The nervous system is made of two main types of cells: neurons, which carry electrical signals, and glial cells, which support and protect them. These cells are organized into the brain, spinal cord, and a branching network of nerves that reach every part of your body. Beyond the cells themselves, the system includes protective membranes, cushioning fluid, connective tissue, and a rich chemical soup of signaling molecules that keep everything running.
Neurons: The Signaling Cells
Neurons are the cells that actually transmit information. Each one has three basic parts. Dendrites are branch-like extensions that receive incoming signals from other neurons. The cell body (sometimes called the soma) houses the neuron’s DNA and manufactures the proteins the cell needs. And the axon is a long, thin fiber that carries outgoing electrical signals, sometimes over surprisingly long distances.
When a neuron “fires,” it generates a brief electrical pulse called an action potential that travels the length of its axon. When that pulse reaches the end, it triggers the release of chemical messengers into the tiny gap between neurons. The next neuron’s dendrites pick up those chemicals, and the cycle continues. This is how every thought, sensation, and movement gets relayed through your body.
Dendrites aren’t smooth. They’re covered in tiny protrusions called spines, each one a contact point where another neuron’s axon can deliver a signal. A single neuron can have thousands of these spines, which means it’s constantly integrating input from many sources at once before deciding whether to fire.
Glial Cells: The Support Network
For a long time, glial cells were considered little more than packing material. That view has changed dramatically. There are several types, each with a distinct job.
Astrocytes are star-shaped cells that maintain the chemical environment neurons need to function. They regulate the concentration of key ions like potassium around synapses, clean up leftover signaling chemicals, and supply neurons with metabolic fuel. Scientists now recognize that astrocytes also actively influence how signals pass between neurons, making them far more than passive caretakers.
Oligodendrocytes produce myelin, a fatty insulating layer that wraps around axons in the brain and spinal cord. Myelin dramatically speeds up electrical transmission and gives white matter its characteristic color. In the peripheral nervous system (the nerves outside your brain and spinal cord), a different cell type called a Schwann cell does the same job. Myelin is roughly 80% fat and 20% protein, which is why it functions so well as insulation.
Microglia are the nervous system’s immune cells. They patrol for damage, infection, or dying cells and mount a response when something goes wrong. They also play a role in development and maintenance: microglia physically eat up synapses that the brain has tagged as unnecessary, a pruning process that helps refine neural circuits throughout life.
The Two Major Divisions
Structurally, the nervous system splits into two parts. The central nervous system (CNS) is the brain and spinal cord. The peripheral nervous system (PNS) is every nerve that branches off the spinal cord and extends to the rest of your body, reaching your organs, muscles, and skin.
Inside the brain, tissue is organized into grey matter and white matter. Grey matter forms a thin layer on the brain’s surface and consists mostly of neuron cell bodies, dendrites, and synapses. This is where the bulk of processing happens. White matter sits deeper and is made up of long axon bundles coated in myelin. These bundles are the brain’s internal wiring, connecting distant regions so they can communicate quickly.
How Peripheral Nerves Are Built
A single peripheral nerve isn’t just a lone axon. It’s a carefully layered cable. Individual axons are wrapped in a thin layer of connective tissue called the endoneurium. Groups of axons are bundled together into fascicles, each surrounded by another connective tissue layer called the perineurium. Finally, the entire nerve is enclosed in a tough outer sheath called the epineurium. This layered design protects the delicate nerve fibers from compression and stretching as you move.
Protective Layers Around the Brain and Spinal Cord
The brain and spinal cord are wrapped in three membranes collectively called the meninges. The outermost layer, the dura mater, is thick and tough, sitting just inside the skull. The middle layer, the arachnoid mater, is thinner and web-like. The innermost layer, the pia mater, clings directly to the surface of the brain tissue.
Between the arachnoid and pia layers is a space filled with cerebrospinal fluid. This clear liquid cushions the brain against impact, anchors it in place so it doesn’t shift around inside the skull, and helps carry away waste products. The meninges also provide a scaffolding for blood vessels that supply the brain with oxygen and nutrients.
The Chemical Ingredients
The nervous system depends on a large toolkit of chemical messengers called neurotransmitters. Scientists have identified at least 100, and likely many more remain undiscovered. These chemicals fall into a few broad categories.
The most common excitatory neurotransmitter is glutamate. It’s involved in most nervous system functions and essentially tells the next neuron to fire. On the other side, GABA is the most common inhibitory neurotransmitter in the brain, telling neurons to quiet down. In the spinal cord, glycine takes over that inhibitory role. The balance between excitation and inhibition is what keeps your nervous system stable rather than chaotic.
A separate group of signaling chemicals regulates mood, attention, and consciousness. Serotonin influences mood and sleep. Dopamine plays a central role in reward and motivation. Adrenaline and noradrenaline drive your fight-or-flight response, raising heart rate and blood pressure when you’re under stress. Endorphins act as the body’s built-in painkillers. And acetylcholine operates in both the central and peripheral nervous systems, playing roles in muscle contraction, memory, and attention.
Beyond neurotransmitters, the system relies on electrically charged particles (ions) like sodium, potassium, and calcium to generate and propagate the electrical signals that neurons carry. Every action potential is ultimately powered by these ions flowing in and out of nerve cells through specialized channels in the cell membrane.
Putting It All Together
The nervous system is not a single material but a composite: billions of neurons wired together by trillions of synaptic connections, supported by glial cells that outnumber them, insulated by fatty myelin, cushioned by cerebrospinal fluid, wrapped in layered membranes, and powered by a constantly shifting mix of electrical charges and chemical signals. Each component is specialized, but none works in isolation. The system’s complexity comes not from any one ingredient but from how all of them interact.