Thursday, February 15, 2007

HYBW #6 - Muscles

Muscles move things. They work with our skeleton to help move us around. They also help us chew our food, and move it through our digestive tracts.

The human body has more than 650 muscles, making up half its mass.

Humans have three different kinds of muscle:

Skeletal muscle holds the skeleton together, give the body shape, and help it with everyday movements. It is can contract quickly and powerfully.

Smooth muscle is controlled by the nervous system. Examples of smooth muscles are the walls of the stomach and intestines. Smooth muscle is also found in the walls of blood vessels.

Cardiac muscle comprises and powers your heart.

Muscles are controlled by the brain and nervous system. Smooth and cardiac muscles are controlled automatically by the brain and the upper part of the spinal cord called the brain stem. Skeletal muscles are voluntarily regulated by the parts of the brain known as the cerebral motor cortex and the cerebellum.

The motor cortex sends electrical signals through the spinal cord and peripheral nerves to the muscles, causing them to contract. The motor cortex on the right side of the brain controls the muscles on the left side of the body and vice versa.

The cerebellum coordinates the muscle movements ordered by the motor cortex. Sensors in the muscles and joints send messages back through peripheral nerves to tell the cerebellum and other parts of the brain what is going on.

Muscles move body parts by contracting and then relaxing. Your muscles can pull, but they can't push. So they work in pairs of flexors and extensors.

The flexor contracts to bend a limb at a joint. Then, when you've completed the movement, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint.

For example, the biceps muscle, in the front of the upper arm, is a flexor, and the triceps, at the back of the upper arm, is an extensor. When you bend at your elbow, the biceps contracts. Then the biceps relaxes and the triceps contracts to straighten the elbow.