On 10/12/2012 4:55 PM, Rich Cooper wrote:
> My limited understanding of biochemistry is that “signaling” would be a
> better word than “message passing”; a message usually has some content
> other than just its presence at the receptor. (01)
I agree that the term 'signal' is common for signs that have a very
specialized meaning, such as traffic lights. Each chemical compound has
a conventional content, which can vary in intensity by concentration.
So its presence can convey more than 1 bit of information. (02)
But if you go to a typical dictionary, say Merriam-Webster, the first
word sense for 'signal' is "sign, indication". And the second word
sense for 'sign' is "signal". (03)
In biosemiotics, it's common to use the word 'sign' for anything that
conveys information of any kind. Then the more complex versions of
signs are classified in various ways -- such as Peirce's. (04)
By the way, sponges are the most primitive animals, and they don't
have neurons. They do have stringy connective fibers that bind them
together. The same genes that control the development of those fibers
control the development of neurons in higher animals. (05)
There is a step-by-step evolution: (06)
1. Message passing by chemical flow or diffusion is slow. (07)
2. Within a single cell, there can be a difference in electrical
charge from one end to the other. (08)
3. Electrical changes through a long stringy cell can transfer
information much faster than chemical movement. (09)
4. Two cells that are close together can transfer information
chemically across the short gap. (010)
5. The connective cells are used for long-distance communication. (011)
6. Jellyfish are the next step. They have cylindrical symmetry
with neurons in a basket-like configuration. They don't form
any central ganglia, but they can coordinate the rhythmic
pulsation of the entire body. (012)
7. Then the worms with bilateral symmetry get a fat ganglion
in the front, where neurons from the whole body converge. (013)
8. Two light-sensitive spots in the front enable the worm to move
toward or away from the light. (014)
9. All higher animals evolve from the bilateral worms. Multiplying
the spots provides more pixels. Putting a lens in front sharpens
the image. Multiple neurons from all the pixels transfer an image
to the ganglion that has become a brain. (015)
And the rest is history. (016)
John (017)
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