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Muscles Physiology
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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
myofilamentsactin and myosin
Muscle terminology is similar
Sarcolemmamuscle plasma membrane
Sarcoplasmcytoplasm of a muscle cell
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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 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 70 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 hearts
pacemaker
Neural controls allow the heart to respond tochanges 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
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Functional Characteristics of Muscle Tissue
Excitability, or irritabilitythe ability to receive
and respond to stimuli
Contractilitythe ability to shorten forcibly
Extensibilitythe ability to be stretched or extended
Elasticitythe ability to recoil and resume theoriginal resting length
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Muscle Function
Skeletal muscles are responsible for all locomotion
Cardiac muscle is responsible for coursing the blood
through the body
Smooth muscle helps maintain blood pressure, and
squeezes or propels substances (i.e., food, feces)
through organs
Muscles also maintain posture, stabilize joints, and
generate heat
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Skeletal Muscle
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Skeletal Muscle
Each muscle is a discrete organ composed of muscle
tissue, blood vessels, nerve fibers, and connectivetissue
The three connective tissue sheaths are:
Endomysiumfine sheath of connective tissuecomposed of reticular fibers surrounding eachmuscle fiber
Perimysiumfibrous connective tissue thatsurrounds groups of muscle fibers called fascicles
Epimysiuman overcoat of dense regular
connective tissue that surrounds the entire muscle
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Skeletal Muscle
Figure 9.2 (a)
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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 nerveending 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: Attachments
Most skeletal muscles span joints and are attached to
bone in at least two places
When muscles contract the movable bone, themuscles insertion moves toward the immovable
bone, the muscles origin
Muscles attach:
Directlyepimysium of the muscle is fused to theperiosteum of a bone
Indirectlyconnective tissue wrappings extendbeyond the muscle as a tendon or aponeurosis
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Microscopic Anatomy of a Skeletal Muscle Fiber
Each fiber is a long, cylindrical cell with multiple
nuclei just beneath the sarcolemma
Fibers are 10 to 100 m in diameter, and up to
hundreds of centimeters long
Each cell is a syncytium produced by fusion ofembryonic cells
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Microscopic Anatomy of a Skeletal Muscle Fiber
Sarcoplasm has numerous glycosomes and a uniqueoxygen-binding protein called myoglobin
Fibers contain the usual organelles, myofibrils,sarcoplasmic reticulum, and T tubules
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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 Abands and light I bands is evident
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Myofibrils
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
Composed of myofilaments made up of contractile
proteins
Myofilaments are of two typesthick and thin
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Sarcomeres
Figure 9.3 (c)
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Myofilaments: Banding Pattern
Thick filamentsextend
the entire length of an Aband
Thin filamentsextend
across the I band andpartway into the A band
Z-disccoin-shaped sheet
of proteins (connectins)that anchors the thin
filaments and connects
myofibrils to one another
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Myofilaments: Banding Pattern
Thin filaments do not
overlap thick filaments
in the lighter H zone
M lines appear darker
due to the presence of
the protein desmin
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Myofilaments: Banding Pattern
Figure 9.3 (c, d)
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Ultrastructure of Myofilaments: Thick Filaments
Thick filaments are
composed of the proteinmyosin
Each myosin molecule has
a rodlike tail and twoglobular heads
Tailstwo interwoven,
heavy polypeptide chains
Headstwo 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 chiefly composed of the proteinactin
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
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Arrangement of the Filaments in a Sarcomere
Longitudinal section within one sarcomere
S l i R i l (SR)
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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
Functions in the regulation of intracellular calcium
levels
Elongated tubes called T tubules penetrate into the cells
interior at each A bandI band junction
T tubules associate with the paired terminal cisternae to
form triads
S l i R ti l (SR)
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Sarcoplasmic Reticulum (SR)
Figure 9.5
T T b l
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T Tubules
T tubules are continuous with the sarcolemma
They conduct impulses to the deepest regions of themuscle
These impulses signal for the release of Ca2+from
adjacent terminal cisternae
T i d R l ti hi
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Triad Relationships
