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Anatomy of Muscles and
Muscle Tissue
Human Anatomy & Physiology, Sixth Edition
Elaine N. Marieb
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
9
Muscle Overview
 The three types of muscle tissue are skeletal, cardiac,
and smooth
 These types differ in structure, location, function, and
means of activation
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Muscle Similarities
Skeletal and smooth muscle cells are elongated and are
called muscle fibers
Muscle contraction depends on two kinds of myofilaments
– actin and myosin
Muscle terminology is similar:
 Sarcolemma – muscle plasma membrane
 Sarcoplasm – cytoplasm of a muscle cell
 Prefixes – myo, mys, and sarco all refer to muscle
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Skeletal Muscle Tissue
 Packaged in skeletal muscles that attach to and cover the
bony skeleton
 Has obvious stripes called striations
 Is controlled voluntarily (i.e., by conscious control)
 Contracts very rapidly, but tires easily
 Is responsible for overall body motility
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Cardiac Muscle Tissue
 Occurs only in the heart
 Is striated like skeletal muscle but is involuntary
(controlled by the autonomic nervous system)
 Contracts at a fairly steady rate set by the heart’s
pacemaker cells
 Neural controls allow the heart to respond to changes in
bodily needs
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Cardiac Muscle Cells
A. General Features
1. involuntary muscle
2. one, centrally located nucleus
3. mitochondria larger and more numerous
B. Structure of Tissue
1. muscle fibers branch and interconnect
2. intercalated disc - thickening of sarcolemma
3. cells connected by gap junctions
a. allow passage of ions like Calcium
b. makes adjacent cells electrically linked
c. allows for rhythmic, domino-like contraction
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Smooth Muscle Tissue
 Found in the walls of hollow visceral organs, such as the
stomach, urinary bladder, and respiratory passages
 Forces food and other substances through internal body
channels
 It is not striated and is involuntary (controlled by the
autonomic nervous system)
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Smooth Muscle
 Composed of spindle-shaped fibers with a diameter of 2-10
m and lengths of several hundred m
 Lack the coarse connective tissue sheaths of skeletal
muscle, but have fine endomysium
 Organized into two layers (longitudinal and circular) of
closely apposed fibers
 Found in walls of hollow organs (except the heart)
 Peristalsis – alternating contractions and relaxations of
smooth muscles that mix and squeeze substances through
the lumen of hollow organs
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Smooth Muscle
Figure 9.24
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Smooth Muscle Cells
A. General Features
1. non-striated
2. 5-10 um in diameter; 30-200 um long
3. thick in middle; thinner, tapering off to the end
4. single oval centrally located nucleus
5. actin and myosin fibers not arranged as sarcomere
6. lack of organization (no bands) --> smooth muscle
7. also contain intermediate filaments
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Smooth Muscle Cells
B. Difference in Contraction
1. intermediate filaments attach to dense bodies
2. as muscle cell contracts, twists like corkscrew
3. caveolae - like T tubules of skeletal muscle
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Smooth Muscle Cells
C. Two Kinds of Smooth Muscle
1. visceral (single unit) muscle
a. small arteries and veins
b. viscera - stomach, intestines, uterus, bladder
c. continuous network with gap junctions
d. action spreads from one cell to another
2. multiunit muscle
a. each fiber (cell) has it own nerve ending
b. no gap junctions
c. large arteries, large airways, arrector pili
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Skeletal Muscle: Nerve and Blood Supply
 Each muscle is served by one nerve, an artery, and one
or more veins
 Each skeletal muscle fiber is supplied with a nerve
ending that controls contraction
 Contracting fibers require continuous delivery of
oxygen and nutrients via arteries
 Wastes must be removed via veins
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Skeletal Muscle
Each muscle is a discrete organ composed of muscle tissue,
blood vessels, nerve fibers, and connective tissue
The three connective tissue sheaths are:
 Endomysium – fine sheath of connective tissue
composed of reticular fibers surrounding each muscle
fiber
 Perimysium – fibrous connective tissue that surrounds
groups of muscle fibers called fascicles
 Epimysium – an overcoat of dense regular connective
tissue that surrounds the entire muscle
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One Skeletal Muscle in Cross Section
Figure 9.2 (a)
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One Skeletal Muscle Cell (Fiber)
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One Skeletal Muscle Cell Myofibril
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Each Myofibril composed of many Sarcomeres
