Muscles Part I H. Biology II Adapted

Muscles Part I H. Biology II Adapted 2014-2015
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

Muscular System Three Types of Muscle Tissues Skeletal Muscle
usually attached to bones voluntary- under conscious control striated Cardiac Muscle wall of heart involuntary - not under conscious control striated Smooth Muscle walls of most viscera, blood vessels, skin involuntary - not under conscious control not striated 9-2

Table 9.3

Special Characteristics of Muscle Tissue
Excitability (responsiveness or irritability): ability to receive and respond to stimuli Contractility: ability to shorten when stimulated Extensibility: ability to be stretched or extended without damage Elasticity: ability to recoil to resting length (bounce back)

Functions of a Muscle Tissue
Movement: Skeletal- locomotion, vision, facial expression Cardiac- blood pumping Smooth- food digestion Posture: (skeletal) Joint Stability: (skeletal) Heat Generation:(skeletal) A. shivering to increase heat production B. contract muscle to provide heat

Structure of a Skeletal Muscle
organs of the muscular system skeletal muscle tissue nervous tissue blood Each muscle is served by one artery, one nerve, and one or more veins Connective tissues and muscle tissue: 1.fascia – covers the muscle 2.tendon – attaches the muscle 3.aponeuroses – muscle to muscle

Macroscopic Anatomy of skeletal muscles
epimysium tendon perimysium Muscle Fascicle Surrounded by perimysium endomysium Skeletal muscle Skeletal muscle fiber (cell) Surrounded by epimysium Surrounded by endomysium Play IP Anatomy of Skeletal muscles (IP p. 4-6)

(wrapped by perimysium)
Epimysium Epimysium Bone Perimysium Tendon Endomysium Muscle fiber in middle of a fascicle (b) Blood vessel Fascicle (wrapped by perimysium) Endomysium (between individual muscle fibers) Perimysium Fascicle Muscle fiber (a) Figure 9.1

Anatomy of a Skeletal Muscle
Coverings of a muscle 1. Epimysium - outer 2. Perimysium - middle 3. Endomysium - inner Organization of Muscle muscle fascicles muscle fibers myofibrils thick and thin filaments

Anatomy of a Skeletal Muscle
Coverings of a muscle 1. Epimysium – dense regular connective tissue surrounding the entire muscle 2. Perimysium – fibrous connective tissue surrounding a fascicle 3. Endomysium – thin areolar connective tissue surrounding each muscle cell Organization of Muscle muscle fascicles – bundle of muscle cells muscle fibers – a muscle cell myofibrils – a long, filamentous organelle found within muscle cells that has a banded appearance thick and thin filaments (myofilament)- actin &myosin filaments sarcomere – contractile unit of muscle 9-4

Skeletal Muscle Tissue
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

Microscopic Anatomy of Skeletal Muscle Fiber
sarcolemma sacroplasm sarcoplasmic reticulum transverse tubule triad cisterna of sarcoplasmic reticulum myofibril actin filaments myosin filaments sarcomere 9-5

Skeletal Muscle Tissue-Microscopic Anatomy
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

Skeletal Muscle Tissue- Microscopic Anatomy
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

Triad Relationships T tubules conduct impulses deep into muscle fiber Integral proteins protrude into the intermembrane space from T tubule and SR cisternae membranes T tubule proteins: voltage sensors SR foot proteins: gated channels that regulate Ca2+ release from the SR cisternae

Sarcomere Smallest contractile unit (functional unit) of a muscle fiber The region of a myofibril between two successive Z discs Composed of thick and thin myofilaments made of contractile proteins Each myofibril contains 10,000 sarcomeres end to end.

Check for Understanding 1:

Check for Understanding 2:

Microanatomy of a Muscle Fiber (Cell)
transverse (T) tubules sarcoplasmic reticulum sarcolemma terminal cisternae mitochondria myoglobin thick myofilament thin myofilament myofibril triad nuclei Play IP Anatomy of Skeletal muscles (IP p. 7)

myofibril Muscle fiber sarcomere Z-line Thin filaments Thick filaments
Myosin molecule of thick myofilament Thin myofilament

Sarcomere I band A band H zone Z line M line 9-6

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 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 Myosin and Actin Actin Myosin

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 Actin 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

Structural Proteins Titin Dystrophin 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 In the relaxed state, thin and thick filaments overlap only slightly During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens

Sliding Filament Mechanism
When sarcomeres shorten, actin and myosin filaments slide past one another VIDEO#1 VIDEO #2

1 2 Figure 9.6 Z H Z I A I Fully relaxed sarcomere of a muscle fiber Z
Fully contracted sarcomere of a muscle fiber Figure 9.6

