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Chapter 10: Muscular Tissue
Copyright 2009, John Wiley & Sons, Inc.
Overview of Muscular Tissue Skeletal Muscle Tissue- (because most skeletal muscles move bones)
Striated: Alternating light and dark bands when examined microscopically Voluntary : its activity can be consciously controlled Some skeletal muscles also are controlled subconsciously to some
extent Ex: diaphragm - alternately contracts & relaxes w/o conscious control
Cardiac Muscle Tissue - found only in the walls of the heart Striated – like skeletal muscle Involuntary - Contraction of the heart is initiated by the “pacemaker”
Smooth Muscle Tissue - located in the walls of hollow internal structures Blood vessels, airways, and many organs Lacks the striations of skeletal and cardiac muscle tissue Spindle shaped – tapered ends Usually involuntary
Copyright 2009, John Wiley & Sons, Inc.
Copyright 2009, John Wiley & Sons, Inc.
Functions of Muscular Tissue Producing Body Movements
Walking and running Stabilizing Body Positions
Posture Moving Substances Within the Body
Heart muscle pumping blood Moving substances in the digestive tract
Generating heat Contracting muscle produces heat Shivering increases heat production
Properties of Muscular Tissue
Excitability - ability to respond to stimuli Contractility - ability to contract forcefully when stimulated Extensibility - ability to stretch without being damaged Elasticity - ability to return to an original length
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue Connective Tissue Components
Fascia Dense sheet or broad band of irregular connective tissue that
surrounds muscles Epimysium - the outermost layer
Separates 10-100 muscle fibers into bundles called fascicles Perimysium- surrounds numerous bundles of fascicles Endomysium - separates individual muscle fibers from one another
Also contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissue
Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis Tendon
Cord that attach a muscle to a bone Aponeurosis
Broad, flattened tendon
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Structure Composed of muscle cells (fibers),
connective tissue, blood vessels, nerves
Fibers are long, cylindrical, and multinucleated
Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length
Develop from myoblasts; numbers remain constant
Striated appearance Nuclei are peripherally located
Skeletal Muscle Tissue Nerve and Blood Supply Somatic Motor neurons - stimulate muscle fibers to contract Neuron axons branch so that each muscle cell (fiber) is innervated Form a neuromuscular junction (= myoneural junction)
Capillary beds surround muscle fibers Muscles require large amts of energy Extensive vascular network delivers necessary oxygen and nutrients
and carries away metabolic waste produced by muscle fibers
Microscopic Anatomy Number of skeletal muscle fibers is set before you are born Most of these cells last a lifetime Muscle growth occurs by hypertrophy - enlargement of existing muscle
fibers Testosterone and human growth hormone stimulate hypertrophy
Satellite cells retain the capacity to regenerate damaged muscle fibersCopyright 2009, John Wiley &
Sons, Inc.
Skeletal Muscle Tissue Sarcolemma- The plasma membrane of a muscle cell Transverse (T tubules)- Tunnel in from the plasma membrane
Muscle action potentials travel through the T tubules Sarcoplasm - the cytoplasm of a muscle fiber
Sarcoplasm includes glycogen used for synthesis of ATP and myoglobin, a red-colored protein that binds oxygen molecules
Myoglobin releases oxygen when it is needed for ATP production Myofibrils- thread like structures which have a contractile function Sarcoplasmic reticulum (SR)- membranous sacs encircling each
myofibril ; it is modified ER Stores calcium ions (Ca++) Release of Ca++ triggers muscle contraction
Filaments- function in the contractile process Two types of filaments (Thick and Thin) There are two thin filaments for every thick filament
Sarcomeres - Compartments of arranged filaments Basic functional unit of a myofibril
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue Z discs
Separate one sarcomere from the next A band
Darker middle part of the sarcomere Thick and thin filaments overlap
I band Lighter, contains thin filaments but no thick filaments Z discs passes through the center of each I band
H zone Center of each A band which contains thick but no thin filaments
M line Supporting proteins that hold the thick filaments together in the H
zone Zone of Overlap
Thick and thin filaments overlap each other
Copyright 2009, John Wiley & Sons, Inc.
Copyright 2009, John Wiley & Sons, Inc.
