6/10/2014
1
<iframe
src="http://www.slideshare.net/slideshow/em
bed_code/35705970" width="479"
height="511" frameborder="0...
6/10/2014
2
 Cardiac MuscleTissue
 Found only in the walls of the heart
 Striated like skeletal muscle
 Action is invo...
6/10/2014
3
 ConnectiveTissue Components
 Fascia
▪ Dense sheet or broad band of irregularconnective tissue that
surround...
6/10/2014
4
 MicroscopicAnatomy
 The number of skeletal muscle fibers is set
before you are born
▪ Most of these cells l...
6/10/2014
5
 Z discs
 Separate one sarcomere from the next
 Thick and thin filaments overlap one another
 A band
 Dar...
6/10/2014
6
6/10/2014
7
 Muscle Proteins
 Myofibrils are built from three kinds of
proteins
▪ 1) Contractile proteins
▪ Generate for...
6/10/2014
8
 Structural Proteins
 Titin
▪ Stabilize the position of myosin
▪ accounts for much of the elasticity and ext...
6/10/2014
9
 The contractioncycle consists of 4 steps
 1) ATP hydrolysis
▪ Hydrolysis ofATP reorients and energizes the ...
6/10/2014
10
 Excitation–ContractionCoupling
 An increase in Ca++ concentration in the muscle starts
contraction
 A dec...
6/10/2014
11
 The NeuromuscularJunction
 Motor neurons have a threadlike axon that extends from the brain or spinal
cord...
6/10/2014
12
 Nerve impulses elicit a muscle action potential in
the following way
 1) Release of acetylcholine
▪ Nerve ...
6/10/2014
13
 Curare
 A plant poison used by SouthAmerican Indians on arrows
and blowgun darts
 Causes muscle paralysis...
6/10/2014
14
 Creatine Phosphate
 ExcessATP is used to synthesize creatine
phosphate
▪ Energy-rich molecule
 Creatine p...
6/10/2014
15
 Muscle Fatigue
 Inability of muscle to maintain force of
contraction after prolonged activity
 Factors th...
6/10/2014
16
 MotorUnits
 Consists of a motor neuron and the muscle fibers it stimulates
 The axon of a motor neuron br...
6/10/2014
17
 Relaxation period (10–100 msec)
▪ Ca++ is transported into the SR
▪ Myosin-binding sites are covered by tro...
6/10/2014
18
 MuscleTone
 A small amount of tension in the muscle due to
weak contractions of motor units
 Small groups...
6/10/2014
19
 Muscle fibers vary in their content of
myoglobin
 Red muscle fibers
▪ Have a high myoglobin content
▪ Appe...
6/10/2014
20
 Fast Oxidative–Glycolytic Fibers (FOG fibers)
 Intermediate in diameter between the other two types of
fib...
6/10/2014
21
 Distribution and Recruitment of Different
Types of Fibers
 Most muscles are a mixture of all three types o...
6/10/2014
22
 Principal tissue in the heart wall
 Intercalated discs connect the ends of cardiac muscle fibers to one
an...
6/10/2014
23
 Microscopic Anatomy of Smooth Muscle
 Contains both thick filaments and thin filaments
▪ Not arranged in o...
6/10/2014
24
 Physiology of Smooth Muscle
 Most smooth muscle fibers contract or relax in
response to:
▪ Action potentia...
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Bio 201 chapter 10 lecture

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Bio 201 chapter 10 lecture

  1. 1. 6/10/2014 1 <iframe src="http://www.slideshare.net/slideshow/em bed_code/35705970" width="479" height="511" frameborder="0" marginwidth="0" marginheight="0" scrolling="no" style="border:1px solid #CCC; border-width:1px 1px 0; margin-bottom:5px; max-width: 100%;" allowfullscreen> </iframe> <div style="margin-bottom:5px"> <strong> <a href="https://www.slideshare.net/DrPearcy/bi o-201-chapter-8-lecture" title="Bio 201 chapter 8 lecture" target="_blank">Bio 201 chapter 8 lecture</a> </strong> from <strong><a href="http://www.slideshare.net/DrPearcy" target="_blank">Matt</a></strong></div>  Overview of MuscularTissue  Skeletal MuscleTissue  Contraction and Relaxation of Skeletal Muscle Fibers  Muscle Metabolism  Control of Muscle Tension  Types of Skeletal Muscle Fibers  Exercise and Skeletal Muscle Tissue  Cardiac MuscleTissue  Smooth Muscle Tissue  Regeneration of MuscleTissue  Aging and MuscularTissue  Types of MuscularTissue  The three types of muscular tissue ▪ Skeletal ▪ Cardiac ▪ Smooth  Skeletal MuscleTissue  So named because most skeletal muscles move bones  Skeletal muscle tissue is striated: ▪ Alternating light and dark bands (striations) as seen when examined with a microscope  Skeletal muscle tissue works mainly in a voluntary manner ▪ Its activity can be consciously controlled  Most skeletal muscles also are controlled subconsciouslyto some extent ▪ Ex: the diaphragm alternately contractsand relaxes without conscious control
  2. 2. 6/10/2014 2  Cardiac MuscleTissue  Found only in the walls of the heart  Striated like skeletal muscle  Action is involuntary ▪ Contraction and relaxation of the heart is not consciouslycontrolled ▪ Contraction of the heart is initiated by a node of tissue called the “pacemaker”  Smooth MuscleTissue  Located in the walls of hollow internal structures ▪ Blood vessels, airways, and many organs  Lacks the striations of skeletal and cardiac muscle tissue  Usually involuntary  Functions of MuscularTissue  Producing Body Movements ▪ Walking and running  Stabilizing Body Positions ▪ Posture  Moving SubstancesWithin the Body ▪ Heart muscle pumping blood ▪ Moving substances in the digestive tract  Generating heat ▪ Contracting muscle produces heat ▪ Shivering increases heat production  Properties of MuscularTissue  Properties that enable muscle to function and contribute to homeostasis ▪ 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
