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Where does excitation of the heart normally originate?

In the pacemaker cells in the sino-atrial node


Where is the SA node located?

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


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

Sinus rhythm


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

Spontaneous pacemaker potential
The 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


What happens to the membrane potential of pacemaker cells?

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


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

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


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

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


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

Activation of K+ channels resulting in K+ efflux


Anatomy of the conducing system of the heart?

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


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

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


Where is the AV node located?

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


How is excitation spread across the atria?

Mainly cell-to-cell conduction via gap junctions


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

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


Why is the conduction delayed by the AV node?

To allow atrial systole to precede ventricular systole


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

Contractile muscle cells


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



What is the resting membrane potential of contractile muscle cells?



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?

Fast Na+ influx
Phase 0


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

Closure of Na+ channels and transient K+ efflux


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

Mainly Ca2+ influx


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

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


What is phase 4 in cardiac muscle cells?

Resting membrane potential


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

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


What provides the parasympathetic supply to the heart?

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


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

The vagal nerve (parasympathetic)
Vagal tone


What is the purpose of the vagal tone?

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


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

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


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

M2 receptor


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

it is a competitive inhibitor of acetylcholine


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?

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