The nervous system has two main structural divisions: the central nervous system, made up of the brain and spinal cord, and the peripheral nervous system, a branching network of nerves that extends from the spinal cord to every other part of the body. Together, these divisions contain roughly 86 billion neurons and billions of supporting cells, all working to process information, coordinate movement, and regulate everything from your heartbeat to your digestion.
The Central Nervous System
The central nervous system (CNS) is the command center. It consists of just two structures, the brain and the spinal cord, but those two structures handle the vast majority of information processing in your body. Both are wrapped in three protective membranes called meninges: a tough outer layer (the dura mater), a web-like middle layer (the arachnoid mater), and a thin inner layer (the pia mater) that sits directly against the tissue.
The Brain
The brain itself has several major regions, each responsible for different functions. The largest is the cerebrum, the wrinkled, folded mass that makes up most of what you see when you picture a brain. It’s divided into four lobes on each side:
- Frontal lobes: thinking, planning, problem-solving, short-term memory, and movement
- Parietal lobes: processing touch, taste, texture, and temperature
- Occipital lobes: processing visual information and matching what you see to stored memories so you can recognize faces, objects, and places
- Temporal lobes: processing smell, taste, and sound, plus storing memories
Below and behind the cerebrum sits the cerebellum, a dense, wrinkled ball of tissue that combines sensory input from your eyes, ears, and muscles to coordinate smooth movement. The brainstem connects the brain to the spinal cord and controls functions essential to survival: heart rate, blood pressure, breathing, and sleep cycles.
The Spinal Cord
The spinal cord is a long column of nerve tissue running through the protective bones of your spine. If you sliced it in cross-section, you’d see two distinct types of tissue. The inner core is gray matter, packed with the cell bodies of neurons. It forms a butterfly or horn shape and handles two main jobs: its front portion processes motor signals that drive voluntary movement, while its rear portion receives sensory signals from your skin, bones, and joints. The outer layer is white matter, made up of insulated nerve fibers that carry signals up to the brain and back down again.
The spinal cord also manages reflexes on its own. When you pull your hand from a hot surface, that response is processed at the spinal cord level before the pain signal ever reaches your brain, shaving precious milliseconds off your reaction time.
The Peripheral Nervous System
Everything outside the brain and spinal cord belongs to the peripheral nervous system (PNS). This includes 12 pairs of cranial nerves that emerge directly from the brain (handling things like vision, facial movement, and hearing) and 31 pairs of spinal nerves that branch out from the spinal cord to reach the rest of the body. Each spinal nerve carries both sensory fibers, which send information inward from the body, and motor fibers, which send movement commands outward.
The peripheral nervous system splits into two functional branches: somatic and autonomic.
The Somatic Nervous System
The somatic nervous system handles voluntary movement. Every time you decide to pick up a cup, type on a keyboard, or take a step, you’re using it. It controls skeletal muscles, the ones attached to your bones. It also mediates reflex arcs, those rapid, involuntary responses like jerking your knee when a doctor taps it. So while the somatic system is primarily about conscious control, it does handle some automatic reactions too.
The Autonomic Nervous System
The autonomic nervous system runs processes you don’t consciously control: heart rate, digestion, blood pressure, pupil dilation, and more. It operates largely below awareness and divides into three branches.
The sympathetic branch triggers the “fight or flight” response. When you’re stressed or in danger, it increases your heart rate, raises blood pressure, releases stored energy into your bloodstream, opens your airways, and slows digestion. Blood vessels in your muscles dilate to deliver more oxygen where it’s needed.
The parasympathetic branch does roughly the opposite, promoting “rest and digest” functions. It slows the heart rate, stimulates saliva production, enhances digestion, and generally steers the body back toward a calm, energy-conserving state. The vagus nerve, the longest cranial nerve, is the main highway for parasympathetic signals. It runs from the brainstem to the abdomen and influences everything from heart rhythm to inflammation levels.
The enteric nervous system is sometimes called the “second brain.” It’s a mesh of neurons embedded in the walls of your digestive tract, and it primarily regulates digestion on its own. One layer controls the movement of water and electrolytes across the intestinal wall. Another layer coordinates the muscular contractions that push food along your gut. When a lump of food arrives at a section of intestine, neurons contract the muscle behind it and relax the muscle ahead of it, creating a wave that propels the food forward.
The Cells That Make It Work
Two broad categories of cells make up the nervous system: neurons and glial cells. Neurons are the signaling cells. Each one has a cell body, branching extensions called dendrites that receive incoming signals, and a long projection called an axon that transmits signals outward to the next cell. The human brain contains somewhere between 67 and 86 billion neurons. For decades, textbooks claimed the brain also held about one trillion glial cells, outnumbering neurons ten to one. More recent counting methods put the real number closer to 40 to 50 billion glial cells, meaning the ratio is roughly one to one.
Glial cells don’t transmit signals themselves, but the nervous system couldn’t function without them. Astrocytes, found only in the brain and spinal cord, maintain the chemical environment that neurons need to fire properly. Oligodendrocytes, also exclusive to the CNS, produce myelin, a fatty insulating material that wraps around axons in a tight spiral. In the peripheral nervous system, a different cell type called Schwann cells performs the same insulating job. Microglia act as the nervous system’s cleanup crew, removing debris from damaged or dying cells much like immune cells do elsewhere in the body.
How Nerve Signals Travel
Neurons communicate at junctions called synapses, and these come in two types. Chemical synapses are by far the more common. When an electrical signal reaches the end of one neuron’s axon, it triggers the release of chemical messengers (neurotransmitters) into the tiny gap between cells. The neighboring neuron detects those chemicals and generates a new electrical signal in response. This process introduces a slight delay, but it allows the signal to be amplified, modified, or blocked, giving the nervous system enormous flexibility.
Electrical synapses work differently. The two cells are physically connected by tiny channels that allow electrical current to pass directly from one to the other. This makes transmission essentially instantaneous, with no delay, but it also means the signal can’t be modified along the way. Electrical synapses are especially common in circuits where speed matters, like escape reflexes.
Why Myelin Matters
Many axons are wrapped in myelin, that fatty insulation produced by oligodendrocytes in the CNS and Schwann cells in the PNS. The myelin sheath doesn’t cover the axon continuously. It leaves small gaps called nodes of Ranvier at regular intervals, and these gaps are the only places where the electrical signal actively fires. The signal effectively jumps from gap to gap, a process called saltatory conduction, which is dramatically faster than signal transmission in bare, unmyelinated fibers. In myelinated axons, conduction speed increases in direct proportion to the axon’s diameter. In unmyelinated fibers, speed increases only with the square root of the diameter, so you’d need a much thicker axon to achieve the same speed without insulation. Myelin lets the nervous system be both fast and compact.