cardiac muscle mechanics Flashcards
do T tubules exist in cardiac muscle?
yes, they invaginate at the z-line
what is cardiac muscle called a “functional syncytium”?
because all the cardiac muscle cells are mechanically and electrically connected to one other- the entire tissue resembles 1 giant cell
why does cardiac muscle sarcoplasm have large numbers of mitrochondria and abundant reserves of myoglobin?
because the cells are dependent on aerobic metabolism to obtain energy needed to continue contracting
does this statement describe skeletal or cardiac muscle
“physical attachment between L type calcium channel and ryanodine receptors (the endfeet)”
skeletal muscle
how are the ryanodine receptors opened in cardiac muscle cells?
calcium induced calcium release
how long does a skeletal AP last? a cardiac AP?
skeletal- less than 5 msec
cardiac- around 250msec
can there be summation in cardiac muscle action potentials?
NO! the refractory period prevents it- heart contracts ONLY by twitch
what triggers phase 1 in a ventricle action potential?
closing of the Na+ channels
what 2 stages in ventricle action potential correspond to the absolute refractory period?
phase 1 & 2
how does phase 1 look on the graph?
slight dip (brief repolarization)
how does phase 2 look on the graph?
plateau phase
how do action potentials move between adjacent cells?
by means of charge displacement through gap junctions
what percentage of the necessary calcium for a twitch in a cardiac cell comes from outside the cell?
20%
what percentage of the necessary calcium for a twitch in a cardiac cell comes from the SR in the cell?
80%
how much external calcium contributes to skeletal muscle transients and twitch?
ZERO percent
what method of calcium extraction from the sarcoplasm contributes the greatest to calcium removal?
sarcoplasmic and endoplasmic retiuculm ATPase (SERCA)
about 80%
what percentage of sarcoplasmic calcium is removed via the sodium/calcium exchanger (NCX)?
15%
besides NCX and SERCA what is the last “major” way to get ride of calcium in the sarcoplasm?
PCMA-pump that removes calcium from the cell
5%
in cardiac muscle, where does calcium bind to initiate the crossbridge cycle?
troponin C
when is the twitch in a cardiac muscle terminated?
when there is calcium clearance from the sarcoplasm
in cardiac muscle is there recruitment?
NO
how long is the refractory period in an cardiac muscle cell action potential
300 msec
what are two ways tension can be affected in cardiac muscle cells?
- by altering the sarcoplasmic [ca]-by an inotropic agent
- altering calcium sensitivity of the myofilament
withOUT vagal stimulation, what is the heart rate triggered spontaneously by the SA node?
100 bpm
with vagal stimulation, what is the heart rate triggered spontaneously by the SA node?
60 bpm
at what sarcomere length is the maximal force generated in the human heart?
2.2 micrometers
an increase in initial length in cardiac muscle will do what to PO (maximum load)?
increase it
an increase in initial length in cardiac muscle will do what to velocity of shortening? amount of shortening?
increase both
an increase in initial length in cardiac muscle will do what to work the heart does? power delivered by the heart?
increase both
what is unique to cardiac muscle compared to skeletal muscle regarding muscle tension?
in cardiac muscle there is an additional mechanism that plays a role in tension besides just myosin/actin overlap
what results in greater sarcomere stretching?
greater ventricle filling during diastole
what is the result of greater sarcomere stretching?
increased troponin C affinity for calcium–>more crossbridge formation per AP–> greater force of contraction during systole
how do inotropic agents increase force of contraction?
by increasing the amount of calcium release from the SR
in the heart what type of receptor does norepi bind to?
beta 1 adrenergic receptor
when NE binds to a beta 1 adrenergic receptor what 4 things increase?
cAMP
PKA
calcium
tension
after the administration of norepi how many beats does it usually take for the heart to reach its new steady state?
8 beats
how does norepi shorten twitch duration?
by accelerating the SR calcium uptake pump and calcium removal from the sarcoplasm
what does an increase in contractility do to twitch duration?
it decreases it
what is the result of PKA phosphorylating phospholamban?
increases SR Ca++ pump sensitivity which increases the rate of Ca+ uptake into the SR and therefore decreasing duration of muscle contraction
what are the two results of positive inotropic effects of beta adrenergic stimulation?
increased magnitude of contraction
decreased twitch duration
what is the positive chronotropic effect of beta adrenergic stimulation?
increased HR
what are the two ways to increase sensitivity of contractile apparatus to calcium?
- use of certain drugs
2. increase initial fiber length (starling mechanism)
how would the curve be shifted in a tension vs calcium graph when NE is added?
the actual curve would not be shifted up but instead the muscle response would be shifted along the calcium tension curve
graph shown on page 369 of notes
would the curve be shifted in a tension vs length graph when NE is added? if so, which way?
YES the whole curve would be shifted up
graph shown on page 397 of notes
what is the effect on isotonic contraction of NE in the muscle?
isotonic contraction shortens twice the distance with the same load.
-the same contraction performed twice the work with NE
