Initiation of the heart beat Flashcards
How does the duration of a neuronal action potential differ with cardiac
Neuronal-500microsecond-1msec
Cardiac-200-400ms
What is different about the cardiac refractory period
very long action pot->very long refractory potential but rather short relative refractory period
what is the duration of cardiac action potential roughly the same as
QT interval of ECG
What happens to the action pot duration as the rate increases
decrases to fit in more beats per sec
why is the cardiac action pot so long
to protect from tetany hence long refractory period
skeletal muscle can contract with temporal summation, fused and unfused tetanic contraction
sketch action pot for atria, ventricles, SAN, AVN. How do they differ
ref. notes. SAN and AVN show diastolic depolarisation. No stable resting memb pot-can depolarise between beats giving pacemaker function
Does the SAN have the highest or lowest intrinsic rate
highest
What is the membrane clock theory
repetitive pacemaker is generated by ion channels in membrane
What is the calcium clock theory
cyclical release of Ca from intracellular stores drive the memb pot up and down in diastole and hence regulates the pacemaker
When does the funny current occur, what is it carried by
inward current activated when the memb pot gets more negative
carried by Na and K
what is the funny current: stimulated by, inhibited by, blocked by
stimulates: adrenaline
inhibits: acetylcholine
blocked: ivabradine
Why is the conduction through AVN slow
1) allows ventricular filling to occur before electrical impulse is passed to the ventricle
2) also prevents transmission of high rates from the atria
Why is conduction through ventricular conduction system fast
allows apex to contract before the base
What do intercalated discs contain
gap junctions (connexons) form low resitance pathway coupling adjacent cells
PQRST what does it represent
P atria depolarisaton Q depolarisation of septum towards atria R depolarisation ventricle towards apex S depolarisation ventricles towards aria T repolarisation of the ventricles
What do the following represent and list e.g. pathologies: P-Q interval QRS duration Q-T interval S-T segment
P-Q interval: atrial condution and A-V nodal delay (AV block)
QRS duration: ventricular conduction velocity (bundle branch block)
Q-T interval: ventricular action potential duration (long QT syndrome)
S-T segment: heterogeneity of ventricular polarisation (myocardial infarction)
Why is the relative height of the ST seg important
because ST occurs once all ventricles depolarised, if parts of ventricles more or less depolarised, the segment becomes elevated or depressed
How does calcium induce cardiac muscle contraction
Calcium enters through voltage gated L-type Ca channel in t tubules and binds to Ryanodine receptor. Receptor opens and lets out lots of calcium to cytoplasm from SR. Calcium binds to muscle filament and induce contraction
How does cardiac muscle relax
active transport of Ca out of cell by SERCA. SERCA returns Ca released from SR back to SR. Small amount of Ca entering through L type calcium channels removed from cell by Na/Ca exchanger
What do the following mean: chronotropy, inotropy, lusitropy, positive, negative
chronotropy: heart rate
inotropy: strength of contraction
lusitropy: rate of relaxation
positive: increase
negative: decrease
What are positive chronotropic agents and what do they do
agents: sympathetic stimulation (adrenaline, noradrenaline)
increases funny current, faster rate of diastolic depolarisation, faster heart rate)
What are negative chronotropic agents and what do they do
Agent: parassympathetic (ACh)
decreases funny current, opens KACh channels, slower rate of diastolic depolarisation, slower heart rate
Draw how lines of positive inotropic and lusitropic will differ from control
ref. notes
Draw a flow chart of beta 1 adrenoreceptor stimulation
ref. notes
where does the PKA phosphorylation and cAMP act on in cardiac muscle cycle
L type Ca channels: increase channel opening for positive chronotropy, positive inotropy
Ryanodine receptor: increase SR Ca release (positive inotropy)
ATPase subunits: phodpholamban and phospholemman o increase SR Ca uptake and cellular Na extrusion (positive lusitropy)
Pacemaker: membrane and calcium clock (positive chronotropy)
myofilaments: troponin I and myosin binding protein C and increase rate of crosss bridge cycling (positive inotropy and lusitropy)
