Ch. 20 - Skeletal Muscle

Front 1

skeletal muscle

Back 1

- strong, quick discontinuous voluntary contraction
- bundle of fibers
- muscle cells
- myofibril

Front 2

cardiac muscle

Back 2

- strong, quick continuous involuntary contraction

Front 3

smooth muscle

Back 3

- weak, slow involuntary contraction

Front 4

myofibril

Back 4

- rods of proteins inside muscle fibers
- myofilaments = thick and thin filaments
- arranged in specific way = thick filament surrounded by six thin filaments

Front 5

sacromere

Back 5

- functional unit of muscle
- z line and I band to the next z line
- I band split between 2 sacromeres
- sacroplasmic reticulum
- terminal sisterny = stores calcium
- I tubule, terminal sisterny, sacroplasmic reticulum = triad

Front 6

thick filament

Back 6

- made of myosin
- myosin composed of 6 polypeptides
- myosin lines up tail to tail = heads pointed in opposite directions
- 4 light chains and 2 heavy chains

Front 7

thin filament

Back 7

- made of many proteins
- actin composed of globular actin
- actin has myosin binding site
- tropomyosin wraps around actin and covers binding site
- troponin has 3 subunits
- tropomyosin exposes binding sites when calcium present

Front 8

troponin 3 subunits

Back 8

- TnT = binds to tropomyosin
- TnC = binds to calcium
- TnI = inhibitory

Front 9

titin

Back 9

- huge protein that helps center thick filament
- elastic element
- links thick filaments to Z lines
- largest protein in human genome

Front 10

nebulin

Back 10

- acts as molecular ruler
- determines how long the thin filament will be
- actin binding protein

Front 11

sliding filaments

Back 11

- produce muscle contraction
- H zone and I band disappear
- sacromere shortens = thin filament pulled over thick filament

Front 12

cap-Z

Back 12

- binds to thin filaments and stabilizes it on the Z line
- caps the plus ends of actin filaments at Z-disk

Front 13

muscle contraction

Back 13

- dystroglycan complex = proteins that interact with extracellular matrix and bound to sacroglycan complex
- sacroglycan = enormous gene that is capable of having mutations
- muscle dystrophy = dystrophin is absent or mutated

Front 14

sequence of muscle contraction

Back 14

- rigor is a transient state
- ATP binding dissociates myosin from actin
- myosin ATPase hydrolyzes ATP to ADP and P
- hydrolization causes myosin to have a cocked position
- myosin binding site on actin binds to actin binding site on myosin making a cross bridge
- conformation change called a power stroke and ADP is released
- binds to ATP and myosin releases from actin

Front 15

power stroke

Back 15

- thin filament is pulled over the thick filament toward the M line

Front 16

regulation of contraction

Back 16

- myosin binding site on actin is controlled
- tropomyosin covers the myosin binding site and won't let myosin connect to actin
- Ca binds to troponin causing a conformational change that pulls down the tropomyosin exposing myosin binding sites

Front 17

vertebrate plan

Back 17

- based on muscles organized into motor units
- motor units = motor neuron and all muscle fibers it innervates

Front 18

motor end-plate

Back 18

- region of the muscle cell membrane covered by the terminal bud
- has clefts and ridges = junctional folds

Front 19

T tubule

Back 19

- contains dihydropyridine receptor (DHPR)
- blocks channel for ryanodine

Front 20

excitation-contraction coupling sequence

Back 20

- action potential in motor neuron triggers exocytosis of ACh
- ligand-gated channels bind ACh and open generating an action potential
- action potential propagates over cell membrane and depolarizes the t-tubules
- depolarization reaches the DHPR and causes a conformation change that opens a RyR calcium channel of the SR and Ca diffuses out of SR into cytoplasm
- Ca ions bind to troponin and tropomyosin moves to expose myosin-binding sites on actin
- acetylcholinesterase in the extracellular matrix of synaptic cleft hydrolyzes ACh to terminate the action potential
- cross bridges go through several cycles as long as Ca remains bound to troponin
- once wave of depolarization ceases, DHPRs return to their original conformation and RyR Ca channels close
- as ATP-depenedent Ca pumps decrease the Ca concentration in cytoplasm, Ca leaves TN, TM blocks myosin binding sites on actin, and contraction ends

Front 21

excitation-contraction coupling

Back 21

- Ca is stored in the SR both free and bound to the protein calsequestrin
- ATP dependent Ca pumps are continuously active, before, during, and after contraction
- each ATP hydrolyzed, 2 Ca are moved from cytoplasm into the SR

Front 22

excitation-contraction coupling diagram

Back 22

excitation-contraction coupling diagram

Front 23

whole skeletal muscles

Back 23

- prime mover
- synergist
- antagonist
- fixator

Front 24

prime mover

Back 24

- agonist
- produces most of force

Front 25

synergist

Back 25

- aids prime mover
- stabilizes the nearby joint
- modifies direction of movement

