Muscle Flashcards
What are the different Muscle tissue types ?
Skeletal, Smooth, and Cardiac
What is the whole muscle surrounded by ?
Epimysium
What are bundles of muscle called ?
Fascicles or Fasciculi
What are the Fascicle surrounded by ?
Perimysium
What do Fascicle consist of ?
Individual muscle cells (muscle fibers)
What are muscle fibers surrounded by ?
Endomysium
What are muscle fibers made up of ?
– Consists of myofibrils divided into sarcomeres
What is the cell membrane of muscle fibers called ? (Fuses with tendon and conducts action potential.
Plasmalemma
What is the role of Plasmalemma ?
– Fuses with tendon
– Conducts action potential
– Maintains pH, transports nutrients
What is the cytoplasm of muscle fibers called ?
Sarcoplasm
What are the unique features of sarcoplasm ?
glycogen storage, myoglobin
What are Transverse Tubules (T-tubules)
– Extensions of plasmalemma
– Carry action potential deep into muscle fiber
What is the Sarcoplasmic Reticulum ?
– Calcium (Ca2+) storage
How many myofibrils are to a single muscle fiber ?
Hundreds to thousands
What are sarcomeres ?
– Basic contractile element of skeletal muscle
– End to end for full myofibril length
How are sarcomeres identified ?
Distinctive striped appearance (striations)
What are the different striations of the sarcomeres ?
– A-bands: red/blue stripes (both colors)
– I-bands: light/pink stripes (gaps)
– H-zone: middle of A-band
– M-line: middle of H-zone
What is the thin protein filament of sarcomeres ?
Actin
– Show up lighter under microscope (red)
– I-band contains only actin filaments
What are the other proteins of the thin filaments ?
– Tropomyosin: covers active site at rest
– Troponin: anchored to actin, moves tropomyosin
What is the thick protein filament of sarcomeres ?
Myosin (thick filaments)
– Show up darker under microscope (blue)
– A-band contains both actin and myosin filaments
– H-zone contains only myosin filaments
Where is Actin (thin filaments) anchored ?
Anchored at Z disk
What is the third myofilament ?
Titiin
What does titin do ?
- Acts like a spring (stiffness
increases with muscle
activation and force
development).
– Extends from Z-disk to M-band. - Stabilizes sarcomeres and
centers myosin. - Prevents overstretching
What innervates muscle fibers ?
a-Motor Neurons
What’s important to keep in mind about motor units ?
– Single α-motor neuron + all
fibers it innervates
– More operating motor units =
more contractile force
What is the sequence of events that fibers contract through called ?
“excitation-contraction coupling”
Muscle -> Fascicle -> Muscle Fiber -> Myofibril -> Sarcomeres -> protein filaments
What are the steps through “excitation-contraction coupling” ?
- Action potential (AP) starts in brain
- AP arrives at axon terminal, releases acetylcholine (ACh)
- ACh crosses synapse, binds to ACh receptors on plasmalemma
- AP travels down plasmalemma, T-tubules
- Triggers Ca2+ release from sarcoplasmic reticulum (SR)
- Ca2+ enables actin-myosin contraction
What happens after action potential arrive at sarcoplasmic reticulum from T tubule ?
– SR sensitive to electrical charge
– Causes mass release of Ca2+ into sarcoplasm
What happens after Ca2+ binds to troponin on thin filament ?
– At rest, tropomyosin covers myosin-binding site
– Troponin-Ca2+ complex moves tropomyosin
– Myosin binds to actin, forming a cross-bridge,
allowing a contraction to occur
How is the muscle at the relaxed state ?
– No actin-myosin interaction at binding site
– Myofilaments overlap a little
How is the muscle at the contracted state ?
– Myosin head pulls actin toward sarcomere center
(power stroke)
– Filaments slide past each other
– Sarcomeres, myofibrils, muscle fiber all shorten
What happens in muscle contraction state after power stroke ends ?
– Myosin detaches from active site
– Myosin head rotates back to original position
– Myosin attaches to another active site farther down
How does muscle contraction state end ?
– Z-disk reaches myosin filaments
Or
– AP stops, Ca2+ gets pumped back into SR
Where does ATP bind to so its energy can be used for muscle contraction ?
- Binds to myosin head
– ATPase on myosin head
– ATP = ADP + Pi + energy
What causes rigor mortis ?
Lack of ATP production
How does muscle relaxation occur ?
