The nervous system is made up of two main types of cells, neurons and glia, organized into two major divisions: the central nervous system (your brain and spinal cord) and the peripheral nervous system (every nerve that branches out from there into your body). Together, these structures form a communication network that lets you think, move, feel, and keep your organs running without conscious effort.
The Two Major Divisions
The central nervous system, or CNS, consists of the brain and spinal cord. These are the command centers where information is processed, decisions are made, and responses are coordinated. The peripheral nervous system, or PNS, is everything else: the nerves that branch out from the brain and spinal cord to reach every part of your body, from your fingertips to your internal organs.
Twelve pairs of cranial nerves emerge from the underside of the brain. These handle things like vision, hearing, facial movement, and the signals that regulate your heart rate and digestion. Another 31 pairs of spinal nerves branch out from the spinal cord, carrying sensory information in and motor commands out. Each spinal nerve connects to the cord through two roots: one that carries incoming sensory signals and one that carries outgoing movement commands.
The peripheral nervous system splits further into two functional branches. The somatic nervous system handles voluntary movement and sensory input like touch, sound, smell, and taste. The autonomic nervous system runs the processes you don’t consciously control, including heart rate, digestion, and breathing. Four of the 12 cranial nerves and most of the 31 spinal nerves carry autonomic fibers. Scattered along these pathways are ganglia, which are clusters of nerve cell bodies that act as relay stations.
Neurons: The Signaling Cells
Neurons are the cells that carry electrical and chemical signals throughout the nervous system. Each one has the same basic architecture, built for receiving, processing, and transmitting information.
The cell body is the neuron’s metabolic center. It produces energy and manufactures the proteins the cell needs to function. It also receives incoming signals from other neurons. Branching out from the cell body are dendrites, tree-like extensions covered in receptors that pick up chemical messages from neighboring cells. Dendrites often have tiny projections called spines that massively increase the surface area available for these connections.
Each neuron has a single axon, a long fiber that carries the outgoing electrical signal. At the base of the axon, a cone-shaped region called the axon hillock tapers into the initial segment, which is where the electrical impulse typically fires. The axon can be very short or stretch more than a meter in the case of nerves running from your spinal cord to your feet.
Many axons are wrapped in myelin, a fatty insulating layer produced by supporting cells. Myelin dramatically speeds up signal transmission. Myelinated axons carrying touch and body-position information send signals at 80 to 120 meters per second. Smaller, unmyelinated fibers, like those carrying pain signals, transmit at roughly 0.5 to 2 meters per second. That speed difference is why you feel a bump before you feel the sting.
At the end of the axon is the synapse, the junction where one neuron communicates with the next. When an electrical impulse reaches the synapse, it triggers calcium to flow into the nerve ending. That calcium causes tiny sacs called vesicles to release chemical messengers (neurotransmitters) into the gap between cells. These chemicals cross the gap, bind to receptors on the receiving cell, and either excite or inhibit the next signal. A single vesicle contains enough neurotransmitter to activate about 1,000 receptor channels on the other side.
Glia: The Support Cells
Glia were once thought to be passive scaffolding, but they perform essential jobs that neurons can’t do alone. Different types of glia serve different roles depending on whether they’re in the central or peripheral nervous system.
Astrocytes are star-shaped cells that maintain the working environment around neurons in the brain and spinal cord. They regulate neurotransmitter levels at synapses, control the concentration of ions like potassium, and supply metabolic fuel. Because they can sense neurotransmitter activity and release signaling molecules of their own, astrocytes actively influence how synapses behave.
Oligodendrocytes produce the myelin that insulates axons in the central nervous system. A single oligodendrocyte can wrap segments of multiple axons. In the peripheral nervous system, Schwann cells do the same job, but each Schwann cell wraps only one segment of one axon.
Microglia function as the brain’s immune cells. They patrol for injury and disease, clear away dead cells, and even participate in “pruning” unnecessary synapses during development by physically consuming connections tagged for removal.
Ependymal cells line the hollow spaces inside the brain and spinal cord, where they help produce cerebrospinal fluid. Satellite cells surround neuron cell bodies in peripheral ganglia and regulate the chemical environment there. Enteric glial cells support the network of nerves embedded in your digestive tract, sometimes called the “second brain.”
Gray Matter and White Matter
If you were to slice into the brain or spinal cord, you’d see two distinct tissue types. Gray matter contains neuron cell bodies, dendrites, and short axon segments. It’s where thinking, memory, decision-making, and sensory processing happen. White matter is made up of long, myelinated axons bundled together. The myelin gives it its pale color. White matter carries messages between different brain regions and between the brain and the rest of the body.
In the brain, gray matter forms the outer cortex and several deeper clusters. In the spinal cord, the arrangement flips: gray matter sits in the center (shaped roughly like a butterfly in cross-section), surrounded by white matter tracts running up and down.
Protective Layers
The brain and spinal cord are fragile, so the nervous system includes several layers of physical protection. Three membranes called meninges wrap the entire central nervous system.
- Dura mater: The outermost layer, a tough, thick membrane made of two layers of connective tissue that sits just inside the skull and vertebral column.
- Arachnoid mater: The middle layer, a thin membrane with a spiderweb-like structure. It contains no blood vessels or nerves of its own and is connected to the innermost layer by delicate projections.
- Pia mater: The innermost layer, a thin membrane that clings tightly to every contour of the brain and spinal cord like shrink wrap.
Between the arachnoid and pia layers, cerebrospinal fluid circulates, cushioning the brain against impact and helping remove waste.
Peripheral nerves have their own protective wrapping. Three concentric layers of connective tissue surround nerve fibers: an outer sheath that bundles the whole nerve, a middle layer with tight junctions that forms a barrier around each bundle of fibers, and an inner layer packed with Schwann cells and collagen that surrounds individual axons. About 80 to 90 percent of the cells in that inner layer are Schwann cells.
How It All Works Together
Every sensation, movement, and unconscious body function depends on these components working in concert. Sensory neurons in the peripheral nervous system detect a stimulus, like heat on your hand, and send electrical signals along their axons toward the spinal cord. There, the signal may trigger an immediate reflex through local circuits in the gray matter, pulling your hand away before you even register pain. Simultaneously, the signal travels up white matter tracts to the brain, where networks of neurons in the gray matter process the experience and store the memory.
Glia support every step: astrocytes fine-tune the chemical environment at each synapse, oligodendrocytes and Schwann cells keep signals moving quickly through myelination, and microglia stand ready to respond if any of these cells are damaged. The meninges and cerebrospinal fluid protect the whole system from mechanical shock, while the layered connective tissue sheaths protect peripheral nerves as they pass through muscles, joints, and other tissues that move and flex throughout the day.