h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

As I understand it, neurons are information processing cells within the (for example) human body. They receive some form of stimulus and that stimulus may result in an output of varying kinds depending on the input. Not a simple on/off necessarily but could be far more complex dealing with thresholds for response and multiple inputs.

I realise that isn't the indepth explanation but I ain;t gonna try and use terms like 'action potential' and descriptions of the various coded outputs without properly understanding it.

My question though is simply this:
If you look at a particular neuron that performs a particular task (say one of the ones that sits immediately behind the photreceptors in the eye and responds to the output from the rods or cones) in two people, will that neuron have the same parameters? I.e. will it perform identical processing, expecting identical input and generating identical output for identical situations?

The first problem is that you can't say that any one particular neuron in one person corresponds to a particular neuron in another person. You can do that for some simple species which only have a couple of hundred neurons, but not for people. You can describe a category of neurons which are present in everyone (for example, neurons that respond to a horizontal change of contrast in the visual field), and then obviously any neuron which belongs to that category will do the same (in some sense) processing because that's how you define it. It's fairly well established that there are such categories at least early in the visual system, whether or not that's true further along the chain is not known (I think).

But, but, but...

In the drawings of the eye the rods and cones (which I understand to be forms of neurons anyway) hook up to other neurons behind them. Can we not compare these ones at the very low level? Or is everyone's eyes wired up differently?

Traveller in Time good at night vision
"Neurons give pulse trains, not a fixed level. The processing goes by mixing several inputs and make a comparison to give a certain output. It would not surprise me if even a single organism has different levels of processing in each individual nerve, depending on time of day, nutrition, health, what the previous actions have been.

Even both eyes will differ, one had to work hadr all day to compensate for the bright sunlight coming through a window where the other had a relaxed day in the shade of a nose . In such case the optical centers in the back of your head will compensate some. "

We don't even have the same set of rods and cones, although the distribution of them is very similar. I'm fairly sure that although they hook up to the next level of neurons in a similar way, it's not completely regular. So they're comparable in the sense that you can say something like "This neuron fires if the rods and cones it's connected to are firing in a pattern that corresponds to a horizontal change in contrast", but you couldn't, say, give each neuron a unique name that is the same for every person and list for each neuron further down the line the names of the neurons it's connected to etc. In one sentence, our eyes our wired up differently but very, very similarly. The connections are comparable, but not exactly comparable.

Did you know that half of all women have four types of cone (instead of the usual three), and 8% of men only have two types? Obviously someone who only has three types can't be wired up in exactly the same way as someone with four types or two types...

Ah, I see. So the functionality is the same but if measured the pulse train (good phrase, I knew that neurons produced pulses but wasn't sure hw to word it!) would probably be different? Different pulses for the same thing, different voltages for the pulses?

Interesting.

p.s. I thought tetrachromats were incredibly rare and only one or 2 had been studied, including Mrs. M the cleaner at oxbridge who is the most famous study.

Ah found it!

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9893829&dopt=Abstract

Although there are lots of women with four different types of photoreceptors, most of them experience colour the same way as everyone else (at least as far as the common tests can determine). I think there's still plenty of debate about it all though - googling for tetrachromacy brings up lots of stuff.

"So the functionality is the same but if measured the pulse train ... would probably be different?"

Yes, there's a lot of randomness in the brain. It's very unlike a computer in this sense, it isn't sensitive to this randomness (which is why we can still think when drunk, although not so well obviously).

The voltages of spikes do vary slightly, but not a great deal I think. It's the connectivity of neurons and their spike trains that vary a lot.

If it's a case of having 4 colour receptor types, but two of them are very similar in terms of receptivity, there may not be much difference compared with someone with the more normal 3 types.

I'd be interested in how the brains of such people are wired up for colour processing. For example, if someone had two subtly different kinds of long (red) receptor in the eye, but both were wired into the 'red' processing channel in the brain, effectively they would have a similar experience to someone with 3 colour receptors.

I'd have thought that a self-wiring brain may just grow to combine signals from two very similar classes of photoreceptors, since they would almost always be firing at the same time, though I'm not sure exactly how the visual brain *does* wire itself.

Yes I think that's right, that's why most of those women with four photoreceptors see colour the same as everyone else. But it illustrates the point that the wiring certainly isn't going to be exactly the same for everyone.

It certainlt does!
I'm obviously a bit out of touch with research on tetrachromats! Fascinating subject, will have to check later if the proposed link with colourblind offspring was found or not!

Although I do remember that it is perfectly feasible for all 4 to be wired in seperately and this is what was presumably the case with Mrs. M as she did indeed appear to eprceive colours differently.

I only found out about it about a week ago - it's quite amazing. I have a mental note to spend some time investigating it properly as soon as I have some time. Maybe I should write a guide entry on it? Haven't written one for ages...

All nerves transmitted signals in the same way using an action potential. Well sort of there aere two speeds of nerve, mylinated ones are faster than demylinated ones - most of the changing of the signal goes on at the synapes which join between nerves.

F'example some synapes need several action potentions in to get one out so modulated it down a bit.