Download muscle fibers

Muscle Tissue
• Nearly half of body's mass
• Three types
– Skeletal
– Cardiac
– Smooth
• Differ in structure, location, function and
activation
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Similarities:
• Skeletal and smooth muscle cells are
elongated and are called muscle fibers
• Muscle contraction depends upon 2 types of
myofilaments: actin and myosin
• Muscle terminology is similar:
– Prefixes for muscle: Myo, mys, and sarco – Sarcolemma: muscle plasma membrane
– Sarcoplasm: cytoplasm of a muscle cell
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Types of Muscle Tissue
• Skeletal muscles
– muscles that are attached to skin and bones and
also cover the bones
– Elongated cells called muscle fibers
– Striated (striped)
– Voluntary (i.e., conscious control)
– Contract rapidly; tire easily; powerful
– Responsible for locomotion and manipulation
– Require nervous system stimulation
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• Cardiac muscle
– Only in heart; bulk of heart walls
– Striated, but involuntary
• we can’t control rate and pace of contraction however
neural controls allow heart to speed up for short
periods
– Can contract without nervous system stimulation;
rate set by the pacemaker of the heart (group of
special cells)
– Muscle is highly resistant to fatigue
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• Smooth muscle-spindle shaped (fusiform) cells; 1
nucleus
– Not striated
– Involuntary
– Requires Autonomic Nervous System stimulation
to contract
– Role is to force substances through body channels
– In walls of hollow organs, e.g., stomach, urinary
bladder, and respiratory passageways
• Muscle contractions are slow and sustained
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Special Characteristics of Muscle Tissue
• Excitability or irritability: ability to receive and
respond to stimuli
• Contractility: ability to shorten forcibly when
stimulated
• Extensibility: ability to be stretched
• Elasticity: ability to recoil and resume original
resting length
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Muscle Functions
1. Skeletal muscle responsible for movement of bones and
manipulation of objects; locomotion
2. Cardiac muscle responsible for sending blood throughout
the body
3. Smooth muscle helps maintain blood pressure and
squeezes or propels substances through organs (i.e. food,
feces)
4. Maintaining posture and body position
5. Stabilize and strengthen joints
6. Heat generation (especially skeletal muscle)
• Additional functions
– Protects organs, forms valves, controls pupil size, causes
"goosebumps"
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Skeletal Muscle
• Each muscle is an organ composed of muscle tissue,
blood vessels, nerves and connective tissue
– Every skeletal muscle fiber supplied by nerve
ending that controls its activity
– Huge nutrient and oxygen need; generates large
amount of waste
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• Connective tissue sheaths of skeletal muscle
– Support cells; reinforce whole muscle
– External to internal
• Epimysium: dense irregular connective tissue
surrounding entire muscle; may blend with fascia
(tendons)
• Perimysium: fibrous connective tissue surrounding
groups of muscle fibers called fascicles
• Endomysium: fine areolar connective tissue
surrounding each muscle fiber; is located immediately
superior to the sarcolemma
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• Nerve and Blood Supply
– Each muscle cell is served by one nerve, an artery
and one or more veins which enter at the central
part of the muscle and then branch
– Every skeletal muscle fiber has a nerve ending
which controls each contraction
– Contracting fibers require a continuous supply of
oxygen and nutrients (via arteries) and
– Waste removal (via veins)
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Attachments
• Attach in at least two places
– Insertion – movable bone
– Origin – immovable (less movable) bone
• Attachments direct or indirect
– Direct—epimysium fused to periosteum of bone
or to cartilage
– Indirect—connective tissue wrappings extend
beyond muscle as ropelike tendon or sheetlike
aponeurosis
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Microscopic Anatomy of A Skeletal Muscle Fiber
• Long, cylindrical cell
– long because embryonic cells fuse to produce each
fiber
• Multiple nuclei just below the sarcolemma
• Sarcoplasm has many glycosomes for glycogen storage,
and myoglobin for O2 storage
• Each fiber is supplied with a nerve ending to control
contraction
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Myofibrils
• Densely packed, rodlike, contractile elements
• ~80% of cell volume
• Contain sarcomeres – smallest contractile units
– Sarcomeres contain myofilaments
• Exhibit striations - perfectly aligned repeating series
of dark A bands and light I bands
• When myofibrils move, they move as a group,
not as individual units
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H zone: lighter region in midsection of dark A band where filaments do not overlap
M line: line of protein myomesin bisects H zone
Z disc (line): coin-shaped sheet of proteins on midline of light I band that anchors actin filaments and
connects myofibrils to one another
Thick filaments: run entire length of an A band
Thin filaments: run length of I band and partway into A band
Sarcomere: region between two successive Z discs
Thin (actin)
filament
Small part of one
myofibril
enlarged to show
the myofilaments
responsible fr the
banding pattern.
