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Introduction to Muscle
• Movement is a fundamental characteristic of all living things.
• Muscles cells are capable of shortening and converting the chemical energy of ATP into mechanical energy.
• Types of muscle– Skeletal – Cardiac – Smooth
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Skeletal Muscle
– Has obvious stripes called striations and multiple nucleuses – Under voluntary control
• Attached to the skeletal system• Responsible for movement.
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Cardiac Muscle
• Striated muscle but is involuntary• Intercalated discs connect adjacent cells together
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Smooth Muscle
• Found in the walls of hollow visceral organs, such as the stomach, intestines ,bladder. – It is not striated and is involuntary
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Characteristics of Muscle Tissue
• Excitability, or responsiveness– the ability to respond to stimuli
• Conductivity– the ability to produce and conduct an action potential
along the cell membrane• Contractility
– the ability to shorten creates movement by:• Skeletal: pulling on bones • Visceral: Movement created by visceral organs.
• Extensibility– the ability to be stretched
• Elasticity– the ability to recoil after being stretched
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Connective Tissues of a Muscle
Perimysium
Epimysium
Endomysium
Tendon
Deep fascia
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Skeletal Muscle• Each muscle is composed of various tissue types. They
include muscle tissue, blood vessels, nerve fibers, and connective tissue.
• The connective tissue located is important for supplying a framework for the blood vessels and nerves. They also contribute to the elastic qualities of muscle aiding in force production.
• The three connective tissue sheaths are:– Endomysium – fine sheath of connective tissue
surrounding each muscle fiber – Perimysium –connective tissue that surrounds groups
of muscle fibers called fascicles– Epimysium – an overcoat of dense regular connective
tissue that surrounds the entire muscle
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The Muscle Fiber
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Anatomy of a Skeletal Muscle Fiber
• Each fiber is surrounded by a sarcolemma • ( cell membrane around the muscle)
– the sarcolemma contains voltage-gated channels able of generating an action potential
– The action potential travels along the sarcolemma and dips into the center of the muscle via transverse-tubules. (T-tubules)
• Sarcoplasmic reticulum (SR) (bathes the muscle fibers)– Extensions of the T-tubules which store intracellular
calcium (Ca+2)• Once an action potential reaches the T-tubules Ca+2
from the terminal cysterna of SR is released into the sarcoplasm triggering a muscle contraction.
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Myofilaments: Banding Pattern
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Myofilaments: Banding Pattern
The thin and thick filaments overlapping forming sarcomeres.
• Z disc – anchors the thin filaments and connects myofibrils
to one another
– Z-disc to Z disc = one sarcomere • A band
– the length of the thick filaments • I band
– the length of thin filaments within a sarcomere that is not overlapping with the thick filaments
• H zone– the length of thick filaments within in a sarcomere
that do not overlapping with the thin filaments
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Resting Muscle • When a muscle is in a relaxed state the
sarcomere is at its normal length.• The H-zone is visible.
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Contracted Sarcomeres• Muscle cells shorten because their individual sarcomeres shorten
as myosin pulls actin toward the center of the sarcomere.– pulling Z discs closer together
• Notice neither thick nor thin filaments change length they overlap as sarcomeres shorten
• H-zone disappears
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Resting muscle/Contracted muscles
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Thick Filaments• Arranged in a bundle
with heads directed outward in a spiral array
• Myosin heads:– Form cross bridges
with actin.– Myosin contains the
enzyme ATPase which hydrolyzes ATP to create movement.
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Thin Filaments• Actin: Two intertwined strands fibrous protein containing the
active site for myosin heads.• Tropomyosin : Prevents actin and myosin cross bridge
formation • Troponin: a protein attached to tropomyosin that calcium
binds to allowing cross bridge formation and muscle contraction.
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The Neuromuscular Junction
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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
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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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Excitation (Steps 1 and 2)
• AP 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 (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 = creates muscle contraction – myosin head pulls the
actin over it.
– With the binding of more ATP, the myosin head will detach and break the cross bridge.
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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– Muscle fiber returns to its resting length
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Motor Units• A motor neuron and the
muscle fibers it innervates.
• Fine control– small motor units contain as few
as 20 muscle fibers per nerve fiber
– eye muscles• Allow for greater dexterity
because of a lower neuron to muscle fiber ratio.
• Strength control– gastrocnemius muscle has 1000
fibers per nerve fiber• One motor neuron controls many
muscle fibers which allows for strength production.
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Recruitment and Stimulus Intensity
• Strength of muscle contraction is dependant of # of motor units recruited
• Multiple motor unit summation– The harder the activity is the
more motor units will be recruited.
• Lifting 1 lb vs. 100 lbs
– How do you explain the rapid strength gains when you first start training?
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Muscle Twitch
• A single stimulus results in a single muscle twitch• Each twitch has time to recover but develops more
tension than the one before (treppe phenomenon)
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Muscle Response: Stimulation Frequency• Higher frequency stimulation generates gradually more strength
– 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
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Isometric and Isotonic Contractions
• Isometric muscle contraction– develops tension without changing length– important in postural muscle function and antagonistic muscle joint
stabilization• Isotonic muscle contraction
– tension while shortening = concentric – tension while lengthening = eccentric
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Metabolism and Skeletal Muscle Fibers Types
• There are 3 different types skeletal muscle fibers based histological differences, duration of a twitch and the method of ATP production– slow oxidative fibers– fast oxidative fibers– fast glycolytic fibers
• Proportions genetically determined
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Fast Glycolytic, Fast-Twitch Fibers
• Fast glycolytic, fast-twitch fibers:– rich in enzymes for phosphagen and glycogen-lactic acid
systems– Limited # of mitochondria and high concentration of
glycogen stores makes it adapted for anaerobic metabolism
• a lack of myoglobin in glycolytic fibers results in a white color
– sarcoplasmic reticulum releases calcium quickly so contractions are quicker which are required for movements that produce speed and power.
• extraocular eye muscles, gastrocnemius and biceps brachii
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Slow- Twitch Fibers
• Slow oxidative, slow-twitch fibers– Oxidative fibers contain greater amounts of
mitochondria and myoglobin which binds oxygen.
– Rich blood supply and high concentration of myoglobin these fibers appear red in color. • adapted for endurance (resistant to fatigue)
– Soleus and postural muscles of the back are predominantly this type.
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Fast Oxidative Fibers
• Fast oxidative fibers: – characteristics of both fast and slow fibers.– have a fast twitch (use ATP quickly)– Increased mitochondria make it moderately
resistant to fatigue – Usually make up 10% of fibers.
• Training will make these fibers adapt to become functionally more fast or slow.
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Cellular Adaptations to Physical Demands
• Strength training: high intensity training stresses anaerobic pathways.– Increased # and size of glycolytic associated
enzymes and substrates • ATP, creatine phosphate and glycogen
• Endurance training: Enhance the aerobic pathways. – increased # and size of mitochondrial
membranes and associated enzymes. – This will increase O2 uptake ( VO2 max) which will
delay the formation of lactic acid.