Principles of Biology By Frank H. Osborne, Ph. D. Muscular System

Non-Muscular Movement It is possible to have motion without muscles. Turgor pressure Cytoplasmic streaming Somersaulting in Hydra

Major Human Muscles

Types of Muscles Smooth muscle Smooth muscle is the type of muscle found in all of the involuntary organs except the heart. "Involuntary" means that you cannot control it. The tissue is made of individual muscle cells, each with its own nucleus.

Types of Muscles Smooth muscle The cells of smooth muscle do not have striations. Striations are parallel lines that are perpendicular to the long axis of the muscle cell. They are found in striated muscle and cardiac muscle but not in smooth muscle.

Smooth muscle

Types of Muscles Striated muscle Striated muscle is also called voluntary muscle. It is found attached to the bones of the skeleton by tendons. The individual cells of striated muscle are very long, as long as the muscle itself. Cells this long must have many nuclei so striated muscle cells are multinucleate. Each individual nucleus controls its own area.

Types of Muscles Striated muscle Striated muscle has striations. These cross- bands result from the inner molecular structure of the muscle. Striated muscle is under voluntary control. These are the muscles that you can move when you want them to do something for you.

Striated muscle

Types of Muscles Cardiac muscle Cardiac muscle is found only in the heart. The cells are striated. The cells are connected to each other at their ends at which are located intercalated discs. Each individual cardiac muscle cell has its own nucleus.

Cardiac muscle

Muscle Structure Structure of muscle Muscle consists of thick fibers called myosin and thin fibers called actin. These are found in all types of muscles. The actin fibers are attached to vertical structures called Z-lines. The actin and myosin fibers overlap and are in close proximity to each other. This is called the actomyosin complex.

Muscle Structure Structure of muscle The bands cause the striations of skeletal muscle and cardiac muscle. The A band is the complete length of myosin. The I band is the space between myosins. The H band is the space between actins.

Structure of muscle (relaxed)

Structure of muscle (contracted)

Muscle Contraction In relaxed muscle, the Z-lines are far apart and the H and I bands are wide. When the muscle contracts, the actin and myosin bands slide over each other and the Z- lines get closer. Also, the H and I bands become much shorter. As the units shorten, the entire muscle shortens. This is how muscles contract.

Attachment of Muscles Muscles are attached in a way as to provide pairs of muscles for each moving part of the body. The forearm is an example. When the biceps muscle contracts, it closes the angle that the arm makes. This muscle is called a flexor. The triceps is used to extend the arm and open the angle. It is called an extensor.

Attachment of Muscles Normally these muscles do not contract simultaneously. When the flexor contracts, the extensor permits itself to become extended. When the extensor contracts, the flexor permits itself to become extended. Any muscle can only contract. It cannot extend itself. To extend it must be pulled by a force from another muscle.

Human Forearm

The End Principles of Biology Muscular System