In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another neuron or to the target effector cell.

Astrocytes, also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of endothelial cells that form the blood–brain barrier, provision of nutrients to the nervous tissue, maintenance of extracellular ion balance, regulation of cerebral blood flow, and a role in the repair and scarring process of the brain and spinal cord following infection and traumatic injuries. The proportion of astrocytes in the brain is not well defined; depending on the counting technique used, studies have found that the astrocyte proportion varies by region and ranges from 20% to around 40% of all glia. Another study reports that astrocytes are the most numerous cell type in the brain. Astrocytes are the major source of cholesterol in the central nervous system. Apolipoprotein E transports cholesterol from astrocytes to neurons and other glial cells, regulating cell signaling in the brain. Astrocytes in humans are more than twenty times larger than in rodent brains, and make contact with more than ten times the number of synapses.