Skeletal Muscle Contraction Flashcards
1
Q
Muscle types
A
- striated muscle has prominant A and I bands and includes skeletal and cardiac muscle
- smooth muscle is non-striated and lacks definite A and I bands
- A-indicates anisotropic wheres I designates isotropic
2
Q
Organization of sarcomeres
A
- Z line marks the boundary between two sarcomeres
- I band- only actin thin filaments, which extend from the Z line toward the center of the sarcomere
- A band- dark, myosin thick filaments
- H zone- no overlap between actin and myosin
- M line- center of the sarcomere and is the site at which the thick filaments are linked with each other
- myosin filaments are packed in a hexagonal pattern, and each myosin filament is surrounded by six actin filaments in region where the filaments overlap
- each filament is surrounded by three myosin filaments
3
Q
Sliding filament model of muscle contraction
A
- thick and thin filaments slide past each other during muscle contraction
- in successive stages of shortening the changes in banding patterns are evident in living muscle viewed in light microscope
During contraction:
- the Z lines move closer together
- the length of A band is constant
- the length of I band shortens
- as the sarcomere shortens, so does the muscle
4
Q
Myosin molecule
A
- each myosin molecule has six polypeptde chains, two heavy chains of about 230,000 Daltons intertwined as a double helix but terminating in two globular heads and four light chains of about 20,000 Daltons each in the head region, for a total molecular weight of 540,000
- thus each myosin molecule molecule has two globular heads and one doubl coiled helix known as the tail
5
Q
Myosin Heavy Chain
A
- myosin heavy chain is a single protein that contains the globular region that both binds actin and hydrolyzes ATP during muscle contraction
- in addition the heavy chain contains the hinge region and extended tail region
- two heavy chains are wound together in each myosin molecule
6
Q
Regulatory Light Chain
A
- regulatory light chain of myosin (RLC) is phosphorylated in striated muscles by Ca2+/calmodulin-dependent myosin light chain kinase
- unique biochemical and cellular properties of this phosphorylation system in fast-twitch skeletal muscle maintain RLC in the phosphorylated form for a prolonged period after a brief tetanus or during low-frequency repetitive stimulation
- this phosphorylation correlates with potentiation of the rate of development and maximal extent of isometric twitch tension
7
Q
Essential light chain or Alkali light chain
A
-the common findings suggest the ELC provides the fine tuning of the myosin motor function, which is regulated in an isoform and tissue-dependent manner
8
Q
Myosin filament or thick filament
A
- the myosin filament is 1.6 micron is length, equal to the length of the A-band, and is composed of 200 or more individual myosin molecules that point outward from the central H-zone
- successive myosin heads are axially displaced from the previous set by 120 degrees, so that the heads protrude in all directions from the tail
-each myosin head contains an ATPase catalytic site and an actin binding site
9
Q
Scheme of excitation-contraction coupling
A
- a motor action potential travels along a motoneuron to the motor endplate at the neuromusclar junction
- the nerve endings secrete Ach which acts in sarcolemma to open Ach gated ion channels
- channels permit Na flow into the muscle, depolarizing the muscle membrane potential, and initiating an AP which propagates along muscle fiber
- the AP goes down the T-tubule membranes into the triad junction where it causes release of Ca ions that have been sequestered into the longitudinal sarcoplasmic reticulum
- increased Ca in sarcoplasm causes actin and myosin filaments to interact with each other resulting in a sliding motion that shortens the length of sarcomere
- calcium ions are pumped back into sarcoplasmic reticulum by Ca-ATPase ion pump, reducing concentration of calcium in the sarcoplasm and allowing muscle fiber to relax
- lengthening of muscle is achieved by contraction of an antagonistic muscle
10
Q
T-tubules
A
- the sarcoplasmic reticulum stores and releases calcium
- 2 cisternae + 1 T-tubule = a triad
- elaboration of the plasma membrane of the muscle fiber that penetrate the interior of the muscle fiber
- AP propagate along the plasma membrane and down into the T-tubule
- the voltage change at T-tubule membrane ultimately uncorks the SR calcium store and causes calcium release and myofibril contraction
- the T-tubule network and the SR ensure that the sarcomeres contract together within a muscle fiber
11
Q
Tropomyosin
A
- a alpha helical rod protein that covers the myosin binding site on the thin filament in resting mucle
- it blocks myosin from binding to the thin filament
12
Q
Troponin
A
- a complex of three protein subunits
- troponin C binds calcium
- troponin I inhibits actomyosin ATPase, binds actin
- troponin T- binds to tropomyosin
- troponin binds tropomyosin to the thin filament
- when troponin C binds calcium tropomyosin moves and uncovers the active sites on actin, thereby permitting interaction between actin and myosin head (globular domain)
- when calcium binds to troponin C the inhibition of tropomyosin is removed and force is then generated by the interaction of actin with myosin
13
Q
Cross bridge cycle
A
1) ATP binds to myosin, causing tilted cross bridge to separate from actin
2) Myosin ATPase cleaves ATP, but hydrolysis products ADP and phosphate remain bound to myosin. Cleavage of ATP causes cross bridge, still separated from actin, to change conformation to a perpendicular cocked orientation
3) A cross bridge forms as the myosin head binds to actin the myosin head then
4) releases of phosphate cause conformational change in cross bridge to tilted position, constituting the power stroke which drags the actin filament toward the M line
5) ADP is then released in the final step of the cycle
30% of energy stored in ATP is transformed into mechanical work
14
Q
Isometric twitch
A
- some length
- to measure the force generated during an isometric twitch, a muscle if fixed in position such that force can be measured without the muscle shortening
- muscle is stimulated electrically with a single action potential
- there is a latent period between the stimulation and contraction and this is the time delay between the AP and activation of crossbridge cycle
- the contraction by one AP is a twitch
15
Q
Fast and Slow Twitch Muscle
A
- a number of factors dictate the different kinetics between these isometric twitches including the type of myosin heavy chain expressed and the speed of sarcoplasmic calcium elevation and clearance
- eye needs to move quickly- fast twitch fibers - very rapid twitches
- soleus muscle (leg muscle) contains more slow twitch fibers and shows a much longer isometric twitch duration
