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Anatomy and Physiology, Seventh Edition

Rod R. SeeleyIdaho State UniversityTrent D. StephensIdaho State UniversityPhilip TatePhoenix College

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

*See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes.

Chapter 09Chapter 09

Lecture OutlineLecture Outline**

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Muscular System:Histology and Physiology

Chapter 9

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Muscular System Functions

• Body movement

• Maintenance of posture

• Respiration

• Production of body heat

• Communication

• Constriction of organs and vessels

• Heart beat

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Properties of Muscle

• Contractility: ability of a muscle to shorten with force

• Excitability: capacity of muscle to respond to a stimulus

• Extensibility: muscle can be stretched to its normal resting length and beyond to a limited degree

• Elasticity: ability of muscle to recoil to original resting length after stretched

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Muscle Tissue Types• Skeletal

– Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement

– Voluntary

• Smooth– Walls of hollow organs, blood vessels, eye, glands, skin

– Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow

– In some locations, autorhythmic

– Controlled involuntarily by endocrine and autonomic nervous systems

• Cardiac– Heart: major source of movement of blood

– Autorhythmic

– Controlled involuntarily by endocrine and autonomic nervous systems

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Skeletal Muscle Structure

• Composed of muscle cells (fibers), connective tissue, blood vessels, nerves

• Fibers are long, cylindrical, 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 due to light and dark banding

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Connective Tissue

• Layers– External lamina. Delicate, reticular fibers.

Surrounds sarcolemma– Endomysium. Loose C.T. with reticular

fibers.– Perimysium. Denser C.T. surrounding a

group of muscle fibers. Each group called a fasciculus

– Epimysium. C.T. that surrounds a whole muscle (many fascicles)

• Fascia: connective tissue sheet– Forms layer under the skin– Holds muscles together and separates them

into functional groups.– Allows free movements of muscles.– Carries nerves (motor neurons, sensory

neurons), blood vessels, and lymphatics.– Continuous with connective tissue of tendons

and periosteum.

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Nerve and Blood Vessel Supply

• Motor neurons: stimulate muscle fibers to contract. Nerve cells with cell bodies in brain or spinal cord; axons extend to skeletal muscle fibers through nerves

• Axons branch so that each muscle fiber is innervated

• Capillary beds surround muscle fibers

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Muscle Fiber Anatomy

• Nuclei just inside sarcolemma

• Cell packed with myofibrils within cytoplasm (sarcoplasm)– Threadlike– Composed of protein threads called

myofilaments: thin (actin) and thick (myosin)– Sarcomeres: highly ordered repeating units of

myofilaments

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Parts of a Muscle

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Structure of Actin and Myosin

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Actin (Thin) Myofilaments

• Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere.– Composed of G actin monomers

each of which has an active site – Actin site can bind myosin during

muscle contraction. • Tropomyosin: an elongated protein

winds along the groove of the F actin double helix.

• Troponin is composed of three subunits: one that binds to actin, a second that binds to tropomyosin, and a third that binds to calcium ions. Spaced between the ends of the tropomyosin molecules in the groove between the F actin strands.

• The tropomyosin/troponin complex regulates the interaction between active sites on G actin and myosin.

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Myosin (Thick)

Myofilament

• Many elongated myosin molecules shaped like golf clubs.

• Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally.

• Myosin heads1. Can bind to active sites on the actin

molecules to form cross-bridges. 2. Attached to the rod portion by a

hinge region that can bend and straighten during contraction.

3. Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

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Sarcomeres: Z Disk to Z

Disk

• Z disk: filamentous network of protein. Serves as attachment for actin myofilaments

• Striated appearance– I bands: from Z disks to ends of

thick filaments– A bands: length of thick filaments– H zone: region in A band where

actin and myosin do not overlap– M line: middle of H zone; delicate

filaments holding myosin in place

• In muscle fibers, A and I bands of parallel myofibrils are aligned.

• Titin filaments: elastic chains of amino acids; make muscles extensible and elastic

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Sliding Filament Model• Actin myofilaments sliding over myosin to

shorten sarcomeres– Actin and myosin do not change length– Shortening sarcomeres responsible for

skeletal muscle contraction• During relaxation, sarcomeres lengthen because

of some external force, like contraction of antagonistic muscles

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Sarcomere Shortening

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Physiology of Skeletal Muscle

• Nervous system controls muscle contractions through action potentials

• Resting membrane potentials– Membrane voltage difference across membranes (polarized)

• Inside cell more negative due to accumulation of large protein molecules. More K+ on inside than outside. K+ leaks out but not completely because negative proteins hold some back.

