There are truisms in brain-science which hide more than they reveal. For example, the old line that there’s 100 billion neurones in the brain and ten times as many non-neurones as neurones. It’s true, and not true, but the details are complicated. From the diagrammed average brain we have the following breakdown…
1500 grams, 170 billion (170 B) cells
86 B neurones
84 B non-neurones
ratio non/neurone: 0.99
Cerebral Cortex: 81.8% mass, 19.0% neurones
1233 grams, 77 B cells
16 B neurones
61 B non-neurones
ratio non/neurone: 3.76
Cerebellum: 10.3% mass, 80.2% neurones
154 grams, 85 B cells
69 B neurones
16 B non-neurones
ratio non/neurone: 0.23
Rest of Brain: 7.8% mass, 0.8% neurones
118 grams, 8.4 B cells
0.7 B neurones
7.7 B non-neurones
ratio non/neurone: 11.35
…which is interesting because the even ratio of neurone to non-neurone (which includes glial cells and blood vessels etc.) is not evenly distributed. Surprisingly the cortex isn’t the main show for neurones – the “Back-up Brain”, the cerebellum, has more. Which makes sense because of its intensive role in fine-motor control and similar real-time computation heavy work. If the cortex is the repository of cognition and memory, with the hippocampus as the “pattern buffer” of memorising processing, then the higher glial component is needed for its support role for the chemical and hormonal changes needed by memory and “higher level” thinking.
The cerebellum is the “robot controller” which has to smooth out the commands from the cortex and monitors them in real time. Thus lots of neuronal circuits working to keep “body programs” running smoothly in dynamic response to external conditions , thus the neurones are all squeezed close together for maximum speed.
More glial are needed in the brain-stem and cortex because they contain more “cabling” – longer neuronal ‘wires’ feed-up from the body, and back to the body, through the brain-stem and fan-out into the different cortical areas, as well as cross-wiring the different cortical areas.
Ok. Enough description. Part 2 will explore some of the implications.