Lecture 2 (Basic Structual And Electricl Characteristics Of Myocardial Cells)

Question 1

Ion fluctuations at rest

Answer 1

K+ channels open at negative potentials
Large negatively charges intracellular proteins
ATP dependent Na+/K+pump
-3 Na+/2K+
- produces a net outward movement of positive charge

Question 2

Phase 0

Answer 2

“Rapid Depolarization”
Opening of “fast type” voltage sensitive Na+ channels
Na+ channels are very abundant
Driving force behind the rapid influx of Na+
- negative RMP
Low intracellular Na+ concentrations

Question 3

Phase 1

Answer 3

Initial Repolarization
Na+ channels rapidly inactivate (channel kinetics)
Transient K+ outward current activated by depolarization
The prominence of the notch depends on the type of myocardium
- greatest in Purkinje fibers

Question 4

Phase 2

Answer 4

Plateau Phase
Ca++ channels begin opening when the membrane potential goes above -35mV
-L type Ca++ channels
- slow to open/close
- influx following concentration gradient
- Ca++ influx critical for subsequent contraction
K+ conductance is decreased from the resting membrane potential
Action potential duration I dependent upon Ca++ channel kinetics, rate of intracellular, sequestering of Ca++

Question 5

Phase 3

Answer 5

Final Repolarization
Ca++ channels slowly become inactivated
- Ca++ conductance decreases
K+ conductance increases (delayed rectifier channel)
- driving force is for K+ exit from the cell

Question 6

Phase 4

Answer 6

Resting membrane potential (RMP)
K+ conductance high, low Na+, Ca++ conductance
Na+/K+ pumps regulating internal Na+ levels
Na+/Ca++ exchanged regulating internal Ca++

Question 7

The unique characteristics of cardiac muscle (relative to skeletal muscle) that enable the whole heart to synchronously and spontaneously depolarize

Answer 7

Myocardial cells are electrically coupled
Long duration action potential with long refractory period associated with cycles of changing Na+, Ca++, and K+ permeability
Capacity of select cells to spontaneously generate action potential

Question 8

Summary of typical cardiac action potential

Answer 8

Excitation of the cell membrane (sarcolemma) results in a prolonged depolarization of the membrane and cycles of changing Na+, Ca++, and K+ permeability
Excitation is followed by a prolonged refractory period

Question 9

Typical cardiac myocyte has a resting membrane potential of

Answer 9

-90mV