Hergest Ridge

Ingenious experiments have shown that nerve cells, when they grow out from the spinal cord, or from the brain, find their way to their end organ not by following any kind of overall plan but by chemical attraction, rather as a dog sniffs around to find a bitch in season. An early classic experiment by the Nobel-Prize-winning embryologist Roger Sperry illustrates the principle perfectly.

Sperry and a colleague took a tadpole and removed a tiny square of skin from the back. They removed another square, the same size, from the belly. They then regrafted the two squares, but each in the other’s place: the belly skin was grafted on the back, and the back skin on the belly.

When the tadpole grew up into a frog, the result was rather pretty, as experiments in embryology often are: there was a neat postage stamp of white belly skin in the middle of the dark, mottled back, and another neat postage stamp of dark mottled skin in the middle of the white belly. And now for the point of the story. Normally, if you tickle a frog on its back with a bristle, the frog will wipe the place with a foot, as if deterring an irritating fly. But when Sperry tickled his experimental frog on the white ‘postage stamp’ on its back, it wiped its belly! And when Sperry tickled it on the dark postage stamp on its belly, the frog wiped its back.

What happens in normal embryonic development, according to Sperry’s interpretation, is that axons (long ‘wires’, each one a narrow, tubular extension of a single nerve cell) grow questingly, out from the spinal cord, sniffing like a dog for belly skin. Other axons grow out from the spinal cord, sniffing like a dog for belly skin. And normally this gives the right result: tickles on the back feel as though they are on the back, while tickles on the belly feel as though they are on the belly. But in Sperry’s experimental frog, some of the nerve cells sniffing out belly skin found the postage stamp of belly skin grafted on the back, presumably because it smelled right. And vice versa.

People who believe in some sort of tabula rasa theory – whereby we are all born with a blank sheet for a mind, and fill it in by experience – must be surprised at Sperry’s result. They would expect that frogs would learn from experience to feel their way around their own skin, associating the right sensations with the right places on the skin. Instead, it seems that each nerve cell in the spinal cord is labelled, say, a belly nerve cell or a back nerve cell, even before it makes contact with the appropriate skin. It will later find its designated target pixel of skin, wherever it may be. If a fly were to crawl up the length of its back, Sperry’s frog would presumably experience the illusion that the fly suddenly leaped from the back to belly, crawled a little further, then instantaneously leaped on the back again.

Experiments like this led Sperry to formulate his ‘chemo-affinity’ hypothesis, according to which the nervous system wires itself up not by following an overall blueprint but by each individual axon seeking out end organs with which it has a particular chemical affinity.

 
“I mean, who the hell writes with a fountain pen anymore?
For Christ’s sake; How fricking pretentious is that?”

Yes, yes I still write with a fountain pen and find the excerpt from Duplicity hilarious. Call it starting to laughing at yourself in a healthy way, or the early stages of oncoming insanity. Either way, I am sure I’ll enjoy it. :)

How are you, GI Joe? It seems to me that most of you are poorly informed about the going of the war, to say nothing about a correct explanation of your presence over here. Nothing is more confused than to be ordered into a war to die or to be maimed for life without the faintest idea of what’s going on.
(Hanoi Hannah, 16 June 1967)

No one takes the threat of pterodactyls serious anymore. Well, except me and the artist behind this and many other interesting t-shirt designs.