Levels of Functional Organization in Skeletal Muscle Fiber
Contraction and Relaxation of Skeletal Muscle
Skeletal Muscle Tissue
Muscle Proteins
1) Contractile proteins- Generate force during contraction Myosin
Thick filaments Functions as a motor protein which can achieve motion Convert ATP to energy of motion Projections of each myosin molecule protrude outward (myosin head)
Actin - composed of G-actin + nebulin Thin filaments Actin molecules provide a site where a myosin head can attach Tropomyosin and troponin are also part of the thin filament In relaxed muscle, myosin is blocked from binding to actin Strands of tropomyosin cover the myosin-binding sites Calcium ion binding to troponin moves tropomyosin away from myosin-
binding sites→ myosin binds to actin →muscle contraction begins
Copyright 2009, John Wiley & Sons, Inc.
Skeletal Muscle Tissue 2) Regulatory proteins- Switch the contraction process on and off
Tropomyosin and Troponin are also part of the thin filament In relaxed muscle, myosin is blocked from binding to actin Strands of tropomyosin cover the myosin-binding sites Calcium ion binding to troponin moves tropomyosin away from
myosin-binding sites→ Allows muscle contraction to begin as myosin binds to actin
3) Structural proteins- Align the thick and thin filaments properly Provide elasticity and extensibility Link the myofibrils to the sarcolemma Titin- stabilize the position of myosin - functions in elasticity and extensibility of myofibrils Dystrophin - links thin filaments to the sarcolemma Myomesin – forms M line ; holds thick filaments together α- actinin and connectins – makes Z discs
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle The Sliding Filament Mechanism- Explains the relationship between
thick and thin filaments as contraction proceeds Cyclic process beginning with calcium release from SR Calcium binds to troponin Troponin moves, moving tropomyosin and exposing actin active site Myosin head forms cross bridge and bends toward H zone ATP allows release of cross bridge
Structure of Actin and Myosin
Contraction and Relaxation of Skeletal Muscle
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle The contraction cycle consists of 4 steps
1) ATP hydrolysis Hydrolysis of ATP reorients and energizes the myosin head
2) Formation of cross-bridges Myosin head attaches to the myosin-binding site on actin
3) Power stroke During the power stroke the crossbridge rotates, sliding the
filaments 4) Detachment of myosin from actin
As the next ATP binds to the myosin head, the myosin head detaches from actin
The contraction cycle repeats as long as ATP is available and the Ca++ level is sufficiently high
Continuing cycles applies the force that shortens the sarcomere
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle
1 Myosin headshydrolyze ATP andbecome reorientedand energized ADP
P
= Ca2+
Key:
Contraction cycle continues ifATP is available and Ca2+ level inthe sarcoplasm is high
1 Myosin headshydrolyze ATP andbecome reorientedand energized
Myosin headsbind to actin,formingcrossbridges
ADP
ADP
P
P
= Ca2+
Key:
2
Contraction cycle continues ifATP is available and Ca2+ level inthe sarcoplasm is high
1 Myosin headshydrolyze ATP andbecome reorientedand energized
Myosin headsbind to actin,formingcrossbridges
Myosin crossbridgesrotate toward center of thesarcomere (power stroke)
Contraction cycle continues ifATP is available and Ca2+ level inthe sarcoplasm is high
ADP
ADP
ADP
P
P
= Ca2+
Key:
2
3
1 Myosin headshydrolyze ATP andbecome reorientedand energized
Myosin headsbind to actin,formingcrossbridges
Myosin crossbridgesrotate toward center of thesarcomere (power stroke)
As myosin headsbind ATP, thecrossbridges detachfrom actin
Contraction cycle continues ifATP is available and Ca2+ level inthe sarcoplasm is high
ADP
ADP
ADP
ATP
P
P
= Ca2+
Key:
ATP
2
3
4
Contraction and Relaxation of Skeletal Muscle Length–Tension Relationship
The forcefulness of muscle contraction depends on the length of the sarcomeres
When a muscle fiber is stretched there is less overlap between the thick and thin filaments and tension (forcefulness) is diminished
When a muscle fiber is shortened the filaments are compressed and fewer myosin heads make contact with thin filaments and tension is diminished
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle
Contraction and Relaxation of Skeletal Muscle Excitation–Contraction Coupling
An increase in Ca++ concentration in the muscle starts contraction
A decrease in Ca++ stops it An Action potential causes Ca++ to be released from the
SR into the muscle cell Ca++ moves tropomyosin away from the myosin-binding
sites on actin allowing cross-bridges to form The muscle cell membrane contains Ca++ pumps to
return Ca++ back to the SR quickly Decreasing calcium ion levels in sarcoplasm
As the Ca++ level in the cell drops, myosin-binding sites are covered and the muscle relaxes
Copyright 2009, John Wiley & Sons, Inc.
