Origin and Conduction of Cardiac Impulse Flashcards
Autorhythmicity
The can heart beat rhythmically without any external stimuli
Where does excitation of the heart normally originate?
The pacemaker cells of the SAN
Sinus rhythm
When the heart is controlled by the SAN
What generates regular spontaneous pacemaker potentials?
The cells in the SAN, which have no stable resting membrane potential;
What does the spontaneous pacemaker potential do?
Takes the membrane to a threshold, thus creating an action potential
This results in regular spontaneous action potentials in the SAN cells
Define pacemaker potential
Slow depolarisation of membrane potential to a threshold
what generates the pacemaker potential?
Decrease in K+ efflux
Na+ influx (If)
Transient Ca2+ influx (T-type Ca2+ channels)
Rising phase of pacemaker action potential
Depolarisation caused by activation of L-type Ca2+ channels once threshold is reached
Result of rising phase of pacemaker AP being reached
Ca2+ influx
Falling Phase of pacemaker Action Potential
Repolarisation, resulting in K+ efflux
what causes falling phase of pacemaker action potential?
Inactivation of L-type Ca2+ channels and activation of K+ channels
Spread of excitation across heart
SAN->AVN->Bundle of His->L+R branches of bundle->Purkinje fibres
How does cell to cell spread of excitation occur from SAN to AVN?
Via gap junctions
Where is the only point of electrical contact between atria and ventricles?
AVN
Why is conduction in AVN delayed?
It allows atrial systole to precede ventricular systole
What allows rapid spread of action potential to ventricles?
Bundle of His, its branches and purkinje fibres
Resting membrane potential on atrial and ventricular myocytes
-90mV
Cause of rising phase of contraction cardiac muscle cell AP (phase 0)
Fast Na+ influx, causing membrane potential to reverse to 20mV
Phase 1 of ventricular muscle AP
Na+ channels close; transient K+ efflux
Phase 2 of ventricular muscle AP
Plateau phase
Ca2+ influx
Phase 3 of ventricular muscle AP
Ca2+ channels close; K+ efflux
Phase 4 of ventricular muscle AP
Resting membrane potential
What causes plateau phase/ phase 2?
Influx of Ca2+ through L-type Ca2+ channels
Main influence of heart rate
Autonomic Nervous system
Which nerve continuously influences SAN under resting conditions?
Vagus (parasympathetic) nerve
Role of vagal tone
Reduces intrinsic HR from around 100bpm to about 70bpm
What does the vagus nerve supply?
SAN and AVN
Neurotransmitter for vagus nerve and which receptor it acts on
Acetyl Choline
M2 muscarinic receptors
Competitive inhibitor of Acetyl Choline, used in extreme cases of bradycardia
Atropine
Effect of vagal stimulation on threshold, slope of PMP, frequency of AP and +/-ve chronotropic effect
Longer to reach threshold
slope decreases
frequency of AP decreases
-ve chronotropic effect
What do cardiac sympathetic nerves supply?
SAN
AVN
Myocardium
Neurotransmitter for cardiac sympathetic nerve and which receptor it acts on
Noradrenaline
B1-ADR
Effect of sympathetic stimulation on threshold, slope of PMP, frequency of AP and +/-ve chronotropic effect
Threshold reached quicker
Slope increases
Frequency of AP increases
+ve chronotropic effect
ECG
Recording of depolarisation and depolarisation cycle of cardiac muscles taken from skin surface
What does LI connect in a 12 lead ECG?
RA-LA
What does LII connect in a 12 lead ECG?
RA-LL
What does LIII connect in a 12 lead ECG?
LA-LL
What does the P wave represent?
Atrial depolarisation
What does the QRS complex represent?
Ventricle depolarisation (masks atrial repolarisation)
What does the T wave represent?
Ventricular repolarisation
What does the PR interval represent?
Largely AVN delay
What does the ST segment represent?
Ventricular systole
What does the TP interval represent?
Diastole
