Cardiac Muscle 1 Flashcards
SA node and AV node has unique ______________ such that there is no true resting potential
-Phase 4 depolarization
What currents initiate phase 4 depolarization in pacemaker cells?
-Funny currents
When membrane is very negative, ion channels open that conduct slow, inward depolarizing Na+ currents
Funny currents cause the membrane to spontaneously depolarize, thereby initiating phase 4. As the membrane potential reaches above -50 mV, _________ channels open.
-T-type Ca2+ channels
What enforces the delay in E-C contraction in the heart?
-AV node
2 ways heart meets necessity for synchronized contraction
- efficient pulse conduction network
2. Gap junctions
What is the purpose of the long plateau phase in ventricular and atrial APs?
- Delayed repolarization produce prolonged period of absolute refractoriness to restimulation
- Prevents tetanic contraction
What channel changes give rise to absolute and relative refractory periods?
Absolute: Na+ channel inactivation
Relative: Na+ channels mostly unblocked, but K+ channels still remain open
Cardiac E-C coupling is a process of _____________
Calcium induced calcium release
Starling’s Law of the heart
- increasing venous return to the heart stretches the ventricle, which in turn results in more forceful ejection of blood at the very next heartbeat
- rise of stroke volume with rise in venous pressure
- larger filling, larger end diastolic volume, larger stroke volume and more the heart ejects
compared to skeletal muscle, cardiac has a ______ resistance to passive force.
Higher
Relevant length-tension relationship for cardiac muscle has little or no __________.
Descending limb
-never decrease force within physiological stretching
3 mechanisms contributing to Starling’s Law of the Heart
- increasing # of possible cross bridges (increased filling stretches cardiac cells to a more favorable position on the length-tension curve)
- calcium sensitivity of contraction is length dependent (dominant factor)
- calcium release is length dependent
Main mechanism underlying Starling’s Law
- increase in responsiveness of cardiac myofilaments to activating Ca2+ ions at longer sarcomere length
- commonly referred to as myofilament length-dependent activation
Length-dependence of Ca2+ sensitivity
relation between intracellular calcium and isometric force at various sarcomere lengths
-as sarcomere length is increased, greater force is generated at same level of calcium compared to shorter sarcomeres
2 ways cardiac cells can alter force responses to a given level of Ca2+ release
- length-dependent calcium affinity of the troponin complex
2. Neuroendocrine modulation
Increasing afterloads tends to restrain contractile performance and _________.
reduce contractile efficiency
sources of series elastic elements cardiac muscles have (produce passive force)
- Titin
- Extracellular matrix collagen
During initial muscle contraction, forces related to __________ must be overcome before there can be external shortening (work).
- Series elastic elements
- force is being generates in early part of contraction, despite there being no external shortening. Then the series elastic element is overcome, there is progressive shortening, but no further increase in the force being generated
Treppe- The force-frequency response
- cardiac force is a function of calcium concentration and thus stimulation frequency
- as rate of stimulation is increased, the tension increases to a new steady state level. tension goes down again when stimulus rate slows
- due to increasing the frequency, increases the amount of calcium stored and available to be released at next stimulus and thus produce more force
Negative force frequency response
- Seen in failing hearts
- Fail to increase force generated with increasing frequency of stimuli
With increased stimulation rate, increase Calcium flux occurs via L-type Ca2+ channel. If SR uptake kinetics > NCX extrusion kinetics, what occurs?
-increased contractility
B1 stimulation of the sinus node _______ the heart rate. What is the effect of this on force?
increases heart rate
- increases contractility via the force-frequency relationships
- phosphorylation of L-type Ca channel, phospholamban, and RyR all favor larger Ca2+ transients and increases force generation
- Phosphorylation of phospholamban and troponin I and troponin T favor faster relaxation needed for faster heart rate
Net effect of b1 adrenergic stimulation
More beats, stronger beats, and faster relaxation
Decrease in calcium sensitivity during Beta-Adrenergic stimulation
-P-lated form of troponin has faster rate of release of ca during relaxation, so relaxation occurs more rapidly
