Wednesday, July 03, 2013

Melzack & Katz, Pain. Part 12c: Motor output and nervous systems - where they EACH came from

The paper, Pain

Most recent blogposts:

Part 12: Action! 12b: Examining the motor system, first pass


We are still working through the Melzack and Katz paper, in the section "Beyond the gate."
We have decided to explore one of the many inlets of this river, a very large one. It's about motor output. So far we have learned that Wolpert argues that movement is what the brain "does." Movement is what it is for. 

Well, I beg to differ a bit with that idea... let me explain:

Life and movement came before brains

We wouldn't need any brain, or any nervous system,  to move, if we were just single celled animals - they can move in all sorts of ways without needing to have a brain. Even some of the multicellular animals, like sea squirts, don't need a
brain (as Wolpert pointed out, they digest the one they have as sea squirt larvae once they land on and attach to a rock). After that, they function pretty much as just little stomachs. They're alive, and everything that is alive eats, digests, expels waste... No brain, but they can still "squirt." It's completely reflexive. One contractile cell pulls upon another, and a chain reaction occurs. No nervous system needed. It's kinda creepy when you think about it, but our own digestive tracts are still like that. Yes, we do have neurons in ours, a  completely autonomous nervous system built from neural crest cells called the enteric nervous system, probably because it's a massive long system being carried around inside a multi-cell body, and neurons do make things more efficient - but it's mostly just reflexive, triggered by food and triggering its own self, to function.

Presumably therefore, you could argue (and I do) that "movement" arose before brains did. You could argue that mobility of multicellular creatures is what needs something more sophisticated, like a nervous system, to cancel out environmental "noise," bring a creature closer to a food source it can smell because of a chemogradient.. and help it move away fast from being some other creature's food source.. You could argue that mobility is something different from mere movement, or mere locomotion even... 

Then, environmental "noise"...

Back to single cell organisms for a minute - these creatures do everything through their membrane. Well, through pores in their membrane. Like ion channels. At some point in far-back evolutionary history, there was nothing but single-cell life on the planet. It got along just fine for billions of years, being single-celled, making and exporting oxygen to the environment as a waste product. Some sort of planetary tipping point occurred where there was too much oxygen floating around and it was starting to be toxic. Cells huddled up together to save themselves. Well, OK, they didn't have brains, so they couldn't experience fear - instead they huddled up because of thermodynamic gradient reasons. See Into the Cool for more on that. 

All cells still do everything through their membranes, including the ones that comprise our bodies, all the way back to the very first cell we individually "were" - our mom's fertilized egg cell. Cells in our bodies might not be able to "move" by themselves anymore - they've become plastered onto each other and entrapped by each others' multitudinousness.. but they can still sense each other through pores in their membranes; they still have lives - metabolism and excretion, growth and reproduction, eventual death or else, splitting into two new cells - inside their own movement confines. The trade for a safer life as part of an oxygen degrading multi-cellular being they could be part of, was to "export" their freedom-to-move to whatever larger 
of PDF
creature they ended up part of. 

Synapses (nervous systems) evolved from single-cell organisms' membrane functions

If you dig deep enough, you'll learn that the nervous system is thought to have evolved by exaptation  of this membrane sensing function. 

If you poke around a little more you'll discover Seth Grant, and his fascinating work unraveling the mysteries of evolution by way of studying synaptic proteins. 

If you check out what happens when a nerve is cut - the cut axon develops a growth cone on the cut end, which searches around for a target tissue! Just like a little single cell autonomous animal! Each neuron has its own sensory-motor function! [.....It's alive!.....]

When you stop to think about it, there really isn't anything mysterious about a nerve cell. It's just a body cell, but programmed to be very long and skinny with a whole lot more sensitive membrane per unit volume, than other body cells which retained their original blob-shape [lot less (metabolic) work to maintain, given the slings and arrows of outrageous environmental forces]. This makes a nerve cell very very vulnerable, and very very sensitive. But evolution took care of that by wrapping nerve cells in neural crest cells to protect them from the rest of the cells, by managing the sensitivity. So that was a nice thing that happened. And not by design, just by fluke. Because... noise cancelling..?

