Neurons and Glia
- Published30 Nov 2022
- Author Diane A. Kelly
- Source BrainFacts/SfN
Every large system is held up by its smaller parts. The functional unit of neural circuits and networks is the neuron, a specialized cell that can transmit electrical signals to other nerve cells, muscles, or glands. Neurons come in a broad range of shapes and sizes, but all of them have a cell body, dendrites, and an axon. The cell body, also called the soma, contains the neuron’s nucleus and most of its cytoplasm, along with molecular machinery for building and transporting proteins critical to the cell’s function. Dendrites are branched projections that extend from the cell body and collect incoming signals from other neurons. The neuron’s electrical signals travel down its axon — another extension from the cell body that may branch before ending in axon terminals, where the signal is passed across a synapse to other cells. In some neurons, axons are only a fraction of a centimeter long; in others, they may extend more than a meter.
Neurons are associated with support cells called glia. Neuroscientists have long believed that glia outnumber neurons by 10:1 (or more). However, recent investigations suggest that in some regions of the brains of humans and other primates, that ratio is closer to 1:1. However, the ratio of glia to neuron from region to region varies considerably.
The central nervous system contains four main types of glial cells: astrocytes, microglia, ependymal cells, and oligodendrocytes. Astrocytes form a network inside the brain that regulates ion concentrations around neurons, provides them with nutrients, and helps regulate the formation of new connections between neurons. Microglia are the main “immune cells” of the brain. They function mainly as phagocytes — helping protect the brain from infections and cellular damage — but can also regulate the formation of new neuronal connections. Ependymal cells make the cerebrospinal fluid that cushions the brain inside the skull, and oligodendrocytes improve neuron function by wrapping axons in a fatty sheath called myelin.
Adapted from the 8th edition of Brain Facts by Diane A. Kelly.
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