Monday, 2 March 2015

BASIC ORGANIZATION OF MUSCLE TISSUE: Sarcoplasm, sarcoplasmic reticulum, histological features of muscle, general mechanism of muscle contraction, sliding filament theory.

About 40 per cent of the body is skeletal muscle, and perhaps another 10 per cent is smooth and cardiac muscle. Some of the same basic principles of contraction apply to all these different types of muscle.

Physiologic Anatomy of Skeletal Muscle

Skeletal Muscle Fiber

Diagram shows the organization of skeletal muscle, demonstrating that all skeletal muscles are composed of numerous fibers ranging from 10 to 80 micrometers in diameter. Each of these fibers is made up of successively smaller subunits, also shown in diagram and described in subsequent paragraphs. In most skeletal muscles, each fiber extends the entire length of the muscle. Except for about 2 per cent of the fibers, each fiber is usually innervated by only one nerve ending, located near the middle of the fiber.


The sarcolemma is the cell membrane of the muscle fiber. The sarcolemma consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with a tendon fiber, and the tendon fibers in turn collect into bundles to form the muscle tendons that then insert into the bones.

Myofibrils; Actin and Myosin Filaments

Each muscle fiber contains several hundred to several thousand myofibrils, which are demonstrated by the many small open dots in the cross-sectional view of diagram. Each myofibril is composed of about 1500 adjacent myosin filaments and 3000 actin filaments, which are large polymerized protein molecules that are responsible for the actual muscle contraction. These can be seen in longitudinal view in the electron micrograph of diagram and are represented diagrammatically in diagram, parts E through L. The thick filaments in the diagrams are myosin, and the thin filaments are actin. Note in diagram that the myosin and actin filaments partially interdigitate and thus cause the myofibrils to have alternate light and formation of portions of the contractile filaments of the sarcomere, especially the myosin dark bands, as illustrated in diagram. The light bands contain only actin filaments and are called I bands because they are isotropic to polarized light. The dark bands contain myosin filaments, as well as the ends of the actin filaments where they overlap the myosin, and are called “A” bands because they are anisotropic to polarized light. Note also the small projections from the sides of the myosin filaments in diagram. These are cross-bridges. It is the interaction between these cross-bridges and the actin filaments that causes contraction. Diagram also shows that the ends of the actin filaments are attached to a so called “Z” disc. From this disc, these filaments extend in both directions to interdigitate with the myosin filaments. 

The Z disc, which itself is composed of filamentous proteins different from the actin and myosin filaments, passes crosswise across the myofibril and also crosswise from myofibril to myofibril, attaching the myofibrils to one another all the way across the muscle fiber. Therefore, the entire muscle fiber has light and dark bands, as do the individual myofibrils. These bands give skeletal and cardiac muscle their striated appearance. The portion of the myofibril (or of the whole muscle fiber) that lies between two successive Z discs is called a sarcomere. When the muscle fiber is contracted, as shown at the bottom of diagram, the length of the sarcomere is about 2 micrometers. At this length, the actin filaments completely overlap the myosin filaments, and the tips of the actin filaments are just beginning to overlap one another. We will see later that, at this length, the muscle is capable of generating its greatest force of contraction.

What Keeps the Myosin and Actin Filaments in Place? Titin Filamentous Molecules. The side-by-side relationship between the myosin and actin filaments is difficult to maintain. This is achieved by a large number of filamentous molecules of a protein called titin. Each titin molecule has a molecular weight of about 3 million, which makes it one of the largest protein molecules in the body. Also, because it is filamentous, it is very springy. These springy titin molecules act as a framework that holds the myosin and actin filaments in place so that the contractile machinery of the sarcomere will work. There is reason to believe that the titin molecule itself acts as template for initial filaments. 


The many myofibrils of each muscle fiber are suspended side by side in the muscle fiber. The spaces between the myofibrils are filled with intracellular fluid called sarcoplasm, containing large quantities of potassium, magnesium, and phosphate, plus multiple protein enzymes. Also present are tremendous numbers of mitochondria that lie parallel to the myofibrils. These supply the contracting myofibrils with large amounts of energy in the form of adenosine triphosphate (ATP) formed by the mitochondria.

Sarcoplasmic Reticulum

Also in the sarcoplasm surrounding the myofibrils of each muscle fiber is an extensive reticulum, called the sarcoplasmic reticulum. This reticulum has a special organization that is extremely important in controlling muscle contraction. The very rapidly contracting types of muscle fibers have especially extensive sarcoplasmic reticula.


General Mechanism of Muscle Contraction

The initiation and execution of muscle contraction occur in the following sequential steps.
1. An action potential travels along a motor nerve to its endings on muscle fibers. 
2. At each ending, the nerve secretes a small amount of the neurotransmitter substance acetylcholine.
3. The acetylcholine acts on a local area of the muscle fiber membrane to open multiple “acetylcholine gated” channels through protein molecules floating in the membrane.
4. Opening of the acetylcholine-gated channels allows large quantities of sodium ions to diffuse to the interior of the muscle fiber membrane. This initiates an action potential at the membrane.
5. The action potential travels along the muscle fiber membrane in the same way that action potentials travel along nerve fiber membranes.
6. The action potential depolarizes the muscle membrane, and much of the action potential electricity flows through the center of the muscle fiber. Here it causes the sarcoplasmic reticulum to release large quantities of calcium ions that have been stored within this reticulum.
7. The calcium ions initiate attractive forces between the actin and myosin filaments, causing them to slide alongside each other, which is the contractile process.
8. After a fraction of a second, the calcium ions are pumped back into the sarcoplasmic reticulum by a Ca++ membrane pump, and they remain stored in the reticulum until a new muscle action potential comes along; this removal of calcium ions from the myofibrils causes the muscle contraction to cease. 

We now describe the molecular machinery of the muscle contractile process.

Molecular Mechanism of Muscle Contraction

Sliding Filament Mechanism of Muscle Contraction

The diagram demonstrates the basic mechanism of muscle contraction. It shows the relaxed state of a sarcomere (top) and the contracted state (bottom).In the relaxed state, the ends of the actin filaments extending from two successive Z discs barely begin to overlap one another. Conversely, in the contracted state, these actin filaments have been pulled inward among the myosin filaments, so that their ends overlap one another to their maximum extent. Also, the Z discs have been pulled by the actin filaments up to the ends of the myosin filaments. Thus, muscle contraction occurs by a sliding filament mechanism. But what causes the actin filaments to slide inward among the myosin filaments? This is caused by forces generated by interaction of the cross-bridges from the myosin filaments with the actin filaments. Under resting conditions, these forces are inactive, but when an action potential travels along the muscle fiber, this causes the sarcoplasmic reticulum to release large quantities of calcium ions that rapidly surround the myofibrils. The calcium ions in turn activate the forces between the myosin and actin filaments, and contraction begins. But energy is needed for the contractile process to proceed. This energy comes from high energy bonds in the ATP molecule, which is degraded to adenosine diphosphate (ADP) to liberate the energy. In the next few sections, we describe what is known about the details of these molecular processes of contraction.



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