Re: Big animals, big brains - big neurons or more neurons?

"Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx> wrote in
news:dhaelh$3no$1@xxxxxxxxxxxxxxxxxxx:

> A question - well, several questions:
>
> Generally, within an order of mammals, say, brain size scales with
> body size (by some power law). Does this mean proportionately larger
> nerve cells, or does it mean proportionately more nerve cells? Or
> both?
>
> I ask, in part, because I seem to recall reading that the axons of
> neurons in the giraffe's spinal cord are exceptionally long -
> presumably because the vertebrae are long.
>
> In humans, say, does variation in size of brain indicate variation in
> neuron count, or variation in neuron size?
>
> At one time, I thought that a way of experimentally addressing this
> question would be simply to extract all of the DNA in a brain
> (presumably posthumously!) and weigh it. But that would be a metric
> of neuron count plus glial cell count - not a count of neurons alone.
> And I presume that a long axon would be sheathed by more glial cells,
> not bigger glial cells.
>
> Has anyone investigated variation (intra-species or inter-species) in
> the ratio of glial to neural cells (mass or count)?
>
> Pointers to web resources discussing these kinds of questions would be
> appreciated.
>
>

Schmidt-Nielsen, in his book "Scaling" discusses a number of scaling
issues but not specifically neurons. He does note that large and small
animals have cells that are roughly the same size, but again does not
specifically mention neurons. He does include a nice figure showing how
brain size scales with body mass for various animal groups, and a table
comparing primates with mammals as a whole: for mammals as a whole brain
size = 0.01*body mass ^0.70. For great apes it is 0.03 to 004*body mass^
0.66. I briefly tried googling Schmidt-Nielsen on scholar.google.com but
did not get anything on neurons. His reference for uniformity of cell
size is Teissier, G. (1939) "Biometrie de la cellule", in case that
helps.

Yours,

Bill Morse

.

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