Contraction and Relaxation of Skeletal Muscle
Figure 10–11
Exposing the Active site
Contraction and Relaxation of Skeletal Muscle The Neuromuscular Junction
Motor neurons have a threadlike axon that extends from the brain or spinal cord to a group of muscle fibers
Neuromuscular junction (NMJ) Action potentials arise at the interface of the motor neuron and muscle
fiber Synapse
Where communication occurs between a somatic motor neuron and a muscle fiber
Synaptic cleft Gap that separates the two cells
Neurotransmitter Chemical released by the initial cell communicating with the second cell
Synaptic vesicles Sacs suspended within the synaptic end bulb containing molecules of
the neurotransmitter acetylcholine (Ach) Motor end plate
The region of the muscle cell membrane opposite the synaptic end bulbs Contain acetylcholine receptors
Copyright 2009, John Wiley & Sons, Inc.
SEM of Neuromuscular Junction
Contraction and Relaxation of Skeletal Muscle Nerve impulses elicit a muscle action potential in
the following way 1) Release of acetylcholine
Nerve impulse arriving at the synaptic end bulbs causes many synaptic vesicles to release ACh into the synaptic cleft
2) Activation of ACh receptors Binding of ACh to the receptor on the motor end plate opens an ion
channel Allows flow of Na+ to the inside of the muscle cell
3) Production of muscle action potential The inflow of Na+ makes the inside of the muscle fiber more positively
charged triggering a muscle action potential The muscle action potential then propagates to the SR to release its
stored Ca++
4) Termination of ACh activity Ach effects last only briefly because it is rapidly broken down by
acetylcholinesterase (AChE)
Copyright 2009, John Wiley & Sons, Inc.
1
Axon terminal
Axon terminal
Axon collateral ofsomatic motor neuron
Sarcolemma
Myofibril
ACh is releasedfrom synaptic vesicle
Junctional fold
Synaptic vesiclecontainingacetylcholine(ACh)
Sarcolemma
Synaptic cleft(space)
Motor end plate
Synaptic cleft(space)
(a) Neuromuscular junction
(b) Enlarged view of the neuromuscular junction
(c) Binding of acetylcholine to ACh receptors in the motor end plate
Synapticend bulb
Synapticend bulb
Neuromuscularjunction (NMJ)
Synaptic end bulb
Motor end plate
Nerve impulse
11
Axon terminal
Axon terminal
Axon collateral ofsomatic motor neuron
Sarcolemma
Myofibril
ACh is releasedfrom synaptic vesicle
ACh binds to Achreceptor
Junctional fold
Synaptic vesiclecontainingacetylcholine(ACh)
Sarcolemma
Synaptic cleft(space)
Motor end plate
Synaptic cleft(space)
(a) Neuromuscular junction
(b) Enlarged view of the neuromuscular junction
(c) Binding of acetylcholine to ACh receptors in the motor end plate
Synapticend bulb
Synapticend bulb
Neuromuscularjunction (NMJ)
Synaptic end bulb
Motor end plate
Nerve impulse
Na+
1
22
1
Axon terminal
Axon terminal
Axon collateral ofsomatic motor neuron
Sarcolemma
Myofibril
ACh is releasedfrom synaptic vesicle
ACh binds to Achreceptor
Junctional fold
Synaptic vesiclecontainingacetylcholine(ACh)
Sarcolemma
Synaptic cleft(space)
Motor end plate
Synaptic cleft(space)
(a) Neuromuscular junction
(b) Enlarged view of the neuromuscular junction
(c) Binding of acetylcholine to ACh receptors in the motor end plate
Synapticend bulb
Synapticend bulb
Neuromuscularjunction (NMJ)
Synaptic end bulb
Motor end plate
Nerve impulse
Muscle action potential is produced
Na+
1
2
3
2
1
Axon terminal
Axon terminal
Axon collateral ofsomatic motor neuron
Sarcolemma
Myofibril
ACh is releasedfrom synaptic vesicle
ACh binds to Achreceptor
Junctional fold
Synaptic vesiclecontainingacetylcholine(ACh)
Sarcolemma
Synaptic cleft(space)
Motor end plate
Synaptic cleft(space)
(a) Neuromuscular junction
(b) Enlarged view of the neuromuscular junction
(c) Binding of acetylcholine to ACh receptors in the motor end plate
Synapticend bulb
Synapticend bulb
Neuromuscularjunction (NMJ)
Synaptic end bulb
Motor end plate
Nerve impulse
Muscle action potential is produced
ACh is broken down
Na+
1
2
4
3
2
Skeletal Muscle Innervation
Nerve impulse arrives ataxon terminal of motorneuron and triggers releaseof acetylcholine (ACh).
