The "Neuron Doctrine", as Ramon y Cajal's view came to be known, became the fundamental idea for our current understanding of how the brain works. All theories of information processing in the brain, and all of our drugs for treating mental illness, are founded on this one essential idea: synaptic transmission. The Neuron Doctrine tells us that all information in the brain is conducted by electrical impulses transmitted through neurons and communicated across synapses. But is this idea correct?
Now at the turn of a new millennium, a new kind of microscope is revolutionizing our understanding of the brain, and revealing a new dimension of brain function that astonishingly--operates beyond neurons.
Setting in a dark room at Colorado State University in the 1980s, crammed with computers and newly designed microscopes that grafted new digital technology developed for the booming video game industry onto the old optical microscopes, I first saw a neuron fire with my own eyes. Together with Stan Kater and Peter Guthrie, who were pioneers in this new method, we flipped a switch delivering electric shocks to a neuron and saw it emit a burst of light. The light was generated by a chemical we had introduced into the living neuron that fluoresced brightly when the nerve impulse forced calcium to flood into the neuron, driving a chemical reaction producing light. Previously neuroscientists had relied on microelectrodes to tap into electrical transmissions in neural circuits. Now using this new method, when a neuron fired we saw it light up as easily as one sees the pilot light illuminate on a TV when it is switched on. The first time we saw a neuron firing with our own eyes, our shouts of joy filled the dark empty halls.
This new technique fueled a flurry of research around the world using this new imaging method to study how nerve cells work, but very soon scientists were confronted with something completely unexpected and entirely beyond the neuron doctrine. Notably in the laboratory of Stephen Smith, now at Stanford University, but in many other laboratories as well, information was seen flowing freely not only through neurons, but through non-neuronal cells as well--glia.
How do glia communicate? What does this glial communication mean? What can glia, "the other brain" do beyond what the neuronal brain can accomplish?
We are rapidly learning the answers to these intriguing questions about the other brain. Glia communicate, and they provide a separate, non-electric network for information flow through our brain that operates independently but cooperatively with neuronal networks. Glia do not use electricity to communicate, so glia have no need for the axon or dendrites or synapses of nerve cells. Glia communicate by broadcasting chemical messages. Moreover, glia can sense information flowing through neural circuits and alter the communications between neurons at synapses! Glia, we now know, have receptors for detecting the flow of ions generated by neurons firing electrical impulses and for sensing the neurotransmitters neurons release at synapses. Glia intercept these signals and act upon them to increase or decrease the transmission of information across synapses and speed or slow the transmission of electrical information through axons.
These recent discoveries open an entirely new dimension into brain function. Glia are involved in all aspects of nervous system health and disease. They can control neuronal communication, development of the fetal brain, generation of new neurons in the adult brain, participate in epilepsy, Alzheimer's disease, mental illnesses such as depression and schizophrenia, and they provide a new mechanism of learning that operates beyond synapses.
I look forward to describing new research on glia in forthcoming articles together with the latest research on how the brain develops and is modified by experience. This is a new brain and new frontier.
Adapted from:
1. The Other Brain, by R. Douglas Fields, published by Simon and Schuster, NY, 2009.
