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Muscle
Written by Tim Sheppard MBBS BSc. Last updated 15/6/10

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What is a muscle?

We've all heard of muscles - they're the things which make us move, they're the things which make us look powerful, the bulge beneath our skin when we flex our arms or legs. But how much do we know about them - how are they arranged, are there different types, how do they work? So many questions; this is quite a big section!

Importantly there are three types of muscle, as each of them performs a different function.


What types of muscle are there?

The three types of muscle are:

Skeletal Muscle - the type found attached to bones (hence the name skeletal), which is involved in moving the bones to perform the wide range of actions capable by the human body.

Cardiac Muscle - the type of muscle found in the heart, which is responsible for causing it to contract, forcing blood around the body.

Smooth Muscle - the type lining the digestive tract and blood vessels.


What is skeletal muscle?

Skeletal muscle is the muscle, unsurprisingly, you find attached to your skeleton. It is largely under voluntary control - that is, in order for the muscle to contract, the individual has to think about it.

When we talk about 'muscles', we are usually talking about skeletal muscles - biceps, for instance, is a skeletal muscle, made up of the characteristic striated muscle.

Skeletal muscle is described as striated because of its appearance. Under the microscope it is possible to see parallel lines - 'striations' - running across the muscle cells. These are due to the myofibrils found in skeletal muscle cells, which will be discussed shortly.

Each muscle, then, can be split into compartments known as fasicles, with each fascicle wrapped in a sheath of connective tissue known as the perimysium. Surrounding the whole bundle of fascicles is the epimysium, keeping the muscle together. Within each fasicle are the muscle fibres or muscle cells, each wrapped in endomysium. Within each of these fibres, are the myofibrils, which are the functional part of muscle contraction.

Skeletal muscle, then, is the tissue that connect bones causing movement, with contraction stimulated voluntarily, and comprising of collections of muscle fibres collected together and cleverly organised to maximise efficiency and power.


What is a skeletal muscle cell?

A skeletal muscle cell is a fibre which, along with many others, comprise a fasicle in a skeletal muscle. It is essentially what muscles are made of, containing the functional part of the muscle, the myofibril. Myofibrils are cleverly organised strands of the proteins actin and myosin, which work together to cause shortening of the muscle.

Peripheral nuclei is a characteristic feature of skeletal muscle cells, and shows how these cells originally came from lots of young muscle cells fused together. They are essential since they contain the code for producing protein, which enables the actin and myosin of the myofibrils to be replaced when necessary.

Mitochondria are rampant in muscle cells as they are the energy factories of the cell, producing lots of energy for the costly process of muscle contraction.

The sacroplasm is the name given to the cytoplasm of the muscle cell - it is simply the mixture which fills the spaces between the other parts of the cell. The sarcolemma is the membrane that surrounds the muscle fibre. It has in-growths known as T-tubules in places, which enables signals in the form of action potentials to penetrate down into the cell and activate the sarcoplasmic reticulum.


How do muscles contract?

The simple answer is that muscles contract via the shortening of muscle fibres - or, microscopically speaking, the shortening of myofibrils. However, the way in which they shorten is a rather complicated - but very impressive - process, which is well worth understanding. It is explained in the rest of this article.


How are myofibrils arranged?

This is essentially the crucial piece of information in understanding muscle contraction - how, and why, myofibrils are arranged in a particular way. Being the functional component of muscle contraction, it is inevitable that their structure is tailored to the task they have to perform.

Myofibrils are made up of units known as sarcomeres. These are formed by the arrangment of the proteins myosin and actin. As hopefully shown in the image on the left, the myofibrils contained in a muscle fibre are made up of filaments of these two proteins; so short, thin strands of myosin and actin are lined up in the myofibril.

The way that these are lined up is very important for understanding how muscles contract. Along each myofibril are a series of Z-discs or Z-lines. These mark the start of a new sarcomere; so a sarcomere is made up of everything between two Z-discs.

Half way between these two Z-discs is an M-line. Myosin (thick) filaments are attached to the M-line, while actin (thin) filaments are attached to the Z-discs.

When a muscle is resting, the myosin and actin are not actually touching. They get quite close, as we will discuss in a moment, but they are not in contact in a fully relaxed muscle. From the thick myosin filament, myosin heads stretch out towards the thin filaments, and when muscles are to contract, these myosin heads form crossbridges with the actin filaments.

The band which has no myosin in it is known as the I-band, while the zone with only myosin in it is known as the H-zone or H-band.

