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Basic Types of Muscle Tissue
Skeletal Muscle Tissue
Mostly voluntary
Actin and Myosin
Fibers
Cardiac Muscle Tissue
Involuntary
Actin and Myosin
Intercalated disc
Short and fat
Smooth Muscle Tissue
Fibers
Involuntary
Basic Characteristics of Muscle Cells• Excitability
• Contractility
• Extensibility
• Elasticity
General Functions of Muscle Tissue
Bone
Endomysium(between individualmuscle fibers)
Muscle fiber
Fascicle(wrapped by perimysium)
Epimysium
Tendon
Blood vessel
General Structure of a Skeletal Muscle
Epimysium
Muscle fiberin middle ofa fascicle
Perimysium
Endomysium
Skeletal Muscle Attachments
Embryonicmesoderm cells
Embryonicmesoderm cellsundergo celldivision (toincrease number)and enlarge.
Severalmyoblastsfuse togetherto form amyotube.
Myotubematures intoskeletal musclefiber.
Myoblasts
Myotube(immaturemultinucleatemuscle fiber)
Satellite cell
MatureSkeletalmusclefiber1 2 3
Skeletal Muscle Fiber Formation
Nucleus
Sarcolemma
Mitochondrion
Myofibril
Skeletal Muscle Fiber Microanatomy
Sarcoplasm
Myoglobin Glycogen
Nuclei
Fiber
Dark A band
Light I band
Skeletal Muscle Fiber Microanatomy
Sarcomeres
I band I bandA bandSarcomere
H zone
Thin (actin)filament
Thick (myosin)filament
Z disc Z disc
M line
Skeletal Muscle Fiber Microanatomy
Skeletal Muscle Fiber Microanatomy
Z disc Z discM line
Sarcomere
Thin (actin)filament
Thick(myosin)filament
Sarcomere Structure
I band H zone Outer edgeof A band
Cross-sectional view of a sarcomere cut through in different locations.
I IA
Z ZH
1 Fully relaxed sarcomere of a muscle fiber
Sliding Filament Model of Muscle Contraction
I IA
Z Z
2 Fully contracted sarcomere of a muscle fiber
Sliding Filament Model of Muscle Contraction
Excitation-Contraction Coupling
Nucleus
Action potential(AP)
Myelinated axonof motor neuron
Axon terminal ofneuromuscular junction
Sarcolemmaof the musclefiber
Action potential arrives at axon terminal of motor neuron.
Voltage-gated Ca2+
channels open and Ca2+ enters the axon terminal.
Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.
Acetylcholine, a neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.
Ca2+
Axon terminalof motor neuron
Synaptic vesiclecontaining ACh
Mitochondrion
Synaptic cleft
Junctionalfolds of
sarcolemma
Fusingsynaptic vesicles
ACh
Sarcoplasm of muscle fiber
Ca2+
1
2
3
4
Excitation-Contraction Coupling
Postsynaptic mem-brane ion channel opens; ions pass.
Na+ K+
5 ACh binding opens ion channels that allowsimultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber.
Excitation-Contraction Coupling
Excitation-Contraction Coupling
ACh
Na+
K+
Degraded ACh
Acetylcholinesterase
Postsynaptic membraneion channel closed;ions cannot pass.
6 ACh effects are terminated by itsenzymatic breakdown in the synaptic cleft by acetylcholinesterase.
Na+ K+
Axon terminal
Synapticcleft
ACh–
ACh
1 Local depolarization: generation of the end plate potential on the sarcolemma
Na+
Open Na+
ChannelClosed K+
Channel
K+
K+
2 Generation and propagation of the action potential (AP)
Sarcoplasm of muscle fiber
Na+
Excitation-Contraction Coupling
Na+ channelsclose, K+ channelsopen
K+ channelsclose
Repolarizationdue to K+ exit
Threshold
Na+
channelsopen
Depolarizationdue to Na+ entry
Calciumions arereleased.
Steps inE-C Coupling:
Terminalcisterna of SR
Voltage-sensitivetubule protein
T tubule
Ca2+
releasechannel
Ca2+
Sarcolemma
Action potential ispropagated along thesarcolemma and downthe T tubules.
