The Theory of Nothing

How do muscles work?

How do muscles work?
This question has to do with how we move. The answer is by means of muscles. That was too easy! There has to be more.
Muscle tissue is a very interesting type of tissue in the human body. It's considered soft tissue composed of protein filaments of actin (a globular protein that has microfilaments) and myosin (a motor protein). The bottom line is that these filament-like cells can change length and shape when stimulated by nerve impulses.
I'm talking about the skeletal muscles in this discussion. There are also smooth involuntary muscles and cardiac muscles. These muscles act automatically by means of the involuntary nervous system.
Skeletal muscles do what their name suggests--they are attached to and move the skeleton. The real action comes from what is called muscle contraction. The scenario goes like this. You decide that you want to move your arm. The brain sends out the appropriate nerve signals on nerves that go down the spinal column and then branch off to the appropriate muscle system. I say system because most movements involve more than one muscle. Also, some muscle contraction signals originate in the spinal nerves in order to save time. These are reflex movements that involve feedback with the brain. Many of the smooth muscles operate in this fashion.
There are two kinds of skeletal muscle movements. One is a twitch, which only causes the muscle to contract for a brief time, so brief that the muscle never gets to its peak force.
The other is called a tetanic contraction. Tetanic muscle contractions are for longer periods and are categorized by the force generated over a specific time. For example, an isometric contraction happens when one holds an object up without moving it. An isotonic contraction is where the muscle remains constant despite having its length changed.
Basically, what makes a muscle contract is a shock, a voltage potential that travels to a motor neuron, which transmits it down its axon. What happens next is down at the cellular level, and it's a series of complicated steps that results in muscular contraction that involves a calcium ion being released at the axon interface and causing a neurotransmitter known as acetylcholine to go over to a receptor that activates a sodium/potassium channel, which causes sodium to go in and potassium to go out, which in turn causes a potential difference that triggers an action potential that involves ATP or adenosine triphosphate. Good grief! I'm lost!
To make this process shorter, the effect is to cause actin filaments to relax into a rigor mortis state and allow myosin filaments to energize or stiffen. This causes the two kinds of filaments to slide over one another, effectively contracting the muscle tissue. This is a simple description of a much more complicated process.
What one should carry away from this is that muscle contraction is a process that consumes energy, energy stored in ATP, and it can only function for a given period of time before calcium, which is acting like a pump, is no longer capable of doing its thing. This causes the contraction to weaken and eventually stop.
The other thing of importance here is the balance between sodium and potassium. This balance is critical for muscles to work and if, for example, potassium goes too high or too low, the heart stops. In other words, muscles move by a very complex chemical process and if it fails, we die.
The energy process that runs muscles involves glycogen, which is made from glucose metabolism. ATP is the energy regulator that is produced when glucose is metabolized. It is this ATP that powers the myosin filaments. Remember that myosin is the motor protein filament. Well, as the muscles contract, the glycogen is converted back to glucose, which forms lactic acid. This is not good and builds up to a level that causes the muscle contraction process to stop. Aerobic exercise is where this process can take place for long periods without lactic acid build up. Where it doesn't work well is in strength exercise where the muscle is not allowed to relax. I believe that this is the way that the human body has evolved to prevent tissue damage.
The bottom line is that we have evolved to be able to move both strenuously and precisely by means of seven hundred or so muscles. Whew! I'm glad that I don't have to name them all.
Thanks for reading.