Synaptic vesiclefilled with ACh
ACh receptor
Muscle action potential
Nerveimpulse
1
ACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).
Nerve impulse arrives ataxon terminal of motorneuron and triggers releaseof acetylcholine (ACh).
Synaptic vesiclefilled with ACh
ACh receptor
Muscle action potential
Nerveimpulse
1
2 ACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).
Nerve impulse arrives ataxon terminal of motorneuron and triggers releaseof acetylcholine (ACh).
Synaptic vesiclefilled with ACh
ACh receptorAcetylcholinesterase insynaptic cleft destroysACh so another muscleaction potential does notarise unless more ACh isreleased from motor neuron.
Muscle action potential
Nerveimpulse
1
2
3
ACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).
Nerve impulse arrives ataxon terminal of motorneuron and triggers releaseof acetylcholine (ACh).
Synaptic vesiclefilled with ACh
ACh receptorAcetylcholinesterase insynaptic cleft destroysACh so another muscleaction potential does notarise unless more ACh isreleased from motor neuron. Ca2+
Muscle action potential
Nerveimpulse
SR
Muscle AP travelling alongtransverse tubule opens Ca2+
release channels in thesarcoplasmic reticulum (SR)membrane, which allowscalcium ions to flood into the sarcoplasm.
1
2
3
4
Transverse tubuleACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).
Nerve impulse arrives ataxon terminal of motorneuron and triggers releaseof acetylcholine (ACh).
Synaptic vesiclefilled with ACh
ACh receptorAcetylcholinesterase insynaptic cleft destroysACh so another muscleaction potential does notarise unless more ACh isreleased from motor neuron. Ca2+
Muscle action potential
Nerveimpulse
SR
Ca2+ binds to troponin onthe thin filament, exposingthe binding sites for myosin.
Muscle AP travelling alongtransverse tubule opens Ca2+
release channels in thesarcoplasmic reticulum (SR)membrane, which allowscalcium ions to flood into the sarcoplasm.
1
2
3
4
5
Transverse tubuleACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).
Nerve impulse arrives ataxon terminal of motorneuron and triggers releaseof acetylcholine (ACh).
Synaptic vesiclefilled with ACh
ACh receptorAcetylcholinesterase insynaptic cleft destroysACh so another muscleaction potential does notarise unless more ACh isreleased from motor neuron. Ca2+
Muscle action potential
Nerveimpulse
SR
Contraction: power strokesuse ATP; myosin heads bindto actin, swivel, and release;thin filaments are pulled towardcenter of sarcomere.
Ca2+ binds to troponin onthe thin filament, exposingthe binding sites for myosin.
Muscle AP travelling alongtransverse tubule opens Ca2+
release channels in thesarcoplasmic reticulum (SR)membrane, which allowscalcium ions to flood into the sarcoplasm.
Elevated Ca2+
1
2
3
4
5
6
Transverse tubuleACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).
Nerve impulse arrives ataxon terminal of motorneuron and triggers releaseof acetylcholine (ACh).
Synaptic vesiclefilled with ACh
ACh receptorAcetylcholinesterase insynaptic cleft destroysACh so another muscleaction potential does notarise unless more ACh isreleased from motor neuron. Ca2+
Muscle action potential
Nerveimpulse
SR
Contraction: power strokesuse ATP; myosin heads bindto actin, swivel, and release;thin filaments are pulled towardcenter of sarcomere.
