How do muscles help the bone?

Answer 1

A muscle expands when it relaxes. The muscles in our bodies are attached to bones. They help the bones move, much like strings help a puppet move. But muscles can pull in only one direction. They cannot push. Each bone is usually attached to a pair of muscles. While one muscle pulls and contracts (shortens), its partner relaxes

This is actually a very complex question to answer, since there are a number of levels to it.

On the most basic level, muscles can move bones because they are an example of contractile tissue, which means they can reduce their length in response to a stimulus (in the case of muscles the stimulus comes from the nervous system). Skeletal muscles have two ends, an origin and an insertion. These ends are each connected to different bones and the bones connected to each other by a joint. The two bones and the muscle can be seen to form a triangle, with the bones making up two edges and the muscle the third. The angle between the two bones is determined by the length of the muscle, so when the muscle contracts the angle between the bones is reduced and the joint is moved. Of course, muscles are contractile, so they can only pull, not push. This means that a second muscle is needed on the opposite side of the joint to move it back again.

The answer to how muscles contract is the next level. If you look at a piece of muscle (such as a chicken breast or a beef steak) there is some structure visible, but no clues as to where the movement comes from. As you look closer and closer, using a microscope in the end, you can see that the tissue is divided into bundles, and the bundles themselves are also divided into smaller bundles. When you magnify a muscle so that you can see the smallest level of bundles, you can see that it is striped across the bundles and when the muscle contacts the stripes move! Some of the stripes are formed because these smallest bundles are formed from many, many microscopic fibres, the ends of which line up very neatly and butt up against the ends of other similarly neatly aligned fibres. Other stripes are formed because there is a second kind of fibre in the bundle, that are placed between the first kind. These fibres are also aligned with each other, but staggered with respect to the first. When the muscle contracts, these two types of fibre (actin and myosin) slide past one-another, causing a shortening of their combined length (a bit like a telescopic car aerial). This shortens the bundle and it is coordinated between all the fibres in all the bundles in all the bundles in all the bundles.... that make up the muscle.

What about how the fibres move past each other? Well, it turns out that the two types of bundle are connected to each other by very small bridges. These bridges are really small (only 10's of a millionth of a millimetre). The bridges between the fibres are not static though, they are the key to the whole thing! Each bridge is solidly attached to the myosin fibre, but can attach and detach from the actin fibres. The bridge can also swing back and forth on the myosin fibre when there are certain molecules present. The key molecule is called ATP. When ATP is present the bridge binds to the actin solidly and swings backwards. When it does this the ATP is destroyed inside the bridge part and no longer does the job of keeping the actin-end of the bridge solidly bound, so it falls off the actin and swings forward again. Then another ATP binds and the whole cycle is repeated. There are a huge number of bridges, all swinging back and forth, binding and unbinding with the actin. The net result of all this action is that the myosin "walks" along the actin fibre! This action is too small to be seen with even the most powerful optical microscopes, but the result of the fibres sliding past each other can be seen, and the contraction of the whole muscle can obviously be seen and felt by anyone.

This process happens EVERY time you contract ANY muscle. That is, every movement you make, every blink of your eye, every breath you take and every beat of your heart, thousands of tiny legs are running up and down the tiny fibres in your muscles. Amazing.