Origin and Conduction of Cardiac Impulse Flashcards
What is the heart?
Electrically controlled muscular pump which sucks and pumps blood
Where are the electrical signals which control the heart generated?
Within the heart
What is autorhythmicity?
Heart is capable of beating rhythmically in the absence of external stimuli
Where does excitation of the heart normally begin?
Pacemaker cells in the sinoatrial node
Clusters of these cells initiate the heart beat
Where is the SA node situated?
Upper right atrium close to where superior vena cava enters the right atrium
What does the SA normally do?
Drives the entire heart rate (sets the pace)
What does it mean if a heart is in sinus rhythm?
Heart controlled by sino atrial node
Role and characteristics of cells in SA node?
No stable resting membrane potential
They generate regular spontaneous pacemaker potentials
What happens when a spontaneous pacemaker potential is generated?
It takes the membrane potential to a threshold and every time a threshold is reached an action potential is generated
What does an action potential being produced result in?
Generation of regular spontaneous action potentials in the SA nodal cells
Permeability to K+ in pacemaker cells?
Does not remain the same between action potentials
Pacemaker potential?
Slow depolarization of membrane potential to a threshold
Pacemaker potential?
Slow depolarization of membrane potential to a threshold
What does pacemaker potential look like on a diagram?
It is the beginning of a slight incline between two rises (-60mV->-40mV)
What does pacemaker potential look like on a diagram?
It is the beginning of a slight incline between two rises (-60mV->-40mV)
Ionic basis for a pacemaker action potential?
Once the threshold is reached:
- The rising phase (depolarization) of action potential is caused by activation of long-lasting L type Ca++ channels
- Resulting in Ca++ influx
- The falling phase (re-polarization) is caused by inactivation of L-type Ca++ channels and
- Activation of K+ channels resulting in K+ efflux
What is threshold?
-40mV
General description of how cardiac excitation spreads through heart?
starts at SA node Passes by cell to cell conduction to get to the AV node Then travels to the Bundle of His then down the left and right branches and to the purkinje fibers
Cell to cell excitation via GAP junctions?
From SA node through both atria
From SA node to AV node
Within ventricles
What is the AV node?
Bundle of specialized cardiac cells located at the base of the right atrium just above junction of atria and ventricles
What is special about AV node?
ONLY point of electrical contact between atria and ventricles
Spread of excitation across atria?
Mainly cell to cell via gap junctions
Spread of excitation from SA node to AV node?
Mainly cell to cell conduction via gap junctions but also some inter-nodal pathways
What happens to conduction in AV node?
It is delayed - this allows atrial systole (contraction) to precede ventricular systole
What do Bundle of His and Purkinje fibres allow?
Rapid spread of action potential to the ventricles
Spread of excitation across ventricular muscle?
Cell to cell conduction
Action potential in contractile cardiac cells and pacemaker cells?
Differ considerably
Resting membrane potential of atrial and ventricular myocytes?
-90mV until excitation
Depolarization of myocytes? and it’s name
- Caused by fast Na+ influx
- Rapidly reverses membrane potential to +20mV
- Phase 0 of AP in contractile cardiac muscle cells
Phases of ventricular muscle AP?
Phase 0- fast influx of Na+
Phase 1- Closure of Na+ channels and transient K+ efflux
Phase 2- Mainly Ca++ influx
Phase 3- Closure of Ca++ channels and K+ efflux
Phase 4- Resting membrane potential
Phase 0=
Fast influx of Na+
Phase 1=
Closure of Na+ channels and transient K+ efflux
Phase 2=
Mainly Ca++ influx
Phase 3=
Closure of Ca++ channels and K+ efflux
Phase 4=
Resting membrane potential
Plateau phase of action potential?
- Membrane potential maintained near the peak of AP for a few 100 milliseconds
- Unique characteristic of contractile cardiac muscle cells
- Mainly due to influx of Ca++ through L type Ca++ channels
Falling phase of ventricular muscle action potential?
Repolarization is caused by inactivation of Ca++ channels and activation of K+ channels= K+ efflux
What is the heart rate mainly influenced by?
Autonomic nervous system
What does sympathetic stimulation do to heart rate?
Increases HR
What does parasympathetic stimulation do to heart rate?
Decreases HR
What do changes of the heart rate usually involve?
Reciprocal action of sympathetics and parasympathetics
What does the vagus nerve do?
Parasympathetic supply to the heart
Exerts a continuous influence on the SA node under resting conditions
Vagal tone under normal resting conditions?
Dominates
What does vagal stimulation do?
Slows the intrinsic heart rate from ~100bpm to produce normal resting HR and increases AV nodal delay
Bradycardia
Resting heart rate less than 60bpm
Tachycardia
Resting heart rate over 100bpm
What does the vagus nerve supply?
AV and SA node
Parasympathetic supply of heart
Vagus nerve
Neurotransmitter= Acetylcholine acting through muscarinic M2 receptors
Atropine?
Competitive inhibitor of acetyl choline used in extreme bradycardia to speed up the heart
What do negative chronotropes do?
Decrease HR
What do cardiac sympathetic nerves supply?
AV node, SA node and myocardium
What does sympathetic stimulation do?
Increases HR
Decrease AV nodal delay
Force of contraction
Sympathetic Neurotransmitter?
Noraderinaline acting through B1 adrenoceptors
Effect of sympathetic nervous system on HR in detail?
Increases=
- Slope of pacemaker potential
- Heart rate
- Pacemaker cell Na+ & Ca++ influx
Decreases=
- Pacemaker cell K+ influx
- AV nodal delay
Effect of parasympathetic nervous system on HR in detail?
Increases=
- Pacemaker cell K+ influx
- AV nodal display
Decreases=
- Slope of pacemaker potential
- Heart rate
- Pacemaker cell Na+ & Ca++ influx
What is the ECG?
Record of depolarization and repolarisation cycle of cardiac muscle obtained from skin surface
How to record ECG?
lead 1: RA to LA
Lead 2: RA to LL
Lead 3: LA to RL