Exam 2 Flashcards
Name the four theories which explain muscle contraction.
sliding-filament theory, excitation-contraction coupling theory, steric-block theory, and cross-bridge theory
Define the sliding filament theory.
Thin and thick myofilaments are fixed in length, but move in relation to each other
What is the result of the sliding filament theory?
Change in sarcomere length and therefore muscle shortening
What zones/bands of the sarcomere are effected in the sliding filament theory?
H zone gets narrower
I band becomes shorter
Does the A band change with the sliding filament theory?
No, the myosin length are constant
Define the excitation-contraction coupling theory.
Process by which the muscle membrane AP leads to the release of intracellular Ca++ from SR, thereby causing muscle contraction
What is considered the second messenger?
Ca++
Give the three basic steps to excitation-contraction coupling.
1) AP propagate along motor neuron to motor end plate
2) ACh binding at motor end plate
3) New AP is initiated on the muscle fiber plasma membrane
What specialized membranes are important to the excitation-contraction coupling theory?
T-tubules and Sarcoplasmic Reticulum
What two functions do the specialized membranes serve during the excitation-contraction coupling?
1) Take the AP from surface down into center of the cell
2) Allow contractile proteins to interact
What is the T tubule system?
Invaginations of surface membrane which transmit excitation signal from along the surface membrane down deep into the muscle fiber.
What is the SR and what channels/receptors are important to the excitation-contraction coupling?
The SR stores and releases Ca++ and has a DHP receptor on the T side of the terminal cisternae and the RYR on the SR side.
What type of channel is the DHPR?
Voltage dependent L type Ca++ channel
What three proteins play a major role in Ca++ binding?
Calmodulin binds Ca++ in the cytosol, Calsequestrin binds Ca++ in the SR, and TnC binds Ca++ on the troponin complex.
Define the Steric Block Theory.
The blocking of binding sites within the thin filament proteins which is required for muscle action.
What are the three subunits of Troponin and what is their function?
TnC: Ca++ binding site
TnI: Helps position Tropomyosin on Actin
TnT: Loose attachment to Tropomyosin blocking binding site
What kind of change does Ca++ binding to TnC cause?
conformational change
Describe tropomyosin structure.
Tubular regulatory protein on actin which blocks myosin binding site
What is the result of Ca++ uptake?
Muscle relaxation
What drives the unbinding of Ca++ and TnC?
SR sequestering Ca++
How long does force generation continue for?
As long as neural impulses are arriving at the NMJ and Ca++ concentrations remain high in the myofilament region
What happens to Ca++ and force when the neural impulse stops?
Reduced Ca++ and reduced force
Define the cross-bridge theory.
Movement of the thin filament across the thick filament via myosin and actin binding
What area of the thick filament is the cross-bridge formed from?
The heavy meromyosin (HMM) of myosin heavy chain (MHC)