Front 26

antagonist

Back 26

- opposes prime mover
- prevents excessive movement and injury

Front 27

fixator

Back 27

- prevents movement of bone

Front 28

force

Back 28

- generated only by contracting
- lengthen passively

Front 29

contraction

Back 29

- tension generated by a muscle during cross bridge activity
- may or may not involve shorteining

Front 30

twitch

Back 30

- mechanical response of a muscle to a single action potential
- latent period
- contraction phase
- relaxation phase

Front 31

latent period

Back 31

- period of time that elapses between the generation of an action potential and the start of the contraction
- Ca release cross bridge formation

Front 32

contraction phase

Back 32

- starts at the end of latent period and ends at tension peak

Front 33

relaxation phase

Back 33

- period of time from end of the tension peak until the end of the contraction

Front 34

types of muscle contraction

Back 34

- isotonic
- isometric

Front 35

isotonic contraction

Back 35

- changes in length
- muscle attempts to move a load that is equal to or less than the tension generated by muscle
- tension in the muscle remains constant despite a change in muscle length
- shortening can occur only when a muscle's maximal force of contraction exceeds the total load on the muscle

Front 36

types of muscle contractions

Back 36

- concentric = shortening
- eccentric = lengthening

Front 37

concentric

Back 37

- tension generated is sufficient to overcome the resistance, and the muscle shortens as it contracts
- occurs throughout the length of the muscle, generating tension at musculo-tendinous junction, causing the muscle to shorten and changing the angle of the joint
- cross-bridge cycling

Front 38

eccentric

Back 38

- tension generated is insufficient to overcome the external load on the muscle and the muscle fibers lengthen as they contract
- an opposing force is greater than the force generated by the muscle
- used as a means of decelerating a body part or object, or lowering a load gently rather than letting it drop, or hiking uphill
- unknown mechanism
- leads to minor muscle damage that causes soreness following exercise

Front 39

recording isometric and isotonic contraction

Back 39

- tension developed is sufficient to move the load, and the muscle shortens
- latent period is followed by rise in tension
- not enough tension to move a load
- plateau = force produced by the muscle remains constant
- isotonic/same tension allowing the muscle to shorten

Front 40

work of contraction

Back 40

- isotonic contractions show that muscle shortens the greatest distance with no load
- shortens progressively shorter distances with increasing loads
- multiply the force developed by distance shortened for each load gives a curve that represents work performed by muscle

Front 41

muscle force

Back 41

- decreases with increased velocity of contraction during concentric contraction
- increases with increased velocity of contraction during eccentric contraction

Front 42

isometric contraction

Back 42

- tension without changing length
- muscle attempts to move a load that is greater than the tension generated by the muscle
- muscles of hand and forearm grip an object, joints of hand don't move but muscles generate sufficient tension to prevent the object from being dropped

Front 43

power

Back 43

- equal to force times velocity
- muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force)
- maximal force produced by a muscle is proportional to cross-sectional area of its contractile elements
- velocity of shortening is influenced by the myosin isoforms expressed by the motor units of muscle

Front 44

tension/muscle force

Back 44

- amount a muscle can do depends on its volume
- force/cross sectional area
- tension generated by a muscle fiber that is directly proportional to number of attached cross bridges

Front 45

threshold

Back 45

- minimal stimulus needed to depolarize the sarcolemma
- point at which sodium ions start to move into the cells = depolarization
- ability to reach threshold is determined by the magnitude of stimulation and duration of stimulation

Front 46

length-tension relationship

Back 46

- tension that a muscle generates varies with its length
- found when a muscle is under isometric contraction and maximum activation of the muscle
- in a singe muscle fiber, peak force is noted at a normal resting length
- bell-shaped curve
- too much overlap of thick and thin filaments results in less tension
- overlap of thick and thin filaments is ideal to generate maximal force
- sacromere set longer than ideal length doesn't have enough overlap so fewer sites of cross sectional formation

Front 47

length-tension for isometric contraction

Back 47

- shows tension produced by a muscle when it is set a different lengths prior to simulation
- shorter = tension drops
- maximal tension was achieved when muscle was set a lengths near normal relaxed lengths

Front 48

motor units

Back 48

- skeletal muscles of vertebrates
- independent
- single alpha motor neuron and all the corresponding muscle fibers it innervates
- when activated all of its fibers contract

Front 49

motor unit recruitment

Back 49

- progressive activation of a muscle by successive recruitment of contractile units/motor units to accomplish increasing gradations of contractile strength
- each vertebrate muscle twitch fiber is innervated by a single axon that branches to make many synaptic contacts at the middle of multiple fibers