T tubules and SR provide tightly linked signals for
muscle contraction
A double zipper of integral membrane proteinsprotrudes into the intermembrane space
T tubule proteins act as voltage sensors
SR foot proteins are receptors that regulate Ca2+
release from the SR cisternae
Slidi Fil t M d l f C t ti
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Sliding Filament Model of Contraction
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 slightly
Upon stimulation, myosin heads bind to actin andsliding begins
Slidi Fil t M d l f C t ti
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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
Sk l t l M l C t ti
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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
N Sti l f Sk l t l M l
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Nerve Stimulus of Skeletal Muscle
Skeletal muscles are stimulated by motor neurons ofthe 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
Ne rom sc lar J nction
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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
Neuromuscular Junction
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Neuromuscular Junction
Figure 9.7 (a-c)
Neuromuscular Junction
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Neuromuscular Junction
When a nerve impulse reaches the end of an axon at
the neuromuscular junction:
Voltage-regulated calcium channels open and allow
Ca2+to enter the axon
Ca2+inside the axon terminal causes axonal
vesicles to fuse with the axonal membrane
Neuromuscular Junction
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Neuromuscular Junction
This fusion releases ACh into the synaptic cleft via
exocytosis
ACh diffuses across the synaptic cleft to ACh
receptors on the sarcolemma
Binding of ACh to its receptors initiates an action
potential in the muscle
Destruction of Acetylcholine
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Destruction of Acetylcholine
ACh bound to ACh receptors is quickly destroyed
by the enzyme acetylcholinesterase
This destruction prevents continued muscle fiber
contraction in the absence of additional stimuli
Action Potential
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Action Potential
A transient depolarization event that includes
polarity reversal of a sarcolemma (or nerve cell
membrane) and the propagation of an action
potential along the membrane
Role of Acetylcholine (Ach)
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Role of Acetylcholine (Ach)
ACh binds its receptors at the motor end plate
Binding opens chemically (ligand) gated channels
Na+and K+diffuse out and the interior of the
sarcolemma becomes less negative
This event is called depolarization
Depolarization
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Depolarization
Initially, this is a local electrical event called end
plate potential
Later, it ignites an action potential that spreads in all
directions across the sarcolemma
Action Potential: Electrical Conditions of a
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The outside
(extracellular) face
is positive, while
the inside face isnegative
This difference in
charge is the restingmembrane potential
Figure 9.8 (a)
Action Potential: Electrical Conditions of a
Polarized Sarcolemma
Action Potential: Electrical Conditions of a
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The predominantextracellular ion isNa+
The predominantintracellular ion isK+
The sarcolemma isrelativelyimpermeable toboth ions
Figure 9.8 (a)
Action Potential: Electrical Conditions of a
Polarized Sarcolemma
Action Potential: Depolarization and Generation
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An axonal terminal of
a motor neuron
releases ACh andcauses a patch of the
sarcolemma to
become permeable to
Na+ (sodium channels
open)
Figure 9.8 (b)
Action Potential: Depolarization and Generation
of the Action Potential
Action Potential: Depolarization and Generation
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Na+enters the cell,and the resting
potential is
decreased(depolarization
occurs)
If the stimulus isstrong enough, an
action potential is
initiatedFigure 9.8 (b)
Action Potential: Depolarization and Generation
of the Action Potential
Action Potential: Propagation of the Action
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Polarity reversal ofthe initial patch ofsarcolemmachanges the
permeability of theadjacent patch
Voltage-regulated
Na+channels nowopen in the adjacentpatch causing it todepolarize
Figure 9.8 (c)
Action Potential: Propagation of the Action
Potential
Action Potential: Propagation of the Action
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Thus, the actionpotential travels
rapidly along the
sarcolemma
Once initiated, the
action potential is
unstoppable, andultimately results in
the contraction of a
muscleFigure 9.8 (c)
Action Potential: Propagation of the Action
Potential
Action Potential: Repolarization
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Action Potential: Repolarization
Immediately after the
depolarization wavepasses, thesarcolemmapermeability changes
Na+channels closeand K+channels open
K+
diffuses from thecell, restoring theelectrical polarity ofthe sarcolemma
Figure 9.8 (d)
Action Potential: Repolarization
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Action Potential: Repolarization
Repolarization occurs
in the same directionas depolarization, andmust occur before themuscle can be
stimulated again(refractory period)
