KIN 305 Mdtm 1 Study Questions
1. Explain the ionic basis of the resting membrane potential of a cardiac myocyte.
• gK>>gNa>gCa at rest in a semipermeable cell membrane
• K concentration is very high intracellular, Na concentration very high extracellular
• Ek = -95 according to Nernst equation (if only K+ was present and pK = 1, fully permeable
membrane) K+ will exit cell, and make it more negative inside
• Ena = +70 (if only Na+ was present and pK = 1, fully permeable membrane) Na+ will enter cell
and make it more positive inside
• Chord conductance equation: shows that the resting membrane potential will be dominated by
the greatest conductance (gK) at rest.
• Hence, in a cardiac myocyte, the resting Vm is -90, closest to gK because K+ will exit the cell
most and make it very negative
• Other ions affect the resting Vm, such as Cl-, and Ca++.
• Na/K pump also maintains the electrochemical gradient by taking out 3 Na+ for 2K+ (makes
inside cell more negative due to the trade off)
2. Differentiate between the absolute and relative refractory period of a cardiac ventricular
• Absolute/Effective Refractory period:
o H-gate channels are closed
o There cannot be any new AP when H-gates are closed b/c it is impossible for Na+
influx through the fast voltage gated Na+ channels
• Relative Refractory period:
o H-gates and M-gates have begun to reset (M-gates closed, H-gates open)
o A stronger stimulation would be needed for a new AP
o The AP will be weaker
o The longer you wait through phase 3, the stronger the new AP will be
o Once phase 4 is reached, you can achieve a full new AP
3. Why does the speed of the AP conduction differ between the AV node and purkinje cells?
Why is this physiologically significant?
• Teliological: Heart needs time for atrial systole to complete contraction before ventricles can
begin contraction otherwise we will have heart arrhythmia
• Physiological reason:
o Lesser diameter in AV node = greater resistance = lesser field effect = slower
o Lack of fast Na channels in AV node, only slower Funny current channels
o Less negative resting Vm produces a weaker AP that has lesser field effect and slower
o Lower density of gap junctions which aid in conduction of APs, so slower conduction 4. Describe the ionic events that occur during Phase 2 of the cardiac ventricular myocyte AP.
Explain their physiological significance.
• Physiological significance: Ca++ needs to be brought into the cell to activate CICR from SR
that will increase [Ca++]i and activate muscle contraction mechanism
• Vm remains fairly constant
• Ik1 is not very significant because of inward rectifying. Its role is reduced at 0mV and more
• Ito, and Ik are actively causing K+ efflux down conc’n gradient and electrical gradient
• Ca++ influx through L-type channel occurring to offset the K+ channels. Ca++ flowing down
conc’n gradient, but against electrical gradient
5. How do pacemaker cells (such as those of the SA node) trigger their own depolarization?
• During phase 4, Funny Current channels activate at -55mV
• This is self activating from its own K+ outward repolarizing currents during phase 4
1. Draw and label a “normal” (Lead II) ECG tracing. Explain why the QRS complex and the T-
wave both produce positive deflections in the ECG.
2. Describe what the electrical vector of Einthoven’s Triangle represents. Describe two situations
that cause this vector to shift.
• Electrical vector is downwards and to the left of the heart. This is the direction the
depolarization is traveling
• Ventricle Hypertrophy and Systemic hypertension causes left axis shift
• Heart may be displaced
3. In the absence of extracellular calcium, explain what happens to cardiac contraction. How (and
why) does this effect differ from skeletal muscle contraction?
• Luminar Ca++ can bind to L-site of a RYR2 channel causing release of Ca++ into cell. Now
that Ca++ can bind to A-sites of RYR2 channels causing further Ca++ release into the cell for
• Skeletal muscle cannot be used in absence of extracellular Ca++ b/c it has no RYR2 channels
4. Draw a “normal” volume-pressure diagram for the left ventrical. Label all values and axes.
Indicate where the following are located or occur:
a. Opening and closing of the AV valve b.