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Chapter 10

The Muscular System

11-2

The Muscular System• Structural and

functional organization of muscles

• Muscles of the head and neck

• Muscles of the trunk

• Muscles acting on the shoulder and upper limb

• Muscles acting on the hip and lower limb

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Organization of Muscles

• 600 Human skeletal muscles• General structural and functional topics

– muscle shape and function– connective tissues of muscle– coordinated actions of muscle groups – intrinsic and extrinsic muscles– muscle innervation

• Regional descriptions

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The Functions of Muscles

• Movement of body parts and organ contents

• Maintain posture and prevent movement

• Communication - speech, expression and writing

• Control of openings and passageways• Heat production

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Connective Tissues of a Muscle

Perimysium

Epimysium

Endomysium

Tendon

Deep fascia

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Connective Tissues of a Muscle

• Epimysium– covers whole muscle belly – blends into CT between muscles

• Perimysium– slightly thicker layer of connective tissue– surrounds bundle of cells called a fascicle

• Endomysium– thin areolar tissue around each cell– allows room for capillaries and nerve

fibers

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Location of Fascia

Superficial Fascia

Deep Fascia

• Deep fascia– found between adjacent muscles

• Superficial fascia (hypodermis)– adipose between skin and muscles

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Muscle Attachments

• Tendon – bone to muscle• Aponeuroses- Flat sheetlike tendon

located beneath the scalp; also includes attachments in the abdominal, lumbar, hand and foot areas.

• Retinaculum – connective tissue that groups together several tendons from separate muscles – like a bracelet at the wrist.

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Chapter 11

Muscle Tissue

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Muscle Tissue

• Types and characteristics of muscular tissue• Microscopic anatomy of skeletal muscle• Nerve-Muscle relationship• Behavior of skeletal muscle fibers• Behavior of whole muscles

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Introduction to Muscle• Movement is a fundamental characteristic

of all living things• Cells capable of shortening and

converting the chemical energy of ATP into mechanical energy

• Types of muscle– skeletal, cardiac and smooth

• Physiology of skeletal muscle– basis of warm-up, strength, endurance and

fatigue

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Characteristics of Muscle• Responsiveness (excitability)

– to chemical signals, stretch and electrical changes across the plasma membrane

• Conductivity– local electrical change triggers a wave of

excitation that travels along the muscle fiber• Contractility -- shortens when stimulated• Extensibility -- capable of being stretched• Elasticity -- returns to its original resting

length after being stretched

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Skeletal Muscle• Voluntary striated muscle attached to

bones• Muscle fibers (myofibers) as long as 30

cm• Exhibits alternating light and dark

transverse bands or striations– reflects overlapping arrangement of

internal contractile proteins• Under conscious

control (voluntary)

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Connective Tissue Elements• Attachments between muscle and bone

– endomysium, perimysium, epimysium, fascia, tendon

• Collagen is extensible and elastic– stretches slightly under tension and recoils

when released• protects muscle from injury• returns muscle to its resting length

• Elastic components– parallel components parallel muscle cells– series components joined to ends of muscle

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The Muscle Fiber

Muscle Fiber

Skeletal Muscle Fiber

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Muscle Fibers

• Multiple flattened nuclei inside cell membrane– fusion of multiple myoblasts during

development– unfused satellite cells nearby can multiply to

produce a small number of new myofibers • Sarcolemma has tunnel-like infoldings or

transverse (T) tubules that penetrate the cell– carry electric current to cell interior

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Muscle Fibers 2

• Sarcoplasm is filled with – myofibrils (bundles of myofilaments)– glycogen for stored energy and myoglobin

for binding oxygen• Sarcoplasmic reticulum = smooth ER

– network around each myofibril– dilated end-sacs (terminal cisternea) store

calcium– triad = T tubule and 2 terminal cisternea

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Thick Filaments

• Made of 200 to 500 myosin molecules– 2 entwined polypeptides (golf clubs)

• Arranged in a bundle with heads directed outward in a spiral array around the bundled tails– central area is a bare zone with no heads

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Thin Filaments• Two intertwined strands fibrous (F) actin

– globular (G) actin with an active site• Groove holds tropomyosin molecules

– each blocking 6 or 7 active sites of G actins• One small, calcium-binding troponin

molecule on each tropomyosin molecule

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Elastic Filaments

• Springy proteins called titin• Anchor each thick filament to Z disc• Prevents overstretching of sarcomere

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Regulatory and Contractile Proteins

• Myosin and actin are contractile proteins• Tropomyosin and troponin = regulatory proteins

– switch that starts and stops shortening of muscle cell– contraction activated by release of calcium into sarcoplasm

and its binding to troponin, – troponin moves tropomyosin off the actin active sites

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Overlap of Thick and Thin Filaments

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Striations = Organization of Filaments• Dark A bands (regions) alternating with lighter I bands (regions)

– anisotrophic (A) and isotropic (I) stand for the way these regions affect polarized light

• A band is thick filament region– lighter, central H band area

contains no thin filaments• I band is thin filament region

– bisected by Z disc protein called connectin, anchoring elastic and thin filaments

