The nervous system is built from two basic types of cells: neurons and glial cells. Neurons are the signaling cells that carry electrical impulses throughout your brain and body. Glial cells support, protect, and maintain the environment neurons need to function. The adult human brain contains roughly 86 billion neurons and about 85 billion glial cells, a near 1:1 ratio that debunks the old claim that glia outnumber neurons 10 to 1.
Neurons: The Signaling Cells
A neuron’s job is to receive information, process it, and pass it along. Every neuron has three main parts that make this possible. The cell body (also called the soma) houses the nucleus and the machinery that keeps the cell alive. Branching off the cell body are dendrites, tree-like extensions that pick up incoming signals from other neurons or from sensory receptors. Finally, a single long fiber called the axon carries the outgoing signal away from the cell body toward its target, which could be another neuron, a muscle, or a gland.
At the tip of the axon sits the axon terminal, where the electrical signal gets converted into a chemical one. The terminal releases chemical messengers called neurotransmitters into the tiny gap between cells, known as a synapse. Those messengers float across the gap and bind to the next cell’s dendrites, passing the message forward. This electrical-to-chemical-to-electrical relay is how your entire nervous system communicates.
How Neurons Fire
Neurons transmit signals through a rapid electrical event called an action potential. When a neuron receives a strong enough stimulus, sodium channels in its membrane snap open, letting positively charged sodium ions rush into the cell. This sudden influx flips the cell’s internal voltage from negative to positive in a chain reaction that travels down the axon. Milliseconds later, potassium channels open, letting potassium ions flow out and restoring the cell’s resting voltage. This whole process is all-or-nothing: once the threshold is reached, the neuron fires at full strength every time.
Three Functional Types of Neurons
Not all neurons do the same work. They fall into three broad categories based on the direction they carry signals.
- Sensory neurons carry information from your body toward the brain and spinal cord. They convert physical stimuli like touch, light, and sound into nerve impulses.
- Motor neurons carry instructions from the brain and spinal cord outward to muscles and glands, triggering movement or the release of hormones.
- Interneurons sit between sensory and motor neurons, mostly within the brain and spinal cord. They process and relay signals, forming the vast networks responsible for thought, memory, and reflexes.
Glial Cells: The Support Network
Glial cells were once thought to be simple filler tissue, but they play active roles in everything from immune defense to signal speed to brain development. The central nervous system (brain and spinal cord) has three major types of glia: astrocytes, oligodendrocytes, and microglia. The peripheral nervous system (nerves outside the brain and spinal cord) has its own versions, including Schwann cells, satellite cells, and others.
Astrocytes
Astrocytes are star-shaped cells found only in the brain and spinal cord. Their primary role is maintaining the chemical environment neurons need to send signals properly. They regulate ion concentrations around neurons, clean up excess neurotransmitters from synapses, and produce antioxidant compounds that protect brain tissue.
One of their most important jobs involves the blood-brain barrier, the tightly sealed layer of cells that prevents most substances in the bloodstream from entering the brain. Astrocytes extend specialized projections called endfeet that wrap around blood vessels and release growth factors essential for keeping the barrier intact. These same endfeet also help match blood flow to brain activity: when a region of your brain is working harder, nearby astrocytes signal blood vessels to deliver more oxygen and glucose to that area.
Astrocytes also play a role in synaptic pruning, the process of eliminating unnecessary connections between neurons. They produce a signaling molecule that causes neurons to tag weak or unused synapses, marking them for removal by microglia.
Oligodendrocytes and Schwann Cells
Both of these cell types produce myelin, a fatty wrapping that insulates axons the way rubber insulates a wire. Myelin dramatically speeds up electrical signals by forcing them to jump between gaps in the coating rather than travel continuously along the axon.
The key difference is location and efficiency. Oligodendrocytes work in the central nervous system, and a single oligodendrocyte can wrap segments of up to 60 different axons. Schwann cells handle myelination in the peripheral nervous system, but each one wraps just a single segment of a single axon. The gaps between myelin segments, called nodes of Ranvier, also differ: in peripheral nerves, Schwann cell processes cover these nodes, while in the brain and spinal cord the axon at each node is left bare.
Microglia
Microglia are the brain’s resident immune cells, making up roughly 10% of all cells in the central nervous system. Unlike other brain cells, they originate from the same stem cells that produce blood and immune cells elsewhere in the body. They act as the brain’s cleanup crew, engulfing dead cells, cellular debris, protein clumps, and invading microbes.
Beyond defense, microglia play a surprising role in brain development and learning. During early life, they physically eat dendritic spines (the small projections on dendrites that form synaptic connections) that aren’t receiving functional input. This synaptic pruning is essential for building efficient neural circuits. In the adult brain, microglia continue to shape synaptic plasticity by releasing molecules that strengthen or weaken connections between neurons depending on how active those connections are.
Ependymal Cells
Lining the fluid-filled cavities inside the brain and spinal cord are ependymal cells, a less well-known but important type of glia. These cells have tiny hair-like projections called cilia that beat in coordinated waves to keep cerebrospinal fluid flowing in the right direction. Cerebrospinal fluid cushions the brain, delivers nutrients, and carries away metabolic waste. Ependymal cells help regulate the production and composition of this fluid, and some subtypes act as chemical sensors that monitor what’s in it. Certain specialized ependymal cells called tanycytes can even generate new neurons and astrocytes in limited brain regions, making them a focus of interest in brain repair.
How Neurons and Glia Work Together
The nervous system doesn’t function as two separate populations of cells doing independent jobs. Neurons depend on astrocytes to keep their chemical surroundings stable, on oligodendrocytes or Schwann cells to insulate their axons for fast communication, and on microglia to prune and refine their connections. Glial cells in turn respond to neuronal activity, adjusting blood flow, recycling neurotransmitters, and modulating how quickly signals propagate. Every thought, sensation, and movement you experience is the product of neurons and glia working as an integrated system.