I guess I used to think that the relaxation of a muscle was a passive thing. The brain sends a signal, “clench!” and the muscle contracts, and when the signal stops the muscle, you know, unclenches. But I don’t think that’s right.
I am asking because of spasticity, one of my issues with multiple sclerosis. Any information would be appreciated.
That’s the way it works. When there’s a problem with the nerves though, an uncontrolled signal to clench can be generated in the nerves or spinal cord without the brain being involved.
The clench signal can also be sent to two opposing muscles at the same time, say the biceps and triceps. Both tighten but your arm doesn’t move because they are counteracting each other.
Hm. I wonder if this is for sure.
So, if I can’t relax a muscle in my forearm, say, that means there is a continuous signal traveling from my brain to that muscle? 'Cause that’s really interesting, especially in light of the fact that, although there are about a bazillion things that MS can cause, “fatigue” is the most commonly reported symptom in MS. Does it take energy to generate a signal? I would think so, although prolly a teeny-tiny amount. But when I add up all the individual muscles that are being constantly stimulated (if that’s indeed what’s going on), the constant feeling of being physically exhausted kinda makes sense.
So, this signal…is it more like a steady current, or like a pulse? And how do we know?
How do we know there’s no signal for “relax”? Can individual signals be measured? Or is the assumption that relaxation is passive based on lack of contrary evidence?
Note that a subset of fibers in all skeletal muscles are taut at all times. The interesting trick of the nervous system is that it constantly cycles thru bundles, relaxing some and tightening others. If a fiber bundles stays tense too long, bad things happen. Flexing involves increasing the percentage tight on one side and decreasing the percentage on the other. A lot of muscle problems stem from when this system goes haywire. Too many staying tight too long causes pains and cramps. Ones not relaxing when they should causing jerky movements. Staying too relaxed is also an obvious problem. Just a huge list of possibilities when you start thinking of all the places the system can break down.
So there are a lot of nerves in each muscle so that all the bundles can be fine tuned. In general most muscle tone is kept in check by a local feedback cycle (to the spinal cord and back) in the nervous system. The brain will get involved when a postural change is needed.
The nerve signal itself is a trivial amount of energy. The key chemicals and receptors at nerve junctions can “wear out”, which might cause depression and other woes in some people. But your tiredness probably is from the muscle fibers not relaxing as regularly as they should. Note that a bundle staying tight a little too long on one side means that another bundle opposite it has to stay tight to balance it. It goes on continuously and it all adds up.
Signals are sent thru nerves as pulses; they can be close enough together that the muscle would see it as essentially continuous. But again, a nerve stimulating a muscle for very long is going to be a problem. See here.
Thanks, guys. This gives me some good information to start with.
ftg, the idea of a local feedback loop is fascinating, and entirely new to me. Must go buy books.
What amazing machines these are, in which we live.