Propagation of the Cardiac Action Potential

Question 1

describe the events that allow and AP stimulus to move along the length of a cardiac myofiber

Answer 1

1. depolarizing stimulus arrives
2. Na+ ions (or Ca2+ in SA and AV nodal cells) at border zone flow toward negatively-charged adjacent membrane, creating a local electrical current that reduces membrane polarity (makes less negative to threshold)
3. at threshold (-70mV) voltage-gated Na+ channels open and adjacent membrane depolarizes as AP is generated
4. AP propagation continues down the length of the fiber as a wave of depolarization

Question 2

identify the major action potential difference that distinguishes rapidly and slowly conducting cells

Answer 2

rapid conductors (myocytes and purkinje fibers) have a fast response AP; rapid phase 0 depolarization because these cells are larger/broader and have more gap junctions;

slow conductors (SA and AV nodal cells) have a slow response AP/slower upstroke of phase 0 with a slow impulse conduction velocity because these cells are smaller in diameter and have fewer gap junctions

Question 3

if given a diagram of the heart be able to draw and label the normal specialized conduction system

Answer 3

be able to label where SA, AV nodes, bundle of His, L and R bundle branches, and purkinje fibers are

Question 4

list all steps comprising the normal sequence of cardiac activation (depolarization)

Answer 4

1. impulse originates in SA node
2. depolarization of atrial syncytium
3. depolarization of AV node; delays transmission of impulse from atria to ventricles with long refractory period
4. rapid conduction down the specialized ventricular conduction (His-purkinje system)

Question 5

describe the His-purkinje system generally

Answer 5

an extensive subendocardial network of rapidly conducting cells (fibers not nerves!) on the inner surface of the ventricles that provides efficiency for synchronous ventricular contraction; once signal reaches ends of purkinje fibers it travels cell-by-cell in an endocardial to epicardial direction through the myocardium

Question 6

describe the divisions of the His-purkinje system

Answer 6

1. Bundle of His/His bundle
2. left and right bundle branches
3. left anterior and left posterior fascicles of the left bundle branch
4. purkinje fibers: terminal fibers of the His-purkinje system that penetrate the inner 1/3 of the myocardium in dogs, cats, and humans, or entire myocardium in horses

Question 7

order the structures of the heart with reference to their relative speed of conduction

Answer 7

1. purkinje fibers are fastest conductors
2. atrial and ventricular myocytes are second fastest
3. AV node is slow
4. SA node is slowest conductor