DNURS 827 Week 2 Resting membrane potential / Excitation-Contraction Coupling / Electrical activity of the heart

Question 1

Resting Membrane Potential

Question 2

1.) Relative to the outside of a cell, is the inside positive or negative in charge?

Answer 2

• Negative
• The diffusion carries positive charges to the outside but leaves the nondiffusible negative anions
  on the inside, thus creating electronegativity on the inside

Question 3

2.) Is potassium higher in concentration inside or outside of a cardiac muscle cell?

Answer 3

• Inside

Question 4

3.) Is sodium higher in concentration inside or outside of a cell?

Answer 4

• High concentration of sodium ions outside the membrane and a low concentration of sodium ions on the inside.

Question 5

4.)What prevents potassium from leaking out of a cell until the concentration is the same on the outside and inside of the cell?

Answer 5

• Electrostatic force
• As K+ leaves cell, negativity increases on the inside of the cell membrane and electrostatically
  attracts K+. This electrostatic force prevents K+ from leaving the cell.

Question 6

5.) What does the Nernst Equation calculate?

Answer 6

• Equilibrium potential = The membrane voltage that perfectly balances an ion’s tendency to diffuse down its concentration gradient
• Chemical force = electrostatic force
• NO net movement of K+= equilibrium

Question 7

Resting Membrane Potential

Answer 7

The Stable electrical potential across the cell membrane caused by ions, predominantly potassium, flowing down the electrochemical gradient that was established by the sodium potassium pump

Question 8

6.) What ion makes the major contribution to the resting membrane of the cardiac muscle cell?

Answer 8

• Potassium
• Potassium is the major determinant of the resting membrane potential
• potassium and sodium ion channels allow leakage of these ions across cell membranes

Question 9

7.) What ion makes a small contribution to the resting membrane potential?

Answer 9

Chloride

Question 10

8.) What ion pump returns ion concentrations back to baseline?

Answer 10

• Na+, K+- ATPase
• sodium-potassium pump
• Metabolic pump which extrudes Na+ from the cells and pumps in K+
• Pumps in 3 sodium to 2 potassium ratio
• Na-Ca Pump
• Electrogenic pump
• partially inhibited by digitalis

Question 11

9.) How does the Na+-K+ - ATPase pump contribute to the resting membrane potential?

Answer 11

• the pumping of more sodium ions to the outside than the potassium ions being pumped to the inside causes continual loss of positive charges from the inside of the membrane creating an additional degree of negativity (about -4 mV) on the inside beyond that which can be accounted for by diffusion alone.

Digoxin reduces effectiveness of pump

Question 12

Action Potential

Answer 12

A mechanism by which excitable cells (neurons, myocytes) can rapidly transmit electrical signals along their cell membranes and to adjacent cells

Consists of a stereotypical change in membrane potential that’s caused by a sequence of ion channels opening and closing

Question 13

1.) What ion moves rapidly into a cell during depolarization?

Answer 13

Sodium

Question 14

2.) What ion exits the cell to restore the baseline electrical charge in a cell during repolarization?

Answer 14

Potassium

Question 15

3.) What restores ion concentrations back to their baseline levels?

Answer 15

The decrease in sodium entry to the cell and the simultaneous increase in potassium exit from the cell combine to speed the repolarization process leading to full recovery of the resting membrane
potential.

Sodium Potassium Pump

Question 16

Electrical Activity of the Heart

Question 17

1.) In what part of the heart are Fast-Response Action Potentials (non-pacemaker action potentials) found?

Answer 17

• Atrial myocardial fibers
• ventricular myocardial fibers
• purkinjefibers

Question 18

2.) In what part of the heart are Slow-Response Action Potentials (pacemaker action potentials) typically found?

Answer 18

• SA and AV node

Question 19

3.) What are some of the differences between Non-pacemaker and Pacemaker action
potentials?

Answer 19

• Resting membrane potential: slow> fast
• slope of upstroke: fast> slow
• Amplitude of action potential: fast >slow
• Overshoot of action potential: fast>slow

Calcium = depolarization in slow
no phase 1 or 2 in slow

Question 20

4.) What are the 5 phases of the Non-pacemaker (Fast-Response) action potential?

Answer 20

• Phase 0: depolarization- sodium in
• Phase 1: partial repolarization- Potassium out
• Phase 2: plateau- calcium in, potassium out- contraction
• Phase 3: repolarization- potassium out
• Phase 4: resting membrane potential- diastole

Question 21

5.) Which ions contribute to the various phases?

Answer 21

• Phase 0: sodium in
• Phase 1: potassium out
• Phase 2: calcium in, potassium out- calcium ions that enter during this plateau phase active the
  muscle contractile process: CICR
• Phase 3: potassium out
• phase 4: sodium-potassium pump, Na+- Ca2+ exchanger, ATP driven Ca++ pump (SERCA)

Question 22

6.) What is another name for Phase 2 in a Non-pacemaker cell?

Answer 22

Contraction, plateau

Question 23

7.) Release of a large amount of calcium from the Sarcoplasmic Reticulum is triggered by the entry of which ion?

Answer 23

Calcium

Question 24

8.) What is the function of the Sarcoplasmic Reticulum within a cardiac muscle cell?

Answer 24

Membrane-bound structure in muscle cells that store and regulate calcium movement

Regulatesintracellular Calcium

Question 25

9.) Which ion is the major determinant of the Resting Membrane Potential in Cardiac cells?