Thick
Each sarcomere
extends from one Z (myosin)
filament
disc to the next.
Z disc
I band
H zone
Z disc
I band
A band
Sarcomere
M line
M-lines are where the
myosin filaments are held in
alignment
© 2013 Pearson Education, Inc.
Sarcomere
• Smallest contractile or functional and
structural unit of muscle fiber
• Composed of thick and thin myofilaments
made of contractile proteins
• Region of myofibril between 2 consecutive Z
discs
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Myofibril Banding Pattern
• Orderly arrangement of actin and myosin
myofilaments within sarcomere
– Actin myofilaments = thin filaments
• Extend across I band and partway in A band
• Anchored to Z discs
– Myosin myofilaments = thick filaments
• Extend length of A band
• Connected at M line
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Thick Filament
• Composed of protein myosin
– Myosin tails-2 twisted, interwoven, rodlike chains;
made up of polypeptides
– Myosin heads, also polypeptide chains, act as
cross bridges during contraction
• Binding sites for actin (thin) filaments
• Binding sites for ATP
• ATPase enzymes
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Thin Filament
• Twisted double strand of fibrous protein
• Contains sites for myosin head attachment during
contraction
• Tropomyosin and troponin - proteins bound to actin
– Tropomyosin in a relaxed fiber blocks the myosin
binding sites
• Twists around actin fibers and help stiffen and
stabilize it
– Troponin binds calcium ions
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Sarcoplasmic Reticulum (SR)
• Network of smooth endoplasmic reticulum
surrounding each myofibril
– Most run longitudinally
• Pairs of terminal cisternae form perpendicular
cross channels throughout SR
• Functions in regulation of intracellular calcium
levels
– Stores and releases Ca2+
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T Tubules
• Continuations of sarcolemma--protrudes
deeply into the interior of the cell
• Increase muscle fiber's surface area
• Conduct nerve impulses to the deepest areas
of the muscle fiber
• Impulses signal calcium to be released from
the adjacent terminal cistermae
• Associated with paired terminal cisterns to
form triads that circle each sarcomere
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Sliding Filament Model of Contraction
• In relaxed state, thin and thick filaments overlap only
at ends of A band
• Sliding filament model of contraction
– Upon muscle stimulation, thin filaments slide past
thick filaments  actin and myosin overlap to a
greater degree
– When myosin heads bind to actin  cross bridges
form and sliding begins
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• Cross bridges form and break several times, ratcheting thin
filaments toward center of sarcomere
• Causes shortening of muscle fiber
• Pulls Z discs toward M line
I bands shorten; Z discs closer; H zones disappear; A bands move closer
(length stays same)
Z disc
H zone
I band
A band
I band
M line
Physiology of Skeletal Muscle Fibers
• For skeletal muscle to contract
– Activation (at neuromuscular junction)
• Requires nervous system stimulation
• AND an electric current or action potential
along sarcolemma
• Intracellular Ca2+ levels must rise briefly (final
step for contraction to begin)
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Nerve Stimulus and the Neuromuscular Junction
• Skeletal muscles stimulated by motor neurons of the
voluntary nervous system
• Axons of motor neurons travel via nerves to skeletal
muscle cells
• Each axon forms several branches as it enters the
muscle
• Each axon ending or branch, forms a neuromuscular
junction with a single muscle fiber
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Neuromuscular Junction (NMJ) is where the axon and
muscle fiber meet.
