Melzack & Katz, Pain. Part 12b: Examining the motor system, first pass

The paper, Pain

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Part 12: Action! 

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We are still working through the Melzack and Katz paper, in the section "Beyond the gate." We

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covered the subsection, Action neuromatrix. Well, sort of. Once through lightly. I told you how I didn't know much about it. The picture to the right, from Wikipedia, is the kind of image that comes to mind. It's decades old and awfully static. 

Because it's been ages since I've read much about the motor output system, I feel obliged to add something about it in here, and this seems a good place. [Remember, I warned you (all three of you..) that there might be side trips up inlets. Get ready for a little review of the motor output system, including pictures of brain bits.]

So, the problem is, how can I study the motor output system in a way that makes it feel more alive, more than just a bunch of pretty pink lines on a picture? 

I like what Peggy Mason has to say in her text, Medical Neurobiology, page 10: 

"The motoneuron is critical to movement but is not active in isolation. Therefore, motoneurons produce muscle contractions only when activated by inputs."

All right! This is starting to sound more neuromatrix-y to me. 

"Furthermore, when a motoneuron is activated in patterns dictated by neurons in the cerebral cortex, cerebellum, and basal ganglia, smooth motions involving multiple joints and muscles are produced. Since all central neurons influence motoneuron activity, directly or indirectly (in many cases, very indirectly), one can view the whole brain as "pre-motor," functioning to control the activity of motoneurons."

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Yes! That's exactly the right lead-in to make the whole project more fun and less daunting. Like the image at the left, motor control starts at the brain (top bowl), flows down through spinal cord (middle bowl) and out to the body (bottom bowl). 

Looked at another way, brain is the pre-motor puppeteer. The body is

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puppet. The motoneurons are strings. 

Mason starts at the lowest level on the hierarchy, reflexive movement. Another text, Mayo Clinic Medical Neurosciences,  has a beautiful image of how interneurons and motor neurons connect in the spinal cord. This would be way way down in the anterior horn of the spinal cord. 

IMAGE: from page 271
MayoClinicMedicalNeurosciences

Note that there are two sets of influences on a motor neuron - descending pathways (i.e., from the brain or at least from more rostral in the system), and primary afferents. E.g., hitting the knee with a reflex hammer. Looks a bit like the gate control diagram Melzack developed way back in the 60's.

Based on Wolpert (from yesterday), maybe the primary afferents are like "noise", and descending pathways are like "noise-cancelling headphones." Or maybe I'm stretching that noise metaphor a bit too much. On second thought, I don't think I am.  This whole nervous system evolved within an environment, a "noisy" environment, subject to upheaval and stress and unpredictability. It evolved from bottom to top, not the other way round. It would have had "noise" to deal with, the entire time it was evolving (and still is). So it makes sense that there would be some sort of rudimentary noise-cancelling "motor control" features at the anterior horn, even at a stage when there wasn't much rostral to descend. The first kind of brain in existence was the "aquatic brain," the original critter brain, 500 million years old, still going strong in there. 

Fish ancestors gathered the nerve net into a spinal cord. It is from them we inherit our vertebrate nervous system layout. Lest we forget. 

According to the link, it contained spinal cord, hindbrain, midbrain and forebrain. All the bits needed to run the system started there, and it has emerged, unfolded itself, ever since.  

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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 7b: Gate 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 9c: Neuromatrix: 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 10d: Conclusion 3: The brain's sense of "Self" can INclude missing parts, or EXclude actual parts, of the biological body Part 10e: The neural network that both comprises and moves "Self" is (only)modified by sensory experience
Part 11: We 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!