Physiology 1 -The Origin and Conduction of Cardiac Impulse Flashcards
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 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
What happens to the membrane potential of pacemaker cells?
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
What causes the pacemaker potential in pacemaker cells (the slow depolarisation of the membrane potential to a threshold)?
Decrease in K+ effluxSlow 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?
-60mV
What is the resting membrane potential of contractile muscle cells?
-90mV
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+30mVPhase 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?
AcetylcholineM2 receptor
What drug is used to speed up the heart?How does it work?
Atropineit 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 decreasesNegative chronotropic effect (decrease in HR)
What do cardiac sympathetic nerves supply in the heart?
SA nodeAV nodeMyocardium
What does sympathetic stimulation of the heart cause?
Increase in heart rateDecrease in AV nodal delayUncreased force of contraciton
What neurotransmitter does the sympathetic supply to the heart use and what receptors on the heart does it bind to?
NoradrenalineB1-adrenoceptors
Effect of noradrenaline on pacemaker cells:-slope of pacemaker potential?-time taken for pacemaker potential to reach threshold?-frequency of action potentials?
Slope increasedReach threshold quickerfrequency increases (positive chronotropic effect)
