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Chapter 10
Muscular Tissue
Copyright 2009, John Wiley & Sons
Introduction
The scientific study of muscles is known as.
Muscular tissue accounts for ~45% of totalbody mass.
Most of the work generated by the body (e.g.,pumping blood, moving food through the GI
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, ,maintaining posture, and producing themovements of our skeletons, is a result of theactivity of muscles.
Overview of muscle tissue
There are three types of muscular tissue:, , .
These types differ from one another in their
anatomy, location, & how they are controlled. Muscle tissue functions by sustained
contractions, or alternating contraction/relaxation.
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usc e unct ons epen upon a num er ophysiological properties.
Types of muscular tissue
Skeletal muscle.
Is striated (microscopic bands) and voluntary(contraction can be voluntarily controlled).
Cardiac muscle Found only in the heart, it is striated and involuntary
(contraction isnt consciously controlled).
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Smooth muscle Found in the walls of tubes such as blood vessels.
Is unstriated & involuntarily controlled by the ANS.
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Functions of muscular tissue
1. Body movements require skeletal muscles,, .
2. Body stability requires skeletal muscle
contractions to help maintain body positions.
3. Storing and moving substances within the bodyrequire both smooth & cardiac muscle.
4. Heat roduction is a b roduct of friction caused
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by muscle tissue contracting, especially skeletalmuscle.
Properties of muscular tissue
Electrical excitability is a property shared.
response to specific stimuli (e.g., hormones).
Contractility is an ability to contractforcefully in response to electrical stimuli.
Extensibility is the ability of a muscle cell to
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. Elasticity is the ability of a muscle cell to
return to its original length and shapefollowing a contraction.
Skeletal muscle organization
(Fig. 10.1)
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Structure of skeletal muscles
A skeletal muscle bundle consists of a body (belly)connected b tendons to the skeleton.
Tendons are a nearly avascular structure,composed of a parallel arrangement of collagen
fibers at the ends of muscle. A tendon is an extension of three connective tissue
layers that surrounds the different organizationallevels of a skeletal muscle e. . trice s .
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Muscle fibers are surrounded by endomysium.
Muscle fascicles are surrounded by perimysium.
Muscle bundles are surrounded by epimysium
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Microscopic anatomy of skeletal muscle
fiber (Fig. 10.2)
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Microscopic anatomy of a skeletal muscle
fiber.
Muscle fibers while very small in diameter~ m , run e eng o e musc e
bundle and are usually measured in inches.
Muscle fibers are multinucleate, form beforebirth, and last a lifetime.
Muscle growth after being born is due to
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hypertrophy(increased cell size) rather thanhyperplasia(increased cell numbers).
Microscopic anatomy Sarcolemma, T
tubules, and sarcoplasm.
Sarcolemma= plasma membrane of a
.
T tubules= extensions of the sarcolemma that
tunnel throughout the muscle fiber, are opento the outside, and are filled with ISF.
Sarcoplasm= cytoplasm of a skeletal muscle
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fiber.
Microscopic anatomy Myofibrils and
sarcoplasmic reticulum.
Myofibrils are nonmembrane bound
.than the muscle fiber (2 m), and extend the
entire length of the muscle fiber. Sarcoplasmic reticulum (SR) is like smooth
endoplasmic reticulum in nonmuscle cells.
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SR stores Ca++ until needed. Release ofCa++ into the sarcoplasm couples electrical
excitation to muscle contraction.
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Microscopic anatomy Muscular atrophy.
Muscular atrophy refers to a wasting of muscle due.
Atrophy from unused muscles is termed disuse
atrophy(reversible) because the flow of nerveimpulses to muscle fibers is greatly reduced.
Atrophy from the loss of the nerve supply to a
muscle is called denervation atrophy(irreversible).
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The muscle will shrink considerably, with muscle
fibers replaced by fibrous connective tissue.
Microscopic anatomy Filaments and the
sarcomere.
Myofibrils are made of thick filaments and thin
.
These filaments are arranged in special compartments
called sarcomeres (the functional unit of contraction).
The pattern of overlap of thick & thin filaments consistsof zones and bands which cause the striated
appearance of skeletal muscle.
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A band:
I band:
H zone:
Muscle proteins general points
Myofibrils are built from three kinds of proteins.
genera e orce ur ng
contraction by sliding past each other rather thanliterally shortening.
Regulatory proteins help switch the contraction
process on and off.
