Download Muscle Overview Function: to transform chemical energy (ATP) into

Muscle Overview
Function: to transform chemical energy (ATP) into
mechanical energy
The three types of muscle tissue are:
Skeletal
Cardiac
smooth
These types differ in structure, location, function,
and means of activation
PLAY
InterActive Physiology ®:
Anatomy Review: Skeletal Muscle Tissue, page 3
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
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
Longest muscle cells
Has obvious stripes called striations
Is controlled voluntarily (i.e., by conscious control)
Contracts rapidly but tires easily
Is responsible for overall body motility
Is extremely adaptable and can exert forces ranging from a
fraction of an ounce to over several hundred pounds
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Cardiac Muscle Tissue
Occurs only in the heart
Is striated like skeletal muscle but is not voluntary
Contracts at a fairly steady rate set by the heart’s
pacemaker
Neural controls allow the heart to respond to
changes in bodily needs
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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
Contractions are slow and sustained
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Functional Characteristics of Muscle Tissue
Muscles respond to neurotransmitter stimulus via nerve
cells generating an electrical impulse that passes along the
sarcolemma (plasma membrane) of the muscle cell causing
it to contract
Excitability, or irritability – the ability to receive and
respond to stimuli
Contractility – the ability to shorten forcibly
Extensibility – the ability to be stretched or extended
Elasticity – the ability to recoil and resume the original
resting length
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Muscle Function
i) Produce movement
ii) Maintain posture
iii) Stabilize joints
iv) Generate heat
v) Protection of viscera
vi) act as valves
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Structure and Organization of Skeletal Muscle
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Table 9.1a
Structure and Organization of Skeletal Muscle
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Table 9.1b
Skeletal Muscle
Each muscle is a discrete organ composed of muscle
tissue, blood vessels, nerve fibers, and connective tissue
Each muscle is served by:
1 nerve
1 artery
1 or more veins (much waste to be removed)
With all 3 entering at the central region of the muscle, then
branching outward
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Skeletal Muscle
The three connective tissue sheaths are (internal to
external):
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.
This may blend with the deep fascia that lies
between neighboring muscles
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Skeletal Muscle
The sheaths are continuous with one another and
with the tendons
Thus, when muscle fibers contract, they pull on the
sheaths which transmit force to the bone
They also contribute to the elasticity of muscles
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Skeletal Muscle
PLAY
InterActive Physiology ®:
Anatomy Review: Skeletal Muscle Tissue, pages 4-6
Figure 9.2a
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Skeletal Muscle: Attachments
Skeletal muscles span joints
Attached to bones in at least 2 places
Upon contraction, the muscle’s insertion (movable bone)
moves towards the muscle’s origin (less movable bone)
Direct attachment- epimysium (see slide 11 again) is fused
to the perichondrium or periosteum
Indirect attachment (more common)- tendon or
aponeurosis anchors muscle to the connective tissue
covering the skeletal element
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Microscopic Anatomy of a Skeletal Muscle
Fiber
Unit = muscle fiber = a skeletal muscle cell
Multinucleated
Plasma membrane = sarcolemma
10x bigger than the average body (epithelial) cell
Up to 30 cm long (e.g. that is 12 inches!)
Cytoplasm = sarcoplasm: contains high stores of glycosomes (glycogen
granules) and myoglobin (an oxygen storing pigment)
Contains modified organelles:
i) Myofibrils
ii) Sarcoplasmic reticulum & T-tubles
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Myofibrils
Densly packed, 80% of cell volume
Contain the contractile elements
Striations are repeating series of A bands (dark) and I bands (light)
The A band possesses an H zone which is bisected vertically by an M line
I bands are interrupted by the Z disc (line)
A, I, H, M, Z
(what’s that now?)
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Sarcomeres
The smallest contractile unit of a muscle
The region of a myofibril between two successive
Z discs
Composed of myofilaments made up of contractile
proteins
Contain: A band & ½ I band
Myofilaments are of two types – thick and thin
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Sarcomeres
PLAY
InterActive Physiology ®:
Anatomy Review: Skeletal Muscle Tissue, page 9
Figure 9.3c
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Myofilaments: Banding Pattern
Thick filaments (myosin) – extend the entire length of an A band
Thin filaments (actin) – extend across the I band and partway into the A
band. Anchored by Z disc
Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin
filaments and connects myofibrils to one another
H band is less dense because actin (thin) filaments do not extend there
M line in the center of H band is dense due to protein (desmin) strands
that bind adjacent myosin filaments together
Myofilaments are attached to the sarcolemma at the Z discs and the M
lines
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Myofilaments: Banding Pattern
Figure 9.3c,d
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Ultrastructure of Myofilaments: Thick
Filaments
Thick filament contains 300 myosin molecules
The tails form the central part of the thick filament and consist of 2
interwoven, heavy polypeptide chains
The heads face outward and in opposite directions and consist of 2 smaller,
light polypeptide chains called cross bridges
The central portion is smooth, and the ends are studded with myosin heads
The heads possess actin binding sites & ATPase activity sites (the latter
splits ATP to generate energy)
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Ultrastructure of Myofilaments: Thick
Filaments
Figure 9.4a,b
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Ultrastructure of Myofilaments: Thin
Filaments
Thin filaments are chiefly composed of the protein actin
Each actin molecule is a helical polymer of globular subunits
called G actin
G actin monomers are polymerized into long actin filaments
called fibrous (F) actin
Each thin filament is formed from 2 intertwined actin
filaments
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 9.4c
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Ultrastructure of Myofilaments: Thin Filaments
Tropomyosin:
2 strands spiral about actin
Help stiffen actin
Block myosin binding to actin in the relaxed state
Troponin:
3 polypeptide complex
Tn1: inhibitory subunit bound to actin
TnT: binds to tropomyosin and helps position it on actin
TnC: binds calcium ions.