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Myofibrils
 Myofibrils are densely packed, rodlike contractile
elements
 They make up most of the muscle volume
 The arrangement of myofibrils within a fiber is such that
a perfectly aligned repeating series of dark A bands and
light I bands is evident
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Many Myofibrils in One Muscle Cell
Figure 9.3 (b)
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Sarcomeres
The smallest contractile unit of a muscle
The region of a myofibril between two successive Z discs
 Myofilaments are of two types – thick and thin
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Many Sarcomeres make up a single Myofibril
Figure 9.3 (c)
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Myofilaments: Banding Pattern
 Thick filaments (myosin filaments) – extend the entire
length of an A band
 Thin filaments (actin filaments) – extend across the I
band and partway into the A band
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Myofilaments: Banding Pattern
Figure 9.3 (c, d)
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Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Ultrastructure of Myofilaments: Thick Filaments
 Thick filaments are composed of the protein myosin
 Each myosin molecule has a rodlike tail and two
globular heads
 Tails – two interwoven, heavy polypeptide chains
 Heads – two smaller, light polypeptide chains called
cross bridges
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Ultrastructure of Myofilaments: Thick Filaments
Figure 9.4 (a)(b)
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Ultrastructure of Myofilaments: Thin Filaments
 Thin filaments are composed of the protein actin
 Each actin molecule is a helical polymer of globular
subunits called G actin
 The subunits contain the active sites to which myosin
heads attach during contraction
 Tropomyosin and troponin are regulatory subunits
bound to actin
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Ultrastructure of Myofilaments: Thin Filaments
Figure 9.4 (c)
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Arrangement of the Filaments in a Sarcomere
 Longitudinal section within one sarcomere
Figure 9.4 (d)
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Sarcoplasmic Reticulum (SR)
 SR is an elaborate, smooth endoplasmic reticulum that
mostly runs longitudinally and surrounds each myofibril
 Functions in the regulation of intracellular calcium
levels
 T tubules associate with the paired terminal cisternae to
form triads
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Sarcoplasmic Reticulum (SR)
Figure 9.5
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Neuromuscular Junction (NMJ)
1. motor neuron - nerve cell that innervates muscle
2. motor end plate - where axon meets muscle cell
3. neuromuscular junction - entire muscle/nerve site
4. synapse – name for where nerve terminal meets target cell
5. synaptic vesicles - inclusions in the axon terminal
a. neurotransmitter - chemical messenger
i. acetylcholine ACh (for muscular synapse)
6. synaptic cleft - space between axon and cell
7. motor unit - a neuron and all muscle cells it stimulates
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings
Muscle Hypertrophy and Atrophy
1. muscular hypertrophy
a. increase in size of myofibers (muscle cells)
b. allows for increased strength
c. adult muscle cells are amitotic (do not divide)
2. muscular atrophy
a. progressive loss of myofibrils
i. disuse atrophy
ii. denervation atrophy (neuron lost)
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Interactions of Skeletal Muscles
 Skeletal muscles work together or in opposition
 Muscles only pull (never push)
 As muscles shorten, the insertion generally moves
toward the origin
 Whatever a muscle (or group of muscles) does, another
muscle (or group) “undoes”
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Muscle Classification: Functional Groups
Prime movers – provide the major force for producing a
specific movement
Antagonists – oppose or reverse a particular movement
Synergists – work cooperatively to achieve a certain action
 Add force to a movement
 Reduce undesirable or unnecessary movement
Fixators – synergists that immobilize a bone or muscle’s origin
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Naming Skeletal Muscles
1. Location of muscle – bone or body region associated with
the muscle
2. Shape of muscle – e.g., the deltoid muscle (deltoid =
triangle)
3. Relative size – e.g., maximus (largest), minimus (smallest),
longus (long)
4. Direction of fibers – e.g., rectus (fibers run straight),
transversus, and oblique (fibers run at angles to an imaginary
defined axis)
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Naming Skeletal Muscles
5. Number of origins – e.g., biceps (two origins) and triceps
(three origins)
6. Location of attachments – named according to point of
origin or insertion
7. Action – e.g., flexor or extensor, as in the names of muscles
that flex or extend, respectively
Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings