Muscle

Muscle is a tissue in animal bodies. Muscles are contractile tissue. This means they get shorter. When they get shorter, this is called contraction.

Muscles help us to move our body parts. Many muscles pull on bones to make arms and legs move.

There are three kinds of muscles:

1. Skeletal muscles
2. Smooth muscles
3. Cardiac muscles

Muscle action can be classified as being either voluntary or involuntary.

The Skeletal muscles move the limbs (arms and legs). They move the jaw up and down so that food can be chewed. Skeletal muscles are the only voluntary muscles. This means they are the only muscle that you can choose to move.

The Cardiac muscle is the muscle in the heart. When this muscle contracts (makes itself smaller) it pushes blood through the circulatory system. The Cardiac muscle is not voluntary. Animals do not choose for their heart to contract.

The Smooth muscles are the other muscles in the body that are involuntary. Smooth muscles are in many places. They are in:

• The gastrointestinal system – this includes the stomach and intestines. This is how food moves through us and we take energy from it.
• Blood vessels – smooth muscles make blood vessels smaller or bigger. This controls blood pressure.
• Hairs – smooth muscle in hair follicles makes your hair stand up when you are scared or get cold.

Muscles are made of many muscle cells. The cells contract together to make the muscle get shorter. The muscle cells know to do this together because many of them get information sent to them by nerves. Then cells that get the message from nerves tell other cells that are near them. They tell the other cells by sending an electrical current.

Muscle cells are filled with proteins called actin and myosin. These are the proteins that make the muscle contract (get shorter.)

When a nerve tells a muscle to contract, the muscle opens holes in its cell membrane. These holes are proteins that are called calcium channels. Then calcium rushes into the cell. Calcium also comes out of a special place in the cell called the sarcoplasmic reticulum. This calcium sticks to the actin and myosin. This makes these proteins contract.

Contraction also needs ATP. This is the energy that your cells use. It is made from using glucose in the cell. It takes a lot of energy to release contracted muscles. They use most of the energy your body uses.

Muscles work in pairs called "antagonistic pairs".

Exercise makes muscles get bigger (see hypertrophy). Exercise also makes muscles stronger. If a person does not exercise, their muscles atrophy. This means their muscles get smaller and weaker.

There are many different kinds of muscle diseases. There are three big groups of diseases:

1. Neuromuscular diseases – these are problems with how the nerves tell the muscles to move. Strokes, cerebral palsy, and Parkinson's disease are neuromuscular diseases.
2. Motor endplate diseases – these are problems with the place where the nerve tells the muscle to move. Tetanus and myasthenia gravis are motor endplate diseases.
3. Myopathies – these are problems with the structure of the muscle. Muscular dystrophy, cancers like Ewing's sarcoma, and cardiomyopathy are myopathies.

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