Muscular System

Muscle tissue is contractile tissue. It is responsible for movement in the organism. There are three types of muscle tissue found in the animal kingdom (striated, cardiac, and smooth). Muscle tissue is grouped in two ways: structure of the cells and whether it is under the control of our conscious or unconscious mind.

Skeletal Muscle. This muscle is attached to the organisms skeleton. Its major responsibility is to move the organism. These muscles are paired up in such a way as there movements oppose each other. This is called working in antagonistic pairs. Skeletal muscles contract actively, while the same muscle will relax passively. If a limb is moved in one direction actively, it cannot get back to its original place by just relaxing. Another muscle must contract to pull it back to its original place. The elbow joint is moved by the alternate contraction and relaxation of the biceps and triceps muscles. These muscles are under the control of the cerebrum. They are called voluntary muscles. Muscle coordination is under the control of the cerebellum. The cerebrum allows us to control the movement of a part of the body but it's the cerebellum that allows us to walk, pick up things, and control these movements in a coordinated manor.

Skeletal Muscle Structure.The cells of skeletal muscles are long fiber-like structures. They contain many nuclei and are subdivided into smaller structures called myofibrils. Myofibrils are composed of 2 kinds of myofilaments. The thin filaments are made of 2 strands of the protein actin and one strand of a regulatory protein coiled together. The thick filaments are staggered arrays of myosin molecules.

Units of organization of skeletal muscle. The filaments are organized into structures called sarcomeres. Sarcomeres are constructed in the following manner:

Z lines are at the borders of the sarcomere. They align in adjacent myofibrils.

I bands are areas near the edge of the sarcomere containing only thin filaments.

A bands are regions where thick and thin filaments overlap and correspond to the length of the thick filaments.

H zones are areas in the center of the A bands containing only thick filaments.

The Mechanism of Muscular contraction. Muscle contraction reduces the length of each sarcomere. The sliding filament theory is explained below.

The thin filaments ratchet across thick filaments to pull the Z lines together and shorten the sarcomere. The myofilaments themselves do not contract.

Myosin molecules on thick filaments attach to the actin on the thin filaments to form a crossbridge. The cross bridges then bend inward pulling the thin filament toward the center of the sarcomere. These cross bridges are broken and reformed further down .

Energy for cross bridge formation comes from the hydrolysis of ATP by the head region of the myosin.

Skeletal muscle contracts when stimulated by motor neurons.

Graded Contraction of Whole Muscles. are due to summation of multiple motor unit activity and wave summation.

Motor neurons usually deliver their stimuli rapidly, resulting in a smooth contraction.

A motor unit consist of a single motor neuron and all the muscle fibers it controls.

As more motor neurons are recruited by the brain, tension in the muscle progressively increases.

Duration of the muscle contraction depends on how long the concentration of calcium remains elevated.

Slow fibers have longer lasting twitches because they have less sarcoplasmic reticulum. The calcium remains in the cytoplasm longer. They have many mitochondria, rich blood supply, and myoglobin.

Fast fibers have short duration twitches and found in fast muscles for rapid, powerful contractions.

Cardiac Muscle. in vertebrates is only found in the heart. It is striated. Muscles cells are branched, and the junctions between the cells contain intercalated discs that electrically connects all heart muscle cells, allowing coordinated action. Cells can also generate their own action potential.

Smooth Muscle. lack striations and contain less myosin; the myosin is not associated with specific actin strands. Contractions are slow but there is a greater range of control. Calcium must enter the cell through the plasma membrane during an action potential. Found in the walls of blood vessels and the digestive organs.