  3. 3. 6/10/2014 3  ConnectiveTissue Components  Fascia ▪ Dense sheet or broad band of irregularconnective 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  Tendon ▪ Cord that attach a muscle to a bone  Aponeurosis ▪ Broad, flattened tendon  Nerve and Blood Supply  Neurons that stimulate skeletal muscle to contract are somatic motor neurons  The axon of a somatic motor neuron typically branches many times ▪ Each branch extending to a different skeletal muscle fiber  Each muscle fiber is in close contact with one or more capillaries
  4. 4. 6/10/2014 4  MicroscopicAnatomy  The number of skeletal muscle fibers is set before you are born ▪ Most of these cells last a lifetime  Muscle growth occurs by hypertrophy ▪ An enlargement of existing muscle fibers  Testosterone and human growth hormone stimulate hypertrophy  Satellite cells retain the capacity to regenerate damaged muscle fibers  Sarcolemma  The plasma membrane of a muscle cell  Transverse (T tubules)  Tunnel in from the plasma membrane  Muscle action potentials travel through theT tubules  Sarcoplasm, the cytoplasm of a muscle fiber  Sarcoplasm includes glycogen used for synthesis of ATP and a red-colored protein called myoglobin which 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 which encircles each myofibril  Stores calcium ions (Ca++)  Release ofCa++ triggers muscle contraction  Filaments  Function in the contractile process  Two types of filaments (Thick andThin)  There are two thin filaments for every thick filament  Sarcomeres  Compartmentsof arranged filaments  Basic functional unit of a myofibril
  5. 5. 6/10/2014 5  Z discs  Separate one sarcomere from the next  Thick and thin filaments overlap one another  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 throughthe center of each I band  H zone  Center of eachA band whichcontains thick but no thin filaments  M line  Supportingproteins that hold the thick filaments together in the H zone
  6. 6. 6/10/2014 6
  7. 7. 6/10/2014 7  Muscle Proteins  Myofibrils are built from three kinds of proteins ▪ 1) Contractile proteins ▪ Generate force during contraction ▪ 2) Regulatory proteins ▪ Switch the contraction process on and off ▪ 3) Structural proteins ▪ Align the thick and thin filaments properly ▪ Provide elasticity and extensibility ▪ Link the myofibrils to the sarcolemma  Contractile Proteins  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 ▪ 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 ▪ Allows muscle contraction to begin as myosin binds to actin
  8. 8. 6/10/2014 8  Structural Proteins  Titin ▪ Stabilize the position of myosin ▪ accounts for much of the elasticity and extensibility of myofibrils  Dystrophin ▪ Links thin filaments to the sarcolemma  The Sliding Filament Mechanism  Myosin heads attach to and “walk” along the thin filaments at both ends of a sarcomere  Progressively pulling the thin filaments toward the center of the sarcomere  Z discs come closer together and the sarcomere shortens  Leading to shortening of the entire muscle  The Contraction Cycle  The onset of contraction begins with the SR releasing calcium ions into the muscle cell  Where they bind to actin opening the myosin binding sites
  9. 9. 6/10/2014 9  The contractioncycle consists of 4 steps  1) ATP hydrolysis ▪ Hydrolysis ofATP 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 1 Myosin heads hydrolyzeATP and become reoriented and energized Myosin heads bind to actin, forming crossbridges Myosin crossbridges rotate toward center of the sarcomere (power stroke) As myosin heads bindATP, the crossbridgesdetach from actin Contraction cycle continues if ATP is available and Ca2+ level in the sarcoplasm is high ADP ADP ADP ATP P P = Ca2+ Key: ATP 2 3 4 Muscle Contraction MuscleContraction
  10. 10. 6/10/2014 10  Excitation–ContractionCoupling  An increase in Ca++ concentration in the muscle starts contraction  A decrease in Ca++ stops it  Action potentials 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 containsCa++ pumps to return Ca++ back to the SR quickly ▪ Decreasing calcium ion levels  As the Ca++ level in the cell drops, myosin-bindingsites are covered and the muscle relaxes  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
  11. 11. 6/10/2014 11  The NeuromuscularJunction  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
  12. 12. 6/10/2014 12  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 theSR to release its stored Ca++  4)Termination of ACh activity ▪ Ach effects last only briefly because it is rapidly broken down by acetylcholinesterase (AChE)  Botulinum toxin  Blocks release of ACh from synaptic vesicles  May be found in improperly canned foods ▪ A tiny amount can cause death by paralyzing respiratory muscles  Used as a medicine (Botox®) ▪ Strabismus (crossed eyes) ▪ Blepharospasm(uncontrollableblinking) ▪ 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 in the lumbar region
