These are small star-shaped cells that give off a number of processes (Fig. 11.11). The processes are often flattened into leaf-like laminae that may partly surround neurons and separate them from other neurons. The processes frequently end in expansions in relation to blood vessels or in relation to the surface of the brain. Small swellings called gliosomes are present on the processes of astrocytes. These swellings are rich in mitochondria.

Astrocytes are of two types: fibrous and protoplasmic.

- Fibrous astrocytes are seen mainly in white matter. Their processes are thin and are asymmetrical.

- Protoplasmic astrocytes are seen mainly in grey matter. Their processes are thicker than those of fibrous astrocytes and are symmetrical.

- Intermediate forms between fibrous and protoplasmic astrocytes are also present. Protoplasmic extensions of astrocytes surround nodes of Ranvier.

The processes of astrocytes are united to those of other astrocytes through gap junctions. Astrocytes communicate with one another through calcium channels. Such communication plays a role in regulation of synaptic activity and in the metabolisms of neurotransmitters and neuromodulators.

Function

- They provide mechanical support to neurons.

- In view of their nonconducting nature, they serve as insulators and prevent neuronal impulses from spreading in unwanted directions.