Wednesday, June 25, 2014

Suzana Herculano-Houzel has novel news about glia in the human brain

I'm way behind in my MOOC these days, but I'm still learning stuff - recently from the interchange between Peggy Mason and other students. Yesterday I spotted this blogpost in my news feed: 

Neurons outnumber glia? Yup, a fact learned from a MOOCer. (1) For the longest time the dogma has been that glia outnumber neurons by a ratio of 10:1. This woman says, no, that's not right

Why is this important to me? Because prominent pain theorists in my profession have been saying, number 1, that glia-neuron ratios in the human brain are more like 20:1 (from a lecture at NOI 2012, scroll down), and number two (and incidentally to this, but incorrectly according to me) that glia are immune cells, instead of glia are non-neuronal neural cells from the neural tube that can exhibit certain immune-like properties at times. Anyway, the gist of the argument is that glia are overwhelmingly numerous and can bother neurons instead of maintaining them.

Well, I don't think the "glia are immune cells" assertion was correct; now it looks like maybe the "overwhelming numbers" assertion is suddenly weaker. Bear in mind meanwhile, that I completely support the move away from biomechanical clinical reasoning in this profession, and toward nervous system-based reasoning, theory, practice methods, explanatory models, treatment models. 

Here are some more links.

This post in this thread

Here is a link to Suzana Herculano-Houzel's lab. Under a section titled "Dogmas that are no more" she states, 
there are not 10 times more glial cells than neurons in the human brain, but at most 1 glial cell to every neuron in the whole brain
Here is a link to the abstract of her paper about this. Not all brains are made the same: new views on brain scaling in evolution. (2)

Here is a link to her TED talk, What is so special about the human brain? 

She points out in the paper that in some areas of the brain,  glial ratio is higher and in other areas (e.g., cerebellum) way lower. 

Here is a recent paper on this:  The glia/neuron ratio: How it varies uniformly across brain structures and species and what that means for brain physiology and evolution.(3)

"It is a widespread notion that the proportion of glial to neuronal cells in the brain increases with brain size, to the point that glial cells represent "about 90% of all cells in the human brain." This notion, however, is wrong on both counts: neither does the glia/neuron ratio increase uniformly with brain size, nor do glial cells represent the majority of cells in the human brain. This review examines the origin of interest in the glia/neuron ratio; the original evidence that led to the notion that it increases with brain size; the extent to which this concept can be applied to white matter and whole brains and the recent supporting evidence that the glia/neuron ratio does not increase with brain size, but rather, and in surprisingly uniform fashion, with decreasing neuronal density due to increasing average neuronal cell size, across brain structures and species. Variations in the glia/neuron ratio are proposed to be related not to the supposed larger metabolic cost of larger neurons (given that this cost is not found to vary with neuronal density), but simply to the large variation in neuronal sizes across brain structures and species in the face of less overall variation in glial cell sizes, with interesting implications for brain physiology. The emerging evidence that the glia/neuron ratio varies uniformly across the different brain structures of mammalian species that diverged as early as 90 million years ago in evolution highlights how fundamental for brain function must be the interaction between glial cells and neurons."

It makes total sense to me that it would be thus - the brain is an evolved phenomenon, not a done deal. It's not homogeneous, not a monolith - more like an ecosystem. 

This makes sense, looking back at the TRPv1 business about how TRPv1 receptors so heavily influence the blood brain barrier (5) in some parts of the brain (spinal cord and just about all the critter brain) but not other parts (cerebellum and frontal lobes).  According to this new information, one might see a pattern here: glia are not as numerous as previously supposed, glia form the blood brain barrier, the more there are in a given location the more affected they would become, presumably, and the less there are in a given region the less affected that region would be by something affecting the bloodbrain barrier. 

1. The Brain is Soooo Cool blog, Peggy Mason, post from June 17/2014

2. Herculano-Houzel S.; Not all brains are made the same: new views on brain scaling in evolution. Brain Behav Evol. 2011;78(1):22-36

5. Simon Beggs, Xue Jun Liu, Chun Kwan and Michael W Salter; Peripheral nerve injury and TRPV1-expressing primary afferent C-fibers cause opening of the blood-brain barrier. Molecular Pain 2010, 6:74

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