The ionic
concentration of theresting state isrestored by theNa+-K+pump
Figure 9.8 (d)
Excitation-Contraction Coupling
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Excitation Contraction Coupling
Once generated, the action potential:
Is propagated along the sarcolemma
Travels down the T tubules
Triggers Ca2+release from terminal cisternae
Ca2+binds to troponin and causes:
The blocking action of tropomyosin to cease
Actin active binding sites to be exposed
Excitation-Contraction Coupling
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Excitation Contraction Coupling
Myosin cross bridges alternately attach and detach
Thin filaments move toward the center of the
sarcomere
Hydrolysis of ATP powers this cycling process
Ca2+
is removed into the SR, tropomyosin blockageis restored, and the muscle fiber relaxes
Excitation-Contraction Coupling
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Excitation Contraction Coupling
Figure 9.9
Role of Ionic Calcium (Ca2+) in the Contraction
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At low intracellular Ca2+concentration:
Tropomyosin blocks the
binding sites on actin
Myosin cross bridges
cannot attach to binding
sites on actin
The relaxed state of the
muscle is enforced
Role of Ionic Calcium (Ca ) in the Contraction
Mechanism
Figure 9.10 (a)
Role of Ionic Calcium (Ca2+) in the Contraction
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Figure 9.10 (b)
At higher intracellular Ca2+
concentrations:
Additional calcium binds
to troponin (inactive
troponin binds two Ca2+)
Calcium-activated
troponin binds an
additional two Ca2+at a
separate regulatory site
Role of Ionic Calcium (Ca ) in the Contraction
Mechanism
Role of Ionic Calcium (Ca2+) in the Contraction
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Calcium-activated
troponin undergoes a
conformational change
This change moves
tropomyosin away fromactins binding sites
Figure 9.10 (c)
Role of Ionic Calcium (Ca ) in the Contraction
Mechanism
Role of Ionic Calcium (Ca2+) in the Contraction
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Myosin head can
now bind and cycle
This permitscontraction (sliding
of the thin filaments
by the myosin crossbridges) to begin
Figure 9.10 (d)
Role of Ionic Calcium (Ca ) in the Contraction
Mechanism
Sequential Events of Contraction
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q
Cross bridge formationmyosin cross bridge
attaches to actin filament
Working (power) strokemyosin head pivots and
pulls actin filament toward M line
Cross bridge detachmentATP attaches to myosin
head and the cross bridge detaches
Cocking of the myosin head energy fromhydrolysis of ATP cocks the myosin head into the
high-energy state
Sequential Events of Contraction
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Myosin cross bridge attaches to
the actin myofilament
1
2
3
4 Working strokethe myosin head pivots and
bends as it pulls on the actin filament, sliding it
toward the M line
As new ATP attaches to the myosin
head, the cross bridge detaches
As ATP is split into ADP and P i,
cocking of the myosin head occurs
Myosin head
(high-energy
configuration)
Thick
filament
Myosin head
(low-energy
configuration)
ADP and Pi(inorganicphosphate) released
q
Figure 9.11
Thin filament
Contraction of Skeletal Muscle Fibers
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Contractionrefers to the activation of myosins
cross bridges (force-generating sites)
Shortening occurs when the tension generated by thecross bridge exceeds forces opposing shortening
Contraction ends when cross bridges become
inactive, the tension generated declines, andrelaxation is induced
Contraction of Skeletal Muscle (Organ Level)
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( g )
Contraction of muscle fibers (cells) and muscles
(organs) is similar
The two types of muscle contractions are:
Isometric contractionincreasing muscle tension
(muscle does not shorten during contraction)
Isotonic contractiondecreasing muscle length(muscle shortens during contraction)
Isotonic and Isometric Contractions
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Motor Unit: The Nerve-Muscle Functional Unit
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A motor unit is a motor neuron and all the muscle
fibers it supplies
The number of muscle fibers per motor unit can varyfrom four to several hundred
Muscles that control fine movements (fingers, eyes)
have small motor units
Motor Unit: The Nerve-Muscle Functional Unit
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Motor Unit: The Nerve-Muscle Functional Unit
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Large weight-bearing muscles (thighs, hips) have
large motor units
Muscle fibers from a motor unit are spread throughout
the muscle; therefore, contraction of a single motor
unit causes weak contraction of the entire muscle
Muscle Spindles
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Are composed of 3-10 intrafusal muscle fibers that
lack myofilaments in their central regions, arenoncontractile, and serve as receptive surfaces
Muscle spindles are wrapped with two types ofafferent endings: primary sensory endings of type Iafibers and secondary sensory endings of type II
fibers
These regions are innervated by gamma () efferentfibers
Note: contractile muscle fibers are extrafusal fibers
and are innervated by alpha () efferent fibers
Muscle Spindles
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Figure 13.15
Operation of the Muscle Spindles