Structural roteins kee thick and thin filaments in
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proper alignment, give the myofibril elasticity andextensibility, and link the myofibrils to the
sarcolemma and extracellular matrix.
Muscle proteins contractile & regulatory
Myosin is the major component of thick filaments.
yos n unc ons as a mo or pro e n a ac evesmovement by converting the chemical energy intomechanical energy.
Actin is the major component of thin filaments.
Thin filaments also contain the regulatory proteins,tro om osin and tro onin.
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.
Tropomyosin and troponin regulate the interactionsbetween actin and myosin.
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Muscle proteins structural Structural proteins contribute to the alignment,
stability, elasticity, and extensibility of myofibrils. Titin, myomesin, and dystrophin are key
structura prote ns. Titin is a huge protein that anchors a thick filament to
both a Z disc and the M line, and it accounts for muchof the elasticity and extensibility of myofibrils.
Myomesin proteins form the M line, bind to titin, andconnect adjacent thick filaments to one another.
Dystrophin reinforces the sarcolemma and helps
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transmit tension generated by the sarcomeres to thetendons. It does so by linking thin filaments toproteins of the sarcolemma. In turn, these membraneproteins attach to proteins in the connective tissue thatsurrounds muscle fibers.
Sarcomere Anatomy
(Fig. 10.3)
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TEM Showing Zones and Bands of a
Sarcomere (Fig. 10.4)
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Structure of Thin and Thick Filaments (Fig.
10.5)
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Contraction of Skeletal Muscle Fibers
Nerve impulses cross the NMJ from the
the actions of acetylcholine (ACh).
Muscle impulses move inside the musclefiber via the T tubules causing Ca++ to bereleased into the sarcoplasm from the SR.
++
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contraction, making it possible for the thickand thin filaments to slide past each other,and shortening the sarcomere.
Sliding Filament Mechanism (Fig. 10.7)
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Muscle Tone
Even while at rest, a skeletal muscle exhibits,
to weak, involuntary contractions of its motorunits.
To sustain muscle tone, motor unit groupsare alternately contract and relax whichkee s skeletal muscles firm, without
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producing bodily movements.
This is very important in maintaining posture(e.g., while sitting).
Types of Skeletal Muscle Fibers
Skeletal muscle fibers vary in their content of
2- .
Muscle fibers high in myoglobin are calledred muscle fibers, while those that are lowin myoglobin are called white muscle fibers.
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Skeletal Muscle Fiber Classification
Skeletal muscle fibers can be categorized accordingto the rate at which they contract and relax. So, afiber may be called slow or fast depending on howrapidly ATP is broken down.
Plus, skeletal muscle fibers are categorized accordingto the metabolic reactions used to generate ATP andin how quickly they fatigue.
Based on these characteristics, skeletal muscle fibers
are classified as:
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slow oxidative fibers, fast oxidative-glycolytic fibers, and fast glycolytic fibers.
Exercise and Skeletal Muscle Tissue
The relative ratio of FG and SO fibers is geneticallydetermined and hel s account for individualdifferences in physical performance. For example, Those with more FG fibers often excel in activities that
require periods of intense activity, such as weight lifting orsprinting.
Those with more SO fibers are better at activities thatrequire endurance, such as long-distance running.
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oug e o a num er o s e e a musc e ersusually does not increase, the characteristics ofthose present can change in response to specificexercise regimes.
Histology of Cardiac Muscle
(Fig. 10.8)
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Cardiac Muscle Cells
Striated, involuntary, branched, & uninucleate
Intercellular connections points (intercalated
discs) contain structural and electrical
connections, called desmosomes and gapjunctions, respectively.
Cardiac tissue is capable of
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autorhythmicity, an ability to spontaneouslygenerate impulses that trigger contraction.
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Smooth Muscle Tissue
Involuntary, non-striate, uninucleate, & tapered cells.
ere are wo ypes o smoo musc e:
Single-unit smooth muscle
More common
Autorhythmic cells linked by gap junctions
Found in the walls of small blood vessels and many visceralorgans
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- Less common
Contraction requires neuronal stimulation
Found in large arteries, and respiratory airways
Smooth Muscle Tissue continued
There a numerous differences between thep ys o ogy o smoo musc e ce s an o ermuscle tissues.
Length of contraction
Cell size
Differences in filament types and ratios
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Greater regeneration capability
Histology of smooth muscle tissue (Fig.
10.9)
Copyright 2009, John Wiley & Sons