Excitation–Contraction Excitation–Contraction CouplingCoupling

• Action potential reaches a triad:Action potential reaches a triad:– releasing Careleasing Ca2+2+

– triggering contraction triggering contraction

• Requires myosin heads to be in Requires myosin heads to be in “cocked” position:“cocked” position:– loaded by ATP energyloaded by ATP energy

Exposing the Active SiteExposing the Active Site

Figure 10–11

The Contraction CycleThe Contraction Cycle

Figure 10–12 (1 of 4)

The Contraction CycleThe Contraction Cycle

Figure 10–12 (2 of 4)

The Contraction CycleThe Contraction Cycle

Figure 10–12 (3 of 4)

The Contraction CycleThe Contraction Cycle

Figure 10–12 (Navigator) (4 of 4)

5 Steps of the Contraction 5 Steps of the Contraction Cycle Cycle

1.1. Exposure of active sitesExposure of active sites

2.2. Formation of cross-bridgesFormation of cross-bridges

3.3. Pivoting of myosin headsPivoting of myosin heads

4.4. Detachment of cross-bridgesDetachment of cross-bridges

5.5. Reactivation of myosinReactivation of myosin

Fiber ShorteningFiber Shortening

• As sarcomeres shorten, muscle pulls As sarcomeres shorten, muscle pulls together, producing together, producing tensiontension

Figure 10–13

Contraction DurationContraction Duration

• Depends on:Depends on:– duration of neural stimulusduration of neural stimulus– number of free calcium ions in number of free calcium ions in

sarcoplasmsarcoplasm– availability of ATPavailability of ATP

RelaxationRelaxation

• CaCa2+2+ concentrations fall concentrations fall

• CaCa2+2+ detaches from troponin detaches from troponin

• Active sites are recovered by Active sites are recovered by tropomyosintropomyosin

• Sarcomeres remain contracted Sarcomeres remain contracted

A Review of Muscle A Review of Muscle ContractionContraction

Table 10–1 (1 of 2)

KEY CONCEPTKEY CONCEPT (Part 1)(Part 1)

• Skeletal muscle fibers shorten as thin Skeletal muscle fibers shorten as thin filaments slide between thick filaments slide between thick filamentsfilaments

• Free CaFree Ca2+2+ in the sarcoplasm triggers in the sarcoplasm triggers contractioncontraction

• SR releases CaSR releases Ca2+2+ when a motor when a motor neuron stimulates the muscle fiberneuron stimulates the muscle fiber

KEY CONCEPTKEY CONCEPT (Part 2)(Part 2)

• Contraction is an active processContraction is an active process

• Relaxation and return to resting Relaxation and return to resting length is passivelength is passive

Tension Production Tension Production

• The The all–or–none principalall–or–none principal::– as a whole, a muscle fiber is either as a whole, a muscle fiber is either

contracted or relaxed contracted or relaxed

Tension of a Single Muscle Tension of a Single Muscle FiberFiber

• Depends on:Depends on:– the number of pivoting cross-bridgesthe number of pivoting cross-bridges– the fiber’s resting length at the time of the fiber’s resting length at the time of

stimulationstimulation– the frequency of stimulationthe frequency of stimulation

Frequency of StimulationFrequency of Stimulation

• A single neural stimulation produces:A single neural stimulation produces:– a single contraction or a single contraction or twitchtwitch – which lasts about 7–100 msecwhich lasts about 7–100 msec

• Sustained muscular contractions:Sustained muscular contractions:– require many repeated stimulirequire many repeated stimuli

3 Phases of Twitch3 Phases of Twitch

1.1. Latent periodLatent period before contraction: before contraction:– the action potential moves through the action potential moves through

sarcolemmasarcolemma– causing Cacausing Ca2+2+ release release

3 Phases of Twitch3 Phases of Twitch

2.2. Contraction phaseContraction phase: : – calcium ions bindcalcium ions bind– tension builds to peak tension builds to peak

3 Phases of Twitch3 Phases of Twitch

3.3. Relaxation phaseRelaxation phase: : – CaCa2+2+ levels fall levels fall– active sites are coveredactive sites are covered– tension falls to resting levelstension falls to resting levels

TreppeTreppe

• Repeated stimulations Repeated stimulations immediately immediately afterafter relaxation phase: relaxation phase:– stimulus frequency < 50/secondstimulus frequency < 50/second

• Causes a series of contractions with Causes a series of contractions with increasing tensionincreasing tension

Wave SummationWave Summation

• Increasing tension or Increasing tension or summation of summation of twitches twitches

Figure 10–16b

Incomplete TetanusIncomplete Tetanus

• Twitches reach maximum tensionTwitches reach maximum tension

Figure 10–16c

Complete TetanusComplete Tetanus

Figure 10–16d

Muscle ToneMuscle Tone

• The normal tension and firmness of a The normal tension and firmness of a muscle at restmuscle at rest

• Muscle units actively maintain body Muscle units actively maintain body position, without motion position, without motion

• Increasing muscle tone increases Increasing muscle tone increases metabolic energy used, even at restmetabolic energy used, even at rest

2 Types of Skeletal 2 Types of Skeletal Muscle TensionMuscle Tension

1.1. Isotonic contraction Isotonic contraction

2.2. Isometric contractionIsometric contraction

Isotonic ContractionIsotonic Contraction

Figure 10–18a, b

Isometric ContractionIsometric Contraction

Figure 10–18c, d

ATP and Muscle ContractionATP and Muscle Contraction

• Sustained muscle contraction uses a Sustained muscle contraction uses a lot of ATP energylot of ATP energy

• Muscles store enough energy to start Muscles store enough energy to start contractioncontraction

• Muscle fibers must manufacture Muscle fibers must manufacture more ATP as neededmore ATP as needed

