3D-HuBEd No.4 -MUSCLE-

Muscles are grouped into three types roughly: the cross-striated muscle, the smooth muscle and the heart muscle. The cross-striated muscle is called the skeletal muscle. Skeltal muscles stick to skeletons and they can move them voluntarily. On the other hand, the smooth muscle and the heart muscle are involuntary muscles.

The automatic nervous system controls their activities. The smooth muscle mainly forms the walls of internal organs. Even if this muscle stretches, tension of the muscle does not change. Compared with cross-striated muscle, the contraction of the smooth muscle is slower and more rhythmical. The heart muscle forms the heart and makes beats of the heart. It has features both of the cross-striated muscle and smooth muscle. This chapter explains mainly about skeletal muscles.

More than 600 skeletal muscles exist in human body and they have various shapes and sizes depending on their functions. Skeltal muscles usually work in pairs, one on each side of the bone. When one musclecontracts, the other skeletal muscle expands. These antagonisms of these pairs of skeletal muscles enable us to make many kinds of movement of the body and expressions.

Refer to chapter 0 for more information on how to use this textbook.

　　4.1 The structure of the sleletal muscle

　　4.2 The contraction of the muscle fiber

4.1 The structure of the skeletal muscle.

The total amount of skeletal muscle makes up about 40% of tissues of whole human body. A skeletal muscle consists of long fiber muscles that have many nucleuses.

A fiber muscle is made up of myofibrils that consist of protein. There are light tone parts and dark tone parts and they are appeared one after the other.

This arrangement makes the cross-striation of a fiber muscle. A kind of separator exists between this light part and dark part, which is called a Z membrane. The myotome is between a Z membrane and a next Z membrane. It is thought to be the structural unit of myofibril.

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4.2 The contraction of the muscle fiber

The contraction of a muscle is performed using the energy of Adenosine Tri-Phosphate(ATP). In a myofibril, the contraction occurs when the actin filament slides into the space between the myosin filaments.

A lot of crosslinks(part of a myosin molecule) project from a myosin filament. And the center of activation of ATPase exists in this part. When a muscle is not stimulated, the actin and the myosin are separated from one another and the ATPase does not work.

When a muscle is stimulated, the contraction occurs through the following steps:

The calcium ions are released from the sarcoplasmic reticulum that is around the muscle fiber.
The calcium ions case the ATPase to combine with the actin.
Using the energy derived from, the ATPase waves and the actin filament slides into the myosin filament, making the muscle contracts.
When the stimulation stops, the calcium ion is taken into the sarcoplasmic reticulum.

Then the actin is separate from the myosin and the actin filament goes back. Considering its features, the ATPase is an enzyme that resolves ATP.

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