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Stretching the muscles activates the muscle spindle
There is an increased rate of action potential in Ia
fibers
Contracting the muscle reduces tension on the
muscle spindle
There is a decreased rate of action potential on Iafibers
Operation of the Muscle Spindles
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Figure 13.16
Muscle Twitch
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A muscle twitch is the response of a muscle to a
single, brief threshold stimulus
The three phases of a muscle twitch are:
Latent period
first few milli-seconds after
stimulation
when excitation-contraction
coupling is
taking placeFigure 9.13 (a)
Muscle Twitch
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Period of contractioncross bridges actively form
and the muscle shortens
Period of relaxation
Ca2+is reabsorbed
into the SR, and
muscle tension
goes to zero
Figure 9.13 (a)
Graded Muscle Responses
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Graded muscle responses are:
Variations in the degree of muscle contraction
Required for proper control of skeletal movement
Responses are graded by:
Changing the frequency of stimulation
Changing the strength of the stimulus
Muscle Response to Varying Stimuli
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A single stimulus results in a single contractile
responsea muscle twitch
Frequently delivered stimuli (muscle does not have
time to completely relax) increases contractile force
wave summation
Figure 9.14
Muscle Response to Varying Stimuli
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More rapidly delivered stimuli result in incomplete
tetanus
If stimuli are given quickly enough, complete
tetanus results
Figure 9.14
Muscle Response: Stimulation Strength
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Threshold stimulusthe stimulus strength at whichthe first observable muscle contraction occurs
Beyond threshold, muscle contracts more vigorously
as stimulus strength is increased
Force of contraction is precisely controlled by
multiple motor unit summation
This phenomenon, called recruitment, brings more
and more muscle fibers into play
Stimulus Intensity and Muscle Tension
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Figure 9.15 (a, b)
Treppe: The Staircase Effect
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Staircaseincreased contraction in response to
multiple stimuli of the same strength
Contractions increase because:
There is increasing availability of Ca2+in thesarcoplasm
Muscle enzyme systems become more efficient
because heat is increased as muscle contracts
Treppe: The Staircase Effect
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Figure 9.16
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Smooth muscle
Smooth muscle
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Nonstriated
Lack sarcomeres
Thin filaments anchored to dense bodies
Involuntary
Structure of smooth muscle
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Shape of muscle fiber
spindle shaped 1-5 m in diameter
20-500 m in length
A single nucleus present in the central thick portion. Sarcolemma (cell membrane).
Cytoplasm appears homogenous without striations.
Fewer mitochondria as compared to the skeletal
muscle. Metabolism mostly glycolytic.
Actin, Myosin & Tropomyosin but NO Troponin
Structure of smooth muscle
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Dense bodies present attached to the cell membranes OR
dispersed throughout the cell
Dense bodies serve the same purpose as the Z-discs
Attached to the dense bodies are numerous numbers of
Actin filaments
Interspersed between the actin filaments are Myosin
filaments (their diameter twice as much as actin
filaments)Usually, 5-10 times as many actin filaments as Myosin
filaments
Smooth Muscle Tissue
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Figure 10.23
Smooth Muscle
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Innervated by autonomic nervous system (ANS)
Visceral or unitary smooth muscle
Only a few muscle fibers innervated in each group
Impulse spreads through gap junctions Contracts as a unit
Often autorhythmic
Multiunit:
Cells or groups of cells act as independent units
Errector pili of skin and iris of eye
Functional characteristics of smooth muscle
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Contract when calcium ions interact with
calmodulin Activates myosin light chain kinase
Functions over a wide range of lengths
Plasticity Multi-unit smooth muscle cells are innervated by
more than one motor neuron
Visceral smooth muscle cells are not always
innervated by motor neuronsNeurons that innervate smooth muscle are not
under voluntary control
Oblique arrangement of sarcomeres
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Smooth Muscle Cell
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Sidepolar cross-bridges
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Myosin filaments have sidepolar cross-bridges
Bridges on one side hinge in one direction & on the other side in the oppositedirection
Allows myosin to pull an actin filament in one direction while simultaneouslypulling it in the other direction on the other side
Allow smooth muscle to contract 80% as compared to only 30 % in theskeletal muscle
(force of contraction in skeletal muscle is limited because of the presence of
the z-disc, against which the thick filament will abutt against and cannotmove any further)
Calcium Pump: pumps Ca back into the SR if present for relaxation totake place. But it is very slow so that duration of cont. is longer.
