Exam 2- Skeletal Muscle Physiology Flashcards
what are the 3 types of muscle
1- skeletal muscle (striated): attached to skeleton/bone
- responsible for general motor movement
2- smooth muscle: associated with body organs (viscera): blood vessels, GI tract, reproductive tract, glands
3- cardiac muscle: heart
- modified form of skeletal, similar striated patterns, but significantly diff patterns in organization and function
describe the structural components of skeletal muscle
- horizontal patterns interrupted by vertical bands
- horizontal are myofilaments- thick & thin filaments (thick= myosin & thin= actin)
- vertical lines are the z-lines
- everything in between 2 successive z lines is the sarcomere - actin filaments are connected to the z line on each side of the sarcomere, but they are discontinuous in the center of sarcomere
- H zone is the space between 2 actin filaments in a sarcomere
- myosin thick filaments are concentrated in middle of sarcomere, not directly connected to z line
- in 2 adjacent sarcomeres, gap b/w myosin filaments called I band
- myosin filament itself is called the A band
what is the basic unit of a muscle cell
sarcomere
the myosin thick filament is called the __ band
A
describe the I and A bands of sarcomere
I stands for “isotropic”- this allows light to pass through (I band is the gap between myosin filaments, across 2 sarcomeres)
A band called “anisotropic”, does not allow light to pass through (A band is myosin filaments)
how many actin surround 1 myosin
6 actin surround every 1 myosin (always 6:1 ratio)
what are the molecular building blocks that make up the sarcomere
myofilaments
“myo” means muscle
define sarcomere
all myofilaments between successive z-lines
describe the 2 places in sarcomere that myosin is anchored directly & indirectly
myosin anchored in middle of sarcomere to the M-line by myomesin
anchored indirectly to either side of Z-line by large spring-like protein called titin
describe how actin extends in the sarcomere and what proteins allow this
actin filaments start at Z-line, add more and more beads and lengthen it to the center, protein called tropomodulin caps the leading edge and keeps the beads from dissociating
at the trailing end is CapZ, which keeps the beads from dissociating at the Z-line
myosin is anchored to M-line by ___
actin is anchored to Z-line by ___
myomesin
alpha-actinin
describe the difference b/w myofibrils and myofiber
(myofilaments make up sarcomere)
- if myofilaments laid end to end, make up myofibril (long chain of sarcomeres laid end to end)
- myofiber is a muscle cell- a long cell made of many myofibrils arranged in parallel, held together by muscle membrane
- muscle cells are kinda like cables, each myofibril is one strand of cable (the more myofibrils you can pack in, the stronger the muscle cell will be)
list the arrangement of organization of skeletal muscle
myofilaments –>
sarcomere —>
myofibril –>
myofiber –>
muscle
there is more to muscle than just myofilaments…what surrounds myofilaments?describe
inside the muscle cell and wrapped around the myofibrils is an internal set of membranes called the sarcoplasmic reticulum (SR)- flimsy membrane that wraps itself around each of the myofibers (2 components of SR: lateral sac & fenestrated collar)
describe the 2 components of SR
1- the lateral sac (terminal cisternae)- associated with the z-line of the sarcomere
2- fenestrated collar- in between the lateral sacs is flimsy membrane to hold everything together
lateral sac associated with T-tubule, T-tubule is an invagination of the muscle membrane itself that goes deep into the muscle cell
describe the 2 components of SR
1- the lateral sac (terminal cisternae)- associated with the z-line of the sarcomere
2- fenestrated collar- in between the lateral sacs is flimsy membrane to hold everything together
lateral sac associated with T-tubule, T-tubule is an invagination of the muscle membrane itself that goes deep into the muscle cell
- on either side of T-tubule is a lateral sac (this T-tubule and 2 lateral sacs forms a structure called a triad)
what is the function of the lateral sac of the SR?
it is an internal storage site for calcium
- calcium gets in lateral sac by SERCA (sarcoplasmic endoplasmic reticulum calcium ATPase)- pumps calcium into lateral sac of SR
- inside lateral sac is another protein called calsequestrin, a calcium binding protein- 1 molecule of calsequestrin can bind 43 molecules of calcium (high volume, low affinity- binds calcium loosely)
describe the mechanism of muscle contraction (which bands/filaments/zones) contract
when a muscle contracts, the sarcomere shortens
- as sarcomere shortens, the H-zone and the I-band get smaller and sometimes completely disappear
- but the length of the A-band and then actin thin filament does not change
- sarcomere gets shorter, but filaments themselves are not contracting –> sliding filament hypothesis (muscle contraction is due to the active inward sliding of the thin filament over the surface of the thick filament towards the center of the sarcomere)
sliding filament hypothesis is based off the molecule interaction b/w ___ and ___
actin and myosin
describe structure of actin
actin is a polymer of individual beads, form long filaments- 2 that wrap together in helical fashion
- each individual bead is globular protein called G-actin (globular actin)
- long filament called F-actin (filamentous actin)
- all along the actin filament, the individual G-actin molecules have binding sites that allow it to attach to myosin (actin pulled across surface of myosin, so has to connect somehow)
2 other proteins in actin polymer:
1- tropomyosin: wraps itself around the surface of the filamentous actin, by doing this, it covers up/masks the binding sites all along F-actin polymer (at rest- actin binding sites are masked and cannot bind w myosin)
2- troponin (globular protein all along tropomyosin), specifically troponin C (c for calcium)- a calcium binding protein
- at rest, actin associated with troponin-tropomyosin complex which sites on surface of actin polymer, inhibiting binding sites of myosin
describe the structure of myosin
myosin has a head and a tail
- the tail called the light meromyosin (LMM)- makes up the central core of the myosin filament- whole bunch of tails wound together, makes the core of the myosin molecule
- the head called heavy meromyosin (HMM)- the physiologically important components are found in the myosin head
- myosin head has complementary binding sites for actin, also has myosin ATPase (needs ATP for muscle contraction)
- regulatory and essential light chains are on the heavy meromyosin head (regulatory light chain is another calcium binding protein, when binds calcium –> activates myosin ATPase)
where are the physiologically important components of myosin found
in myosin head (HMM)
after AP is transmitted from motor neuron to muscle membrane and sodium channel of nicotinic receptor depolarizes membrane to threshold, what happens?