1
2
Excitation-Contraction Coupling
Flexible hinge region
Tail
Myosin head
ATP-bindingsite
Actin-binding sites
Myosin molecule
Heads
Structure of Myosin
Tropomyosin Troponin Actin
Active sitesfor myosinattachmentActin subunits
Actin subunits
Structure of Actin
Calcium binds totroponin and removesthe blocking action oftropomyosin.
Contraction begins
Troponin Tropomyosinblocking active sitesMyosin
Actin
Active sites exposed andready for myosin binding
Ca2+
Myosincross bridge
3
4
Excitation-Contraction Coupling
What’s Next?• Sarcoplasmic reticulum Ca++ ATPase
• Troponin and tropomyosin revert
• No more cross bridges
• Recoil of the connective tissue
• Gravity
• Return to resting length
Spinal cord
Motor neuroncell body
Muscle
Nerve
Motorunit 1
Motorunit 2
Musclefibers
Motorneuronaxon
Axon terminals atneuromuscular junctions
Motor Units
All or None
# of motor units stimulated ∝ tension developed
Stimulus strength
Proportion of motor units excited
Strength of muscle contraction
Maximal contraction
Maximalstimulus
Thresholdstimulus
Motor Units
Latentperiod
Singlestimulus
Period ofcontraction
Period ofrelaxation
Muscle Twitch
Latent period
Extraocular muscle (lateral rectus)
Gastrocnemius
Soleus
Singlestimulus
Muscle Twitch
Contraction
Relaxation
Stimulus
Single stimulus single twitch
Muscle Twitch
Stimuli
Partial relaxation
Low stimulation frequencyunfused (incomplete) tetanus
Wave Summation – Incomplete Tetanus
Stimuli
High stimulation frequencyfused (complete) tetanus
Complete Tetanus
3 kg
3 kg
Musclecontracts(isotoniccontraction)
Tendon
Tendon
Isotonic Concentric Muscle Contraction
Amount ofresistance
Peak tensiondeveloped
Resting length
Musclestimulus
Musclerelaxes
Isotonic Concentric Muscle Contraction
Isotonic Eccentric Muscle Contraction
6 kg 6 kg
Musclecontracts(isometriccontraction)
Isometric Muscle Contraction
Amount of resistance
Peak tensiondeveloped
Resting length
Musclestimulus
Musclerelaxes
Isometric Muscle Contraction
Short-duration exercise Prolonged-durationexercise
ATP stored inmuscles isused first.
ATP is formedfrom creatinePhosphateand ADP.
Glycogen stored in muscles is brokendown to glucose, which is oxidized togenerate ATP.
ATP is generated bybreakdown of severalnutrient energy fuels byaerobic pathway. Thispathway uses oxygenreleased from myoglobinor delivered in the bloodby hemoglobin. When itends, the oxygen deficit ispaid back.
Energy Supply for Muscle Activity
Coupled reaction of creatinephosphate (CP) and ADP
Energy source: CP
(a) Direct phosphorylation
Oxygen use: NoneProducts: 1 ATP per CP, creatineDuration of energy provision:15 seconds
Creatinekinase
ADPCP
Creatine ATP
Energy Supply for Muscle Activity
Energy source: glucose
Glycolysis and lactic acid formation
(b) Anaerobic pathway
Oxygen use: NoneProducts: 2 ATP per glucose, lactic acidDuration of energy provision:60 seconds, or slightly more
Glucose (fromglycogen breakdown ordelivered from blood)
Glycolysisin cytosol
Pyruvic acid
Releasedto blood
net gain
2
Lactic acid
O2
O2
ATP
Energy Supply for Muscle Activity
Energy source: glucose; pyruvic acid;free fatty acids from adipose tissue;amino acids from protein catabolism
(c) Aerobic pathway
Aerobic cellular respiration
Oxygen use: RequiredProducts: 32 ATP per glucose, CO
2, H
2O
Duration of energy provision: Hours
Glucose (fromglycogen breakdown ordelivered from blood)
32
O2
O2
H2O
CO2
Pyruvic acidFattyacids
Aminoacids
Aerobic respirationin mitochondriaAerobic respirationin mitochondria
ATP
net gain perglucose
Energy Supply for Muscle Activity
Muscle Fatigue
Na+ K+
Axon terminal
ACh–
AChK+Na+
Oxygen Debt
Muscle Fiber Types
Muscle Fiber Types
Capillary densityMitochondriaMyoglobin
Endurance Training
Resistance Training
Nucleus
SarcolemmaMitochondrion
Myofibril