The presence of glial cells may increase the capacity for signaling in the brain by a small factor, but is unlikely to qualitatively change the nature or extent of signaling in the brain.
Number of glial cells
Azevado et al. physically count the number of cells in a human brain and find about 10¹¹ each of neurons and glial cells, suggesting that the number of glia is quite similar to the number of neurons.
References to much larger numbers of glial cells appear to be common, but we could not track down the empirical research supporting these claims. For example the Wikipedia article on neuroglia states “In general, neuroglial cells are smaller than neurons and outnumber them by five to ten times,” and an article about glia in Scientific American opens “Nearly 90 percent of the brain is composed of glial cells, not neurons.” An informal blog post suggests that the factor of ten figure may be a popular myth, although that post also draws on Azevado et al. so should not be considered independent support.
Nature of glial signaling
Sandberg and Bostrom write: “…the time constants for glial calcium dynamics is generally far slower than the dynamics of action potentials (on the order of seconds or more), suggesting that the time resolution would not have to be as fine” (p. 36). This suggests that the computational role of glial cells is not too great.
Newman and Zahs 1998 mechanically stimulate glial cells in a rat retina, and find that this stimulation results in slow-moving waves of increased calcium concentration.1 These calcium waves had an effect on neuron activity (see figure 4 in their paper, which also provides some indication concerning the characteristic timescale). For reference, these speeds are about a million times slower than action potential propagation (neuron firing). These figures support Sandberg and Bostrom’s claims, and as far as we are aware they are consistent with the broader literature on calcium dynamics.
Astrocytes—a type of glial cell—take in information from action potentials (from neurons).2 There is some evidence that a small fraction of glia can generate action potentials, though such cells are “estimated to represent 5–10% of the cells” and so unlikely to substantially change calculations based on neurons.
It seems possible that further study or a more comprehensive survey of the literature would reveal other high-bandwidth signaling between glial cells, or that timescale-based estimates for the bandwidth of calcium signaling are too low, but at the moment we have little reason to suspect this.
Energy of glial signaling
If glia were performing substantially more computation than neurons, we would weakly expect them to consume more (or at least comparable) energy for a number of reasons:
- The energy demands of the brain are very significant. If glia could perform comparable computation with much lower energy, we would expect them to predominate in terms of volume, whereas this does not seem to be the case.
- It would be surprising if different computational elements in the brain exhibited radically different efficiency.
However, the majority of energy in the brain is used to maintain resting potentials and propagate action potentials, for example a popularization in Scientific American summarizes “two thirds of the brain’s energy budget is used to help neurons or nerve cells “fire” or send signals.”
Although we can imagine many possible designs on which glia would perform most of the information transfer in the brain while neurons provided particular kinds of special-purpose communication at great expense, this does not seem likely given our current understanding. This provides further mild evidence that the computational role of glial cells is unlikely to substantially exceed the role of neurons.
- “The resulting Ca2+ waves, traveling through astrocytes and Muller cells, were similar to those observed previously in the isolated retina (Newman and Zahs, 1997), although the propagation velocities were somewhat slower: 13.8 + 0.4 micrometers/sec (57) compared with 23.1 micrometers/sec in the isolated retina (where the bathing solution was supplemented with glutamate and ATP). In the eyecup, the largest Ca2+ waves attained a diameter of about 400 micrometers.” – Newman and Zahs 1998
- “Instead of integrating membrane depolarization and hyperpolarization into action potential output, like neurons do, astrocytes sense and integrate information mainly through the generation of intracellular calcium (Ca2+) signals (Figure 1). It is now well-established that astrocytes are able to sense transmitters released by neurons and other glial cells (either astrocytes or microglia)” – Min, Santello, and Nevian, 2012