Physiology 1 -The Origin and Conduction of Cardiac Impulse

Question 1

Where does excitation of the heart normally originate?

Answer 1

In the pacemaker cells in the sino-atrial node

Question 2

Where is the SA node located?

Answer 2

In the upper right atrium close to where the SVC enters the right atrium

Question 3

What is the phrase used to describe a heart controlled by the SA node?

Answer 3

Sinus rhythm

Question 4

What do pacemaker cells in the SA node exhibit which leads to the membrane potential depolarising?What does this do

Answer 4

Spontaneous pacemaker potentialThe spontaneous pacemaker potential is a gradual drift of the membrane potential towards depolarisation This takes the membrane potential to threshold where an action potential is generated in the SA nodal cells

Question 5

What happens to the membrane potential of pacemaker cells?

Answer 5

They gradually drift (pacemaker potential) until it reaches threshold value causing the cell to fireThe firing is followed by repolarisation before another action potential occurs

Question 6

What causes the pacemaker potential in pacemaker cells (the slow depolarisation of the membrane potential to a threshold)?

Answer 6

Decrease in K+ effluxSlow Na+ influx (the funny current)(this creates a net movement of positively charged ions into the cell which happens spontaneously)

Question 7

What causes the rising phase of the action potential once the threshold has been reached? (pacemaker cells)

Answer 7

Activation of voltage-gated Ca2+ channels resulting in Ca+ influx

Question 8

What is the falling phase of the action potential (depolarisation) caused by? (pacemaker cells)

Answer 8

Activation of K+ channels resulting in K+ efflux

Question 9

Anatomy of the conducing system of the heart?

Answer 9

Sino-atrial (SA) node - Atrioventricular node - bundle of His - left and right branches - purkinje fibres

Question 10

What does cell-to-cell spread of excitation occur between?

Answer 10

Adjacent myocytes through gap junctions within intercalated discs (intercalated discs are made up of gap junctions, fascia adherents and desmosomes)

Question 11

Where is the AV node located?

Answer 11

It is a small bundle of specialised cardiac cells located at the base of the right atrium just above the junction of atria and ventricles

Question 12

How is excitation spread across the atria?

Answer 12

Mainly cell-to-cell conduction via gap junctions

Question 13

How is excitation spread from the SA node to the AV node?

Answer 13

Mainly cell-to-cell conduction via gap junctions but there are also some internal pathways

Question 14

Why is the conduction delayed by the AV node?

Answer 14

To allow atrial systole to precede ventricular systole

Question 15

Does the action potential in pacemaker cells or contractile muscle cells have a plateau phase?

Answer 15

Contractile muscle cells

Question 16

What is the most depolarised membrane potential of the pacemaker potential?

Answer 16

-60mV

Question 17

What is the resting membrane potential of contractile muscle cells?

Answer 17

-90mV

Question 18

What causes the rising phase of the action potential in contractile muscle cells?what does this rapidly reverse the membrane potential to?What phase of action potential in contractile cardiac muscle is this?

Answer 18

Fast Na+ influx+30mVPhase 0

Question 19

What causes phase 1 of action potential in contractile cardiac muscle?

Answer 19

Closure of Na+ channels and transient K+ efflux

Question 20

What causes phase 2 of action potential in contractile cardiac muscle?

Answer 20

Mainly Ca2+ influx

Question 21

What causes phase 3 of action potential in contractile cardiac muscle?

Answer 21

Closure of Ca2+ channels and activation of K+ channels resulting in K+ efflux

Question 22

What is phase 4 in cardiac muscle cells?

Answer 22

Resting membrane potential

Question 23

What is the name for the period where the action potential is maintained near the peak of action potential for a few hundred milliseconds?

Answer 23

Plateau phase (this is a unique characteristic of contractile cardiac muscle cells)

Question 24

What provides the parasympathetic supply to the heart?

Answer 24

The vagus nerve which exerts a continuous influence on the SA node (and AV node) under resting conditions (the vagal tone)

Question 25

What part of the autonomic nervous system controlling the heart dominates under resting conditions?Name for this?

Answer 25

The vagal nerve (parasympathetic)Vagal tone

Question 26

What is the purpose of the vagal tone?

Answer 26

To slow the intrinsic heart rate from approx. 100 bp to produce a normal resting heart rate of about 70bpm

Question 27

What effect does stimulation of the vagal nerve have on the heart?

Answer 27

Supplies the SA node and the AV node causing slowing of the heart rate and increased AV nodal delay

Question 28

What neurotransmitter does the vagus nerve use and what receptor on the heart does this attach to?

Answer 28

AcetylcholineM2 receptor

Question 29

What drug is used to speed up the heart?How does it work?

Answer 29

Atropineit is a competitive inhibitor of acetylcholine

Question 30

What is the effect of vagal stimulation on pacemaker potential slope?What happens to the frequency of AP?What is the name given to this?

Answer 30

Decreases (cell hyper polarises for longer to reach threshold)Frequency of action potentials decreasesNegative chronotropic effect (decrease in HR)

Question 31

What do cardiac sympathetic nerves supply in the heart?

Answer 31

SA nodeAV nodeMyocardium

Question 32

What does sympathetic stimulation of the heart cause?

Answer 32

Increase in heart rateDecrease in AV nodal delayUncreased force of contraciton

Question 33

What neurotransmitter does the sympathetic supply to the heart use and what receptors on the heart does it bind to?

Answer 33

NoradrenalineB1-adrenoceptors

Question 34

Effect of noradrenaline on pacemaker cells:-slope of pacemaker potential?-time taken for pacemaker potential to reach threshold?-frequency of action potentials?

Answer 34

Slope increasedReach threshold quickerfrequency increases (positive chronotropic effect)