Ca2+ activetransport pumps
Ca2+ release channels inSR close and Ca2+ activetransport pumps use ATPto restore low level of Ca2+ in sarcoplasm.
Ca2+ binds to troponin onthe thin filament, exposingthe binding sites for myosin.
Muscle AP travelling alongtransverse tubule opens Ca2+
release channels in thesarcoplasmic reticulum (SR)membrane, which allowscalcium ions to flood into the sarcoplasm.
Elevated Ca2+
1
2
3
4
5
67
Transverse tubuleACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).
Nerve impulse arrives ataxon terminal of motorneuron and triggers releaseof acetylcholine (ACh).
Synaptic vesiclefilled with ACh
ACh receptorAcetylcholinesterase insynaptic cleft destroysACh so another muscleaction potential does notarise unless more ACh isreleased from motor neuron. Ca2+
Muscle action potential
Nerveimpulse
SR
Contraction: power strokesuse ATP; myosin heads bindto actin, swivel, and release;thin filaments are pulled towardcenter of sarcomere.
Troponin–tropomyosincomplex slides back into position where it blocks the myosinbinding sites on actin.
Ca2+ activetransport pumps
Ca2+ release channels inSR close and Ca2+ activetransport pumps use ATPto restore low level of Ca2+ in sarcoplasm.
Ca2+ binds to troponin onthe thin filament, exposingthe binding sites for myosin.
Muscle AP travelling alongtransverse tubule opens Ca2+
release channels in thesarcoplasmic reticulum (SR)membrane, which allowscalcium ions to flood into the sarcoplasm.
Elevated Ca2+
1
2
3
4
5
67
8
Transverse tubuleACh diffuses acrosssynaptic cleft, bindsto its receptors in themotor end plate, andtriggers a muscle action potential (AP).
Nerve impulse arrives ataxon terminal of motorneuron and triggers releaseof acetylcholine (ACh).
Synaptic vesiclefilled with ACh
ACh receptorAcetylcholinesterase insynaptic cleft destroysACh so another muscleaction potential does notarise unless more ACh isreleased from motor neuron. Ca2+
Muscle action potential
Nerveimpulse
SR
Contraction: power strokesuse ATP; myosin heads bindto actin, swivel, and release;thin filaments are pulled towardcenter of sarcomere.
Troponin–tropomyosincomplex slides back into position where it blocks the myosinbinding sites on actin.
Muscle relaxes.
Ca2+ activetransport pumps
Ca2+ release channels inSR close and Ca2+ activetransport pumps use ATPto restore low level of Ca2+ in sarcoplasm.
Ca2+ binds to troponin onthe thin filament, exposingthe binding sites for myosin.
Muscle AP travelling alongtransverse tubule opens Ca2+
release channels in thesarcoplasmic reticulum (SR)membrane, which allowscalcium ions to flood into the sarcoplasm.
Elevated Ca2+
1
2
3
4
9
5
67
8
Transverse tubule
Muscle Relaxation
After contraction, a muscle fiber returns to resting length by: elastic forces (tendons & ligaments) opposing muscle contractions gravity
A Review of Muscle Contraction
Table 10–1 (1 of 2)
A Review of Muscle Contraction
Table 10–1 (2 of 2)
Contraction and Relaxation of Skeletal Muscle Botulinum toxin- Blocks release of ACh from synaptic vesicles
May be found in improperly canned foods Tiny amount can cause death by paralyzing respiratory muscles
Used as a medicine (Botox®) Strabismus (crossed eyes) Blepharospasm (uncontrollable blinking) Spasms of the vocal cords that interfere with speech Cosmetic treatment to relax muscles that cause facial wrinkles Alleviate chronic back pain due to muscle spasms (lumbar region)
Curare- Plant poison used American Indians on arrows & blowgun darts Blocks ACh receptors & inhibits Na+ channels→ muscle paralysis
Curare derivatives used during surgery to relax skeletal musclesAnticholinesterase - Slows actions of acetylcholinesterase and removal of ACh→ can strengthen weak muscle contractions
Ex: Neostigmine- treatment for myasthenia gravis - Antidote for curare poisoning - Terminate the effects of curare after surgery
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism Production of ATP in Muscle Fibers
A huge amount of ATP is needed to: Power the contraction cycle Pump Ca++ into the SR
The ATP inside muscle fibers will power contraction for only a few seconds
ATP must be produced by the muscle fiber after reserves are used up
Muscle fibers have three ways to produce ATP 1) From creatine phosphate 2) By anaerobic cellular respiration 3) By aerobic cellular respiration
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism
Muscle Metabolism Creatine Phosphate
Excess ATP is used to synthesize creatine phosphate Energy-rich molecule
Creatine phosphate transfers its high energy phosphate group to ADP regenerating new ATP
Creatine phosphate and ATP provide enough energy for contraction for about 15 seconds
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism Anaerobic Respiration
