Deoxyribose said:
From what I understand, nerve impulses don't necessarily cause muscles to contract, but they greatly increase the probability of muscle contraction. Because of the stochastic movements of ions and molecules at the molecular level, sometimes an ion channel may open even though there was no nerve impulse preceding it. Just because muscle contraction is more likely to occur following a nerve impulse does not mean that muscle contraction will not occur in the absence of a stimulus. Granted, it is far less likely to occur without a stimulus, but the likelihood of contraction without a stimulus is not 0. Did that make sense?
That's not sufficient to account for what Sameeralord is describing.
The misunderstanding is the assumption that 1) postural muscle tone is passive and 2) there is no stimulus. When you're sitting or standing, you're not doing so involuntarily; it's a voluntary action not to fall limply to the ground (hence, why the person who faints and loses voluntary control of their muscles falls down).
The "stimulus" helping you stand still (for example) is proprioceptive sensory information about your current posture, as well as the active decision to remain standing, and should you start to lose your balance, you correct that by contracting muscles (in that case, by a reflex arc rather than voluntary control).
The force of contraction of a muscle is determined by the rate of action potential firing in the motor neurons, with increasing rates of action potentials leading to increasing forces of contraction. When you were studying your neuro unit, you may have seen a maximal firing rate of action potentials referred to as tetani, which is full force contraction (I don't know which came first, the name of the tetanus bacteria, which causes spastic muscle contractions, or the name of the action potential firing pattern...something to look up another day). So, for postural muscle tone, the motor neurons are just firing action potentials at a slower rate than during active contraction, such as walking.
Afterall, even just remaining standing or sitting is a weight-bearing activity.
If you're referring to maintaining muscle tone without any weight-bearing, like lying flat in bed, then you really don't maintain muscle tone. The activity needs to be weight-bearing to maintain muscle activity and hence tone (using the term to mean the opposite of muscle atrophy rather than talking about tonic action potential firing). You can look for NASA funded studies for more background about the effects of weightlessness on muscle atrophy if you're interested in looking into it further. If you've ever watched a space shuttle mission returning, particularly for the longer mission, you may have noticed the astronauts are often carried off the shuttle, or given a great deal of assistance walking. This is because of the muscle atrophy they experience in a reduced gravity environment.
