Lecture 12 Flashcards
Describe a typical action potential in a ventricular muscle
Describe how ionic currents contribute to the four phases of the cardiac action potential.
Use this information to explain differences in shapes of the action potentials of different cardiac cells.
Ventricular Muscle cell:
• Rapid depolarization:
(Phase 0)
-Ca2+ and Na+ responsible for depolarization
-Threshold reached, Opening of voltage-gated Na+ channels = Na+ entering the cell = + inside cell
• Initial Rapid Repolarization:
(Phase 1)
-K+ channel opens –> K+ rushes out of cell = decrease AP, then channel closes rapidly
• Plateau phase: (Phase 2) -Inward flow of Ca2+ -K+ leaving -Slows down the repolarization due to the outward K+ current
• Rapid Repolarization
(Phase 3)
-Eventually Na+ inactivation occurs/channels close, triggering opening of voltage gated K+ channels (repolarization)
• Resting:
-K+ responsible for resting potential
Describe the ion channels that contribute to each phase of the cardiac action potential.
How do differences in channel population influence the shape of the action potential in the nodal, atrial muscle, ventricular muscle, and Purkinje fiber
cardiac cells?
1) Sodium:
- Fast Na+ (INa)
- Phase 0 of myocytes
2) Calcium:
- L-type (ICa)
- phase 2 of myocytes
- phases 4 and 0 of SA and AV nodal cells
3) Potassium:
- Transient outward (Ito)
- Phase 1 in myocytes
- Delayed rectifier (IKr)
- Phase 3 repolarization
Explain what accounts for the long duration of the cardiac action potential and the resultant long refractory period.
What is the advantage of the long plateau of the cardiac action potential and the long refractory period?
Cardiac AP and Long refractory Period:
This sustained sodium channel inactivation, combined with activation of calcium channels
and the delay in opening of potassium channels, accounts for the long plateau phase andthe long cardiac refractory period, which lasts until the end of phase 3 (repolarization)
Advantage:
-No tetanus –> Heart cannot fill if in continuous and max contracted state
Beginning in the SA node, diagram the normal sequence of cardiac activation (depolarization)
1) AP generated in the SA node
2) AP spreads through atrial muscle –> Av nodes
3) Av node delays AP
(Delays atrial and ventricular contraction)
4) AP goes from bundle of HIS –> Right and left branches
5) (Ventricle) purkinje fibers
Explain why the AV node is the only normal electrical pathway between the atria and the ventricles and
Explain the functional significance of the slow conduction through the AV node.
• AV node electrical pathway between atria and ventricle:
-Electrical impulses cannot pass Right atrium and right atrium directly, would results in pathologies
-Only way for impulses to pass through is through the bundle of HIS that comes from AV node
• Significance of slow conduction in AV node:
-Allows time for the atria to contract before ventricles contract –> Prevents pathologies
Describe a the following of a pacemaker cell
1) Location
2) Typical action potential in a pacemaker cell
3) Explain the ionic mechanism of pacemaker automaticity and rhythmicity
5) Factors that influence their rate.
Location:
-SA node, AV node (Right atrium), ventricular conduction system
Typical AP in pacemaker cell
Phase 4: Slow diastolic potential
-Na+ in, Ca2+ in
-Na channels unusually open when membrane is at rest/relatively polarized
Phase 0: Rapid depolarization
-40 mV reaches, L-type ca2+ channels open–> ca+, channels then close
Phase 1:
-Rapid repolarization
Phase 2: Plateau
Phase 3: Repolarization
- Ca2+ channels close
- K+ channels open
Phase 4: Pacemaker potential
-Na+ channels open SPONTANEOUSLY
Explain automaticity:
-Pacemakers = Cells able to generate an AP –> automaticity
(Happens in phase 4)
Determinants of rate:
-Pacemaker cells are driven by the higher rate of the SA = Overdrive suppression
Contrast the sympathetic and parasympathetic nervous system influence on heart rate and cardiac excitation in general.
Identify which arm of the autonomic nervous system is dominant at rest and during exercise
- Symp and Parasym on HR:
1) Sym –> Increases - Norepinephrine increases depolarization = prolong phase 4
2) Para –> decreases
- ACh causes a slower rise of the pacemaker potential = decrease HR
- Sym and Parasym on cardiac excitation:
1) Sym - increases SA firing node
- Decreases vagal tone
- Increase conduction velocity
- Increase atrial intropy
2) Para
- Vagal tone dom
- Increases hyperpolarization
- Decreases slope phase 4 AP
- Decrease conduction velocity
- Decreases intropy
Dominance:
-At rest: (vagal tone dom)
Para
-Exercise:
Sym
ION CONCENTRATION INSIDE AND OUTSIDE OF RESTING MYOCYTE
ION INSIDE OUTSIDE mEq/L:
1) K+, 140 mEq/L, 4 mEq/L
2) Na+, 10 mEq/L, 142 mEq/L
3) Ca++, 0.0001 mEq/L, 2.4 mEq/L
4) Cl-, 4 mEq/L, 103 mEq/L
What does the sodium-potassium pump do?
- Pumps 2 K+ in
- Pumps 3 Na+ out
This establishes gradient at resting
Potassium leak channel
- Always open
- Allows K+ to leak out of the cell
- Helps establish diffusion potential
Describe how Diffusion Potential occurs
At resting K+ will leak out of pores of the cell, but - ions cannot do that
–> generating diffusion potential = electronegativity inside the cell (Bc K+ is positive)
- Eventually K+ will be stopped from leaving the cell
- Diffusion Potential across the membrane exactly opposes the net diffusion of a particular ion through the membrane is called: NERST POTENTIAL for that ion
What’s the order and speed of contraction in the heart?
Order:
1) Atria = ventricle fills
2) Ventricle = contracts septum then apex at base
Speed:
-Slower conduction is in the AV node
What’s the resting potential of the cardiacmyocyte? What’s it determined by?
-90mV
Determinants: 1) [ ] + and - ions 2) permeability of the cell membrane to these IONS 3) Ionic pumps that transport IONS across the membrane
