Once thought to be mere “filler” for the space around neurons, astrocytes are star-shaped glial cells found throughout the brain and spinal cord that are now known to perform many important functions, such as regulating the transmission of ions and glucose between blood vessels and the brain.
When Cajal drew these protoplasmic astrocytes, the prevailing theory was that astrocytes only provided structural support for neurons. He rejected this idea and instead hypothesized that all the astroglia in the brain formed a sort of gland, which would release substances that affect brain function; it is now known that astroglia do indeed release substances that affect neuronal signaling, including glutamate, GABA, and ATP.
Having observed the close association of astrocytes with blood vessels and neurons, as well as the fact that all astrocytes appeared to have a prominent appendage, or “foot,” Cajal further hypothesized that astrocytes used this “foot” to stimulate blood vessel dilation. Astrocyte “feet” do indeed regulate blood vessel diameter, albeit via the release of signaling molecules rather than through physical manipulation, as Cajal imagined. Astrocyte-evoked changes in blood flow, and thus in oxygenation, constitute the signal that is measured by fMRI, a tool used to image brain activity.