Glial cells are the support cells of the nervous system, but they do much more than hold neurons in place. This cheat sheet helps students compare the major glial cell types in the central nervous system and peripheral nervous system. It is useful for understanding brain function, nerve repair, myelination, and how neurons stay healthy.
Students in grades 10 to 12 need these comparisons because glial cells often appear together but have different locations and jobs.
The core idea is that neurons send signals, while glia protect, nourish, insulate, clean, and regulate the environment around neurons. CNS glia include astrocytes, oligodendrocytes, microglia, and ependymal cells. PNS glia include Schwann cells and satellite cells.
Myelin increases signal speed by insulating axons and allowing action potentials to jump between nodes of Ranvier.
Key Facts
- CNS glia are found in the brain and spinal cord, while PNS glia are found in nerves and ganglia outside the brain and spinal cord.
- Astrocytes support neurons, help form the blood-brain barrier, regulate ions and neurotransmitters, and guide repair after injury.
- Oligodendrocytes form myelin in the CNS, and one oligodendrocyte can myelinate segments of several different axons.
- Schwann cells form myelin in the PNS, and one Schwann cell myelinates one segment of one axon.
- Microglia act as immune cells in the CNS by detecting damage, removing debris, and responding to infection or inflammation.
- Ependymal cells line brain ventricles and the central canal of the spinal cord, and they help produce and circulate cerebrospinal fluid.
- Myelination rule: more myelin generally means faster nerve impulse conduction along an axon.
- Saltatory conduction means an action potential jumps from node of Ranvier to node of Ranvier, which increases conduction speed and saves energy.
Vocabulary
- Glial cell
- A non-neuron nervous system cell that supports, protects, nourishes, or insulates neurons.
- Astrocyte
- A star-shaped CNS glial cell that supports neurons, regulates the extracellular environment, and contributes to the blood-brain barrier.
- Oligodendrocyte
- A CNS glial cell that forms myelin around axons in the brain and spinal cord.
- Schwann cell
- A PNS glial cell that forms myelin around peripheral axons and helps damaged peripheral nerves regenerate.
- Microglia
- A CNS immune glial cell that removes pathogens, damaged cells, and cellular debris.
- Myelin sheath
- A fatty insulating layer around an axon that increases the speed of electrical signal transmission.
Common Mistakes to Avoid
- Saying glial cells are just filler cells is wrong because glia actively regulate neuron survival, signaling, immunity, and insulation.
- Confusing oligodendrocytes with Schwann cells is wrong because oligodendrocytes myelinate axons in the CNS, while Schwann cells myelinate axons in the PNS.
- Assuming one myelinating cell always covers an entire axon is wrong because myelin is divided into segments separated by nodes of Ranvier.
- Forgetting that microglia are immune cells is wrong because their main role is defense, cleanup, and response to injury in the CNS.
- Thinking myelin creates the nerve impulse is wrong because neurons generate action potentials, while myelin mainly speeds conduction and reduces signal loss.
Practice Questions
- 1 A neuron in the spinal cord has myelin on its axon. Which glial cell most likely made that myelin?
- 2 A peripheral nerve axon has 12 myelinated segments. If each segment is made by one Schwann cell, how many Schwann cells are needed?
- 3 One oligodendrocyte myelinates 4 axon segments. How many axon segments could 6 oligodendrocytes myelinate in total?
- 4 Explain why damage to myelin can slow communication in the nervous system even if the neuron itself is still alive.
Understanding Nervous System Glial Cells Reference
Neurons work only when the fluid around them stays within narrow limits. Astrocytes help control this local environment. When neurons fire repeatedly, potassium ions build up outside their membranes.
Too much potassium can make nearby neurons fire at the wrong time. Astrocytes absorb extra potassium and help return it to safer levels. They also take up neurotransmitters after a signal crosses a synapse.
This prevents a chemical signal from lasting too long. At blood vessels, astrocyte processes help control which substances move from blood into nervous tissue. This matters because many chemicals, germs, and sudden changes in blood composition could disrupt delicate neural circuits.
Myelin changes the electrical behavior of an axon. Without myelin, charged particles can leak across much of the axon membrane, so the signal must be rebuilt continuously. A myelin covering reduces this leakage.
The exposed gaps, called nodes of Ranvier, contain many sodium channels. The action potential is regenerated mainly at these gaps, rather than along every tiny part of the axon. This makes signaling faster and reduces the energy needed to restore ion balance afterward.
Myelin is not the only factor that affects speed. A wider axon usually carries impulses faster, and low temperature can slow ion movement. Damage to myelin can cause weakness, numbness, poor coordination, or vision problems because messages no longer reach their targets reliably.
Multiple sclerosis involves damage to CNS myelin. Guillain Barre syndrome can damage myelin in peripheral nerves.
Glial responses after injury show why the nervous system does not repair equally well everywhere. In a damaged peripheral nerve, Schwann cells clear parts of the injured axon and form a pathway that can guide regrowth toward the original target. Recovery is still difficult and depends on the size of the injury, but this pathway gives peripheral axons a better chance to reconnect.
In the CNS, repair is more limited. Microglia remove dead cells and detect infection, yet a strong or long lasting inflammatory response can harm healthy tissue. Other cells can form a scar around the injury.
The scar helps isolate damaged tissue, but it can block axon regrowth. Cerebrospinal fluid adds another layer of protection by cushioning the brain and spinal cord, carrying nutrients, and helping remove wastes.
When studying glial cells, focus first on location, cell shape, and the exact object each cell interacts with. Astrocytes contact neurons and blood vessels. Oligodendrocytes extend processes to several axons.
A Schwann cell wraps one section of one peripheral axon. Satellite cells surround neuron cell bodies in peripheral ganglia, where they help regulate the nearby chemical environment. Microglia are best remembered as the resident immune defenders of nervous tissue.
Ependymal cells are linked to fluid filled spaces. Students often mix up oligodendrocytes and Schwann cells because both make myelin. The reliable distinction is CNS versus PNS, followed by how many axon segments each cell can wrap.