Cells of the Nervous System: Neurons and Glial Cells
Neuron structure, nerve and tract organization, and the five types of glial cells — explained for the MCAT.
The nervous system is built from two broad categories of cells: neurons, which generate and transmit signals, and glial cells, which support and maintain the environment neurons need to function. This page walks through the structure of a typical neuron, the terminology used to describe how neurons are organized, and the five main types of glial cells.
Key Takeaways
Neurons transmit electrical impulses and convert them into chemical signals; their key parts are the soma, dendrites, axon hillock, axon, myelin sheath, and nerve terminal.
The axon hillock acts as the threshold gatekeeper that decides whether an action potential fires.
Myelin — produced by oligodendrocytes (CNS) or Schwann cells (PNS) — speeds up signal transmission via saltatory conduction, jumping between the nodes of Ranvier.
In the PNS, bundled axons form nerves and their cell bodies cluster into ganglia; in the CNS, bundled axons form tracts and their cell bodies cluster into nuclei.
The five main glial cell types are astrocytes (blood-brain barrier, nutrient support), ependymal cells (cerebrospinal fluid production), microglia (immune defense), oligodendrocytes, and Schwann cells (myelin production).
Oligodendrocytes can myelinate multiple axons at once; Schwann cells myelinate only one axon each.
Neurons: The Signaling Units
A neuron is a specialized cell built to transmit electrical impulses and then translate those impulses into chemical signals — carrying information quickly and precisely throughout the body. Each part of a neuron plays a distinct role in receiving, processing, and passing along that signal.
Soma (Cell Body)
The soma, or cell body, is the central part of the neuron. It houses the nucleus and most of the cell’s organelles, and it’s responsible for maintaining the cell and processing incoming information — essentially, the neuron’s command center.
Dendrites
Dendrites are branch-like structures extending from the soma. They receive signals — usually in the form of neurotransmitters — from other neurons and carry those signals toward the soma, so information generally flows from the dendrites inward.
Axon Hillock
Once enough stimulation converges on the soma, it reaches the axon hillock, the region where the cell body transitions into the axon. This site acts as a gatekeeper: if the incoming electrical signal is strong enough to reach a certain threshold, the axon hillock initiates an action potential, the electrical signal that will travel down the neuron.
Axon and the Myelin Sheath
The axon is the long, slender projection that carries the action potential away from the soma toward its target — essentially the neuron’s transmission wire.
Surrounding many axons is the myelin sheath, a fatty insulating layer. Myelin isn’t part of the neuron itself — it’s produced by separate support cells. In the central nervous system (brain and spinal cord), myelin is produced by oligodendrocytes. In the peripheral nervous system, it’s produced by Schwann cells, which wrap themselves around the axon multiple times to build the sheath.
Nodes of Ranvier and Saltatory Conduction
Myelin insulates the axon and speeds up impulse travel. Instead of moving continuously down the axon, the signal “jumps” between exposed segments in a process called saltatory conduction. Those exposed segments are the nodes of Ranvier — at each node, ion channels open and regenerate the electrical signal, allowing it to travel rapidly down the axon.
Nerve Terminal
At the end of the axon is the nerve terminal, also called the axon terminal or synaptic bouton. This is where the electrical signal converts into a chemical message: when the action potential arrives, it triggers the release of neurotransmitters into the synaptic cleft, the tiny space between neurons. These chemical messengers then bind to receptors on the next cell — another neuron, a muscle cell, or a gland.
Signal flow through a neuron: dendrites receive the signal → the soma processes it → the axon hillock decides whether to fire → the axon carries the signal down the line → myelin speeds up transmission → the nerve terminal passes the message to the next cell.
Organizing Neurons: Nerves and Tracts
The peripheral and central nervous systems use different terms to describe how neurons are bundled together.
PNS vs. CNS organizational terms:
Peripheral nervous system: bundled axons form a nerve, which can carry sensory information, motor information, or both. Cell bodies of the same neuron type cluster together into ganglia.
Central nervous system: bundled axons form a tract, which — unlike a peripheral nerve — carries only one type of information, either sensory or motor, but not both. Cell bodies of neurons in the same tract are grouped into nuclei.
Glial Cells: Supporting the Nervous System
Neurons don’t work alone. Glial cells are support cells that don’t send signals themselves but instead create the right environment for neurons to function — nourishing them, providing physical support, maintaining chemical stability, and in some cases producing the myelin sheath.