- AP ends, electrical stimulation of SR stops
- Ca2+ pumped back into SR
– Stored until next AP arrives
Referring to muscle relaxation what happens without Ca 2+ ?
- troponin and tropomyosin
return to resting conformation
– Covers myosin-binding site
– Prevents actin-myosin cross-bridging
What is a muscle biopsy ?
– Small (10-100 g) piece of muscle removed
– Frozen, sliced, examined under microscope
What fiber has slow form of myosin ATPase ?
Type 1
What fiber has fast form of myosin ATPase ?
Type 2
How are type 1 fiber described ?
High aerobic endurance
* Efficiently produce ATP from oxidation of fat and carbohydrate
* Can maintain exercise for prolonged periods
* Require oxygen for ATP production
* Recruited most often during low-intensity
– aerobic exercise (e.g., marathon running)
– daily activities (walking)
– posture
When is peak tension in type 1 fibers ?
– Peak tension in 110 ms (slow twitch)
– ~50% of fibers in an average muscle
When is peak tension in type 2 fibers ?
– Peak tension in 50 ms (fast twitch)
– Type IIa (~25% of fibers in an average muscle)
– Type IIx (~25% of fibers in an average muscle)
How are type 2 fiber described ?
- Type II fibers in general
– Poor aerobic endurance, fatigue quickly
– Produce ATP through anaerobic pathways - Type IIa
– More force, faster fatigue than type I
– Short, high-intensity endurance events (1,600
m run) - Type IIx
– Seldom used for everyday activities
– Short, explosive sprints (100 m)
How do Type 1 compare to Type 2 in Variable speed of myosin ATPase ?
– Fast myosin ATPase = fast contraction cycling.
– Slower myosin ATPase = slower contraction cycling.
How do Type 1 compare to Type 2 in Sarcoplasmic reticulum ?
– Type II fibers have a more highly developed SR.
– Ca2+ release is faster
How do Type 1 compare to Type 2 in motor units ?
– Type I motor unit: smaller neuron, <300 fibers
– Type II motor unit: larger neuron, >300 fibers
How do Type 1 compare to Type 2 in Peak power ?
– Type IIx > type IIa > type I
What are muscle Fiber type determinants ?
- Each person has unique ratios.
- Genetic factors
– Most influential in determining percentage - Arm and leg ratios are similar in one
person.
– Type I predominates in endurance athletes.
– Type II predominates in power athletes. - Soleus is type I in everyone.
- Aging: muscles lose type II motor units
What is muscle fiber recruitment also called ?
motor unit recruitment
– When a motor unit is recruited,
ALL of its fibers are activated
What is the method of muscle fiber recruitment for altering force production ?
– Less force: fewer or smaller
motor units (type I)
– More force: more or larger
motor units (type II)
What is the orderly recruitment (Size Principle) ?
- Recruitment order:
– First: Smallest (type I) motor units
– Next: Midsized (type IIa) motor units
– Last: Largest (type IIx) motor units - Recruited in same order each time
What is the Principle of orderly recruitment
(size principle) ?
order of recruitment
of motor units directly related to size
of α-motor neuron
How do thresholds change in motor units ?
- Smaller MUs are low-threshold, recruited first
- Progressively higher threshold MUs are
recruited based on the increasing demands of
the activity
What are the types of dynamic contraction ?
– Concentric
– Eccentric
What is dynamic contraction ?
– Muscle produces force and changes length
– Joint movement produced
How is concentric contraction described ?
- Muscle shortens while
producing force - Sarcomere shortens,
filaments slide toward center
How is eccentric contraction described ?
- Muscle lengthens while
producing force - Cross-bridges form but
sarcomere lengthens - Ex) lowering heavy weight
How is Static (isometric) contraction described ?
– Muscle produces force but does not change length
– Joint angle does not change
– Myosin cross-bridges form and recycle, no sliding
What are the dependents of the amount of force developed ?
- Number and type of motor units activated
- Size of the muscle
- Frequency of stimulation of each motor unit
- Muscle fiber and sarcomere length
- Speed of contraction
When is more force generated and how is it related to the type of fibers ?
More force generated when more motor units
activated
* Type I motor units = less fibers = less cross-bridges = less force
* Type II motor units = more fibers = more cross-bridges = more force
How can a motor unit exert varying level of force ?
Single motor unit can exert varying levels of force
dependent on frequency of stimulation
What are the frequency of stimulations called regarding motor units based on rate coding ?
