Astrocytes, also called astroglia, are the most abundant cells in the human brain and spinal cord. They are one of the many types of neuroglial cells that surround neurons (nerve cells) in the central nervous system; other neuroglia include oligodendrocytes, microglia, satellite cells, and Schwann cells. Like all neuroglia, astrocytes provide structural and metabolic support for neurons and insulate their electrical activity. Yet astrocytes also perform a number of unique functions, such as the removal of excess glutamate and other neurotransmitters from the brain, the digestion of dead neurons, the transport of nutrients and other extracellular contents to neurons, the maintenance and control of synapses and neuronal circuits, and the regulation of blood flow. Astrocytes, which derive their name from their frequently star-shaped appearance, are much smaller than neurons, but far more numerous. Unlike neurons, they do not conduct electrical impulses (action potentials) and they do not participate directly in the propagation and integration of information. Studies have demonstrated that astrocytes can detect neuronal activity and modulate neuronal networks, thereby affecting brain function. Hence, the functioning of the brain, including thinking and emotions, requires the proper interaction and integration of the neurons and the neuroglial cells. Recent investigations have also indicated that astrocytes play an important role in neurogenesis, the formation and growth of nerves from neural stem cells. See also: Brain; Central nervous system; Nerve; Nervous system (vertebrate); Neurogenesis; Neuron; Spinal cord

Astrocytes (bright green) in the brain of a mouse. These typically star-shaped glial cells, which are found in the brain and spinal cord, outnumber neurons by more than fivefold. (Photo courtesy of the Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD)

Astrocytes also assist in the intrinsic defense system of the brain, limiting the amount of damage after a traumatic brain injury and helping with the recovery of neural function. This process is called astrogliosis and involves the proliferation of astrocytes to form a scar-like tissue that walls off the areas of injury. Conversely, the dysfunction of astrocytes often plays a role in the pathophysiology of neurodegenerative disorders. For example, astrocyte degeneration and atrophy have been noted in the early stages of conditions including Parkinson's disease, Alzheimer's disease and other forms of dementia, Huntington's disease, multiple sclerosis, and amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease). Astrocyte aberrations may also be involved in autism spectrum conditions and schizophrenia. Researchers are investigating the possibility of stimulating astrocytes to spur the transformation of stem cells into functional neurons, thereby potentially preventing or at least circumscribing the devastating effects of the aforementioned diseases. See also: Alzheimer's disease; Autism; Degenerative neural diseases; Dementia; Huntington's disease; Multiple sclerosis; Nervous system disorders; Parkinson's disease; Schizophrenia; Stem cells