Download Human Anatomy: Skeletal Muscle Tissue

Human Anatomy:
Skeletal Muscle Tissue
© 2014 Pearson Education, Inc.
6 Functions of Skeletal Muscles
1. Produce skeletal movement
2. Maintain body position and posture
3. Support soft tissues
4. Guard body openings (entrance/exit)
5. Maintain body temperature
6. Store Nutrient reserves
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Properties of Muscle Tissue
• Contractility
• Myofilaments are responsible for shortening of
muscles cells
• Actin and myosin are two type of myofilaments
• Excitability
• Nerve signals excite muscle cells, causing
electrical impulses to travel along the
sarcolemma (plasma membrane of a muscle
cell).
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Properties of Muscle Tissue
• Extensibility
• Contraction of a skeletal muscle stretches the
opposing muscle
• Smooth muscle is stretched by substances
within that hollow organ
• Food in stomach; urine in urinary bladder
• Elasticity
• Recoils after being stretched
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Terminology Specific to Muscle Tissue
• Myo and mys—prefixes meaning “muscle”
• Sarco—prefix meaning “flesh”
• Sarcolemma—plasma membrane of muscle
cells
• Sarcoplasm—cytoplasm of muscle cells
• Sarcasm – flesh-eating words!
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Types of Muscle Tissue
Skeletal muscle tissue
• Makes up 40% of body weight
• Cells are striated and multinucleated
• Skeletal muscle is innervated & voluntary
Cardiac muscle tissue
• Occurs only in the walls of the heart
• Cells are striated
• Contraction is involuntary
Smooth muscle tissue
• Occupies the walls of hollow organs
• Cells lack striations
• Innervated by involuntary division of the nervous
system
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Sheaths of connective tissue bind a skeletal muscle and
its fibers together
Epimysium—dense regular connective tissue
Bone
Epimysium
surrounding entire muscle
Perimysium—surrounds each fascicle
(group of muscle fibers)
Epimysium
Endomysium—a fine sheath of connective tissue
wrapping each muscle cell
Perimysium
Endomysium
Tendon
Connective tissue sheaths
Muscle fiber
are continuous with
in middle of
a fascicle
tendons
Blood vessel
Fascicle
(wrapped by perimysium)
Endomysium
(between individual
muscle fibers)
Epimysium
PerimysiumFascicle
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Endomysium
Muscle fiber
Gross Anatomy of a Skeletal Muscle
• Muscle attachments
• Most skeletal muscles run from one bone to
another
• One bone will move; other bone remains fixed
• Origin—less movable attachment
• Insertion—more movable attachment
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Figure 10.3 Muscle attachments (origin and insertion).
Muscle contracting
Origin
by direct
attachment
Brachialis
Tendon
Insertion by
indirect attachment
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Gross Anatomy of a Skeletal Muscle
• Muscle attachments (continued)
• Muscles attach to origins and insertions by
connective tissue (CT)
• Bone markings are present where tendons meet
bones
• E.g. Tubercles, trochanters, and crests
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Microscopic and Functional Anatomy of Skeletal Muscle
Tissue
• The skeletal muscle fiber
• Fibers are long and cylindrical
• Are huge cells—diameter is 10–100 µm
• Length—several centimeters to dozens of
centimeters
• Each cell formed by fusion of embryonic cells:
myoblast cells
• Cells are multinucleate
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Anatomy of Skeletal Muscle Cells
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Figure 10–3
Myofibrils and Sarcomeres
• Striations result from internal structure of
myofibrils
• Myofibrils are long row of repeating segments
called sarcomeres (functional unit of skeletal
muscle tissue)
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Anatomy of a Sarcomere
•Z disc (Z line)—boundaries of each sarcomere
•Thin (actin) filaments—extend from Z disc
toward the center of the sarcomere
•Thick (myosin) filaments—located in the center
of the sarcomere
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• A bands—full length of the thick filament
• H zone—center part of A band where no thin
filaments occur. Shortens during a contraction.
• M line—in center of H zone
• I band—region with only thin filaments
• Spans two adjacent sarcomeres
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Titin and Other Myofibrils
• Titin is a springlike protein in sarcomeres
Titin molecules extend from the Z disc to thick filaments
Function of titin:
• Resists overstretching
• Holds thick filaments in place
• Allows for recoil during relaxation state
Z disc
M line
Z disc
Thin (actin)
filament
Elastic (titin)
filaments
Thick
(myosin)
filament
Myosin
heads
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The Sarcolemma
• Cell membrane of a muscle cell
• Surrounds the sarcoplasm (cytoplasm of muscle
fiber)
• A change in transmembrane potential (VOLTAGE)
begins contractions. (know this!)
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Transverse Tubules (T tubules)
• Directly connected to sarcolemma
• Transmit action potential (VOLTAGE) deep inside the
cell.
• Allow entire muscle fiber to contract simultaneously
• Have same properties as sarcolemma: able to
transmit voltage.
