Skeletal muscle Flashcards
Structural hierarchy of skeletal muscle
Muscle-> fascicle-> Muscle cell/ fiber-> myofibril made of many sarcomeres
Sarcomere anatomy:
Z-line: where the thin filaments attach
I band: only thin filaments (actin , tropomysoin, troponin)
A band: thick and thin
H band: only thick
M line: structureal proteins to keep from buldging
H and I dissappear when contracted
Molecular mechanism of force generation
in skeletal and cardiac muscle, force generated by slinding filament mechanism (the force comes from ATP hydroysis on the myosin head) and active sites on the actin. in skeletal and cardiac the interaction of myosin cross briges with active sites is initiated by an increase in Ca intracellular concentraion and regulated by the troponin and tropomyosin
Myosin structure (thick filament)
a large protein with a pair of large heavy chains and two light chains, heavy chains have wound alpha helix, and the N terminus makes the large head and is able to hydrolyze ATP, two light chains are associated with the large globular head (one of the light chains is essential for ATPase activity) regulatory light chains serve the purpose of smooth muscle contraction
Thin filaments
consists of two intertwined strains of G-actin monomers, with tropomyosin and troponin, F actin contains the active sites for myosin. Tropomyosin is a filamentous protein, theres 2 for every 7 actin. Troponin is globular with 3 subunits a pair of troponins is found at every 7 actin monomers, when Ca binds to the troponin, it causes tropomyosin molecules to shift, exposing the active sites
How does myosin move
mysin walks along the actin.
- ATP binds to myosin heavy chain and cocks the myosin 2. (light chain converts to ADP and PPi and puts it on the next active site
- when Pi leaves myosin-> power stroke
- Adp releases and the Myosin is in the second actin active site
Role of ATP in contraction of striated muscle
4 roles:
- source of energy that is converted to mechanical energy
- release of myosin from actin (relaxation)
- SERCA (ATPase) ca storage in the endoplasmic reticulum
- maintanence of the normal cell membrane potential (NA/K ATPase)
Neuromuscular Transmission and excitation- contraction coupling in skeletal muscle
- Ach activates ionotropin (nicotinic) receptors on the end plate-> AP that spreads to the T tubule
- T tubule activation (of DHP) -> release of the Ca stored in the Sarcoplasmic reticulum (terminal cisternae) via
- Ca flows and binds to troponin
Sarcoplasmic reticulum
when the t tuble is activated the DHP receptor is activated (dihydropyrimidine) conformational change in DHP leads to the unplugging of the junctional foot protein connected to the Ryanodine receptor (unplugs the terminal cisternae of SR)
SERCA transports Ca back into the SR
inside the SR theres a protein called calsequesterin that binds to Ca to reduce the Electrochemical gradient
isometric vs isotonic contraction
isometric: muscle is fixed and cannot shorten (when weight is too heavy)
isotonic: muscel can shorten
the larger the weight, the slower the contraction. Maximum velocity is determined by the rate of ATP hydrolysis and bridge cycling. In skeletal muscle, Vmax is constant for a given muscle type but can change in cardiac and smooth muscle
Temporal summation, Quantal summation tetanus and fatigue
Quantal summation (the number of stimuli/ time) Temporal summation (force of contraction due to more units being activated)
As stimulus frequency increases (twitch contractions summate b/c the muscle doesnt relax before the next contraction)- with high rates of stimulation-> tetanic contraction -> fatigue (in the muscle itself because AP frequency doesnt decrease, and Ach levels are not depleted)
Length tension relationship
The ability of the muscle to develop active contractile force is determined by the degree of overlap of thick and thin filaments in individual sarcomeres
At the whole muscle level: amount of active force in one contraction= passive Elastic elements of the muscle+ active force of muscle cells
Lo= optimal length for contraction (maximum active force length of muscle)
Le= equilibrium length (no passive force, tendons cut) sarcomere is very short so you cant really add much tension
Force velocity relationship
when velocity is zero with a heavy load contractions are isometric
celovity of contraction at zero load= Vmax (rate of cross bridging and ATP hydrolysis
skeletal muscle= Vmax is constant