ATP and CP ReservesATP and CP Reserves

• Adenosine triphosphateAdenosine triphosphate (ATP)(ATP): : – the active energy moleculethe active energy molecule

• Creatine phosphateCreatine phosphate (CP)(CP)::– the storage molecule for excess ATP the storage molecule for excess ATP

energy in resting muscleenergy in resting muscle

ATP GenerationATP Generation

• Cells produce ATP in 2 ways:Cells produce ATP in 2 ways:– aerobic metabolismaerobic metabolism of fatty acids in the of fatty acids in the

mitochondriamitochondria– anaerobic glycolysisanaerobic glycolysis in the cytoplasm in the cytoplasm

Aerobic MetabolismAerobic Metabolism

• Is the primary energy source of Is the primary energy source of resting musclesresting muscles

• Breaks down fatty acids Breaks down fatty acids

• Produces 34 ATP molecules per Produces 34 ATP molecules per glucose moleculeglucose molecule

Anaerobic GlycolysisAnaerobic Glycolysis

• Is the primary energy source for peak Is the primary energy source for peak muscular activitymuscular activity

• Produces 2 ATP molecules per Produces 2 ATP molecules per molecule of glucosemolecule of glucose

• Breaks down glucose from glycogen Breaks down glucose from glycogen stored in skeletal muscles stored in skeletal muscles

Energy Use and Muscle Energy Use and Muscle ActivityActivity

• At peak exertion:At peak exertion:– muscles lack oxygen to support muscles lack oxygen to support

mitochondriamitochondria– muscles rely on glycolysis for ATPmuscles rely on glycolysis for ATP– pyruvic acid builds up, is converted to pyruvic acid builds up, is converted to

lactic acidlactic acid

Results of Muscle FatigueResults of Muscle Fatigue

1.1. Depletion of metabolic reservesDepletion of metabolic reserves

2.2. Damage to sarcolemma and Damage to sarcolemma and sarcoplasmic reticulumsarcoplasmic reticulum

3.3. Low pH (lactic acid)Low pH (lactic acid)

4.4. Muscle exhaustion and painMuscle exhaustion and pain

The The Recovery PeriodRecovery Period

• The time required after exertion for The time required after exertion for muscles to return to normal muscles to return to normal

• Oxygen becomes availableOxygen becomes available

• Mitochondrial activity resumes Mitochondrial activity resumes

Oxygen DebtOxygen Debt

• After exercise:After exercise:– the body needs more oxygen than usual the body needs more oxygen than usual

to normalize metabolic activities to normalize metabolic activities – resulting in heavy breathing resulting in heavy breathing

Hormones andHormones and Muscle Metabolism Muscle Metabolism

• Growth hormoneGrowth hormone

• TestosteroneTestosterone

• Thyroid hormonesThyroid hormones

• EpinephrineEpinephrine

Structure of Cardiac TissueStructure of Cardiac Tissue

• Cardiac muscle is Cardiac muscle is striated, found only in striated, found only in the heartthe heart

Figure 10–22

Intercalated DiscsIntercalated Discs

• Are specialized contact points Are specialized contact points between cardiocytesbetween cardiocytes

• Join cell membranes of adjacent Join cell membranes of adjacent cardiocytes (gap junctions, cardiocytes (gap junctions, desmosomes) desmosomes)

Functions of Intercalated Functions of Intercalated DiscsDiscs

• Maintain structureMaintain structure

• Enhance molecular and electrical Enhance molecular and electrical connectionsconnections

• Conduct action potentials Conduct action potentials

Smooth Muscle in Body Smooth Muscle in Body Systems (Part 1)Systems (Part 1)

• Forms around other tissues Forms around other tissues

• In blood vessels:In blood vessels:– regulates blood pressure and flowregulates blood pressure and flow

• In reproductive and glandular In reproductive and glandular systems:systems:– produces movements produces movements

Smooth Muscle in Body Smooth Muscle in Body Systems (Part 2)Systems (Part 2)

• In digestive and urinary systems:In digestive and urinary systems:– forms sphinctersforms sphincters– produces contractionsproduces contractions

• In integumentary system:In integumentary system:– arrector piliarrector pili muscles cause goose bumps muscles cause goose bumps

Structure of Smooth Muscle Structure of Smooth Muscle

• NonstriatedNonstriated tissue tissue

Figure 10–23

Comparing Smooth Comparing Smooth and Striated Muscleand Striated Muscle

• Different internal organization of Different internal organization of actin and myosin actin and myosin

• Different functional characteristicsDifferent functional characteristics

8 Characteristics of8 Characteristics of Smooth Muscle Cells Smooth Muscle Cells

1.1. Long, slender, and spindle shapedLong, slender, and spindle shaped

2.2. Have a single, central nucleusHave a single, central nucleus

3.3. Have no T tubules, myofibrils, or Have no T tubules, myofibrils, or sarcomeressarcomeres

4.4. Have no tendons or aponeurosesHave no tendons or aponeuroses

8 Characteristics of 8 Characteristics of Smooth Muscle CellsSmooth Muscle Cells

5.5. Have scattered myosin fibersHave scattered myosin fibers

6.6. Myosin fibers have more heads per Myosin fibers have more heads per thick filamentthick filament

7.7. Have thin filaments attached to Have thin filaments attached to dense bodiesdense bodies

8.8. Dense bodiesDense bodies transmit contractions transmit contractions from cell to cellfrom cell to cell

Functional Characteristics Functional Characteristics of Smooth Muscleof Smooth Muscle

1.1. Excitation–contraction couplingExcitation–contraction coupling

2.2. Length–tension relationshipsLength–tension relationships

3.3. Control of contractionsControl of contractions

4.4. Smooth muscle toneSmooth muscle tone