Skeletal Muscle Contraction Flashcards
What is the structure of skeletal muscle?
Composed of cylindrical muscle fibers
Bundle of fibers - fascicle
Endomysium surrounds each fascicle
Where do the striations arise from?
Repeating pattern of light and dark bands due to arrangement of thick and thin filaments within myofibril
What do thin filaments contain?
Actin
Anchored to Z lines at each end of a sacromere
What do thick filaments contain?
Myosin
In A band at center of sacromere
Which band is the dark and light band?
A - dark band
I - light band
What is the mechanism of contraction?
Thin filaments propelled toward center of their sarcomere by movements of myosin cross bridges that bind to actin
What do the 2 globular heads of each cross bridge contain?
Binding site for actin and enzymatic site that splits ATP
What are the 4 phases of the cross bridge cycle?
- Cross-bridge formation; release of Pi
- Power stroke, ADP released, myosin undergoes conformational change
- ATP binds myosin, detachment of myosin from actin, cross-bridge dissociates
- ATP hydrolysis occurs, cocking myosin head
What is the function of tropomyosin?
Tropomyosin protein coils around Actin helically and attaches via Troponin complex
Ca2+ is trigger
What is the function of the Ca2+ ions?
Bind to troponin, producing a change in shape transmitted via tropomyosin to uncover binding sites on actin, allowing cross bridges to bind to thin filaments
How is the action potential propagated?
Along t tubules to sarcoplasmic reticulum where it releases Ca2+
What is relaxation caused by?
Active transport of cytosolic calcium ions back into SR
What is an adaption to t tubules?
Presence of continuous invaginations along SR allows for effective contraction
What is the structure of the triad junction?
Central t-tubular element flanked on either side by a terminal cisternae
Occurs at every Z line
How does depolarization occur?
Voltage dependent activation and conformational change of subunits of the L-type Ca2+ channel or DHPR
What are L-type voltage gated Ca2+ channels linked to?
RyR receptors
What is the role of DHPR?
Voltage sensor
Transmits signal to RyR four subunits
What is the role of the alpha 1 subunits?
Voltage sensing and pore forming subunit
What is the role of the beta 1a subunit?
Indispensible for EC coupling
Surface expression of DHPRs and tetrad formation
What are the 4 main proteins responsible for Ca2+ cycling and storage within SR?
DHPR, RyR1, SERCA pump and calsequestrin
What is calsequestrin?
High capacity Ca2+ binding protein found in terminal cisternae of SR
What is the Ryanodine receptor?
Ligand gated channel and forms a homotetramer
Largest known ion channel
Highly clustered
What does the N-terminal domain of RyR receptors contain?
Binding sites
What does the C-terminal domain of RyR receptors contain?
Transmembrane domains
Ion-conducting pore
What are the junctional feet of the SR?
Possibly communicate between surface and internal membranes
4 DHPRs link to a subunit of an underlying foot
