Origin And Conduction Of Cardiac Impulses Flashcards
The heart can beat without stimuli what is this
Autorhytmicicty
What su the sino- atrial node
Cluster of specialised cells in Ra close to superior vena cava
Pacemaker cells initiate heart beat
Drives entire heart beat - sinus rhythm
How does cardiac excitation normally originate
Cells in sa node do not have stable mem potentials and exhibit spontaneous pacemaker potential which takes the me pot to threshold and gen an action potentail
What is the pottasium permeability of the pacemaker cells like
Does not remain stable between action potentials
What is the pacemaker potentail
Slow depo of mem potential to a threshold due to
Decrease in pottasium efflux
Slow sodium influx
What causes the falling phase of action potential (repolarisation)
Caused by activation of pottasium channels resulting in pottasium efflux
How does the Cardiac excitation normally spread across the heart
Sa node Cell - cell conduction Av node Bundle of his Left and right branches Purkinje fibres
What is cell-cell conduction
Gap junctions
What su the av node
Small bundle of specialised cells
Located at base Ra
ONLY point of electrical contact between atria and ventricles
The av node cells are smaller and has slower conduction velocity
What happens to conduction in the av node
Delayed which allows atrial systole contraction to proceed ventricular systole
What is the resting mem pot
-90
What sit the rising phase of action potential (depo) caused by
Fast sodium influx which causes mem pot to be +30
Phase 0
What are the phases of ventricular muscle action potential
Phase 0 - fast sodium influx
Phase 1 - closure of sodium channels and transient pottasium efflux (repo)
Phase 2 - mainly calcium influx makes plateau stay
Phase 3 - closure of calcium channels and pottasium efflux
Phase 4 - resting mem pot
What is the plateau phase of action potential
Mem pot maintained near peak for Ms
Unique characteristic of contractile cells cardiac
Due to calcium influx though calcium voltage hated channels
What causes the falling phase potential (repo)
Inactivation of calcium channels and activation of pottasium channels
Pottasium efflux
What Ns mainly influences the heart
Autonomic Ns
What different stimulations affect the heart
Symp increases heart rate
Parasymp decreases heart rate
What does the vagus nerve affect the heart
Parasymp
Exerts continuous influence on sa node under resting conditions
Vagal tone dominates resting conditions
Vagal tone slows intrinsic heart rate to normal rest rate approx 70 bpm
What are the conditions if the heart rate is over/under norm
Over 100 bpm - tachycardia fast
Under 60 bpm - Bradycardia
What does the vagus nerve stimulate in the Heart
The sa and av node
Vagal Stim slows heart rate and increases av nodal delay
Neurotransmitter - ACh Acting on M2 receps
What is a comp inhib of ACh
Atropine used in extreme bradycardia to speed up heart
What su the effect of vagal stimulation on pacemaker potentials
Cell hyper polarises Longer to reach threshold Slope of pacemaker potentials decreases Frequency of ap decreases Negative chronotrooic effect
What is the symp supply to the heart
Cardiac symp nerves Stim sa and av node and myocardium
Symp increases heart rate and decreases av nodal delay
Increases force of contraction
Neurotransmitter - noradrenaline acting through beta adenoreceps
What is the effect of noradrenaline on pacemaker cells
Slope of pace,aker portantisla increases
Pacemaker potentials reaches threshold quicker
Frequency of action potentials increases
Positive chronotrooic effect