• Outside cell more positive and more Na+ on outside than inside. Na/K pump maintains this situation.

– Must exist for action potential to occur

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Ion Channels

• Types– Ligand-gated. Ligands are

molecules that bind to receptors. Receptor: protein or glycoprotein with a receptor site

• Example: neurotransmitters• Gate is closed until

neurotransmitter attaches to receptor molecule. When Ach attaches to receptor on muscle cell, Na gate opens. Na moves into cell due to concentration gradient

– Voltage-gated• Open and close in response to

small voltage changes across plasma membrane

• Each is specific for one type of ion

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Action Potentials

• Phases– Depolarization: Inside of plasma

membrane becomes less negative. If change reaches threshold, depolarization occurs

– Repolarization: return of resting membrane potential. Note that during repolarization, the membrane potential drops lower than its original resting potential, then rebounds. This is because Na plus K together are higher, but then Na/K pump restores the resting potential

• All-or-none principle: like camera flash system

• Propagate: Spread from one location to another. Action potential does not move along the membrane: new action potential at each successive location.

• Frequency: number of action potential produced per unit of time

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Gated Ion Channels and the Action Potential

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Action Potential Propagation

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Neuromuscular Junction

• Synapse: axon terminal resting in an invagination of the sarcolemma

• Neuromuscular junction (NMJ):– Presynaptic terminal:

axon terminal with synaptic vesicles

– Synaptic cleft: space– Postsynaptic

membrane or motor end-plate

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Function of Neuromuscular Junction

• Synaptic vesicles– Neurotransmitter:

substance released from a presynaptic membrane that diffuses across the synaptic cleft and stimulates (or inhibits) the production of an action potential in the postsynaptic membrane. Acetylcholine

– Acetylcholinesterase: A degrading enzyme in synaptic cleft. Prevents accumulation of ACh

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Excitation-Contraction Coupling

• Mechanism where an action potential causes muscle fiber contraction

• Involves– Sarcolemma

– Transverse (T) tubules: invaginations of sarcolemma

– Terminal cisternae

– Sarcoplasmic reticulum: smooth ER

– Triad: T tubule, two adjacent terminal cisternae

– Ca2+

– Troponin

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Action Potentials and Muscle Contraction

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Cross-Bridge Movement

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Relaxation

• Ca2+ moves back into sarcoplasmic reticulum by active transport. Requires energy

• Ca2+ moves away from troponin-tropomyosin complex

• Complex re-establishes its position and blocks binding sites.

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Muscle Twitch

• Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers

• Phases– Lag or latent

– Contraction

– Relaxation

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Stimulus Strength and Muscle Contraction

• All-or-none law for muscle fibers– Contraction of equal force in response to

each action potential

• Sub-threshold stimulus: no action potential; no contraction

• Threshold stimulus: action potential; contraction

• Stronger than threshold; action potential; contraction equal to that with threshold stimulus

• Motor units: a single motor neuron and all muscle fibers innervated by it

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Contraction of the Whole Muscle• Strength of contraction is

graded: ranges from weak to strong depending on stimulus strength

• Multiple motor unit summation: strength of contraction depends upon recruitment of motor units. A muscle has many motor units– Submaximal stimuli

– Maximal stimulus

– Supramaximal stimuli

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Multiple-Wave Summation

• As the frequency of action potentials increase, the frequency of contraction increases– Incomplete tetanus:

muscle fibers partially relax between contraction

– Complete tetanus: no relaxation between contractions

– Multiple-wave summation: muscle tension increases as contraction frequencies increase

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Treppe

• Graded response

• Occurs in muscle rested for prolonged period

• Each subsequent contraction is stronger than previous until all equal after few stimuli

• Possibly explanation: more and more Ca2+ remains in sarcoplasm and is not all taken up into the sarcoplasmic reticulum

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Types of Muscle Contractions

• Isometric: no change in length but tension increases– Postural muscles of body

• Isotonic: change in length but tension constant– Concentric: overcomes opposing resistance and

muscle shortens

– Eccentric: tension maintained but muscle lengthens

• Muscle tone: constant tension by muscles for long periods of time

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Muscle Length and Tension

• Active tension: force applied to an object to be lifted when a muscle contracts

• Stretched muscle- not enough cross-bridging

• Crumpled muscle- myofilaments crumpled, cross-bridges can't contract

• Passive tension: tension applied to load when a muscle is stretched but not stimulated

• Total tension: active plus passive

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Fatigue

• Decreased capacity to work and reduced efficiency of performance

• Types– Psychological: depends on emotional state of

individual– Muscular: results from ATP depletion – Synaptic: occurs in NMJ due to lack of

acetylcholine

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Physiological Contracture and Rigor Mortis

• Physiological contracture: state of fatigue where due to lack of ATP neither contraction nor relaxation can occur

• Rigor mortis: development of rigid muscles several hours after death. Ca2+ leaks into sarcoplasm and attaches to myosin heads and crossbridges form. Rigor ends as tissues start to deteriorate.

