Cells of the nervous system that support the neurons. There are four classes of glial cells: astrocytes are star-shaped cells that contribute to the blood-brain barrier by surrounding the smallest blood vessels in the brain and modulating the endothelial cells of the vessels. This barrier restricts the passage of various potentially noxious substances from the blood to neurons in the central nervous system. Each astrocyte makes contact with numerous synapses, typically over 100 000, and modulates their activity; the astrocytes can themselves take up and release neurotransmitter and thereby regulate communication between neurons. They have stores of glycogen with which they supply energy to the neurons, and they also take part in the repair and regeneration of neurons. Oligodendrocytes wrap around the axons of neurons in the brain and spinal cord, covering them with layers of insulating plasma membrane, similar to the role of Schwann cells, their glia counterparts in the peripheral nervous system. This prevents impulses from ‘short-circuiting’ between adjacent neurons. Ependymal cells line the ventricles of the brain and central canal of the spinal cord, where the beating of their hairlike cilia helps circulate the cerebrospinal fluid through these cavities. They also give rise to the epithelial layer surrounding the choroid plexus. Microglia are the main immune defence cells of the nervous system, acting as macrophages and able to mount inflammatory responses.