– from one Z disc (Z line) to the next is a sarcomere

Sliding Filament

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Striations and Sarcomeres

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Relaxed and Contracted Sarcomeres

• Muscle cells shorten because their individual sarcomeres shorten – pulling Z discs closer together– pulls on sarcolemma

• Notice neither thick nor thin filaments change length during shortening

• Their overlap changes as sarcomeres shorten

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Nerve-Muscle Relationships

• Skeletal muscle must be stimulated by a nerve or it will not contract

• Cell bodies of somatic motor neurons in brainstem or spinal cord

• Axons of somatic motor neurons = somatic motor fibers– terminal branches supply one muscle fiber

• Each motor neuron and all the muscle fibers it innervates = motor unit

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Motor Units• A motor neuron and the muscle

fibers it innervates– dispersed throughout the muscle– when contract together causes weak

contraction over wide area– provides ability to sustain long-term

contraction as motor units take turns resting (postural control)

• Fine control– small motor units contain as few as

20 muscle fibers per nerve fiber– eye muscles

• Strength control– gastrocnemius muscle has 1000

fibers per nerve fiber

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Neuromuscular Junctions (Synapse)

• Functional connection between nerve fiber and muscle cell

• Neurotransmitter (acetylcholine/ACh) released from nerve fiber stimulates muscle cell

• Components of synapse (NMJ)– synaptic knob is swollen end of nerve fiber (contains

ACh)– junctional folds region of sarcolemma

• increases surface area for ACh receptors• contains acetylcholinesterase that breaks down ACh and

causes relaxation– synaptic cleft = tiny gap between nerve and muscle

cells– Basal lamina = thin layer of collagen and glycoprotein

over all of muscle fiber

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The Neuromuscular Junction

Neuromuscular Junction

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Neuromuscular Toxins• Pesticides (cholinesterase inhibitors)

– bind to acetylcholinesterase and prevent it from degrading ACh

– spastic paralysis and possible suffocation• Tetanus or lockjaw is spastic paralysis

caused by toxin of Clostridium bacteria– blocks glycine release in the spinal cord and

causes overstimulation of the muscles• Flaccid paralysis (limp muscles) due to

curare that competes with ACh– respiratory arrest

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Electrically Excitable Cells• Plasma membrane is polarized or charged

– resting membrane potential due to Na+ outside of cell and K+ and other anions inside of cell

– difference in charge across the membrane = resting membrane potential (-90 mV cell)

• Stimulation opens ion gates in membrane– ion gates open (Na+ rushes into cell and K+

rushes out of cell)• quick up-and-down voltage shift = action potential

– spreads over cell surface as nerve signal

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Muscle Contraction and Relaxation

• Four actions involved in this process– excitation = nerve action potentials lead to

action potentials in muscle fiber– excitation-contraction coupling = action

potentials on the sarcolemma activate myofilaments

– contraction = shortening of muscle fiber – relaxation = return to resting length

• Images will be used to demonstrate the steps of each of these actions

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Excitation of a Muscle Fiber

Excitation of a Muscle Fiber

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Excitation (steps 1 and 2)

• Nerve signal opens voltage-gated calcium channels. Calcium stimulates exocytosis of synaptic vesicles containing ACh = ACh release into synaptic cleft.

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Excitation (steps 3 and 4)

Binding of ACh to receptor proteins opens Na+ and K+ channels resulting in jump in RMP from -90mV to +75mV forming an end-plate potential (EPP).

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Excitation (step 5)

Voltage change in end-plate region (EPP) opens nearby voltage-gated channels producing an action potential

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Excitation-Contraction Coupling

Excitation and Contraction - animation

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Excitation-Contraction Coupling (steps 6 and 7)

Action potential spreading over sarcolemma enters T tubules -- voltage-gated channels open in T tubules causing calcium gates to open in SR

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Excitation-Contraction Coupling (steps 8 and 9)

• Calcium released by SR binds to troponin• Troponin-tropomyosin complex changes shape

and exposes active sites on actin

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Contraction (steps 10 and 11)

• Myosin ATPase in myosin head hydrolyzes an ATP molecule, activating the head and “cocking”it in an extended position

• It binds to actin active site forming a cross-bridge

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Contraction (steps 12 and 13)• Power stroke =

myosin head releasesADP and phosphate as it flexes pulling the thin filament past the thick

• With the binding of more ATP, the myosin head extends to attach to a new active site– half of the heads are bound to a thin

filament at one time preventing slippage– thin and thick filaments do not become

shorter, just slide past each other (sliding filament theory)

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Relaxation (steps 14 and 15)

Nerve stimulation ceases and acetylcholinesterase removes ACh from receptors. Stimulation of the muscle cell ceases.

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Relaxation (step 16)

• Active transport needed to pump calcium back into SR to bind to calsequestrin

• ATP is needed for muscle relaxation as well as muscle contraction

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Relaxation (steps 17 and 18)

• Loss of calcium from sarcoplasm moves troponin-tropomyosin complex over active sites– stops the production or maintenance of tension

• Muscle fiber returns to its resting length due to recoil of series-elastic components and contraction of antagonistic muscles

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Rigor Mortis

• Stiffening of the body beginning 3 to 4 hours after death

• Deteriorating sarcoplasmic reticulum releases calcium

• Calcium activates myosin-actin cross-bridging and muscle contracts, but can not relax.

