In essential partnerships with neurons // Implicated in learning and memory // Likely involved in regulating the sleep cycle // Far from “useless, stupid cells.”
Glia’s Hidden Talents
In 1888, when the Spanish anatomist Santiago Ramón y Cajal began almost obsessively drawing every kind of brain tissue he could obtain—from humans, rabbits, pigs, dogs, fish, frogs, mice and other creatures—he depicted many hundreds of glia, a newly discovered cell type whose fine tendrils radiating in all directions from a small cell body fascinated (and, as an artist, delighted) him. Glia were thought to serve chiefly as “glue” (glia is Greek for “glue”), cementing the brain’s other cells, but to Ramón y Cajal they looked like spiders, and he dubbed them células aracneiformes—spider cells.
Yet it was a second type of brain cell, the neuron, that ultimately captivated him. Neurons had been discovered only a few decades earlier, and anatomists were still trying to puzzle out how they functioned. Studying their form, Ramón y Cajal began to decipher how they communicated, and he developed a theory that would become known as the neuron doctrine, whose basic tenets continue to inform neuroscience. Ramón y Cajal surmised, correctly, that neurons relay electrical signals from one cell to the next throughout the nervous system. The signals are called action potentials, and they travel in one direction only, from the cell’s rootlike dendrites to its axon, a long, slender central section that ends at a tiny gap, the synapse, across which lies the dendrite of the next neuron.
The electrical activity spurs the release of chemicals called neurotransmitters. This happens when gated calcium ion channels open and calcium flows into the presynaptic neuron, triggering the release of neurotransmitters. The receiving, or postsynaptic, neuron has specialized protein receptors in its dendrites that accept the neurotransmitters, causing the neuron’s voltage to drop, firing an impulse down its axon. This in turn releases neurotransmitters from the second neuron and transmits them across the synapse to the next neuron’s dendrites, all at speeds as fast as 200 miles per hour.
Glia, in contrast, were considered incapable of communicating with one another, let alone with neurons. Besides holding neurons in place, glia seemed to fulfill just a few mundane housekeeping tasks, all in the service of the more exalted neurons. This apparently subservient role meant that, for more than a century, glia have gotten little respect and drawn scant attention from scientists, who, in any event, lacked the tools to decipher what these brain cells are doing.
Researchers have long been able to measure the electrical potential of neurons. But glia communicate more subtly, so even though they make up approximately half the volume of the human brain, researchers couldn’t gauge the impact of their role. Only gradually, with the development of sophisticated imaging techniques for calcium, which glia use to communicate, has it become clear that glia handle far more complex jobs than previously thought. Even their housekeeping functions, it turns out, are extremely important to the nervous system’s well-being.
Now, in fact, a growing cadre of neuroscientists is beginning to realize that glia are as fundamental a part of neural circuitry as neurons, probably involved in the development of the brain’s wiring and in learning and memory, blood and energy supply, neurotransmitter balance and more. And as these scientists start to more fully understand the connections between glia and neurons, they anticipate that glia could become important targets of therapy for many disorders of the nervous system, from brain cancers and spinal cord injuries to such neurological diseases as multiple sclerosis and Parkinson’s.