Neurons are the cells that carry signals, and glial cells far outnumber them and keep them working. Most mature neurons cannot divide, so lost neurons are rarely replaced.
Parts of a neuron
Dendrites receive signals and pass them to the cell body, which holds the nucleus. The axon carries the signal away, sometimes for more than a meter, to its terminals.
Myelin
Many axons are wrapped in myelin, a fatty insulating layer with small gaps called nodes of Ranvier. Myelin makes conduction much faster and saves energy.
Glial cells
Glial cells make myelin, control the chemical environment around neurons, clear debris and line the fluid spaces of the brain. Without them neurons cannot survive.
Repair
Peripheral nerves can regrow slowly, about 1 mm a day, if the sheath formed by Schwann cells stays intact. Damage in the brain and spinal cord heals poorly and often leaves a scar.
| Cell | Where | Main job |
|---|---|---|
| Astrocytes | CNS | Support neurons, help form the blood-brain barrier, balance ions |
| Oligodendrocytes | CNS | Make myelin for several axons each |
| Microglia | CNS | Immune cleanup of debris and microbes |
| Ependymal cells | CNS (ventricles) | Line the ventricles and help make CSF |
| Schwann cells | PNS | Make myelin for one axon segment and guide regrowth |
At the bedside
Loss of myelin explains multiple sclerosis in the CNS and Guillain-Barré syndrome in the PNS. Both slow or block conduction and show up on exam as weakness, numbness or changed reflexes.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).