Cardiac Muscle 1

Question 1

SA node and AV node has unique ______________ such that there is no true resting potential

Answer 1

-Phase 4 depolarization

Question 2

What currents initiate phase 4 depolarization in pacemaker cells?

Answer 2

-Funny currents

When membrane is very negative, ion channels open that conduct slow, inward depolarizing Na+ currents

Question 3

Funny currents cause the membrane to spontaneously depolarize, thereby initiating phase 4. As the membrane potential reaches above -50 mV, _________ channels open.

Answer 3

-T-type Ca2+ channels

Question 4

What enforces the delay in E-C contraction in the heart?

Answer 4

-AV node

Question 5

2 ways heart meets necessity for synchronized contraction

Answer 5

1. efficient pulse conduction network

2. Gap junctions

Question 6

What is the purpose of the long plateau phase in ventricular and atrial APs?

Answer 6

• Delayed repolarization produce prolonged period of absolute refractoriness to restimulation
• Prevents tetanic contraction

Question 7

What channel changes give rise to absolute and relative refractory periods?

Answer 7

Absolute: Na+ channel inactivation
Relative: Na+ channels mostly unblocked, but K+ channels still remain open

Question 8

Cardiac E-C coupling is a process of _____________

Answer 8

Calcium induced calcium release

Question 9

Starling’s Law of the heart

Answer 9

• 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

Question 10

compared to skeletal muscle, cardiac has a ______ resistance to passive force.

Answer 10

Higher

Question 11

Relevant length-tension relationship for cardiac muscle has little or no __________.

Answer 11

Descending limb

-never decrease force within physiological stretching

Question 12

3 mechanisms contributing to Starling’s Law of the Heart

Answer 12

1. increasing # of possible cross bridges (increased filling stretches cardiac cells to a more favorable position on the length-tension curve)
2. calcium sensitivity of contraction is length dependent (dominant factor)
3. calcium release is length dependent

Question 13

Main mechanism underlying Starling’s Law

Answer 13

• increase in responsiveness of cardiac myofilaments to activating Ca2+ ions at longer sarcomere length
• commonly referred to as myofilament length-dependent activation

Question 14

Length-dependence of Ca2+ sensitivity

Answer 14

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

Question 15

2 ways cardiac cells can alter force responses to a given level of Ca2+ release

Answer 15

1. length-dependent calcium affinity of the troponin complex

2. Neuroendocrine modulation

Question 16

Increasing afterloads tends to restrain contractile performance and _________.

Answer 16

reduce contractile efficiency

Question 17

sources of series elastic elements cardiac muscles have (produce passive force)

Answer 17

• Titin

- Extracellular matrix collagen

Question 18

During initial muscle contraction, forces related to __________ must be overcome before there can be external shortening (work).

Answer 18

• 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

Question 19

Treppe- The force-frequency response

Answer 19

• 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

Question 20

Negative force frequency response

Answer 20

• Seen in failing hearts

- Fail to increase force generated with increasing frequency of stimuli

Question 21

With increased stimulation rate, increase Calcium flux occurs via L-type Ca2+ channel. If SR uptake kinetics > NCX extrusion kinetics, what occurs?

Answer 21

-increased contractility

Question 22

B1 stimulation of the sinus node _______ the heart rate. What is the effect of this on force?

Answer 22

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

Question 23

Net effect of b1 adrenergic stimulation

Answer 23

More beats, stronger beats, and faster relaxation

Question 24

Decrease in calcium sensitivity during Beta-Adrenergic stimulation

Answer 24

-P-lated form of troponin has faster rate of release of ca during relaxation, so relaxation occurs more rapidly

Question 25

Phospholamban role

Answer 25

• inhibits SERCA
• P-lation of it during B1 adrenergic stimulation relieves some of this inhibition so relaxation can occur more quickly
• also results in Ca2+ increasing in SR to increase the force developed