Skeletal Muscle Contraction Flashcards
what percentage of body mass is muscle and what is the breakdown of muscle types?
50%
skeletal 40% and cardiac and smooth is 10%
what types of muscle are striated? Which bands appear dark and which light?
cardiac and skeletal muscle
a bands are darker and i bands are lighter
describe the organization of skeletal muscle
muscle composed of fasicles that are composed of multinucleated muscle fibers. fibers are composed of myofibrils. myofibrils are composed of linear sarcomeres which are the contracting portion of muscle
describe a sarcomere.
Z lines delineate borders of sarcomere
I band is the light band surrounding the Z line and the A band is the darker band between the Z lines. The H zone isthe lighter portion of the A band and the M line is at the center of the sarcomere
what is the only component of the i band?
actin filaments
what are the components of the A band? what is lacking at the H zone?
myosin and actin
actin is not in the H zone (it is the bare zone of myosin with no heads)
what is the m line?
where the thick filaments are linked with each other
how are thick and thin filaments arranged in the myofibril?
in double hexogonal array
mysoin fibers in a hexagonal pattern. each myosin filament is surrounded by six actin filaments (each actin filament surrounded by 3 myosin filaments)
what is the sliding filament theory?
describes the contraction of skeletal muscle as thick and thin filaments sliding past each other during contraction
during contraction, what are the changes to the sarcomere appearance?
Z lines move closer together
A band is constant but I band gets smaller
describe a myosin protein.
6 polypeptides: 2 heavy chains intertwined in the tail and extending to the globular regions and 2 light chains in each of the two globular regions.
describe the myosin heavy chain.
contains globular region portion that binds actin and hydrolyzes ATP. also makes up the hinge and tail regions.
describe the regulation of myosin
regulated by the regulatory light chain phosphorylated in striated muscle by a calcium/calmodulin-dependent kinase
what does the alkali light chain of myosin do?
it fine tunes moor function
describe a myosin filament.
1.6 micron length, composed of >200 myosin molecules that point outward from H zone
myosin are displaced by 120 degrees from the one preceding it
what is the sequence of events in excitation-contraction coupling?
1) signal travels neuron to neuromuscular junction
2) secretes ACh and opens gated ion channels
3) sodium flows in and depolarizes muscle->AP
4) muscle AP travels down T tubule->release of Ca
5) Ca concentration causes interaction at sarcomere
6) Ca pumped back into SR, muscle relaxes
7) muscle lengthened by antagonistic muscle
where is the sarcoplasmic reticulum located and what does it do?
SR surrounds each myofibril and stores and releases Ca
what is the triad in muscle fibers?
two terminal cisternae that travel with a T tubule and penetrate the interior of the muscle fiber.
what are T tubules? how do they cause calcium influx into the cell?
extension of the muscle cell membrane
action potentials propagate along the membrane and down into the T tubule, activating Ca channels in the adjacent SR cisternae
what is the function of the coupling of T tubules to the SR?
to ensure that sarcomeres contract together within the muscle fiber
what is tropomyosin?
a helical rod protein that covers myosin binding sites on actin in a resting muscle
what is troponin and what are its subunits?
protein that binds tropomyosin to actin
C subunit binds Ca
I subunit binds actin
T subunit binds tropomyosin
what is the mechanism of Ca induced contraction?
calcium binds to C subunit of troponin and causes a conformational change that uncovers the active sites on actin
describe the movement of myosin on actin filaments.
ATP hydrolysis fueled “walking” along the actin
what are the steps of the cross bridge cycle of muscle contraction?
1) ATP binds and myosin separates from actin
2) ATP cleaved and myosin head is “cocked”
3) myosin attaches to actin
4) phosphate released and power stroke occurs
5) ADP is released and myosin remains attached
when do isometric contractions occur? what is the contraction generated by a single action potential called?
at a constant muscle length
a twitch
how is an isometric contraction measured?
muscle cell is stimulated electrically and the force generated is measured as a function of time
what is the latent period between stimulation and isometric contraction?
the time period between the action potential and activation of the crossbridge cycle
what do the contraction and relaxation periods closely parallel in isometric contraction?
the levels of calcium in the sarcoplasm that surrounds the sarcomere
what are the causes in twitch properties in different muscles?
type of myosin expressed and speed of sarcoplasmic calcium elevation and clearance
what is an example of a fast twitch muscle and a slow twitch muscle?
fast- extraocular muscles
slow- soleus muscle
what are synonyms for slow and fast twitch fibers? which of these are fatigable?
slow- type 1
fast- type 2a and 2b
2b is fatigable
2a is a slower fast twitch fiber
describe a type 1 muscle fiber
slow twitch with MYH7 myosin
fatigue resistant, red from myoglobin
oxidative metabolism with many mitochondria and little glycogen
describe a type 2a muscle fiber
fast twitch with MYH2 myosin
fatigue resistant, red from myoglobin
oxidative metabolism with many mitochondria and abundant glycogen
describe a type 2b muscle fiber
slow twitch with MYH4 myosin
fatigable, white from lack of myoglobin
glycolytic metabolism with few mitochondria and high amounts of glycogen
what is the difference between the myoglobin subtypes?
different heavy chains and different rates of ATP hydrolysis
how is a length tension diagram obtained for an isometric contraction?
muscle is stretched and passive tension is measured. cell is depolarized, causing it to contract and the additional tension is measured
in a length tension diagram, what does the space between the curves represent?
active tension generated by the muscle contraction
what does the length tension diagram show us?
there is an optimal muscle length for active tension generation-greatest number of active cross bridges
what happens if a muscle fiber is stretched too far? not stretched enough
too far-there is not enough crossbridges formed and less tension is generated
not far enough- thin filaments overlap each other
when thick filaments reach Z line, no more tension
what is th enormal range of muscle length?
100-115% of resting muscle length
what do satellite cells mediate? where are they located?
normal repair and exercise induced cellular hypertrophy
located at the periphery of mature skeletal muscle fibers
how do satellite cells mediate muscle growth?
when cell is injured, satellite cells proliferate, differentiate and fuse into multinucleate myofibers
these fuse to existing
what does the fusion of satellite cells contribute to the muscle cell?
more nuclei, myofibril protein is increased, amount of sarcoplasm and muscle fiber connective tissue is increased
what occurs with the loss of myostatin? what is the result of heterozygosity with one normal myostatin gene?
uncontrolled muscle hypertrophy by altered satellite cell activation
heterozygotes have an intermediate condition
what correlation has been found between exercise, growth hormone and myostatin?
exercise and growth hormone decrease myostatin in the muscle
what is one way that testosterone increase muscle mass?
increases the number of satellite cells
what is MYO-029 and what does it do?
it is a recombinant human antibody that inhibits myostatin. can be used to treat muscular dystrophy
