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16 Cards in this Set

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  • Back
describe the DHP receptor in skeletal muscle
dihydropyridine R is a VOLTAGE SENSOR in the T tubule whose foot processes open a ryanodine receptor on the sarcoplasmic reticulum and releases Ca
describe the DHP receptor in cardiac muscle
a Ca influx channel, NOT a voltage sensor, but is voltage gated
what are the differences between cardiac and skeletal muscle
Ca release
diameter: cardiac has a smaller diameter allowing greater diffusion
SR: cardiac has a less extensive SR, ie dyad instead of triad
Ca release: Skeletal is ONLY Ca from the SR through ryanodine R, Cardiac is both Ca from the SR and Ca through the DHP receptor
calcium induced calcium release, in cardiac muscle an ap travels down the T tubule and opens voltage gated Ca channels (DHP R), Ca then diffuses across a short distance to the terminal cisternae of the SR and opens the ryanodine R
AP, Ca release, Ca re-uptake, and actual twitch force in skeletal muscle
First the AP spikes, then myoplasmic Ca spikes as Ryanodine R open, the spike rapidly decreases as Ca-ATPase's on the SR quickly eat up cytoplasmic Ca back into the SR (these ATPases are always on),

twitch force lags behind an ap, and also behind the myoplasmic Ca increase
twitch gradually increases as myoplasmic Ca increases and spikes while myoplasmic Ca is decreasing
AP, Ca release, Ca uptake, cardiac muscle
AP travels down T-tubules and causes DHP Ca channels to open allowing a small influx of Ca not enough to initiate contraction, But this Ca is enough to open Ryanodine R. Note: Ca that enters during the plateau phase of a cardiac AP is not enough to cause twitch either.
SR releases Ca and causes twitch
Ca uptake = Ca-ATPase's on the SR AND the sarcolemma. The sarcolemma also has Na/Ca 2o active exchanger to get Ca out of the cell
What happens if you increase the influx of Ca in cardiac muscles through the DHP R , say through an elongated plateau phase of an AP?
Ca will rush into the cell from the outside and some of it will be sequestered in the SR, this will increase the [Ca] in the SR. This doesnt happen in skeletal b/c skeletal only recycles Ca from within itself, doesnt get anything from the outside
Does skeletal, cardiac, or both respond to an increase in Ca as an all-or-none response
only skeletal, cardiac is graded

skeletal AP is all or none and looks the same no matter what

cardiac AP has a threshold, but you can prolong the plateau phase, this will prolong Ca release via DHP R, causing Ca to diffuse faster(faster rate of contraction) and more Ca will activate more ryanodine R(greater force of contraction)

in addition it will increase the amount of Ca taken up by the SR and available for subsequent contractions
describe the graded response in cardiac muscle
Cardiac muscle responds to an increase in Ca with a graded increase in force,
Also the rate of force production is graded - get a faster contraction with more Ca

low Ca = slow diffusion to ryanodine
High Ca = quicker diffusion, plus binds MORE ryanodine
describe extracellular Ca effects on skeletal muscle vs cardiac
skeletal muscle: not much

cardiac: profound effect, decrease in EC Ca will decrease influx during AP and increase efflux due to decrease [grad]
Sensitivity of contractile proteins
any event that prolongs the lifespan of the TnC conformation will facilitate contraction
motor unit
motor nn plus all the muscle cells it innervates
skeletal muscle motor unit
neural control, each muscle fiber is electrically and mechanically independent
graded force in skeletal muscle
3 mechanisms
1) frequency of nerve stimulation, more AP can produce more force by releasing more total calcium, repetitive stimulation at a f that doesnt allow relacation results in summation of twitch response. note the max freq at which a membrane can be excited is set by the duration of the refractory period

2) gradation by number of motor units

3)gradation by activation of larger motor units,

repetitive stimulation to achieve max activation of muscle, physiologically occurs rarely

skeletal and smooth muscle can undergo tetaus becaue the refractory period is over before peak force is reached

cardiac can't because electrical and mechanical events follow a similar time course
hennemen Size principle
motor neurons with the smallest cell bodies are the easiest to bring to threshold and will be recruited first. These will have the smallest # of muscle fibers per muscle unit. Larger motor units will be recruited later, and so the percent increase in force ensures a smooth gradation.