Excitation–Contraction Excitation–Contraction Coupling Coupling • Action potential reaches a Action potential reaches a triad: triad: – releasing Ca releasing Ca 2+ 2+ – triggering contraction triggering contraction • Requires myosin heads to be in Requires myosin heads to be in “cocked” position: “cocked” position: – loaded by ATP energy loaded by ATP energy
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Excitation–Contraction Coupling Action potential reaches a triad: Action potential reaches a triad: –releasing Ca 2+ –triggering contraction Requires myosin.
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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
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