Lecture 10: Cardiac Action Potential and Conduction Flashcards
what are the two types of cardiac muscle cells
contractile muscle fibers (99%) - do the mechanical pumping activity of the heart
autorhythmic cells (1%) organized into the conducting system of the heart
- don’t do physical work
- initiate and propagate APs that trigger contraction of contractile cells
why is the heart not innervated by motor neurons
b/c cardiomyocytes can contract autonomously
what is the sequence of excitation in the conducting system?
- APs generated in SA node pacemaker cells
- AP spreads across walls of both atria and into AV node
- AP spreads from AP node to apex of heart via Bundle of His and Bundle branches in the interventricular septum
- AP spreads via purkinje fibers to the contractile cells of the ventricle
- Ventricular contraction begins at the apex, squeezing blood towards aorta or pulmonary trunk
what prevents the ventricles and atria from contracting at the same time
- atrial cells are insulated from ventricular
- all currents to the ventricles have to pass through AV node and Bundle of His
- the spread of the depolarization is delayed at the AV node
what are the intrinsic properties of the AV node that cause it to have a delayed depolarization?
- small cell diameter of AV node cells
- slow response cardiac fibers with long refractory periods limit max rate at which atria can drive ventricles
- regulated by parasympathetic
why is the SA node the pacemaker
b/c it has the fastest rhythm
what assures contraction of ventricles begins at the apex
conducting cells in the interventricular septum are insulated from contractile cells
Purkinje fiber cells are electrically coupled to contractile cells
what is the purpose of the contraction of papillary muscle?
stiffens cordae tendinae, which prevents mitral valve prolapse (valves don’t collapse back into atrium)
what sets the heart rate
the slope of pacemaker potential in pacemaker cells sets the frequency of APs (and HR)
how are autorhythmic cells electrically coupled to each other (and to some contractile cells) ?
via gap junctions
what is the plateau in cardiac muscle AP
plateau corresponds to the refractory period
- during which the second muscle contraction cannot occur
what causes the long refractory period / plateau
the Ca+ influx that extends the depolarization and the delayed opening of the K+ channels
what signifies the end of the absolute refractory period
the opening of the k+ channels
why is the long refractory period important in cardiac muscle?
ensures time for the rhythmic pumping action
explain the effect of norepinephrine on contractile tissues
NE gets released at SA node and binds to B1-Adrenergic receptors, increasing cAMP
↑ cAMP –> ↑ activity of funny Na+ channels –> decreased hyperpolarization bw APs and and ↑ slope of pacemaker potentials (HR)
