Lecture 24 10/18/23 Flashcards
What does it mean for cardiomyocytes to be excitable?
an electrochemical gradient is maintained across their cell membranes
What is the stimulation of cardiomyocytes?
momentary reversal of trans-membrane potential (an action potential)
What are the characteristics of the action potential?
-initiates excitation-reaction coupling
-stimulus for myocardial shortening
What determines membrane potential?
conductance of ions (Na+, K+, Ca2+)
What are the two electrophysiologically distinct populations of cells?
-specialized conduction cells
-working cardiomyocytes
What are the characteristics of specialized conduction cells?
-depolarize spontaneously
-exhibit automacity
What is the role of working cardiomyocytes?
responsible for contraction
When do working cardiomyocytes get excited?
when there is an action potential to stimulate them
How does the AV node give rise to perkinje fibers?
-AV node gives rise to common bundle/His
-common bundle splits into bundle branches
-bundle branches give rise to perkinje fibers
Which structures have cells that exhibit normal automaticity?
-SA node
-AV node
-His/purkinje system
What is the role of the fibrous skeleton?
to electrically isolate the atria and ventricles
Why is it important for the atria and ventricles to be electrically separated?
allows for the atria and ventricles to depolarize separately
What is unique about the AV bundle?
it penetrates the fibrous skeleton
What is the role of gap junctions?
rapid diffusion of ions between cells
What things contribute to the negative resting membrane potential of the cell?
-Na/K pump
-Donnan effect
-selective permeability of membrane to K, but not Na (K leaks)
What type of current is associated with potassium?
outward current
What are the two types of channels for potassium and their functions?
-voltage gated: repolarization
-inward rectifier: maintain resting membrane potential
What type of current is associated with sodium?
inward current
What are sodium channels responsible for?
depolarization
What type of current is associated with calcium?
inward current
What are the two types of Ca channels?
-L type “slow” channels
-T type channels
What happens during the 0 phase?
Na+ in and depolarization
What happens during the 2 phase?
Ca2+ in and plateau
What happens during the 3 phase?
K+ out and repolarization
When do the refractory periods occur?
during repolarization
What is an absolute refractory period?
when the cell cannot be re-stimulated
What is a relative refractory period?
when the cell can be re-stimulated only by a strong stimulus; causes a less robust AP
Which ion is associated with depolarization of atrial/ventricular cells?
sodium
Which ion is associated with depolarization of nodal cells?
calcium
Which type of cells do not spontaneously depolarize?
atrial/ventricular cells
Which type of cells do spontaneously depolarize?
nodal cells
Why is the SA node the dominant pacemaker?
most rapid inherent rate of discharge of the conduction system
What is overdrive suppression?
when depolarization occurs at a rate that exceeds inherent rate, spontaneous depolarization is inhibited
Why is overdrive suppression important?
it prevents competition of automatic tissues
What are the mechanisms of spontaneous depolarization?
-deactivation of K+ channels
-influx of Na+
-influx of Ca+
How does the sympathetic nervous system impact spontaneous depolarization and heart rate?
increases rate
How does the parasympathetic nervous system impact spontaneous depolarization and heart rate?
decreases rate
What are the characteristics of sympathetic cardiac innervation?
-diffuse distribution
-norepinephrine is main transmitter
What are the characteristics of parasympathetic cardiac innervation?
-vagal innervation of conduction system
-acetylcholine is main transmitter
Why is it important that APs in cardiac cells have a greater duration than in nerve cells?
prevents development of physiologic tetanus