Longitudinal section of filaments within one sarcomere of a myofibril
Thick filament Thin filament In the center of the sarcomere, the thick filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap. Thick filament Thin filament Each thick filament consists of many myosin molecules whose heads protrude at opposite ends of the filament. A thin filament consists of two strands of actin subunits twisted into a helix plus two types of regulatory proteins (troponin and tropomyosin). Portion of a thick filament Portion of a thin filament Myosin head Tropomyosin Troponin Actin Actin-binding sites Heads Tail Active sites for myosin attachment ATP- binding site Actin subunits Flexible hinge region Myosin molecule Actin subunits Figure 9.3

Contraction & Relaxation of Skeletal Muscle

Microscopic Contraction Events

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

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

Cross Bridge Cycle Continues as long as the Ca2+ signal and adequate ATP are present Cross bridge formation—high-energy myosin head attaches to thin filament Working (power) stroke—myosin head pivots and pulls thin filament toward M line Cross bridge detachment—ATP attaches to myosin head and the cross bridge detaches “Cocking” of the myosin head—energy from hydrolysis of ATP cocks the myosin head into the high-energy state

1 Myosin heads hydrolyze ATP 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 bind ATP, the crossbridges detach from actin Contraction cycle continues if ATP is available and Ca2+ level in the sarcoplasm is high ADP ATP P = Ca2+ Key: 2 3 4 1 Myosin heads hydrolyze ATP and become reoriented and energized Myosin heads bind to actin, forming crossbridges Myosin crossbridges rotate toward center of the sarcomere (power stroke) Contraction cycle continues if ATP is available and Ca2+ level in the sarcoplasm is high ADP P = Ca2+ Key: 2 3 1 Myosin heads hydrolyze ATP and become reoriented and energized Myosin heads bind to actin, forming crossbridges ADP P = Ca2+ Key: 2 Contraction cycle continues if ATP is available and Ca2+ level in the sarcoplasm is high 1 Myosin heads hydrolyze ATP and become reoriented and energized ADP P = Ca2+ Key: Contraction cycle continues if ATP is available and Ca2+ level in the sarcoplasm is high

Figure 9.12 Thin filament Actin Ca2+ Myosin cross bridge Thick
ADP Pi Thick filament Myosin 1 Cross bridge formation. ADP ADP Pi ATP hydrolysis Pi 4 Cocking of myosin head. 2 The power (working) stroke. ATP ATP 3 Cross bridge detachment. Figure 9.12

Role of Calcium (Ca2+) in Contraction
At low intracellular Ca2+ concentration: Tropomyosin blocks the active sites on actin Myosin heads cannot attach to actin Muscle fiber relaxes

Role of Calcium (Ca2+) in Contraction
At higher intracellular Ca2+ concentrations: Ca2+ binds to troponin Troponin changes shape and moves tropomyosin away from active sites Events of the cross bridge cycle occur When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends

Muscle Contraction The generation of force Does not necessarily cause shortening of the fiber Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening

Requirements for Skeletal Muscle Contraction
Activation: neural stimulation at a neuromuscular junction Excitation-contraction coupling: (not heavily discussed 2015) Generation and propagation of an action potential along the sarcolemma Final trigger: a brief rise in intracellular Ca2+ levels

Skeletal Muscle Contraction
? How does a muscle contract?

Sequence of a Muscle Contraction (Summary) Neuromuscular Junction
Brain Spinal Cord Nerve (Action potential) Motor Unit Neuromuscular Junction (Calcium is released) Acetylcholine (Neurotransmitter) Contraction

Motor Unit single motor neuron (a single nerve)
one motor neuron and many skeletal muscle fibers 9-9

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

Neuromuscular Junction
site where a motor nerve fiber and a skeletal muscle fiber meet 9-8

Muscle Contraction Action potential causes the release of Ca++ at the NMJ. a neurotransmitter releases a chemical substance from the motor end fiber, causing stimulation of the muscle fiber That substance is called acetylcholine (ACh) ACh causes the muscle fibers to become stimulated and contract (shorten). 9-10

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 (Relaxation) Ach effects last only briefly because it is rapidly broken down by acetylcholinesterase (AChE)

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

Pearson NMJ

Relaxation of a Muscle- Review
acetylcholinesterase – an enzyme that breaks down acetylcholine. NMJ muscle impulse stops calcium moves back into sarcoplasmic reticulum myosin and actin action prevented muscle fiber relaxes Cd 9-14

Recruitment of Motor Units
Recruitment - increase in the number of motor units activated whole muscle composed of many motor units as intensity of stimulation or contraction increases, recruitment of motor units continues until all motor units are activated = all or none principle 9-22

Applications for M. Contraction/Relaxation
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

Applications for Muscle Contraction/Relaxation
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 Myasthenia Gravis Common symptoms can include: A drooping eyelid Blurred or double vision Slurred speech Difficulty chewing and swallowing Weakness in the arms and legs Chronic muscle fatigue Difficulty breathing

Question ???? We now know how a muscle contracts and relaxes, so is energy needed for that to happen? NO or YES ?

