Heart Phys Flashcards
major targets of cardiovascular drugs
voltage gated ion channels. anti-arrhythmic, anti-hypertensive, anti-angina.
three major channel conformations
resting (closed), activated (open), inactivated (closed). there is an inactivation gate as well as an activation gate.
phases 0-4 of the major active ionic current
0: depolarization
1: early repolarization
2: plateau
3: repolarization
4: resting (diastole)
slow response action potential
occurs in the nodes (SA and AV). only stages 0, 3, and 4. no plateau.
autonomic control of heart rate
Norepi and Epi increase heart rate (positive chronotropy)
ACh decreases heart rate (negative chronotropy)
threshold for excitation
amount of membrane depolarization required to produce an action potential
what does hypokalemia do to threshold?
hyperpolarizes, increases the threshold. this should decrease excitability, but gk decreases with low potassium causing increased excitability
what does hyperkalemia do to threshold?
depolarizes, decreases the threshold.
sodium current availability
resting membrane potential directly affects cardiac excitability. more depolarizing current required means increase in threshold, and vice versa.
resting membrane potential
can’t be determined by the Nernst equation. determined by the balance of inward Na+ and outward K+ ionic currents. resting myocardial membrane is 20 times more permeable to K than Na ions.
what happens in severe hyperkalemia?
depolarizes Vm, increases threshold through Ina inactivation, and decreases excitability
functional refractory period
minimum time duration after an action potential required for a threshold stimulus to produce a full response again
effective refractory period
no action potential may be elicited no matter how strong the stimulus
relative refractory period
higher than normal stimulus will elicit an action potential with reduced amplitude and duration
3 major determinants of conduction velocity
rate of phase 0 depolarization, resting membrane potential (more negative Vth = faster conduction), threshold potential (less negative Vth = slower conduction)