• Muscle relaxation requires ATP and ATP production is no longer produced after death

• Fibers remain contracted until myofilaments decay

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Length-Tension Relationship• Amount of tension generated depends on length

of muscle before it was stimulated– length-tension relationship (see graph next slide)

• Overly contracted (weak contraction results)– thick filaments too close to Z discs and can’t slide

• Too stretched (weak contraction results)– little overlap of thin and thick does not allow for very

many cross bridges too form• Optimum resting length produces greatest force

when muscle contracts– central nervous system maintains optimal length

producing muscle tone or partial contraction

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Length-Tension Curve

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Muscle Twitch in Frog

• Threshold = voltage producing an action potential– a single brief stimulus at that

voltage produces a quick cycle of contraction and relaxation called a twitch (lasting less than 1/10 second)

• A single twitch contraction is not strong enough to do any useful work

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Muscle Twitch in Frog 2

• Phases of a twitch contraction– latent period (2 msec delay)

• only internal tension is generated• no visible contraction occurs since

only elastic components are being stretched

– contraction phase• external tension develops as muscle

shortens– relaxation phase

• loss of tension and return to resting length as calcium returns to SR

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Contraction Strength of Twitches

• Threshold stimuli produces twitches• Twitches unchanged despite increased

voltage• “Muscle fiber obeys an all-or-none law”

contracting to its maximum or not at all– not a true statement since twitches vary in

strength• depending upon, Ca2+ concentration, previous stretch

of the muscle, temperature, pH and hydration

• Closer stimuli produce stronger twitches

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Recruitment and Stimulus Intensity

• Stimulating the whole nerve with higher and higher voltage produces stronger contractions

• More motor units are being recruited– called multiple motor unit summation– lift a glass of milk versus a whole gallon of milk

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Twitch and Treppe Contractions

• Muscle stimulation at variable frequencies– low frequency

• each stimulus produces an identical twitch response– moderate frequency

• each twitch has time to recover but develops more tension than the one before (treppe phenomenon)

– calcium was not completely put back into SR– heat of tissue increases myosin ATPase efficiency

11-53

Incomplete and Complete Tetanus

• Higher frequency stimulation – generates gradually more strength of contraction– each stimuli arrives before last one recovers

• temporal summation or wave summation– incomplete tetanus = sustained fluttering contractions

• Maximum frequency stimulation– muscle has no time to relax at all– twitches fuse into smooth, prolonged contraction called

complete tetanus– rarely occurs in the body

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ATP Sources

• All muscle contraction depends on ATP• Pathways of ATP synthesis

– anaerobic fermentation (ATP production limited)• without oxygen, produces toxic lactic acid

– aerobic respiration (more ATP produced)• requires continuous oxygen supply, produces H2O and

CO2

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Immediate Energy Needs

• Short, intense exercise (100 m dash)– oxygen need is supplied by

myoglobin• Phosphagen system

– myokinase transfers Pi groups from one ADP to another forming ATP

– creatine kinase transfers Pi groups from creatine phosphate to make ATP

• Result is power enough for 1 minute brisk walk or 6 seconds of sprinting

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Short-Term Energy Needs

• Glycogen-lactic acid system takes over– produces ATP for 30-40 seconds of

maximum activity• playing basketball or running around baseball

diamonds – muscles obtain glucose from blood and

stored glycogen

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Long-Term Energy Needs

• Aerobic respiration needed for prolonged exercise– Produces 36 ATPs/glucose molecule

• After 40 seconds of exercise, respiratory and cardiovascular systems must deliver enough oxygen for aerobic respiration– oxygen consumption rate increases for first 3-4

minutes and then levels off to a steady state• Limits are set by depletion of glycogen and

blood glucose, loss of fluid and electrolytes

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Oxygen Debt• Heavy breathing after strenuous exercise

– known as excess postexercise oxygen consumption (EPOC)

– typically about 11 liters extra is consumed• Purposes for extra oxygen

– replace oxygen reserves (myoglobin, blood hemoglobin, in air in the lungs and dissolved in plasma)

– replenishing the phosphagen system– reconverting lactic acid to glucose in kidneys and

liver– serving the elevated metabolic rate that occurs as

long as the body temperature remains elevated by exercise

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Slow- and Fast-Twitch Fibers

• Slow oxidative, slow-twitch fibers– more mitochondria, myoglobin and

capillaries– adapted for aerobic respiration and

resistant to fatigue– soleus and postural muscles of the

back (100msec/twitch)

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Slow and Fast-Twitch Fibers

• Fast glycolytic, fast-twitch fibers– rich in enzymes for phosphagen and

glycogen-lactic acid systems– sarcoplasmic reticulum releases calcium

quickly so contractions are quicker (7.5 msec/twitch)

– extraocular eye muscles, gastrocnemius and biceps brachii

• Proportions genetically determined