How is energy that is stored in food released?
Cellular Respiration Oxygen Glucose H2O + CO2 Energy ATP

ENERGY The energy used to power the interaction between actin and myosin filaments comes from ATP (useable chemical energy) produced by cellular respiration. ATP stored in skeletal muscle last only about six seconds. ATP must be regenerated continuously if contraction is to continue

Ways to Make Energy Sources for Contraction
1) Creatine phosphate (ADP) 2) Cellular respiration (Aerobic or Anaerobic) Creatine phosphate – stores energy that quickly converts unusable energy (ADP) to usable energy (ATP) 6 Seconds!! Cellular respiration is the process by which the chemical energy of "food" molecules is released and partially captured in the form of ATP.

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

Cellular Respiration (CR)
THREE SERIES OF REACTIONS in CR Glycolysis Citric acid cycle Electron transport chain Produces carbon dioxide water ATP (chemical energy) heat Two Types of Reactions Anaerobic Respiration (without O2) - produce little ATP Aerobic Respiration (requires O2) - produce most ATP 4-11

Anaerobic Reaction (Glycolysis)
Recall that glycolysis results in pyruvate acid. If O2 is not present, pyruvate can be fermented into LACTIC ACID. Lactic Acid It is a waste product of pyruvate acid. Occurs in many muscle cells. Accumulation causes muscle soreness and fatigue.

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

Oxygen Supply & Cellular Respiration
Anaerobic Phase Steps are called glycolysis. occur in the cytoplasm no oxygen produces pyruvic acid and produces lactic acid little ATP Aerobic Phase Steps are called citric acid cycle and electron transport chain. occur in the mitochondrion oxygen produces most ATP / CO2/ H2O

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

Summary of Cellular Respiration
Total ATP Production 2 ATP – Glycolysis 2 ATP – Citrus Acid Cycle 34 ATP – Electron Transport Chain 38 ATP – Total energy released from one molecule of glucose.

Oxygen Debt Oxygen debt – amount of oxygen needed by liver to convert lactic acid to glucose oxygen not available glycolysis continues pyruvic acid converted to lactic acid

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

What happens to the lactic acid once it has accumulated?
The liver filters the blood and rids the body of toxins. Lactic acid is a toxin. liver converts lactic acid to glucose

Muscle fatigue- is a state of physiological inability to contract
commonly caused from decreased blood flow ion imbalances accumulation of lactic acid cramp – sustained, involuntary contraction 9-18

Muscle Fatigue Muscle Metabolism
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

Control of Muscle Tension

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

Two Types of Isotonic Contractions
Types of Contractions Two Types 1. Isometric – muscle contracts but does not change length 2. Isotonic – muscle contracts and changes length Two Types of Isotonic Contractions 1. Eccentric – lengthening contraction 2. Concentric – shortening contraction 9-24

Muscle Tone Muscle tone – continuous state of partial contraction
Even when a muscle appears to be at rest, a certain amount of sustained contraction is occurring in its fibers. Atrophy – a wasting away or decrease in size of an organ or tissue. Hypertrophy – Enlargement of an organ or tissue. 9-23

Types of Contractions

Smooth and Cardiac Muscle

Compared to skeletal muscle fibers shorter single nucleus
Smooth Muscle Fibers Compared to skeletal muscle fibers shorter single nucleus elongated with tapering ends myofilaments randomly organized no striations 9-26

Two Types of Smooth Muscle
SEM: Arteriole Multiunit Smooth Muscle irises of eye walls of blood vessels contractions are rapid and vigorous similar to skeletal muscle tissue Visceral Smooth Muscle Location - walls of most hollow organs (intestine) contractions are slow and sustained exhibit rhythmicity – pattern of repeated contractions exhibit peristalsis – wave-like motion that helps substances through passageways. SEM: Stomach

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

Smooth Muscle Tissue Pupil constriction due to increased light energy
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

Smooth Muscle Contraction
Resembles skeletal muscle contraction interaction between actin and myosin both use calcium and ATP both depend on impulses Different from skeletal muscle contraction hormones affect smooth muscle stretching can trigger smooth muscle contraction smooth muscle slower to contract and relax smooth muscle more resistant to fatigue 9-28

Cardiac Muscle Anatomy only in the heart
striated uninuclear cells join end-to-end forming a network arrangement of actin and myosin are not as organized as skeletal muscle Physiology self-exciting tissue (Pacemaker) rhythmic contractions involuntary, all or nothing contractions Pumps blood to: 1. lungs for oxygenation 2. body for distribution of O2 and nutrients 9-29

Cardiac Muscle Tissue Principal tissue in the heart wall
Intercalated discs Allow muscle action potentials to spread from one cardiac muscle fiber to another autorhythmic muscle fibers Continuous, rhythmic activity 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

Aging and Muscular Tissue
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

Of Note: 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)

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