Series of ATP producing reactions that do not require oxygen
Glucose is used to generate ATP when the supply of creatine phosphate is depleted
Glucose is derived from the blood and from glycogen stored in muscle fibers
Glycolysis breaks down glucose into molecules of pyruvic acid and produces two molecules of ATP
If sufficient oxygen is present, pyruvic acid formed by glycolysis enters aerobic respiration pathways producing a large amount of ATP
If oxygen levels are low, anaerobic reactions convert pyruvic acid to lactic acid which is carried away by the blood
Anaerobic respiration can provide enough energy for about 30 to 40 seconds of muscle activity
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism Aerobic Respiration
Activity that lasts longer than half a minute depends on aerobic respiration
Pyruvic acid entering the mitochondria is completely oxidized generating ATP carbon dioxide Water Heat
Each molecule of glucose yields about 36 molecules of ATP Muscle tissue has two sources of oxygen
1) Oxygen from hemoglobin in the blood 2) Oxygen released by myoglobin in the muscle cell
Myoglobin and hemoglobin are oxygen-binding proteins Aerobic respiration supplies ATP for prolonged activity Aerobic respiration provides more than 90% of the needed ATP
in activities lasting more than 10 minutes
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism Muscle Fatigue
Inability of muscle to maintain force of contraction after prolonged activity
Factors that contribute to muscle fatigue Inadequate release of calcium ions from the SR Depletion of creatine phosphate Insufficient oxygen Depletion of glycogen and other nutrients Buildup of lactic acid and ADP Failure of the motor neuron to release enough
acetylcholine
Copyright 2009, John Wiley & Sons, Inc.
Muscle Metabolism Oxygen Consumption After Exercise
After exercise, heavy breathing continues and oxygen consumption remains above the resting level
Oxygen debt The added oxygen that is taken into the body after
exercise This added oxygen is used to restore muscle
cells to the resting level in three ways 1) to convert lactic acid into glycogen 2) to synthesize creatine phosphate and ATP 3) to replace the oxygen removed from myoglobin
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension The tension or force of muscle cell
contraction varies
Maximum Tension (force) is dependent on The rate at which nerve impulses arrive The amount of stretch before contraction The nutrient and oxygen availability The size of the motor unit
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension Motor Units
Consists of a motor neuron and the muscle fibers it stimulates The axon of a motor neuron branches out forming neuromuscular
junctions with different muscle fibers A motor neuron makes contact with about 150 muscle fibers Control of precise movements consist of many small motor units
Muscles that control voice production have 2 - 3 muscle fibers per motor unit
Muscles controlling eye movements have 10 - 20 muscle fibers per motor unit
Muscles in the arm and the leg have 2000 - 3000 muscle fibers per motor unit
The total strength of a contraction depends on the size of the motor units and the number that are activated
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension Twitch Contraction
The brief contraction of the muscle fibers in a motor unit in response to an action potential
Twitches last from 20 to 200 msec L Latent period (2 msec)
A brief delay between the stimulus and muscular contraction
The action potential sweeps over the sarcolemma and Ca++ is released from the SR
Contraction period (10–100 msec) Ca++ binds to troponin Myosin-binding sites on actin are exposed Cross-bridges form
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension Relaxation period (10–100 msec)
Ca++ is transported into the SR Myosin-binding sites are covered by tropomyosin Myosin heads detach from actin
Muscle fibers that move the eyes have contraction periods lasting 10 msec
Muscle fibers that move the legs have contraction periods lasting 100 msec
Refractory period When a muscle fiber contracts, it temporarily cannot
respond to another action potential Skeletal muscle has a refractory period of 5 milliseconds Cardiac muscle has a refractory period of 300 milliseconds
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension Muscle Tone
A small amount of tension in the muscle due to weak contractions of motor units
Small groups of motor units are alternatively active and inactive in a constantly shifting pattern to sustain muscle tone
Muscle tone keeps skeletal muscles firm Keep the head from slumping forward on the
chest
Copyright 2009, John Wiley & Sons, Inc.