Astrocytes
Astrocytes are star-shaped cells with several essential roles. They help induce and maintain the blood-brain barrier — a selective filter, formed primarily by tight junctions between the endothelial cells lining brain capillaries, that controls what substances pass from the bloodstream into brain tissue. Astrocytes also supply nutrients to neurons and help regulate the extracellular environment, especially ion and neurotransmitter concentrations.
Ependymal Cells
Ependymal cells line the ventricles of the brain — its fluid-filled cavities — and produce cerebrospinal fluid. This fluid cushions the brain, helps transport nutrients and waste, and acts as a shock absorber protecting the brain from injury.
Microglia
Microglia act as the immune defense of the central nervous system. These phagocytic cells engulf and break down waste products, cellular debris, and invading pathogens, keeping the CNS clean and protected.
Oligodendrocytes and Schwann Cells
Oligodendrocytes and Schwann cells are the two glial cell types responsible for producing myelin, but they work differently:
Feature | Oligodendrocytes | Schwann Cells |
|---|---|---|
Location | Central nervous system | Peripheral nervous system |
Axons myelinated per cell | Multiple — one oligodendrocyte extends processes to several axons | One — each Schwann cell wraps a single axon |
Wrapping pattern | Reaches out to multiple axons at once | Wraps around its one axon segment by segment along its length |
Common MCAT Mistakes
Reversing dendrite and axon signal direction. Dendrites carry signals toward the soma; the axon carries the signal away from the soma toward the nerve terminal — mixing these up flips the entire signal-flow diagram.
Confusing oligodendrocytes with Schwann cells. Both myelinate axons, but an oligodendrocyte (CNS) extends processes to multiple axons at once, while a Schwann cell (PNS) wraps only a single axon.
Mixing up nerve/ganglia (PNS) with tract/nuclei (CNS) terminology. A peripheral nerve can carry mixed sensory and motor information, but a central tract carries only one type — sensory or motor, not both.
Assuming all glial cells produce myelin. Only oligodendrocytes and Schwann cells do; astrocytes, ependymal cells, and microglia serve entirely different support roles (blood-brain barrier/nutrient support, cerebrospinal fluid production, and immune defense, respectively).
MCAT-Style Concept Check
Question: A single glial cell is observed wrapping its plasma membrane around one axon segment at a time, moving along the length of a single axon in the peripheral nervous system. Which cell type is this, and how does it differ from the glial cell responsible for myelination in the central nervous system?
A) Oligodendrocyte — it myelinates multiple axons simultaneously, unlike this cell
B) Schwann cell — it myelinates only one axon, unlike an oligodendrocyte, which extends processes to several axons at once
C) Astrocyte — it forms the blood-brain barrier rather than myelin
D) Microglia — it acts as the immune defense of the nervous system rather than producing myelin
Answer: B
Explanation: The description — one glial cell wrapping a single axon segment by segment, located in the peripheral nervous system — matches a Schwann cell. This distinguishes it from an oligodendrocyte, the CNS counterpart, which extends processes to myelinate multiple axons at once rather than committing to just one. Astrocytes and microglia are ruled out because neither produces myelin — astrocytes maintain the blood-brain barrier and regulate the extracellular environment, while microglia handle immune defense.
FAQ
What’s the difference between a neuron and a glial cell?
A neuron generates and transmits electrical and chemical signals. A glial cell doesn’t signal in that way — instead, it supports neurons by nourishing them, providing physical structure, maintaining chemical stability, or producing the myelin sheath.
What’s the difference between oligodendrocytes and Schwann cells?
Both produce myelin, but oligodendrocytes work in the central nervous system and can extend processes to myelinate multiple axons at once, while Schwann cells work in the peripheral nervous system and each wraps only a single axon.
What’s the difference between a nerve and a tract?
A nerve is a bundle of axons in the peripheral nervous system that can carry sensory information, motor information, or both. A tract is a bundle of axons in the central nervous system that carries only one type of information — sensory or motor, never both.
What is saltatory conduction and why does it matter?
Saltatory conduction is the way an action potential “jumps” between the exposed nodes of Ranvier along a myelinated axon instead of traveling continuously. Because myelin insulates the axon between nodes, this jumping process lets the signal travel much faster than it would along an unmyelinated axon.
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