• Axon terminal and muscle fiber separated by gelfilled space called synaptic cleft
• Synaptic vesicles from the axon terminal contain the
neurotransmitter acetylcholine (ACh)
• Sarcolemma in the NMJ contain ACh receptors
• ACh diffuses across the synaptic cleft and attaches to
the ACh receptors in the sarcolemma
• ACh binding triggers the electrical events to generate
an action potential
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• Action potential causes changes in the properties of
the cell membrane channels
• Ca2+ channels open  Ca2+ moves into the nerve
axon and this causes release of the ACh into the
synaptic cleft
• ACh diffuses across the cleftsarcolemma
initiates an action potential in the muscle
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Contraction of Skeletal Muscles
• Contraction produces muscle tension, force exerted
on load or object to be moved
– Refers to the activation of the sliding filaments
and the forming of cross bridges between the
actin and myosin
• Contraction ends when cross bridges are deactivated
because of a lack of nerve stimulation or not enough
calcium present
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• Rigor mortis
– Cross bridge detachment requires ATP
– 3–4 hours after death muscles begin to stiffen
with peak rigidity at 12 hours post mortem
• Dying cells take in calcium  cross bridge
formation
• No ATP generated to break cross bridges
– Muscles relax as muscle proteins break down after
death
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2 types of muscle contraction are:
– Isometric contraction: no shortening; muscle
tension increases
– Isotonic contraction: muscle shortens because
muscle tension is high enough
– Force and duration of contraction vary in response to stimuli of
different frequencies and intensities
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Motor Unit: The Nerve-Muscle Functional Unit
• Each muscle is served by at least one motor nerve
– A motor nerve contains axons of many motor
neurons
– Axons branch into terminals, each of which form a
NMJ with single muscle fiber
• Motor unit = motor neuron and all the muscle fibers
it supplies
– For fine control—each motor neuron supplies a
smaller number of fibers
– For large, weight bearing muscles, each motor
neuron supplies a lot of muscle fibers
• The muscle’s response to a single threshold stimulus
is called a Muscle Twitch
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Muscle Twitch
• Three phases of muscle twitch
– Latent period: sequence of events involved in
spreading the action potential along the
sarcolemma leads to the sliding of the muscle
filaments (excitation-contraction coupling); no
muscle tension seen yet
– Period of contraction: cross bridge formation;
tension increases
– Period of relaxation: Ca2+ reentry into SR; tension
declines to zero
• Muscle contracts faster than it relaxes
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• Graded muscle responses
– Varying strength of contraction for different
demands
• Required for proper control of skeletal
movement
• Responses graded by
1. Changing frequency of stimulation
2. Changing strength of stimulation
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Response to Change in Stimulus Frequency
• Single stimulus results in single contractile response—
muscle twitch
– We typically don’t see this in healthy people
• Wave summation
– Increased stimulus frequency (muscle does not
completely relax between stimuli) causes a more
forceful contraction
• Additional Ca2+ release with second stimulus
stimulates more shortening
• Produces smooth, continuous contractions
• Further increase in stimulus frequency causing a
sustained but quivering contraction incomplete
tetanus
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Response to Change in Stimulus Frequency
• If stimuli are given quickly enough, muscle
reaches maximal tension  fused (complete)
tetanus results
– Smooth, sustained contraction
– Do not confuse with “tetanus”-the bacterial
disease
– See fused tetanus only in cases of super-human
strength—lifting a car off someone
– No muscle relaxation  muscle fatigue
• Muscle cannot contract; zero tension
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Response to Change in Stimulus Strength
• Threshold stimulus: stimulus strength causing first
observable muscle contraction
• Recruitment (multiple motor unit summation)
controls force of contraction
• Subthreshold stimuli – no observable contractions
• Maximal stimulus – strongest stimulus that increases
contractile force
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Muscle Fatigue
• Physiological inability to contract despite continued
stimulation
• Occurs when
– ATP production fails to keep pace with ATP use
– Ionic imbalances
– Lactic acid accumulates in the muscle
– Prolonged exercise damages SR and interferes
with Ca2+ regulation and release
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Muscle Fiber Type
• Classified according to two characteristics
– Speed of contraction: slow or fast fibers according
to
• Speed at which myosin ATPases split ATP
• Pattern of electrical activity of motor neurons
– Metabolic pathways for ATP synthesis
• Oxidative fibers—use aerobic pathways
• Glycolytic fibers—use anaerobic glycolysis
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LABWORK
1. Identify and describe the three kinds of muscle tissue (3
microscope slides).
2. Identify and describe all the components of a muscle,
including the connective tissue wrappings around each
part (models and muscle cross section slide).
3. Identify and describe the microstructure of skeletal
muscle cells, and the basics involved in the contraction
mechanism (models and neuromuscular junction slide).
4. Explain the concepts covered about contraction and
muscle physiology (the frog muscle video).
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