Electrical Activity Of The Heart Flashcards
Mention the tertiary pacemaker of heart & its rhythm
Bundle of His & Purkinje fibers
20 bpm, idioventricular rhythm
Mention the rate of discharge of AVN & role of its autorhythmicity
40-60 bpm Secondary pacemaker (it is an alternative pacemaker in case of SAN destruction)
Mention effect of temperature on SAN firing rate
Decreases by cooling
Increases by heating
Mention the effect of drugs on SAN firing rate
Parasymoatholytics & sympathomimetics, inc heart rate & vice versa
Mention effect of autonomic nervous system on cardiac autorhythmicity
Sympathetic, acts by norepinepherine on B1 receptors,
1. Increase inward Na+ current
2. Increase cAMP which facilitates opening if L-Ca++ channels
3. Inactivates K+ channels
Parasympathetic, acts by acetylcholine of M2 receptos
Opposite effect to 1,2 & 3
GR: Normal heart rate is the less than firing rate of SAN
Due to inhibitory vagal tone
Define autorhythmicity
It is the ability of the heart to initiate its own regular repetitive bears independant of any external stimuli.
When do muscle fibers of heart regain auto-rhythmicity?
This is an imdication that such contractile cells are severely damaged.
GR: SAN is the natural pacemaker of the heart
Becuase its slow response action potential has:
- RMP with the highest slope
- RMP is nearer to threshold
- Phase 0 of low amplitude
- Repolarization is rapid
- High repetitive rhythm of discharge
Define conductivity
The conduction of a cardiac impulse through the cardiac tissue.
What is the importance of slow conductivity of SAN
It prevents any ectopic focus from depolarizing it.
The zone of maximal delay is …., while zone of slowest conduction velocity is ….. .
A-N zone
N zone
N-H zone has …., it is not affected by ….
Typical slow response
TTX
Describe characteristic function of AVN
- A-V nodal delay
- A-V nodal block, refractoriness is both voltage and time dependant, it doesn’t allow except 180-200 bmp to pass from atria to ventricles
- Decrease chance of retrograde conduction, to protect SAN from ventricular ectopic focus
GR, Conduction though left wall is slower than Rt
Due to thickness of wall
The first part to be excited is …., while last part to be excited is …..
Left ventricular septum
Epicardial surface at the base of left ventricle
Characteristics of slow and fast response action potentials and their site
S, unstable RMP, slow AP
SAN and AVN
F, stable RMP and fast AP
His bundle, bundle branches amd Purkinje fibers
Mention value of RMP in fast respinse and reason.
-90 mV, stable
Due to outward K+ current which is equal to and compensating inward Na+ current
Ustroke in fast response reaches …., due to ….. , though …. channels which all open at …. .
+30 mV, rapid Na+ entry acconding to electric and concentration gradients at depolarozation but conc grad alone in overshoot
Voltage gated Na+
-70 to -65 mV
….. gate opens during depolarization.
Activation m gate
H gate start to reopen at ….
Middle of repolarization (-50 mV)
Early phase of repolarization in fast response is due to:
- Fall in membrane permeability to sodium
2. Opening of voltage gater K channels
GR: Presence of plateau in fast response potential, mention its voltage
Due to Ca++ influx 2ry to opening od slow L-type Ca channels. Ca++ influx balances the increasing K+ efflux.
At 0 mV
The unstable RMP is also called ….., its value is ….., theshold for firing is …… .
Pacemaker potential, prepotential diastolic depolarization
-50 mV to -60 mV
GR, the RMP of nodal tissue is unstable
Due to imbalance between more Na and Ca inward currents, and weark inward K current.
In prepotential, first inward current is by …. through …. . This starts during ….. .
Na+
Funny channels
Late part of repolarization phase of AP
The second current of preoptential occurs through …. channels
Transient type Ca++
During prepotential, there ia progressive ….. in outward K+ current
Decline
Phase 0 of slow response AP is from …. to ….
-60 mV to (+1)-(+10) mV
Deploarization in slow response is by …. channels
Slow L-type Ca++
Repolarization in fast response is …., while in slow response is ….
Both are due to …..
Triphasic
Monophasic
K+ efflux
Compare extension of absolute refractory period in fast and slow response AP
F, from phase 0 to middle of phase 3
S, from phase 0 to most of phase 3
What is the significance of long ARP in fast response AP
The cardiac muscle cannot be re-stimulated during systole, tgis prevents tetanization of cardiac muscle which is fatal.
Whta does RRR represent?
A state of partial recovery of fast Na channels due to opening of some not all of the h gates.
Compare extension of RRR in slow and fast response AP
F, just after ARP till end of repolarization
S, from late phase 3 and extends particularly in AV node will beyond phase 3 even after complete repolarization (post-repolarization refractoriness)
Drefin3 effective refractory period
A propagated AP cannot be elicited but a localized AP can be generated by strong stimulus.
ARP + first 10 mV of RRP
Mention significance of ERP
- Prolongs the absolute refratoriness of the heart for another impulse originating from within the heart.
- It allows cardioversion in case of ventricular fibrillation by external defibrillating electric pulse.
It induces localized AP in each cell which represents a real unified ARP for all cells thus give the SAN the chance to be the pacemaker again.