starts AP on muscle membrane –> once AP generated on muscle membrane, starts process called excitation-contraction coupling (cause & effect: once AP generated on muscle membrane, will cause cell to contract, the 2 processes are coupled, AP will always lead to contraction)
- AP travels along muscle membrane and then reaches T-tubules (transverse tubules)- T-tubules are deep infoldings of membrane, go deep into center of muscle cell
- muscle cells are big, large diameter, so T-tubules go to center of thick muscle cell (when AP hits T-tubule, travels down T-tubule to center of muscle) –> allows entire muscle to be stimulated simultaneously
membrane of T-tubule is very closely associated with membrane of the lateral sac of the SR…describe receptors
there’s a pair of receptors (one in t-tubule and on in lateral sac of SR)
- t-tubule receptor called DHP receptor (gets name from dihydropyridine- pharm agent)
- lateral sac receptor called ryanodine receptor (pharm agent)
these 2 receptors are physically linked - as AP sweeps down membrane of T-tubule, hits DHP receptor –> energy from AP causes DHP receptor to change shape –> this causes change in shape of ryanodine receptor (ryanodine receptor forms a calcium channel in the SR lateral sac membrane) –> this calcium channel opens up when ryanodine activated
what kind of channel is the calcium channel of the ryanodine receptor
mechanically-gated channel
activated b/c of change in shape of receptor
what protein is associated with the inside of the ryanodine receptor and what does it do
calsequestrin (calcium binding protein, binds 43 calciums loosely)- as soon as ryanodine calcium channel opens, calsequestrin gives up its calcium, flows out of SR and into sarcomere
after ryanodine receptor is activated and calsequestrin gives up its calcium which flows into sarcomere, calcium has 2 functions in sarcomere:
1- calcium binds to troponin C of troponin-tropomyosin complex –> this alters conformation of the complex –> complex becomes buried deep into grooves of F-actin helix –> exposes binding sites all along actin polymer
2- simultaneously, calcium binds to regulatory light chain of myosin head –> changes conformation of myosin head, allows ATP to bind to active site of myosin ATPase –> starts process of powerstroke
describe process of muscle contraction- power stroke
calcium floods sarcomere and allows ATP to bind to myosin head –> ATP split by myosin ATPase and energy is used to pull back myosin head, bringing myosin close to actin, as soon as myosin head pulled back, active sites on myosin come into contact with binding sites on actin –> actin and myosin form cross-bridges (link/hook together) –> cross-bridge formation triggers release of stored mechanical energy, myosin head rotates inward (power stroke), pulling actin into surface toward the center –> new conformation causes release of ADP and Pi, myosin head disengages from actin, myosin head back into normal conformation, waiting for another ATP to bind
in the power stroke muscle contraction, converting __ energy of ATP to __ energy, pulling myosin head back
chemical
mechanical
as long as you have __ and __, can get another rest and powerstroke (each powerstroke shortens the sarcomere and contracts the muscle a little bit more)
ATP and calcium
how does a muscle relax? relaxation starts with…
a 2-step process that clears calcium from the sarcomere
1- soluble protein in sarcomere called parvalbumin- another high volume, low affinity calcium-binding protein (will bind a lot of Ca quickly, this drops the level of Ca in sarcomere very quickly –> when drop levels of calcium, it comess off other calcium binding proteins (troponin) by diffusion –> troponin-tropomyosin complex goes back to original position and masks the actin binding site –> no more cross-bridge formation or contraction
- at same time, calcium comes off myosin head (myosin ATPase goes back to inactivated state), myosin head goes back to its resting position
2- transfer of calcium from parvalbumin to the SERCA, pumps calcium back into the inside of lateral sac of SR (pump has much higher affinity for Ca, why its able to pull Ca off parvalbumin)
what is a secondary way to get calcium out of sarcomere for muscle relaxation?
sodium pump –> pumping sodium out of sarcomere (sets up inward sodium gradient- this drives a sodium-calcium exchange protein), sodium in and calcium out
once cross-bridges are broken, muscles relax to normal length, the mechanism involve is due to what?
series elastic elements/components (sarcolemma, z-line, endomysium, perimysium, epimysium, ligaments, tendons)
- series elastic elements are cellular molecular components of muscle that are stretching/elastic, but not contractile (myofilaments are contractile)
- non-contractile, but elastic- they stretch/compress during contraction and spring back to their normal length during relaxation
