In what ways is the frog heart different from the human heart?
- 3-chambered: 2 atria, 1 ventricle
- Ventricle is trabeculated (grooved) inside
- Sinus venosus = primary pacemaker
- AV myocardium forms a funnel b/t atria and ventricle (similar to AV node)
- No specialized ventricular conduction system (no purkinje fibers)
- No Bundle of His
- Less developed SR
- No coronary circulation
Since a frog heart has a less developed SR, what is its primary source of calcium?
Extracellular sources
Since a frog heart has no coronary circulation, how it the heart oxygenated?
Direct diffusion
What is the function of the sinus venosus?
Channels oxygenated and deoxygenated blood
What is overdrive suppression?
When faster pacemaker cells suppress the activity of slower pacemaker cells
What is the effective refractory period?
An absolute refractory period in the nerves during which the ventricles can’t be activated
What is the relative refractory period?
Period of time during which an AP can occur but it takes longer or greater stimulation
What are the phases of an SA node AP?
- Phase 0: depolarization caused mostly by Ca2+ influx
- No plateau phase
- Phase 3: repolarization from K+ influx
- Phase 4: unstable resting potential due to opening of a non-specific cation channel
What are the phases of a ventricular AP?
- Phase 0: rapid depolarization due to influx of Na+ and to a lesser extent Ca2+
- Phase 1: rapid repolarization from inactivation of Na+ channels
- Phase 2: plateau caused by continued Ca2+ influx
- Phase 3: repolarization due to K+ efflux and Ca2+ channel inactivation (sets duration of AP)
- Phase 4: resting membrane potential
What are intercalated discs composed of? Function?
- Cardiac muscle cells are connected end-to-end by intercalated discs
- Contain:
- desmosomes: mechanically bind cells together w/ adherin proteins
- gap junctions: electrically couple cells together
What makes L-type Ca2+ channels in cardiac muscle different than skeletal muscle?
Not mechanically linked to RyR
Through what type of channel does an increase in Ca2+ in cardiac muscle occur? Location?
L-type Ca2+ channels on the T-tubules
What are the steps of the cardiac cycle? What steps make up systole? Diastole?
- Ventricular filling: AV valves open, semilunar valves close
- Isovolumetric contraction: all valves close –> increase in pressure but no change in volume
- Ventricular ejection: semilunar valves open, ejection of blood causes an increase in pressure and a decrease in volume
- Isovolumetric relaxation: all valves close, ventricles relax –> decrease in pressure but no change in volume
- Systole: phases 2 and 3 (contraction and emptying)
- Diastole: phases 4 and 1 (relaxation and filling)
Draw a Pressure-Volume Loop.
What is the Frank-Starling Law of the Heart?
Heart will contract w/ more force during systole if it’s filled to a greater extent during diastole
- increased preload = increased EDV = increased SV
What is extrasystole? Cause?
A premature ventricular contraction often caused by depolarization in the ventricle rather than at the SA node
What would cause an extrasystolic beat to be smaller than the beat preceding it? Larger?
- Smaller: Frank Sterling Law (reduced filling time)
- Larger: increased Ca2+ buildup
What is a compensatory pause? What happens to the beat following the pause?
- A skipped beat sometimes caused by extrasystole to resume the proper timing of the SA node
- Beats following the pause are usually larger in force than the previous beat
What does the vagus nerve send?
Parasympathetic efferents to the heart?
What does vagal stimulation cause in the heart?
Bradycardia
What does prolonged vagal stimulation cause?
Cardiac arrest
What is vagal escape?
Other pacemakers take over generating the HR at the next fastest pace when SA node fails
How does vagal stimulation affect spontaneous AP generation of the pacemakers?
Slows/halts spontaneous AP generation of SA node, but has less effect on the other pacemakers
What are the effects of epinephrine on the heart?
- Increased HR
- Decreased AP duration
- Increase contractility
- Increased SV