Front 50

tension varies

Back 50

- varying the frequency of impulse in a single motor unit
- varying number of active motor units

Front 51

multiple fiber summation

Back 51

- weak signal is sent by the CNS to muscle
- smaller motor units are stimulated first
- smaller motor units are more excitable than the larger ones
- as strength of signal increases = more motor units are excited in addition to larger ones
- largest motor units having as much as 50 times the contractile strength as the smaller ones
- as more and larger motor units are activated = force of muscle contraction becomes progressively stronger

Front 52

size principle

Back 52

- allows for a gradation of muscle force during weak contraction
- occur in small steps which then become progressively larger when greater amounts of force are required

Front 53

contractile and elastic components

Back 53

- to achieve maximal tension all structures in series must be stretched taut
- sustained high calcium concentration must be present from multiple individual twitches

Front 54

summation

Back 54

- muscle is stimulated repeatedly
- stimuli arrive one after another within a short period of time
- twitches can overlap and result in a stronger muscle contraction

Front 55

tetanus

Back 55

- stimuli continue to be applied frequently to a muscle over a prolonged period of time
- muscle will eventually reach a plateau
- twitches fuse

Front 56

tension force

Back 56

- minimum produced by single twitch in smallest motor unit
- maximum produce by simultaneous fused tetanic contraction in all motor units

Front 57

fiber firing

Back 57

- 1/3 of fibers in muscle firing at once under conscious muscle exertion
- actual number firing affected by various physiological and psychological factors = Golgi tendon organs and Renshaw cells
- low level of contraction is protective mechanism to prevent avulsion of tendon
- force generated by 95% contraction of all fibers is sufficient to damage the body

Front 58

mechanical model of muscle

Back 58

- contractile component = muscle fiber
- series elastic component = tendon
- parallel elastic component = muscle membrane

Front 59

elastic elements

Back 59

- series elastic elements
- non-contractile component of muscles
- lies in series
- store energy when stretched
- tendons
- cross bridges between actin and myosin also contribute

Front 60

parallel elastic elements

Back 60

- non-contractile component of a muscle
- provides resistive tension when muscle is passively stretched
- muscle membranes which lie in parallel to muscle
- Hook's law

Front 61

Hook's law

Back 61

- F=KL
- F is force exerted on spring
- K is constant
- L is displacement

Front 62

viscous resistance

Back 62

- muscle cells contents become compressed
- increase with maximal force
- contributes to passive resistance
- produce by parallel elastic components

Front 63

parallel fibers

Back 63

- parallel to longitudinal axis of the muscle
- sartorius, masseter, biceps branchi
- each fiber attached to its own tendon with tendons converging on a common point
- gets shorter and increases diameter when contracts
- fibers shorten in direction parallel to direction shortening of muscle
- located in positions requiring longer movements with less power of faster movements
- greater length and less cross sectional area = greater velocity

Front 64

convergent fibers

Back 64

- fibers spread over a broad area
- all the fibers converge at one common attachment site
- fibers typically spread out, like a fan or a broad triangle, with tendon at apex
- pectoralis major muscle
- versatility because stimulation of only one portion of muscle can change direction of pull
- pull in different directions
- allow maximum force production

Front 65

pennate fibers

Back 65

- at angle to longitudinal axis of muscle
- rectus femoris, deltoid
- attach to a common tendon
- direction of shortening of individual fibers is different from direction of shortening of whole muscle
- fewer sarcomeres in series
- cannot shorten as much as parallel
- located in positions requiring small but powerful movements
- fatigue quickly
- force produced is greater than force produced by parallel
- greater cross sectional area and less length = greater force

Front 66

angle of pennation

Back 66

- greater it is the smaller the amount of effective force transmitted to tendon
- increases as tension progressively increases in the muscle fibers
- contains more muscle fibers
- produces more tension

Front 67

muscle architecture

Back 67

- muscle force is proportional to physiologic cross-sectional area
- muscle velocity is proportional to muscle fiber length

Front 68

contraction and temperature

Back 68

- most efficient at 38.5 C
- elevated muscle temperature causes a shift in force-velocity curve
- increased maximum isometric tension
- increased maximum velocity of muscle shortening
- requiring less motor unit to sustain a given load
- body temperature too high = heat exhaustion or heat stroke

Front 69

2 types of skeletal muscles

Back 69

- red muscles
- white muscles

Front 70

red muscles

Back 70

- contain myoglobin
- highly vascularized
- receiving and using more oxygen than white muscles
- require lower minimum rate of stimulation for tetanic fusion
- contain many mitochondria
- gets most of its ATP form oxidative phosphorylation = rapid
- sustain contraction longer without fatiguing
- produce large tension