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Sarcoplasmic Reticulum (SR)
• A membranous structure surrounding each myofibril. (Like ER)
• Responds to the action potential
• Contains chambers (terminal cisternae) attached to T tubules
• Stores and releases Ca2+ when an action potential is received
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Cisternae
• Concentrate Ca2+
• Release Ca2+ into cytosol and sarcomeres to begin
muscle contraction.
• A Triad - is formed by 1 T tubule and 2 terminal
cisterna
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So…how do the sarcomeres actually contract?
Thick (myosin)
filament
Thin (actin) filament
Thin (actin)
filament
Movement
Myosin
head
Thick (myosin) filament
Myosin heads attach to actin in the thin filaments,
then pivot to pull the thin filaments inward.
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Sliding Filament Mechanism
• Contraction changes the striation pattern
• Fully relaxed—thin filaments partially overlap thick filaments
• Contraction - see next slide
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Contraction
Z discs move closer together
Sarcomere shortens
I bands shorten; H zone disappears
A band remains the same length
1 Fully relaxed sarcomere of a muscle fiber
Z
I
H
A
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2 Fully contracted sarcomere of a muscle fiber
Z
Z
I
I
Z
A
I
Innervation of Skeletal Muscle Cells
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Figure 10.8 The neuromuscular junction.
Nerve
impulse
Myelinated axon
of motor neuron
Terminal bouton of
neuromuscular junction
Nucleus
Sarcolemma of
the muscle fiber
1 Nerve impulse
stimulates the release
of the neurotransmitter
acetylcholine (ACh) into
the synaptic cleft.
Synaptic
cleft
Terminal bouton
of motor neuron
Synaptic vesicle
containing ACh
Sarcolemma
2 ACh stimulates
changes in the sarcolemma
that excite the muscle fiber.
This stimulus is carried
down the T tubules to
initiate fiber contraction.
Terminal
cistern of SR
Triad
Muscle fiber
Ca2+
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3 Enzymes in the synaptic cleft
break down ACh and thus limit its
action to a single muscle twitch.
Types of Skeletal Muscle Fibers
• Skeletal muscle fibers are categorized
according to two characteristics
• How they manufacture energy (ATP)
• How quickly they contract
• Oxidative fibers—produce ATP aerobically
• Glycolytic fibers—produce ATP anaerobically
by glycolysis
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3 CLASSES OF
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Types of Skeletal Muscle Fibers
1) Slow oxidative fibers
• Red color due to abundant myoglobin
• Obtain energy from aerobic metabolic reactions
• Contain a large number of mitochondria
• Contract slowly and resistant to fatigue
• Fibers are small in diameter
Long distance runner
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Types of Skeletal Muscle Fibers
2) Fast oxidative fibers
• Contract quickly like fast glycolytic fibers
• Are oxygen-dependent: aerobic
• Have high myoglobin content
• Somewhat fatigue resistant
• More powerful than slow oxidative fibers
1500 m race
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Types of Skeletal Muscle Fibers
3) Fast glycolytic fibers
• Contain little myoglobin and few mitochondria
• About twice the diameter of slow oxidative fibers
• Contain more myofilaments and generate more
power
• Depend on anaerobic pathways
• Contract rapidly and tire quickly
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Three Types of Muscle Cells
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Structure of skeletal muscle
• Each cell (fibre) is long and cylindrical
• Muscle fibres are multi-nucleated
• Typically 50-60mm in diameter, and up to 10cm
long
• The contractile elements of
skeletal muscle cells are
myofibrils
• Found in limbs and connected to bones by
ligaments
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Structure of smooth muscle
• Spindle shaped
• Uni-nucleated cells
• Striations not observed
• Actin and myosin filaments are
present(protein fibers)
• Found in visceral organs
• Involuntary
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Structure of cardiac muscle
• Only found in the heart
• Cardiac muscle cells (fibers) are
short, branched and interconnected
• Cells are striated & usually have 1
nucleus
• Adjacent cardiac cells are joined via
electrical synapses (gap junctions)
• These gap junctions appear as dark
lines and are called intercalated
discs
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Muscle Type Overview
Type of
muscle
Nervous
control
Type of
control
Example
Skeletal
Skeletal
Controlled
by CNS
Voluntary
Lifting a
glass
Cardiac
Regulated
by ANS
Involuntary Heart
beating
Smooth
Controlled
by ANS
Involuntary Peristalsis
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Comparison of Muscle Types
Muscle Type
Skeletal
Cardiac
Smooth
Location
Attached to
bone
Heart
Walls of internal
organs + in skin
Function
Movement of
bone
Beating of heart
Movement of
internal organs
Control Mode
Voluntary
Involuntary
Involuntary
Long + slender
Branching
Spindle shape
Striated- light
and dark bands
Many nuclei
Striated
One or two nuclei,
intercalated disks
Non-striated
One nucleus
(visceral)
Shape
Characteristi
cs
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