Control of Muscle Tension Types of Contractions
Isotonic contraction The tension developed remains constant while the
muscle changes its length Used for body movements and for moving objects Picking a book up off a table
Isometric contraction The tension generated is not enough for the object
to be moved and the muscle does not change its length
Holding a book steady using an outstretched arm
Copyright 2009, John Wiley & Sons, Inc.
Isotonic and Isometric Contractions
Types of Skeletal Muscle Fibers Muscle fibers vary in their content of
myoglobin Red muscle fibers
Have a high myoglobin content Appear darker (dark meat in chicken legs and
thighs) Contain more mitochondria Supplied by more blood capillaries
White muscle fibers Have a low content of myoglobin Appear lighter (white meat in chicken breasts)
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers Muscle fibers contract at different speeds, and
vary in how quickly they fatigue Muscle fibers are classified into three main types
1) Slow oxidative fibers 2) Fast oxidative-glycolytic fibers 3) Fast glycolytic fibers
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers Slow Oxidative Fibers (SO fibers)
Smallest in diameter Least powerful type of muscle fibers Appear dark red (more myoglobin) Generate ATP mainly by aerobic cellular respiration Have a slow speed of contraction
Twitch contractions last from 100 to 200 msec Very resistant to fatigue Capable of prolonged, sustained contractions for
many hours Adapted for maintaining posture and for aerobic,
endurance-type activities such as running a marathon
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers Fast Oxidative–Glycolytic Fibers (FOG fibers)
Intermediate in diameter between the other two types of fibers
Contain large amounts of myoglobin and many blood capillaries
Have a dark red appearance Generate considerable ATP by aerobic cellular
respiration Moderately high resistance to fatigue Generate some ATP by anaerobic glycolysis Speed of contraction faster
Twitch contractions last less than 100 msec Contribute to activities such as walking and sprinting
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers Fast Glycolytic Fibers (FG fibers)
Largest in diameter Generate the most powerful contractions Have low myoglobin content Relatively few blood capillaries Few mitochondria Appear white in color Generate ATP mainly by glycolysis Fibers contract strongly and quickly Fatigue quickly Adapted for intense anaerobic movements of short
duration Weight lifting or throwing a ball
Copyright 2009, John Wiley & Sons, Inc.
Fast versus Slow Fibers
Copyright 2009, John Wiley & Sons, Inc.
Types of Skeletal Muscle Fibers Distribution and Recruitment of
Different Types of Fibers Most muscles are a mixture of all three types
of muscle fibers Proportions vary, depending on the action of
the muscle, the person ’s training regimen, and genetic factors Postural muscles of the neck, back, and legs have
a high proportion of SO fibers Muscles of the shoulders and arms have a high
proportion of FG fibers Leg muscles have large numbers of both SO and
FOG fibers
Copyright 2009, John Wiley & Sons, Inc.
Exercise and Skeletal Muscle Tissue Ratios of fast glycolytic and slow oxidative
fibers are genetically determined Individuals with a higher proportion of FG
fibers Excel in intense activity (weight lifting, sprinting)
Individuals with higher percentages of SO fibers Excel in endurance activities (long-distance
running)
Copyright 2009, John Wiley & Sons, Inc.
Exercise and Skeletal Muscle Tissue Various types of exercises can induce
changes in muscle fibers Aerobic exercise transforms some FG fibers
into FOG fibers Endurance exercises do not increase muscle mass
Exercises that require short bursts of strength produce an increase in the size of FG fibers Muscle enlargement (hypertrophy) due to increased
synthesis of thick and thin filaments
Copyright 2009, John Wiley & Sons, Inc.