Front 71

white muscles

Back 71

- contain little myoglobin
- more rapidly contracting fast muscles
- poorly vascularized
- contain few mitochondria
- ATP from glycolysis
- produce small tension

Front 72

muscle fibers specialized

Back 72

- different muscles contain different types of fibers
- specialized for slow or quick response

Front 73

3 different forms of myosin

Back 73

- type 1
- type 2a
- type 2x

Front 74

type 1

Back 74

- slow oxidative
- hydrolyze ATP slowly
- slow Ca uptake by SR
- slow contraction
- long duration of twitches
- high resistance to fatigue
- many mitochondria, myoglobin, and capillaries
- red muscle
- small fiber diameter
- posture
- slow myosin ATPase activity
- specialized for endurance
- require constant oxygen = no lactate dehydrogenase
- energy from aerobic metabolism
- slow cross-bridge formation

Front 75

type 2a

Back 75

- fast oxidative glycolytic
- hydrolyze ATP rapidly
- fast Ca uptake by SR
- fast contraction
- short twitch duration
- intermediate resistance to fatigue
- many mitochondria, myoglobin, and capillaries
- red muscle
- intermediate fiber diameter
- standing, walking, rapid repetitive movements
- generate great deal of power
- myosin ATPase activity is high
- energy from glycolysis and oxidative phosphorylation

Front 76

type 2x

Back 76

- fast glycolytic
- hydrolyze ATP rapidly
- fast Ca uptake by SR
- fast contraction
- short duration of twitch
- low resistance to fatigue
- few mitochondria, myoglobin, and capillaries
- white muscle
- large fiber diameter
- jumping, bursts of high speed locomotion
- generate great deal of power and force
- myosin ATPase activity high
- energy from glycolysis

Front 77

motor unit 1

Back 77

- small alpha motor neurons
- innervate relatively few muscle fibers
- form motor units that generate small forces
- innervate small red muscle fibers = slow oxidative
- small motor units
- especially important for activities that require sustained muscular contraction
- more excitable

Front 78

motor unit 2

Back 78

- intermediate alpha motor neurons
- properties that lie between those for the other two
- use type 2a
- fast fatigue resistance motor units
- not quiet as fast as FF units
- generate about twice the force of a slow motor unit

Front 79

motor unit 3

Back 79

- large motor neurons
- innervate large, more powerful motor units
- type 2x
- fast fatigable motor units
- especially important for brief exertions that require large forces
- fire last
- small axon diameter
- least excitable

Front 80

plasticity

Back 80

- muscle capable of changing in both mass and cellular characteristics
- change mass by hypertrophy
- atrophy occurs when muscle fibers lose actin and myosin or from loss of cells

Front 81

endurance training

Back 81

- elicits changes in fiber type, capillary density, mitochondrial density
- proportion of type1 fibers remain unchanged
- proportion of type 2a fibers increase
- proportion of type 2x fibers decrease
- changes in gene activity
- increase density of capillaries
- exercised muscle produce and release cytokine vascular endothelial growth factor
- increases aerobic capacity of muscle fibers by increasing mitochondria and lipid droplets
- increased Mhc 2a isoform and decreased amount of Mhc 2x isoform

Front 82

vascular endothelial growth factor

Back 82

- increases with exercise
- increases less in trained muscles than in untrained muscles

Front 83

hypertrophy

Back 83

- adding structural proteins
- occurs in cardiac muscle
- doesn't add cell numbers by mitosis

Front 84

regulating muscle mass

Back 84

- myostatin
- PI3-K-Akt1 pathway

Front 85

myostatin

Back 85

- prevents from getting to much muscle
- decreases PI3 - Akt1 pathway
- negative growth regulator
- binds to a R on muscle PM
- initiates an intercellular signaling pathway
- controls cell growth
- decreases amount of fat deposited between muscle fibers
- limits protein production and satellite cell activation

Front 86

PI3-K-Atk1 pathway

Back 86

- provides molecular signals that regulate balance between synthesis and degradation
- insulin like growth factor (IGF-1) secreted when struiated (skeletal and cardiac) muscle exerts force against a load
- insulin also activates
- IGF-1 binds to R
- activates phophoinositol 3-kinase (PI3-K) that phophorylates Akt-1 making it active
- increased protein synthesis by entering nucleus and binding to genes that make products that control protein degradation

Front 87

muscle energetics

Back 87

- ATP is immediate source of energy for powering muscle contraction
- ATP binding required for detachment of myosin and actin
- ATP hydrolysis activates acting binding site on myosin
- ATP drives the ATPase-Ca pump that transports Ca into the SR

Front 88

3 biochemical mechanisms produce ATP in muscle

Back 88

- use of phosphagen creatine phosphate
- aerobic glycolysis
- aerobic catabolism