Answer 25

Potassium

Question 26

10.) What are the various phases found in Pacemaker (Slow Response) Action Potentials?

Answer 26

• Phase 4: due to inward sodium current (funny current) (spontaneous depolarization)
• Phase 0: inward calcium current after threshold is reached (depolarization)
• Phase 3: repolarization due to outward potassium current (repolarization)
• The process by which pacemaker cells in the SA node spontaneously depolarize is automaticity

Question 27

11.) Which ions contribute to the various phases in the Pacemaker Action Potential?

Answer 27

• Phase 4: sodium (funny current)
• Phase 0: inward calcium
• Phase 3: outward potassium

Question 28

12.) What are the different refractory periods associated with the cardiac action potentials?

Answer 28

• Absolute refractory period/Effective refractory period- no subsequent action potential can occur. Largely due to the inactivated state of the sodium channel. (0, 1, 2, 3, early phase 4)
• Relative refractory period-
• Supranormal period- the cell is more excitable than normal - it requires less of an electrical (slow, 0, 2, early 3)

Question 29

13.) What is automaticity?

Answer 29

• The process by which pacemaker cells in the SA node spontaneously depolarize is automaticity
• Ability of focal area of the heart to generate pacemaking stimuli.

Question 30

14.) What is diastolic depolarization?

Answer 30

• Inward Na+ (via funny sodium channels)
• Ca2+ influx
• K+ efflux (opposes effects of other ions

Question 31

15.) What effects on aspects of diastolic depolarization will cause changes in heart rate?

Answer 31

• Vagal stimulation releases ACh at the SA node decreases the slope of phase 4 by inhibiting funny currents thereby causing the pacemaker potential to take longer to reach threshold and slowing HR (negative inotropy)
• The steeper the slope of phase 4, the faster the threshold potential will be reached, and the faster the HR
• Neurotransmitters, acetylcholine, succinylcholine, T1-T4 cardiac accelerators

Hypoxia
Bradycardia

Question 32

16.) Which pacemaker region of the heart is typically dominant?

Answer 32

SA Node

Question 33

17.) What is Overdrive Suppression?

Answer 33

• Sinus node controls beat of heart bc it discharge is faster than any other part of the heart.

Question 34

Excitation-Contraction Coupling:

Question 35

1.) What is a sarcomere?

Answer 35

• defined as the region of the myofilament structure between two Z-lines.
• Repeating micro-anatomical units- the basic contractile unit of the myocyte
• Structural unit of striated muscle tissue between two Z-lines

The sarcomere is defined as the region of myofilament structures between two Z-lines. The sarcomere is composed of thick and thin filaments – myosin and actin, respectively. Chemical and physical interactions between the actin and myosin cause the sarcomere length to shorten, and therefore the myocyte to contract during the process of excitation-contraction coupling.

Question 36

2.) Cardiac cells are arranged in a branching network that is known as what?

Answer 36

Synctium

Question 37

3.) What are T-tubules in a cardiac muscle cell?

Answer 37

a. The T tubules are small and run transverse to the myofibrils. They begin at the cell membrane and penetrate all the way from one side of the muscle fiber to the opposite side. Not shown in the figure is the fact that these tubules branch among themselves and form entire planes of T tubules interlacing among all the separate myofibrils.

b. Also, where the T tubules originate from the cell membrane, they are open to the exterior of the muscle fiber. Therefore, they communicate with the extracellular fluid surrounding the muscle fiber and contain extracellular fluid in their lumens.

c. When an action potential spreads over a muscle fiber membrane, a potential change also spreads along the T tubules to the deep interior of the muscle fiber. The electrical currents surrounding these T tubules then elicit the muscle contraction.

• internal extensions of the cell membrane that penetrate into the center of cardiac (or skeletal muscle cells)
• Open to exterior of muscle fiber- can communicate with the extracellular fluid surrounding the muscle fiber

Question 38

4.) What is the Sarcoplasmic Reticulum?

Answer 38

• Membrane-bound structure in muscle cells that store and regulate calcium movement.

Question 39

5.) What is Excitation-Contraction coupling?

Answer 39

• Coupling between myocyte action potentials and contraction

Question 40

6.) What is the trigger for the release of large amounts of calcium from the Sarcoplasmic Reticulum?

Answer 40

• Influx of Ca2+ from the interstitial fluid during the action potential triggers the release of Ca2+
  from the sarcoplasmic reticulum

Question 41

7.) What is this process called whereby large amounts of calcium are released from the Sarcoplasmic Reticulum into the cell?

Answer 41

• Calcium-Induced calcium release (CICR)

Question 42

9.) The presence of what ion allows binding between actin and myosin?

Answer 42

Calcium

Question 43

10.) What is the function of Troponin in the binding between actin and myosin?

Answer 43

• Inhibit actin and myosin from binding
• Calcium binds to troponin c- conformational change
• myosin heads bind to actin leading to cross-bridge movement and shortening of the sarcomere
• Calcium goes back into the SR
• Calcium is removed from troponin-c, and myosin unbinds from actin

Question 44

11.) What is the Sliding Filament Model ?

Answer 44

• Mechanism of muscle contraction
• myosin (thick) filaments slide past actin (thin) filaments during muscle contraction

Question 45

12.) Is ATP required for cardiac muscle relaxation?

Answer 45

Yes