  13. 13. 6/10/2014 13  Curare  A plant poison used by SouthAmerican Indians on arrows and blowgun darts  Causes muscle paralysis by blockingACh receptors inhibiting Na+ ion channels  Derivatives of curare are used during surgery to relax skeletal muscles  Anticholinesterase  Slow actions of acetylcholinesterase and removal of ACh  Can strengthen weak muscle contractions ▪ Ex: Neostigmine ▪ Treatment for myasthenia gravis ▪ Antidotefor curare poisoning ▪ Terminate the effects of curare after surgery  Production of ATP in Muscle Fibers  A huge amount ofATP is needed to: ▪ Power the contraction cycle ▪ Pump Ca++ into the SR  TheATP 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 produceATP ▪ 1) From creatine phosphate ▪ 2) By anaerobic cellular respiration ▪ 3) By aerobic cellular respiration
  14. 14. 6/10/2014 14  Creatine Phosphate  ExcessATP is used to synthesize creatine phosphate ▪ Energy-rich molecule  Creatine phosphate transfers its high energy phosphate group to ADP regenerating newATP  Creatine phosphate andATP provide enough energy for contraction for about 15 seconds  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  Aerobic Respiration  Activity that lasts longer than half a minute depends on aerobic respiration  Pyruvic acid entering the mitochondriais completely oxidized generating ▪ ATP ▪ carbon dioxide ▪ Water ▪ Heat  Each molecule of glucose yields about 36 molecules ofATP  Muscle tissue has two sources of oxygen ▪ 1) Oxygen from hemoglobin in the blood ▪ 2) Oxygen released by myoglobin in the muscle cell  Myoglobinand hemoglobinare oxygen-bindingproteins  Aerobic respiration suppliesATP for prolongedactivity  Aerobic respiration provides more than 90% of the neededATP in activities lasting more than 10 minutes
  15. 15. 6/10/2014 15  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 andADP  Failure of the motor neuron to release enough acetylcholine  OxygenConsumptionAfter 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 andATP ▪ 3) to replace the oxygen removed from myoglobin  The tension or force of muscle cell contraction varies  MaximumTension (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
  16. 16. 6/10/2014 16  MotorUnits  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  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  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
  17. 17. 6/10/2014 17  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 periodof 300 milliseconds
  18. 18. 6/10/2014 18  MuscleTone  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  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
  19. 19. 6/10/2014 19  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)  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-glycolyticfibers  3) Fast glycolytic fibers  Slow Oxidative Fibers (SO fibers)  Smallest in diameter  Least powerful type of muscle fibers  Appear dark red (more myoglobin)  GenerateATP 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
  20. 20. 6/10/2014 20  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  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  GenerateATP mainly by glycolysis  Fibers contract strongly and quickly  Fatigue quickly  Adapted for intense anaerobic movementsof short duration ▪ Weight lifting or throwinga ball
  21. 21. 6/10/2014 21  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  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)  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
  22. 22. 6/10/2014 22  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  Mitochondriaare large and numerous  Depends on aerobic respiration to generateATP ▪ Requires a constant supply of oxygen ▪ Able to use lactic acid produced by skeletal muscle fibers to makeATP  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 holloworgans ▪ Walls of airways to the lungs ▪ Muscles that attach to hair follicles ▪ Muscles that adjust pupildiameter ▪ Muscles that adjust focus of the lens in the eye
  23. 23. 6/10/2014 23  Microscopic Anatomy of Smooth Muscle  Contains both thick filaments and thin filaments ▪ Not arranged in orderly sarcomeres  No regular pattern of overlap thus not striated  Contain only a small amount of stored Ca++  Filaments attach to dense bodies and stretch from one dense body to another  Dense bodies ▪ Function in the same way as Z discs ▪ During contraction the filaments pull on the dense bodies causing a shortening of the muscle fiber  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 ▪ Prolongedpresence ofCa++ 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
  24. 24. 6/10/2014 24  Physiology of Smooth Muscle  Most smooth muscle fibers contract or relax in response to: ▪ Action potentials from the autonomic nervous system ▪ Pupilconstriction due to increased light energy ▪ In response to stretching ▪ Food in digestive tract stretches intestinal walls initiating peristalsis ▪ Hormones ▪ Epinephrinecauses relaxation of smooth muscle in the air-ways and in some blood vessel walls ▪ Changes in pH, oxygen and carbon dioxide levels  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

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