The A-band is the band which contains the myosin - including both the H-zone, and the area containing both myosin and actin.

The terminology is quite complicated - it makes understanding muscles a lot more complicated, but it helps enormously to understand what happens when a muscle contracts. It is also helpful to recognise these areas, because it is these bands which give skeletal muscle its striated appearance.

To really understand how a crossbridge is formed, we really need to take an even closer look. If we take a glance at where the myosin head stretches towards the actin filament, we can see that the actin filament is made up of lots of molecules of actin joined together. These are wrapped in another protein called tropomyosin, which prevents the myosin from reaching the actin. Dotted along these strands of tropomyosin are molecules of yet another protein, known as troponin.

The crossbridge is formed between the myosin head and the actin when tropomyosin moves out of the way, in a process known as the crossbridge cycle.

To fully understand how crossbridges are formed, there is another important feature of muscles which must be understood: the sarcoplasmic reticulum. Crossbridges are initiated by the addition of calcium, as is discussed later, and it is through the sarcoplasmic reticulum that this calcium is added to the system at the right time. Importantly, the myofibrils shorten the muscle fibres, but rely on stimulation from other parts of the muscle cell.


What is the sarcoplasmic reticulum?

As mentioned above, contraction of muscles requires addition of calcium from somewhere. As it happens, most of the calcium comes from a large store in muscle cells called the sarcoplasmic reticulum (SR).

The SR forms a network of tubes around the myofibrils, and is filled with a high concentration of calcium. It has special areas called the cisterna which comes into contact with T-tubules.

T-tubules are in-growths of the membrane of the muscle cell (the sarcolemma). When an action potential travels across the membrane of the muscle cell, it travels down T-tubules into the heart of the muscle cell, enabling crossbridge cycles to be initiated simultaneously.

It is at the cisterna that calcium is able to flood out of the sarcoplasmic reticulum. There are special proteins on the membrane of the T-tubule which act as gates on the SR cisterna. When the membrane is depolarised by the action potential (b in the image below), these proteins change shape (c), opening the channels in the cisterna, and letting calcium diffuse down a large concentration gradient (d).

When the action potential leaves, the proteins change back, stopping further calcium exit from the SR. The calcium, after it has been used, is pumped back into the SR using pumps on its membrane (f).

This method cleverly regulates contribution of calcium into sacroplasm - calcium only leaves the SR when an action potential passes along the sarcolemma. Through this mechanism, muscles will only contract when stimulated by a nerve.

The calcium released from the SR goes into the sarcoplasm in order to act on proteins in the myofibril and intiate the crossbridge cycle.


What is the crossbridge cycle?

The crossbridge cycle involves a series of steps that lead to the protein myosin pulling on actin filaments. To understand how this shortens muscles, a full understanding of myofibril's arrangement is needed.

When an action potential passes along the sacrolemma, the sarcoplasmic reticulum releases calcium into the sarcoplasm. This binds to the protein troponin, which causes tropomyosium to move out of the way.

Myosin has two sites for binding: one for actin and another for ATP. Previously blocked from actin by the tropomyosin, it is now free to bind to the thin filament. As it does this, a molecule of ATP which is bound to the myosin head is hydrolysed. As the products are released, the shape of the myosin head changes, and the actin is pulled by the myosin head.

As another ATP molecule binds to the myosin head, the shape changes again, and the filaments return to their original positions.


How do muscles grow?

Anyone who has done any exercise knows that the purpose is either to lose weight, or to help muscles to grow. The thing is, after you've reached around half way through pregnancy, the number of muscle fibres doesn't increase - so throughout your life, what's the point in doing exercise?

There are two ways that muscles grow. Either they increase in length (which needs to happen as you get bigger) or they increase in diameter (which is what happens in exercise, as muscles get bulkier).

If a muscle increases in length, this means that it has more sarcomeres added on to the end of each muscle fibre. Each muscle fibre is made up of a number of sarcomeres placed end-to-end; the more sarcomeres you have, the longer the fibre is going to be. The number of fibres hasn't increases, only the number of subunits. That is the first way that muscle growth occurs.

If a muscle increases in diameter, this means that it has more sarcomeres in each cell. A cell will be more bulky the more sarcomeres it has squeezed in. This doesn't mean that it has any more muscle fibres, only that it has more subunits squeezed around. That is what happens when you do enough exercise to build up your muscles.


Further Reading