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.
Sarcolemma
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.
Sarcoplasm
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.
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FUNDAMENTALS OF ARRANGEMENT OF MUSCLE TISSUE |
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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SLIDING FILAMENT THEORY |
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ACTIN & MYOSIN FILAMENT INTERACTION |
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