, and upper limbs.
- Thoracic (12 nerve pairs): extending through your chest, upper back, and abdomen.
- Lumbar (5 nerve pairs): running from your lower back down to your legs and feet.
- Sacral (5 nerve pairs): branching from the base of the spine into the pelvis.
Major Regions of the Brain
The brain’s outer layer, the cerebral cortex, holds about 16 billion neurons and is divided into four lobes, each handling different tasks. The frontal lobe sits behind your forehead and plays a central role in speech, decision-making, and recognizing smells. Behind it, the parietal lobe helps you interpret touch and pain, understand spatial relationships (knowing where your body is relative to the objects around you), and comprehend spoken language. The temporal lobes, located on each side of the brain near your ears, house the hippocampus, a seahorse-shaped structure critical for memory. And the occipital lobe, at the very back of the skull, processes vision.
Beneath these lobes sit deeper structures like the brainstem, which controls breathing and heart rate, and the cerebellum, which coordinates balance and movement. These regions work together constantly, routing signals at high speed through dense networks of interconnected neurons.
The Peripheral Nervous System
Everything outside the brain and spinal cord belongs to the peripheral nervous system, or PNS. This network of nerves reaches every corner of your body, from your fingertips to your internal organs. It’s divided into two functional branches based on what kind of activity they control.
The somatic nervous system handles conscious, voluntary actions. It carries signals to your skin and skeletal muscles, letting you feel a tap on your shoulder, pick up a cup, or pull your hand away from a hot surface. If you’re choosing to do it and you’re aware of the sensation, the somatic system is involved.
The autonomic nervous system manages everything you don’t have to think about: heart rate, breathing, blood pressure, digestion, and body temperature. It runs automatically and continuously, keeping your internal environment stable. This system further splits into two branches that act as counterweights to each other.
Sympathetic vs. Parasympathetic Systems
The sympathetic division is your “fight or flight” system. When you sense danger or stress, it raises your heart rate, opens your airways, dilates your pupils, and redirects blood flow away from digestion and toward your muscles. It prepares your body to act fast.
The parasympathetic division does the opposite, often called “rest and digest.” It slows your heart rate, constricts your pupils, ramps up digestion, triggers saliva production, and relaxes the muscles involved in urination and bowel movements. It also manages aspects of sexual arousal. Many organs receive signals from both branches simultaneously, with one stimulating and the other inhibiting. This push-pull dynamic is what keeps your body in balance.
The Enteric Nervous System
Your digestive tract contains its own nervous system, sometimes called your “second brain.” The enteric nervous system lines the walls of your gastrointestinal tract and contains hundreds of millions of neurons, more than the entire spinal cord. It doesn’t just relay information to the brain. It independently coordinates the movement of food through your intestines, managing digestion on its own without needing instructions from above. This is why gut function continues even when communication with the brain is limited.
Neurons: The Basic Unit
Every part of the nervous system is built from neurons, the specialized cells that carry electrical and chemical signals. A typical neuron has three main components: dendrites (small branching fibers that receive incoming signals), a cell body containing the nucleus, and an axon (a single long fiber that transmits the signal outward to the next cell). Many axons are wrapped in a fatty coating called myelin, which insulates the nerve fiber the way rubber insulates a power cable. Myelin dramatically speeds up signal transmission, and its white color is why certain brain tissue is called “white matter.”
How Signals Travel Between Neurons
Neurons don’t physically touch each other. Between one neuron’s axon and the next neuron’s dendrites sits a tiny gap called a synapse. When an electrical signal reaches the end of an axon, it triggers the release of chemical messengers called neurotransmitters into that gap. These molecules drift across the synapse and lock onto receptors on the receiving neuron, like keys fitting into locks. Once attached, they pass the message forward, and the electrical signal continues down the next cell. Afterward, the neurotransmitters either break down in the synapse or get recycled back into the original neuron to be used again.
This chemical relay happens billions of times per second across your entire nervous system, enabling everything from a thought to a heartbeat.
Support Cells That Keep Neurons Working
Neurons get most of the attention, but roughly half the cells in your brain aren’t neurons at all. They’re glial cells, support cells that maintain the environment neurons need to function. There are several types, each with a distinct job.
Astrocytes are star-shaped cells that regulate the chemical environment around synapses. They control levels of neurotransmitters and important ions like potassium, provide energy to neurons, and can even influence how strongly neurons signal to each other. Microglia serve as the brain’s immune system. They patrol for injury or disease, clear away dead cells, and prune unnecessary connections between neurons during development and learning.
Oligodendrocytes are the cells that produce myelin in the central nervous system, wrapping layers of insulation around long-distance axons in the brain and spinal cord. In the peripheral nervous system, Schwann cells do the same job, coating nerve fibers outside the brain and spine. Damage to either type of cell can slow or block signal transmission, which is what happens in conditions like multiple sclerosis.