origin and conduction of cardiac impulse

Question 1

where in the heart does excitation originate?

Answer 1

• in the pacemaker cells in the SA node (a cluster of specialised pacemaker cells)

Question 2

where is the SA node located?

Answer 2

upper right atrium (close to where the SVC enters the right atrium)

Question 3

what does sinus rhythm mean?

Answer 3

when the heart is controlled by the SA node

Question 4

how does cardiac excitation originate?

Answer 4

• the cells in the SA node have no stable resting membrane potential and generate spontaneous pacemaker potentials
• the spontaneous pacemaker potential takes the membrane potential to a threshold and an action potential is generated, resulting in the generation of regular spontaneous action potentials in the SA nodal cells

Question 5

what is the pacemaker potential due to?

Answer 5

• a decrease in K+ efflux
• Na+ and K+ influx
• transient Ca++ influx

(pacemaker potential is the upward stroke at the bottom)

Question 6

what happens once the threshold is reaches?

Answer 6

• the rising phase of action potential (depolarisation) is caused by activation of L-type Ca++ channels which results in Ca++ influx

Question 7

what causes the falling phase of action potential? (repolarisation)

Answer 7

• inactivation of L-type Ca++ channels and activation of K+ channels which result in K+ efflux

Question 8

what is the AV node, where is it located and what does it do?

Answer 8

• bundle of specialised cardiac cells
• located at the base of the right atrium, above junction of arita and ventricles
• AV node is the only point of electrical contact between atria and ventricles
• AV node cells are small in diameter and have slow conduction velocity

Question 9

how does cardiac excitation spread?

Answer 9

• across the atria (cells to cell conduction via gap junctions)
• from SA node to AV node (cells to cell via gap junctions but also some internodal pathways)
• the conduction is delayed in the AV node (this allows artial systole to precede ventricular systole = contraction)
• the bundle of his and its branches and the network of purkinje fibres allows rapid spread of action potential to the ventricles
• ventricular muscle (cells to cell)

Question 10

what happens in phase 0 of action potential in contractile cardiac muscle cells?

Answer 10

• the resting membrane = -90mV
• rising phase of action potential (depolarisation) is caused by fast Na+ influx
• membrane potenetial is then +20mV

Question 11

what are the phases of ventricular muscle action potential?

Answer 11

0 = fast Na+ influx
1 = closure of Na+ channels and transient K+ efflux
2 = mainly Ca++ influx
3 (falling phase)= closure of Ca++ channels and K+ efflulx
4 = resting membrane potential

Question 12

what is the plateau phase?

Answer 12

• the membrane potential is . maintained near the peak of action potential for a few hundred milliseconds
• it is a unique characteristic of contractile cardiac muscle cells
• the plateau phase is mainly due to influx of Ca++ through L-type Ca++ channels

Question 13

what is the falling phase of ventricular muscles action potential?

Answer 13

• (repolarisation)
• caused by inactivation of Ca++ channels and activation of K+ channels
• resulting in K+ efflux

Question 14

what changes the heart rate?

Answer 14

• HR is mainly influenced by the autonomic nervous system
• sympathetic stimulation increases the heart rate
• parasympathetic stimulation decreases the heart rate
• changes in HR involve a reciprocal action of sympathetics and parasympathetics

Question 15

how does the autonomic nervous system influence the normal HR?

Answer 15

• the vagus nerve (para) exerts a continuous influence on the SA node under resting conditions
• vagal tone dominates under normal resting conditions
• vagal tone slows the intrinsic HR from 100bpm to 70bmp

Question 16

what is normal resting HR?

Answer 16

between 60 and 100 bpm

Question 17

what is bradycardia?

Answer 17

resting HR under 60bpm

Question 18

what is tachycardia?

Answer 18

resting HR over 100bpm

Question 19

how does parasympathetic influence the heart?

Answer 19

• vagus nerve supplies SA node and AV node

- vagal stimulation slows heart rate and increase aV nodal delay

Question 20

how does sympathetic influence the heart?

Answer 20

• cardiac sympa nerve supply the SA node and AV node and myocardium
• sympa stimulation increases HR and decreases AV nodal delay
• also increases the force of contraction

Question 21

what is the neurotransmitter for parasympathetic?

Answer 21

• neurotransmitter is acetyle choline acting through M2

- atropine is a competitive inhibitor of acetylcholine - used in extreme bradycardia to speed up the heart rate

Question 22

what is the neurotransmitter for sympathetic?

Answer 22

neurotransmitter is noradrenaline acting through B1 adenoceptors.

Question 23

what are the standard leads?

Answer 23

1 = RA - LA
2 = RA - LL
3 = LA - LL

Question 24

what are the ECG records?

Answer 24

P = atrial depolarisation (first small bump)
ORS = ventricular depolarisaiton (spike)
T = ventricular repolarisation (seond large bump)
PR = largery AV node delay (line after spike)
ST = segment ventricular systole (line after spike)
TP = diastole (long line)