Glia are brain cells that cannot generate electrical impulses. As a consequence glia were thought to have no function in information processing or transmission. In fact glia were communicating with themselves and with neurons all along, but without using electricity. For a century neuroscientists were deaf to glial communication as they passionately studied neurons, because they were using the wrong tools for the job. Probing the brain with electrodes, the way neuroscientists do to understand neuronal communication, is useless to intercept glial communications. What revealed glia communicating was a new technique called calcium imaging, developed in the 1980s and 90s. These videos will allow you to see with your own eyes glia communicating using waves of calcium.
What you are seeing is a type of glia called astrocytes taken from a rat brain and growing in a culture dish in my laboratory at the NIH. I have added a dye to the cultures that enters the cytoplasm of cells, and fluoresces brightly when the concentration of calcium ions in the cell rises. You can see that waves of calcium are sweeping through the cytoplasm of astrocytes and passing through networks of astrocytes in complex ways. Imagine the shock of this discovery the first time this was done (or read about it in my book The Other Brain.)
How astrocytes communicate
Astrocytes (and other types of glial cells) communicate using neurotransmitters. When a neurotransmitter binds a receptor on the cell membrane this causes a rise in concentration of calcium in the cytoplasm of the cell. The astrocyte releases neurotransmitters in response to the rise in calcium, which spreads to other cells exciting a chain reaction throughout the population of astrocytes.
Neurons also use neurotransmitters to communicate across synapses, and this allows glia to respond to neuronal signaling and to control transmission of information between neurons by releasing or taking up neurotransmitters near the synapse. Whereas neurons signal serially, like land-line telephones, glia broadcast their signals like cell phones. Notice in the videos that the calcium signals do not spread symmetrically like a shock wave surrounding an astrocyte, they propagate through astrocyte networks that are tuned with different types neurotransmitter receptors to respond to specific types of signaling compounds released by different cells.
In the second video you can see the astrocytes respond after I stimulate nerve axons to fire electrical impulses. The axons are seen as bundles of fibers traversing the microscope field that suddenly glow when I give them a brief electric shock to make them fire impulses. Glia sense neuronal firing at synapses (and along axons), control the transmissions of information between neurons across synapses, communicate the neuronal firing through a non-neuronal network without using electricity to subsequently control the transmission of information through a distant synapse somewhere else in the brain that is not necessarily hardwired into a network of neurons. This is the other brain at work.
Note that these time-lapse videos have been sped up. Calcium signaling is much slower than electrical signaling.
Fields, R.D. (2010) Visualizing calcium signaling in astrocytes. Sci. Signal. 3, 147, tr5.
Fields, R.D. and Burnstock, G. (2006) Purinergic signaling in neuron-glia interactions. Nature Reviews Neurosci. 7, 423-436.
Fields, R.D. (2009) The Other Brain, Simon and Schuster, NY.