Chapter 10: Muscular Tissue. Copyright 2009, John Wiley & Sons, Inc.

Transcription

1 Chapter 10: Muscular Tissue

2 Muscular Tissue Chapter 10 Overview of Muscular Tissue Skeletal Muscle Tissue Contraction and Relaxation of Skeletal Muscle Fibers Muscle Metabolism Control of Muscle Tension Types of Skeletal Muscle Fibers Exercise and Skeletal Muscle Tissue Cardiac Muscle Tissue Smooth Muscle Tissue Regeneration of Muscle Tissue Development of Muscle Aging and Muscular Tissue

3 Overview of Muscular Tissue Types of Muscular Tissue The three types of muscular tissue Skeletal Cardiac Smooth Skeletal Muscle Tissue 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 subconsciously to some extent Ex: the diaphragm alternately contracts and relaxes without conscious control

4 Overview of Muscular Tissue Cardiac Muscle Tissue Found only in the walls of the heart Striated like skeletal muscle Action is involuntary Contraction and relaxation of the heart is not consciously controlled Contraction of the heart is initiated by a node of tissue called 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 Usually involuntary

5 Overview of Muscular Tissue

6 Overview of Muscular Tissue 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

7 Overview of Muscular Tissue Properties of Muscular Tissue 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

8 Skeletal Muscle Tissue Connective Tissue Components Fascia Dense sheet or broad band of irregular connective tissue that surrounds muscles Epimysium The outermost layer Separates 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

9 Skeletal Muscle Tissue

10 Skeletal Muscle Tissue 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

11 Skeletal Muscle Tissue Microscopic Anatomy 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

12 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 a red-colored protein called myoglobin which binds oxygen molecules Myoglobin releases oxygen when it is needed for ATP production

13 Skeletal Muscle Tissue Myofibrils Thread like structures which have a contractile function Sarcoplasmic reticulum (SR) Membranous sacs which encircles each myofibril 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

14 Skeletal Muscle Tissue Z discs Separate one sarcomere from the next Thick and thin filaments overlap one another A band I band Darker middle part of the sarcomere Thick and thin filaments overlap Lighter, contains thin filaments but no thick filaments Z discs passes through the center of each I band H zone M line Center of each A band which contains thick but no thin filaments Supporting proteins that hold the thick filaments together in the H zone

15 Skeletal Muscle Tissue

16 Contraction and Relaxation of Skeletal Muscle

17 Skeletal Muscle Tissue 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

18 Skeletal Muscle Tissue 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

19 Skeletal Muscle Tissue 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

20 Contraction and Relaxation of Skeletal Muscle 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

21 Contraction and Relaxation of Skeletal Muscle

22 Contraction and Relaxation of Skeletal Muscle

23 Contraction and Relaxation of Skeletal Muscle

24 Contraction and Relaxation of Skeletal 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

25 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

26 Contraction and Relaxation of Skeletal Muscle

27 Contraction and Relaxation of Skeletal Muscle

28 Key: = Ca 2+ 1 Myosin heads hydrolyze ATP and become reoriented and energized ADP P P 2 Myosin heads bind to actin, forming crossbridges ATP Contraction cycle continues if ATP is available and Ca 2+ level in the sarcoplasm is high ADP 4 As myosin heads bind ATP, the crossbridges detach from actin ATP ADP 3 Myosin crossbridges rotate toward center of the sarcomere (power stroke)

29 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 Action potentials causes Ca ++ to be released from the SR into the muscle cell Ca ++ moves tropomyosin away from the myosinbinding 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 As the Ca ++ level in the cell drops, myosin-binding sites are covered and the muscle relaxes

30 Contraction and Relaxation of Skeletal Muscle

31 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

32 Contraction and Relaxation of Skeletal Muscle

33 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

34 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)

35 Contraction and Relaxation of Skeletal Muscle

36 Sarcolemma Myofibril Axon collateral of somatic motor neuron Sarcolemma Axon terminal Synaptic end bulb Motor end Neuromuscular plate junction (NMJ) Axon terminal Nerve impulse Synaptic vesicle containing acetylcholine (ACh) Synaptic end bulb Synaptic cleft (space) (a) Neuromuscular junction (b) Enlarged view of the neuromuscular junction 1 1 ACh is released from synaptic vesicle Synaptic cleft (space) Synaptic end bulb 4 ACh is broken down 2 2 ACh binds to Ach receptor Na + Motor end plate Junctional fold 3 Muscle action potential is produced (c) Binding of acetylcholine to ACh receptors in the motor end plate

37 Contraction and Relaxation of Skeletal Muscle

38 Nerve impulse 1 Nerve impulse arrives at axon terminal of motor neuron and triggers release of acetylcholine (ACh). Muscle action potential ACh receptor Synaptic vesicle filled with ACh 3 2 ACh diffuses across Transverse tubule synaptic cleft, binds to its receptors in the motor end plate, and triggers a muscle 4 Muscle AP travelling along action potential (AP). transverse tubule opens Ca 2+ release channels in the sarcoplasmic reticulum (SR) membrane, which allows Acetylcholinesterase in calcium ions to flood into the synaptic cleft destroys sarcoplasm. ACh so another muscle action potential does not arise unless more ACh is SR released from motor neuron. Ca 2+ 9 Muscle relaxes. 8 Troponin tropomyosin complex slides back into position where it blocks the myosin binding sites on actin. 5 Ca 2+ binds to troponin on the thin filament, exposing the binding sites for myosin. Elevated Ca 2+ Ca 2+ active transport pumps 7 Ca 2+ release channels in SR close and Ca 2+ active transport pumps use ATP to restore low level of in sarcoplasm. Ca 2+ 6 Contraction: power strokes use ATP; myosin heads bind to actin, swivel, and release; thin filaments are pulled toward center of sarcomere.

39 Contraction and Relaxation of Skeletal Muscle 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 (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 in the lumbar region

40 Contraction and Relaxation of Skeletal Muscle Curare A plant poison used by South American Indians on arrows and blowgun darts Causes muscle paralysis by blocking ACh 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 Antidote for curare poisoning Terminate the effects of curare after surgery

41 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

42 Muscle Metabolism

43 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

44 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

45 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

46 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

47 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

48 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

49 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 muscle fibers per motor unit Muscles in the arm and the leg have 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

50 Control of Muscle Tension

51 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 ( msec) Ca ++ binds to troponin Myosin-binding sites on actin are exposed Cross-bridges form

52 Control of Muscle Tension Relaxation period ( 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

53 Control of Muscle Tension

54 Control of Muscle Tension

55 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

56 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

57 Control of Muscle Tension

58 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)

59 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

60 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

61 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

62 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

63 Types of Skeletal Muscle Fibers

64 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

65 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)

66 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

67 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

68 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

69 Smooth Muscle Tissue 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

70 Smooth Muscle Tissue

71 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

72 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 airways and in some blood vessel walls Changes in ph, oxygen and carbon dioxide levels

73 Smooth Muscle Tissue

74 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

75 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

76 Development of Muscle

77 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

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