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Energy Sources• ATP provides immediate energy for muscle

contractions. Produced from three sources– Creatine phosphate

• During resting conditions stores energy to synthesize ATP

– Anaerobic respiration• Occurs in absence of oxygen and results in breakdown of

glucose to yield ATP and lactic acid

– Aerobic respiration• Requires oxygen and breaks down glucose to produce ATP,

carbon dioxide and water• More efficient than anaerobic

• Oxygen debt: oxygen taken in by the body, above that required for resting metabolism after exercise. ATP produced from anaerobic sources contributes

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Slow and Fast Fibers• Slow-twitch or high-oxidative

– Contract more slowly, smaller in diameter, better blood supply, more mitochondria, more fatigue-resistant than fast-twitch, large amount of myoglobin.

– Postural muscles, more in lower than upper limbs. Dark meat of chicken.• Fast-twitch or low-oxidative

– Respond rapidly to nervous stimulation, contain myosin that can break down ATP more rapidly than that in Type I, less blood supply, fewer and smaller mitochondria than slow-twitch

– Lower limbs in sprinter, upper limbs of most people. White meat in chicken.

• Distribution of fast-twitch and slow-twitch– Most muscles have both but varies for each muscle

• Effects of exercise: change in size of muscle fibers– Hypertrophy: increase in muscle size

• Increase in myofibrils• Increase in nuclei due to fusion of satellite cells• Increase in strength due to better coordination of muscles, increase in

production of metabolic enzymes, better circulation, less restriction by fat– Atrophy: decrease in muscle size

• Reverse except in severe situations where cells die

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Heat production

• Exercise: metabolic rate and heat production increase.

• Post-exercise: metabolic rate stays high due to oxygen debt.

• Excess heat lost because of vasodilation and sweating

• Shivering: uncoordinated contraction of muscle fibers resulting in shaking and heat production

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Smooth Muscle

• Not striated, fibers smaller than those in skeletal muscle

• Spindle-shaped; single, central nucleus

• More actin than myosin• Caveolae: indentations in

sarcolemma; may act like T tubules• Dense bodies instead of Z disks as in

skeletal muscle; have noncontractile intermediate filaments

• Ca2+ required to initiate contractions; binds to calmodulin which regulates myosin kinase. Cross-bridging occurs

• Relaxation: caused by enzyme myosin phosphatase

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Types of Smooth Muscle

• Visceral or unitary: cells in sheets; function as a unit– Numerous gap junctions; waves of contraction– Often autorhythmic

• Multiunit: cells or groups of cells act as independent units– Sheets (blood vessels); bundles (arrector pili

and iris); single cells (capsule of spleen)

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Electrical Properties of Smooth Muscle

• Slow waves of depolarization and repolarization transferred from cell to cell

• Depolarization caused by spontaneous diffusion of Na+ and Ca2+ into cell

• Does not follow all-or-none law

• May have pacemaker cells• Contraction regulated by

nervous system and by hormones

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Functional Properties of Smooth Muscle

• Some visceral muscle exhibits autorhythmic contractions

• Tends to contract in response to sudden stretch but not to slow increase in length

• Exhibits relatively constant tension: smooth muscle tone

• Amplitude of contraction remains constant although muscle length varies

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Smooth Muscle Regulation

• Innervated by autonomic nervous system• Neurotransmitters are acetylcholine and

norepinephrine• Hormones important as epinephrine and

oxytocin• Receptors present on plasma membrane

which neurotransmitters or hormones bind determines response

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Cardiac Muscle

• Found only in heart• Striated• Each cell usually has one nucleus• Has intercalated disks and gap junctions• Autorhythmic cells• Action potentials of longer duration and longer

refractory period• Ca2+ regulates contraction

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Effects of Aging on Skeletal Muscle

• Reduced muscle mass

• Increased time for muscle to contract in response to nervous stimuli

• Reduced stamina

• Increased recovery time

• Loss of muscle fibers

• Decreased density of capillaries in muscle