Tn1
TnC
TnT
Figure 9.4c
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Arrangement of the Filaments in a Sarcomere
Longitudinal section within one sarcomere
Figure 9.4d
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Elastic filament
Elastic filaments are composed of titin
Extends from the Z disc to the thick filament and
runs through the thick filament and is attached to
the M line
2 functions:
Holds the thick filament in place thus
maintaining the A band
Recoils the muscle cell after contraction
terminates. Helps to resist excessive stretching
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Elastic filament
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Sarcoplasmic Reticulum (SR)
SR is an elaborate, smooth endoplasmic reticulum that
mostly runs longitudinally and surrounds each myofibril
Paired terminal cisternae form perpendicular cross
channels at the A band – I band junction
Functions in the regulation of intracellular calcium levels
Stores Ca++ and releases it on demand during contraction
(Ca++ is the final “Go” signal for contraction…are you
thinking about the troponin subunit TnC…you should be!)
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Sarcoplasmic Reticulum (SR)
Elongated tubes called T (transverse) tubules
penetrate into the cell’s interior at each A band–I
band junction (where the sarcolemma penetrates
into the cell interior
Lumen of T-tubule is continuous with the
extracellular space
They conduct nerve impulses from the sarcolemma
deep into the cell interior and to every sarcomere
Impulses signal for the release of Ca++ from
adjacent terminal cisternae
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Sarcoplasmic Reticulum (SR)
Figure 9.5
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Sliding Filament Model of Contraction
During contraction the thin filaments slide past the
thick ones so that the actin and myosin filaments
overlap to a greater degree
In the relaxed state, thin and thick filaments
overlap only at the A bands
Upon stimulation (contraction), the myosin heads
bind and “ratchet” actin toward the center of the
sarcomere
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Sliding Filament Model of Contraction
Z discs are pulled toward the thick filaments
I bands shorten
Distance between Z bands is reduced
H zone disappears
A bands move closer together,
but do not change length
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Sliding Filament Model of Contraction
Each myosin head binds and detaches several
times during contraction, acting like a ratchet to
generate tension and propel the thin filaments to
the center of the sarcomere
As this event occurs throughout the sarcomeres,
the muscle shortens
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Skeletal Muscle Contraction
In order to contract, a skeletal muscle must:
Be stimulated by a nerve ending
Propagate an electrical current, or action potential,
along its sarcolemma
Have a rise in intracellular Ca2+ levels, the final
trigger for contraction
Linking the electrical signal to the contraction is
excitation-contraction coupling
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Nerve Stimulus of Skeletal Muscle
Skeletal muscles are stimulated by motor neurons
of the somatic nervous system
Axons of these neurons travel in nerves to muscle
cells
Axons of motor neurons branch profusely as they
enter muscles
Each axonal branch forms a neuromuscular
junction with a single muscle fiber
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Neuromuscular Junction
The neuromuscular junction is formed from:
Axonal endings, which have small membranous sacs
(synaptic vesicles) that contain the neurotransmitter
acetylcholine (ACh)
The motor end plate of a muscle, which is a specific part of
the sarcolemma that contains ACh receptors and helps form
the neuromuscular junction
Though exceedingly close, axonal ends and muscle fibers
are always separated by a space called the synaptic cleft
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Neuromuscular Junction
Figure 9.7 (a-c)
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Neuromuscular Junction Events
i) Nerve impulse reaches axon terminus
ii) voltage-gated Ca++ channels in membrane open
iii) Ca++ flows into axon from extracellular fluid
iv) ACh is released into synaptic cleft after synaptic vesicles fuse w/
axonal membrane (exocytosis)
v) ACh diffuses across cleft and attaches to receptors on sarcolemma
vi) sarcolemma “fires” and creates an action potential
vii) ACh splits into acetic acid and choline via acetylcholine esterase
(located in the synaptic cleft). This destruction prevents continued
muscle fiber contraction in the absence of additional stimuli
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings