Maya BluittThroughout much of neuroscience’s history, neurons have been the star of the show. These are the cells which send electrical signals to communicate with one another, giving rise to the many complex abilities of the brain. However, astrocytes have gained more and more attention over the past few decades – and rightfully so. While originally thought to simply support neurons as they carry out the heavy lifting, astrocytes perform a broad range of functions that are fundamental to the brain.
Astrocytes are the most abundant cell type in the central nervous system (CNS), which includes the brain and the spinal cord. They are a subtype of glial cells, a type of cell that plays various roles to support neurons (see Figure 1). Astrocytes are found throughout the entire CNS and are organized in a spread out, non-overlapping manner. This organization pattern contributed to their presumed role of providing a structural framework for neurons.
Early studies into their function demonstrated that astrocytes are much more than their structure. One of the notable capabilities of astrocytes is regulating blood flow in the brain, particularly in response to changes in neuronal activity. They can produce and release molecules like prostaglandins, nitric oxide, and arachidonic acid, which modulate the diameter of blood vessels in the CNS in a coordinated manner. Astrocytes also play major roles in energy supply and protecting the brain by contributing to the formation of the blood-brain barrier.
In the late 20th century, scientists began to find evidence that astrocytes contribute to neuronal transmission themselves – and in a myriad of ways. For example, parts of astrocytes near synapses express high levels of transporters for the most abundant types of neurotransmitters in the CNS, including glutamate, GABA, and glycine. Via these transporters, astrocytes are able to remove leftover neurotransmitters from the synapse and avoid overstimulation of neurons.
While neurons can fire action potentials down their cell bodies to transmit information to other neurons, astrocytes cannot. They do, however, use chemicals like potassium, sodium, and calcium to both regulate their own activity and communicate with other astrocytes and neurons. In this way, they control neuronal excitability by maintaining ion homeostasis in the space between cells.
Astrocytes even participate in neuronal transmission themselves by releasing neurotransmitters (see Figure 2). They have been found to release glutamate, the major excitatory neurotransmitter in the brain, and GABA, the major inhibitory neurotransmitter in the brain; as well as adenosine triphosphate (ATP), adenosine, and D-serine. The release of such ‘gliotransmitters’ occurs in response to changes in neuronal activity and can affect the likelihood of a given neuron to fire.
Astrocytes also release growth factors to participate in the formation, maintenance, and pruning of synapses. Even more heroically, astrocytes store glycogen, and can use that glycogen to sustain neuronal activity when the body has low blood sugar or when neurons are overactive.
With the many functions and abilities of astrocytes, it’s easy to imagine how indispensable they are to keeping the CNS running smoothly. From regulating blood flow in the brain and providing metabolic support to managing neurotransmitter levels and even participating in neuronal transmission themselves, these cells are critical to the function and health of the CNS.
Peer Editors: Sy’Keria Garrison and Shea Ricketts