– Twitch: contractile response to single electrical stimulus
– Summation: consecutive stimuli for greater force
– Tetanus: continued stimulation resulting in peak force
What is rate coding ?
- Process of varying levels is rate coding
What happens if fibers are too short or too stretched ?
Too short or too stretched = little or no force develops
Why is having fibers at optimal length important ?
Fibers have optimal length for force production
* Optimal sarcomere length = optimal overlap of
actin/myosin
– Maximizes cross-bridge interaction
Why is speed important to producing force ?
- Concentric: maximal force development decreases at higher speeds
- Eccentric: maximal force development increases at higher speeds
After 3 to 6 months of resistance training
– 25 to 100% strength gain
– Learn to more effectively produce force
– Learn to produce true maximal movement
Strength gains similar as a percent of initial
strength
– Young men experience greatest absolute gains
versus young women, older men, children
– Due to incredible muscle plasticity
Hypertrophy
Muscle size increase = Muscle Strength Increase
Atrophy
Muscle size Decrease = Muscle Strength Decrease
Strength gains result from…..
– Altered neural control
– Increase Muscle size
What has to occur for strength gain ?
neural adaptations via plasticity
Strength gain can occur without hypertrophy
True
What factors are important for strength gains ?
– Increase motor unit recruitment
– Decrease autogenic inhibition
– Reduced coactivation
How are motor units generally recruited?
asynchronously; not all engaged at the same instant
Resistance training = synchronous recruitment
Synchronous recruitment = strength gains
Synchronicity = more motor units fire at the same time
This improves rate of force development and increase capability to exert force.
What are Normal intrinsic inhibitory mechanisms ?
– Inhibit muscle contraction if tendon tension too high
– Prevent damage to bones and tendons
Why does training decrease inhibitory impulses ?
So muscles can generate more force.
What may reducing coactivation do ?
Cause strength gain
What do antagonist (coactivation) normally do ?
Oppose agonist force.
Definition of muscle hypertrophy.
Increase in muscle size.
What is Transient hypertrophy (after exercise bout) ?
– Due to edema formation from plasma fluid
– Disappears within hours
What is Chronic hypertrophy (long term)?
– Structural change in muscle
– Fiber hypertrophy
How is chronic hypertrophy maximized ?
– High-velocity eccentric training
– Disrupts sarcomere Z-lines (protein remodeling)
What may Fiber Hypertrophy be the result of ?
- More actin, myosin filaments
- More myofibrils
- More sarcoplasm
- More connective tissue
Resistance training = Increase in protein synthesis
– During exercise: synthesis decreases , degradation increases
– After exercise: synthesis increases , degradation decreases
How does Testosterone facilitates fiber hypertrophy ?
– Natural anabolic steroid hormone
– Massive doses of anabolic steroids + resistance
training = large increases in muscle mass
We don’t gain muscle fiber (hyperplasia). We increase our fiber size. What does hyperplasia ?
Animals
Short term increase in muscle strength is due to what ?
– Neural factors critical in first 8 to 10 weeks
– Substantial Increase in 1RM
– Due to increase in voluntary neural activation
Long-term Increase in muscle strength is due to what ?
– Hypertrophy major factor after first 10 weeks
– Associated with significant fiber hypertrophy
– Net Increase protein synthesis takes time to occur
What does Reduction or cessation of activity do to muscle ?
major change in muscle structure and function
How much is muscle reduced in immobilization in the first 6 hrs ?
– Lack of muscle use = reduced protein synthesis
How much is muscle reduced in immobilization in First week ?
-strength loss of 3 to 4% per day
– DECREASE Size/atrophy
– DECREASE Neuromuscular activity
How are the effects of immobilization reversed ?
- Recover when activity is resumed
– But recovery period longer than immobilization
What happens in detraining ?
Leads to decrease in 1RM
– Strength losses can be regained (~6 weeks)
– New 1RM matches or exceeds old 1RM
What happens after muscle goal met ?
Once training goal met, maintenance
resistance program prevents detraining
- maintain intensity but reduce frequency
Fiber type alterations Training regimen may not outright change fiber type but …..
– Type II become more oxidative with aerobic training
– Type I become more anaerobic with anaerobic training
Type IIx = type IIa transition common in RT (Fiber Type Alterations)
– 20 weeks of heavy RT program showed
* Static strength Increase, cross-sectional area increases
* Percent type IIx decrease, percent type IIa Increase
Increases in strength dependent primarily on
neural adaptations
True