Cardiac Muscle Tissue Principal tissue in the heart wall
Intercalated discs connect the ends of cardiac muscle fibers to one another Allow muscle action potentials to spread from one cardiac muscle
fiber to another Cardiac muscle tissue contracts when stimulated by its own
autorhythmic muscle fibers Continuous, rhythmic activity is a major physiological difference
between cardiac and skeletal muscle tissue Contractions lasts longer than a skeletal muscle twitch Have the same arrangement of actin and myosin as skeletal
muscle fibers Mitochondria are large and numerous Depends on aerobic respiration to generate ATP
Requires a constant supply of oxygen Able to use lactic acid produced by skeletal muscle fibers to make
ATP
Copyright 2009, John Wiley & Sons, Inc.
Figure 10–22
Smooth Muscle Tissue Usually activated involuntarily Action potentials are spread through the fibers
by gap junctions Fibers are stimulated by certain
neurotransmitter, hormone, or autorhythmic signals
Found in the Walls of arteries and veins Walls of hollow organs Walls of airways to the lungs Muscles that attach to hair follicles Muscles that adjust pupil diameter Muscles that adjust focus of the lens in the eye
Copyright 2009, John Wiley & Sons, Inc.
Smooth Muscle • Cells are not striated• Fibers smaller than those in
skeletal muscle• Spindle-shaped; single,
central nucleus• More actin than myosin• No sarcomeres
– Not arranged as symmetrically as in skeletal muscle, thus NO striations.
• Caveolae: indentations in sarcolemma;– May act like T tubules
• Dense bodies instead of Z disks – Have noncontractile
intermediate filaments• Free Ca2+ in cytoplasm
triggers contraction→Ca2+ binds w/ calmodulin: – in the sarcoplasm→ activates
myosin light chain kinase, an enzyme that breaks down ATP→ initiates contraction
Smooth Muscle Tissue
Copyright 2009, John Wiley & Sons, Inc.
Smooth Muscle Tissue Physiology of Smooth Muscle
Contraction lasts longer than skeletal muscle contraction
Contractions are initiated by Ca++ flow primarily from the interstitial fluid
Ca++ move slowly out of the muscle fiber delaying relaxation
Able to sustain long-term muscle tone Prolonged presence of Ca++ in the cell provides for a state of
continued partial contraction Important in the:
Gastrointestinal tract where a steady pressure is maintained on the contents of the tract
In the walls of blood vessels which maintain a steady pressure on blood
Copyright 2009, John Wiley & Sons, Inc.
Smooth Muscle Tissue Physiology of Smooth Muscle
Most smooth muscle fibers contract or relax in response to: Action potentials from the autonomic nervous
system Pupil constriction due to increased light energy
In response to stretching Food in digestive tract stretches intestinal walls initiating
peristalsis Hormones
Epinephrine causes relaxation of smooth muscle in the air-ways and in some blood vessel walls
Changes in pH, oxygen and carbon dioxide levels
Copyright 2009, John Wiley & Sons, Inc.
Regeneration of Muscular Tissue Hyperplasia
An increase in the number of fibers Skeletal muscle has limited regenerative abilities
Growth of skeletal muscle after birth is due mainly to hypertrophy
Satellite cells divide slowly and fuse with existing fibers Assist in muscle growth Repair of damaged fibers
Cardiac muscle can undergo hypertrophy in response to increased workload Many athletes have enlarged hearts
Smooth muscle in the uterus retain their capacity for division
Copyright 2009, John Wiley & Sons, Inc.
Development of Muscle Muscles of the body are derived from mesoderm As the mesoderm develops it becomes arranged on
either side of the developing spinal cord Columns of mesoderm undergo segmentation into
structures called somites The cells of a somite differentiate into three regions:
1) Myotome Forms the skeletal muscles of the head, neck, and limbs
2) Dermatome Forms the connective tissues, including the dermis of the skin
3) Sclerotome Gives rise to the vertebrae
Cardiac muscle and smooth muscle develop from migrating mesoderm cells
Copyright 2009, John Wiley & Sons, Inc.
Development of Muscle
Copyright 2009, John Wiley & Sons, Inc.
Aging and Muscular Tissue Aging
Brings a progressive loss of skeletal muscle mass A decrease in maximal strength A slowing of muscle reflexes A loss of flexibility
With aging, the relative number of slow oxidative fibers appears to increase
Aerobic activities and strength training can slow the decline in muscular performance
Copyright 2009, John Wiley & Sons, Inc.
End of Chapter 10
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