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Structure of smooth muscle
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Neuromuscular Junction:
Does not occur in smooth muscleInstead the
autonomic nervesmake diffuse junctions that
secrete NT into the matrix coating of smooth muscle
a few micrometers away from the muscle fiber
Also the axons supplying them do not have terminalbuttons but varicositieson their terminal axons that
contain the vesicles containing the NT
Neurotransmitter (NT)
Apart from Ach, norepinephrinecan also bereleased
Instead of synaptic clefts, smooth muscles have
contact junctions
Excitation-Contraction Coupling
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Single unit
Multi unit
Classification of smooth muscles
MULTI UNIT
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SINGLE UNIT
1. Muscles of visceral organs.e.g. GIT, uterus, ureters &some of the smaller bloodvessels.
2. Form a sheet or bundles oftissue.
3. Cell membranes show gapjunctions that allows AP topass rapidly from cell to cell.
4. AP spreads rapidlythroughout the sheet of cellscells contract as a single unit.
MULTI-UNIT
1. Iris & Ciliary body of the
eye, large arteries,Piloerector muscles
2. Showing discrete, individualsmooth muscle fibers.
3. Smooth muscle cells notelectrically linked. Eachmuscle fiber innervated by asingle nerve ending. NTitself can spread and lead to
an AP.4. Selective activation of each
muscle fiber that can thencontract independently of
each other.
Single-Unit Muscle
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Properties of Single-Unit Smooth Muscle
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Gap junctions
Pacemaker cells
with spontaneous
depolarizations
Innervation to few cells
Tone = level of
contraction withoutstimulation
Increases/decreases
in tension
Graded Contractions
No recruitment
Vary intracellular
calcium
Stretch Reflex
Relaxation in
response to suddenor prolonged stretch
Multi-Unit Muscle
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M lti Si l U it M l
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Multi vs. Single-Unit Muscle
Comparisons Types of Muscle
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Single muscle twitch
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Single muscle contraction (muscle twitch) developsmore slowly & relaxes even more slowly longer
sustained contraction without fatigue
Advantage:This ability allows the walls of the organsto maintain tension with a continued load .e.g.urinary bladder filled with urine
A TYPICAL SMOOTH MUSCLE HAS A TOTALCONTRACTION TIME OF 1-3 SECONDS (about30 times as long as single skeletal muscle
contraction)
Single muscle twitch
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Action potential
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In the normal resting state, the membrane potential isabout -50 to -60 mv.
The AP of visceral smooth muscle is of 2 types:
Typical spike potentials: (similar to skeletalmuscles) -mostly seen in the unitary smooth
muscles
AP with Plateaus: Starts like a typical spikepotential but repolarization delayed for severalhundred to as many as 1000 msec -accounts forthe prolonged contraction that occurs in certainorgans
Action potential - Slow wave potentials
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Without an external stimulus membrane potential isoften associated with a basic slow wave rhythm. This is
itself not an AP but a local property of the smoothmuscle fibers.
Cause:
Waxing & waning of the pumping of Na ions Conductance of the ion channels increase &decrease rhythmically
When the peak of the slow wave reaches about -35 mv,
threshold is reached and an AP develops & leads to acontraction at peak of the slow waves an AP canoccur Pacemaker waves
Slow wave potentials Pacemaker potentials
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When unitary (visceral) smooth m. is stretched,
spontaneous AP is usually generated, because:1. Normal slow potentials caused by stretch
2. Overall in membran negativity caused by stretch
Role of calcium
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Poorly developed SR
Presence of small invaginations abut the SR whichrelease Ca when AP reaches it.
Smooth muscle contraction is highly dependent on
Extracellular Calcium concentration
The main source of Calcium ions in smooth muscle is togreater extent ECF and to a lesser extent SR as
compared to the skeletal muscles where greatest source
of Calcium is SR.
Calcium plays the main role in the prolongedcontraction process.
Smooth muscle contraction sequence
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Binding of Ach to the receptors
Increased Influx of Ca into the cell from the following sources:1. ECF thru Ca channels
2. Ca released from SR
3. Stretch-activated Ca channels when memb. Deformed
4. Chemical-gated Ca channels by NT & hormones
Ca binds to Calmodulin
Ca-Calmodulinactivates the enzyme: Myosin light chain kinase MLCK or
simply Myosin kinase
Phosphorylation of myosin, using energy & Pi from ATP
Increased ATPase activity & binding of myosin to actin
Contraction of smooth muscle
Smooth Muscle Contraction
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Smooth muscle relaxation sequence
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Dephosphorylation of Myosin by myosin phosphatase/ MLCP
Decreases its ATP activity
Ca removed from cytoplasm using Ca-Na antiport protein & Ca-ATPase
Calmodulin releases Ca & uncomplexes from MK
MK is phosphorylated by Protein kinase, inactivating it
Relaxation OR sustained contraction
Smooth Muscle Relaxation
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Latch system
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It is a state in which the dephosphrylated myosin
remains attached to actin for prolonged period oftime. This produces sustained contraction without
consuming ATP & thus enables the smooth muscle
to sustain long-term maintenance of tone without
fatigue. E.g. urinary bladder full of urine.