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From MyMCAT

Contents 1 Introduction 2 Types of Muscle 2.1 Skeletal Muscle 2.2 Smooth 2.3 Cardiac 3 Muscle Contraction 3.1 The Different Bands 3.2 The molecular Components

Introduction

Muscle exists in a variety of types, each suited for the job they perform. In all of them, Calcium is used and ATP is consumed during contraction.

Types of Muscle

Skeletal Muscle

Skeletal muscle, as its name is implies, is found throughout the body attached to the skeleton. These muscles are under voluntary control and allow one to control the body's major movements. It is called striated muscle due to its appearance. Each muscle fiber is actually a fusion of muscles cells forming a long tubule and as such has many nuclei.

Smooth

Smooth muscle is found in the walls of all the hollow organs of the body except the heart. These muscles are under involuntary control and continuously act to regulate blood flow in arteries, move food along the digestive pathway, expel urine, and control intake/exhaust of air in the lungs. Unlike skeletal muscles, they are nonstriated, and there is only one nuclei per muscle fiber. Smooth muscle contracts slowly and rhythmically.

Cardiac

Cardiac, or heart muscle, is found in the walls of the heart and is also under control of the autonomic nervous system. Cardiac muscle cells have one central nucleus, like smooth muscle, but it also is striated, like skeletal muscle. The cardiac muscle cell is rectangular in shape. The contraction of cardiac muscle is involuntary, strong, and rhythmical.

Muscle Contraction

Sarcomeres are the basic unit of contraction in a muscle fiber and they are arranged into myofibrils. Sarcomeres are multi-protein complexes composed of three different filament systems:

* The thick filament system is composed of myosin protein.
* The thin filaments are assembled by actin monomers bound to Nebulin.
* Nebulin and Titin gives stability and structure to the sarcomere.

The sarcomeres are what give skeletal and cardiac muscles their striated appearance and as expected, the myofibrils of smooth muscles are not arranged into sacromeres.

The Different Bands

Under a light microscope, the organization of the different protein filaments can be seen. The thick and thin filaments overlap repeatedly throughout the myfibril and during contraction they are pulled past each other, increasing the overlapping regions.

The A band, is the region that covers the whole length of the thick (myosin) filaments. Because of the overlap, the ends of the thin filaments are also covered by this region. The I band, is the region covered by only the thin (actin) filaments and does not include the ends which are part of the A band. The H band, is the portion of the A band which does not include the overlapping region.

During contraction, the thin and thick filaments are pulled past each other, resulting in a larger overlap. As such, the I and H bands shorten since more of their length becomes overlapped. Only the A band remains the same as it is defined as the length of the thick filaments, which does not change during contraction.

An additional band, the Z line, is used to describe the region of the thin filaments which are interconnected. This line separates the start of one sarcomere and the end of another.

The molecular Components

The protein tropomyosin covers myosin binding sites one the actin molecules in the muscle cell. To allow the muscle cell to contract, tropomyosin must be moved to uncover the binding sites on the actin. Calcium ions bind with troponin molecules (which are dispersed throughout the tropomyosin protein) and alter the structure of the tropomyosin, forcing it to reveal the cross bridge binding site on the actin. The concentration of calcium within muscle cells is controlled by the sarcoplasmic reticulum, a unique form of endoplasmic reticulum. Muscle contraction ends when calcium ions are pumped back out of the sarcomere.

At rest, the myosin head is bound to an ATP molecule in a low-energy configuration and is unable to access the cross bridge binding sites on the actin. However, the myosin head can hydrolyze ATP into ADP and an inorganic phosphate ion. A portion of the energy released in this reaction changes the shape of the myosin head and promotes it to a high-energy configuration. Through the process of binding to the actin, the myosin head releases ADP and inorganic phosphate ion, changing its configuration back to one of low energy. As the filament of actin moves away from the myosin head and back toward the center of the sarcomere, the myosin head is unable to preserve its bond with the actin. After cross bridge dissociation, ATP binds with the myosin head and the head is ready for another cycle of muscle contraction.

Remember that neither filament shortens during contraction, only their overlap increases as they are pulled together.