Electrical Activity Of The Heart

Question 1

Mention the tertiary pacemaker of heart & its rhythm

Answer 1

Bundle of His & Purkinje fibers

20 bpm, idioventricular rhythm

Question 2

Mention the rate of discharge of AVN & role of its autorhythmicity

Answer 2

40-60 bpm
Secondary pacemaker (it is an alternative pacemaker in case of SAN destruction)

Question 3

Mention effect of temperature on SAN firing rate

Answer 3

Decreases by cooling

Increases by heating

Question 4

Mention the effect of drugs on SAN firing rate

Answer 4

Parasymoatholytics & sympathomimetics, inc heart rate & vice versa

Question 5

Mention effect of autonomic nervous system on cardiac autorhythmicity

Answer 5

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

Question 6

GR: Normal heart rate is the less than firing rate of SAN

Answer 6

Due to inhibitory vagal tone

Question 7

Define autorhythmicity

Answer 7

It is the ability of the heart to initiate its own regular repetitive bears independant of any external stimuli.

Question 8

When do muscle fibers of heart regain auto-rhythmicity?

Answer 8

This is an imdication that such contractile cells are severely damaged.

Question 9

GR: SAN is the natural pacemaker of the heart

Answer 9

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

Question 10

Define conductivity

Answer 10

The conduction of a cardiac impulse through the cardiac tissue.

Question 11

What is the importance of slow conductivity of SAN

Answer 11

It prevents any ectopic focus from depolarizing it.

Question 12

The zone of maximal delay is …., while zone of slowest conduction velocity is ….. .

Answer 12

A-N zone

N zone

Question 13

N-H zone has …., it is not affected by ….

Answer 13

Typical slow response

TTX

Question 14

Describe characteristic function of AVN

Answer 14

1. A-V nodal delay
2. 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
3. Decrease chance of retrograde conduction, to protect SAN from ventricular ectopic focus

Question 15

GR, Conduction though left wall is slower than Rt

Answer 15

Due to thickness of wall

Question 16

The first part to be excited is …., while last part to be excited is …..

Answer 16

Left ventricular septum

Epicardial surface at the base of left ventricle

Question 17

Characteristics of slow and fast response action potentials and their site

Answer 17

S, unstable RMP, slow AP
SAN and AVN
F, stable RMP and fast AP
His bundle, bundle branches amd Purkinje fibers

Question 18

Mention value of RMP in fast respinse and reason.

Answer 18

-90 mV, stable

Due to outward K+ current which is equal to and compensating inward Na+ current

Question 19

Ustroke in fast response reaches …., due to ….. , though …. channels which all open at …. .

Answer 19

+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

Question 20

….. gate opens during depolarization.

Answer 20

Activation m gate

Question 21

H gate start to reopen at ….

Answer 21

Middle of repolarization (-50 mV)

Question 22

Early phase of repolarization in fast response is due to:

Answer 22

1. Fall in membrane permeability to sodium

2. Opening of voltage gater K channels

Question 23

GR: Presence of plateau in fast response potential, mention its voltage

Answer 23

Due to Ca++ influx 2ry to opening od slow L-type Ca channels. Ca++ influx balances the increasing K+ efflux.
At 0 mV

Question 24

The unstable RMP is also called ….., its value is ….., theshold for firing is …… .

Answer 24

Pacemaker potential, prepotential diastolic depolarization

-50 mV to -60 mV

Question 25

GR, the RMP of nodal tissue is unstable

Answer 25

Due to imbalance between more Na and Ca inward currents, and weark inward K current.

Question 26

In prepotential, first inward current is by …. through …. . This starts during ….. .

Answer 26

Na+
Funny channels
Late part of repolarization phase of AP

Question 27

The second current of preoptential occurs through …. channels

Answer 27

Transient type Ca++

Question 28

During prepotential, there ia progressive ….. in outward K+ current

Answer 28

Decline

Question 29

Phase 0 of slow response AP is from …. to ….

Answer 29

-60 mV to (+1)-(+10) mV

Question 30

Deploarization in slow response is by …. channels

Answer 30

Slow L-type Ca++

Question 31

Repolarization in fast response is …., while in slow response is ….
Both are due to …..

Answer 31

Triphasic
Monophasic
K+ efflux

Question 32

Compare extension of absolute refractory period in fast and slow response AP

Answer 32

F, from phase 0 to middle of phase 3

S, from phase 0 to most of phase 3

Question 33

What is the significance of long ARP in fast response AP

Answer 33

The cardiac muscle cannot be re-stimulated during systole, tgis prevents tetanization of cardiac muscle which is fatal.

Question 34

Whta does RRR represent?

Answer 34

A state of partial recovery of fast Na channels due to opening of some not all of the h gates.

Question 35

Compare extension of RRR in slow and fast response AP

Answer 35

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)

Question 36

Drefin3 effective refractory period

Answer 36

A propagated AP cannot be elicited but a localized AP can be generated by strong stimulus.
ARP + first 10 mV of RRP

Question 37

Mention significance of ERP

Answer 37

1. Prolongs the absolute refratoriness of the heart for another impulse originating from within the heart.
2. 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.