Anyway, enough of the long digression. 
We learned where the vertebrate arrangement of our nervous system, the one we share with all other land animals - from frogs to snakes to birds to cows- came from; ancient fish, 500 million years ago. Without this arrangement it's unlikely we could ever have evolved a way to grow large (think dinosaurs) or even grow limbs in the first place, to move around on land. Then we had to evolve brain parts to help us coordinate all the extra limbs etc. Not that fish hadn't already developed the essential bits first..

Peggy Mason says this about the cerebellum, on p. 11:
"This mini-brain piggybacks on the back of the brainstem and ensures that the movements we make are those we intend to make."
  • acts on movements involving several muscles and acting across several joints, like a conductor of an orchestra
  • learns to smooth and coordinate voluntary action
  • learns what signals to send to which muscles to generate the forces necessary to achieve a designated action
  • learns to anticipate and prevent errors before they occur
  • adjusts gait and other movements to an ever-changing environment

But c'mon. 
We're talking about human brains now. 
Well, not more than a page turn away... 

Peggy Mason says (p. 13) the basal ganglia* choose which movements occur. 

"The only obstacle to performing multiple actions simultaneously is within the brain." Apparently the brain chooses to do one or a very few related actions while simultaneously suppressing all other movement. Within the human brain, the "chooser" is the basal ganglia. The basal ganglia are the ultimate arbiter, deciding which movement occurs and whether a movement continues or is interrupted by a more pressing action. But that's not all they do! Just as the number of actions that we make at one time is limited, you can only have one perception at a time. In any of many visual illusions, a person switches between seeing two images but does not see both images at any one instant. For instance, in one popular illusion, one sees either a white vase or two silhouetted profiles facing each other, but not both images simultaneously. As we explore in Chapter 25, just as the basal ganglia choose movements, they are also critical for choosing perceptions, thought, and emotions."

Allright!! Now I feel like we're getting somewhere! Basal ganglia are totally part and parcel of the neuromatrix. If I perceive pain, I can blame them

We'll look around for a little while in Chapter 25 tomorrow.  
* It turns out fish evolved basal ganglia too
"Modern evidence, in fact, shows that both striatum and pallidum have been the basal ganglia constituents since the earliest jawed fish."


Previous blogposts

Part 1 First two sentences Part 2 Pain is personal Also Pain is Personal addendum., Neurotags! Pain is Personal, Always.

Part 3a Pain is more than sensation: Backdrop Part 3b Pain is not receptor stimulation Part 3c: Pain depends on everything ever experienced by an individual

Part 4: Pain is a multidimensional experience across time

Part 5: Pain and purpose

Part 6a: Descartes and his era; Part 6b: History of pain - what’s in “Ref 4”?; Part 6c: History of pain, Ref 4, cont.. : There is no pain matrix, only a neuromatrix; Part 6d: History of Pain: Final takedown Part 6e: Pattern theories in the history of pain Part 6f: Evaluation of pain theories Part 6g: History of Pain, the cautionary tale. Part 6h: Gate Control Theory.

Part 7: Gate control theory has stood the test of time: Patrick David Wall;  Part 7bGate control: "The theory was a leap of faith but it was right!"
Part 8: Beyond the gate: Self as mayor Part 8b: 3-ring circus of self Part 8c: Getting objective about subjectivity
Part 9: Phantom pain - in the brain! Part 9b: Dawn of the Neuromatrix model Part 9cNeuromatrix: MORE than just spinal projection areas in thalamus and cortex Part 9d: More about phantom body pain in paraplegics
Part 10: "We don't need a body to feel a body." Part 10b: Conclusion1: The brain generates its own experience of being in a body Part 10c:Conclusion 2: Your brain, not your body, tells you what you're feeling Part 10dConclusion 3: The brain's sense of "Self" can INclude missing parts, or EXclude actual parts, of the biological body Part 10eThe neural network that both comprises and moves "Self" is (only)modified by sensory experience
Part 11We need a new conceptual brain model! Part 11b: Intro to a new conceptual nervous system Part 11c: Older brain models just don't cut it Part 11d: The NEW brain model!

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