Origin and conductance of cardiac impulse Flashcards
What does autorhythmicity mean?
The heart is capable of beating rhythmically in the absence of external stimuli
Where does excitation of the heart normally originate?
In the pacemaker cells in the sino-atrial node (SA node)
Where exactly is the sino-atrial node?
In the right atrium, close to where the superior vena cava enters the right atrium
What do you call a heart that is controlled by the sino-atrial node?
The heart is said to be in sinus rhythm
What two features of the cells in the sino-atrial node are responsible for their ability to start excitation in the heart?
The cells in the SA node do not have a stable resting membrane potential
The cells exhibit spontaneous membrane potential
How does the cardiac excitation normally originate?
The spontaneous pacemaker potential takes the membrane potential to a threshold to generate an action potential in the SA nodal cells
What happens to the movement of ions that causes the pacemaker potential?
A decrease in the K+ efflux on top of a slow Na+ influx
What happens when the threshold is reached?
The rising phase of the action potential is caused by activation of voltage-gated Ca++ channels. This results in a Ca++ influx
Is the rising phase of the action potential polarisation or depolarisation?
Depolarisation
What is the falling phase of the action potential?
Repolarisation
Caused by the activation of K+ channels, resulting in efflux of K+
How does cardiac excitation normally spread across the heart?
It originates in the SA node then spreads to the right atrium, then to the atrio ventricular node. Then through the Bundle of His the excitation spreads to the apex of the heart. The wave of the contraction starts at the bottom of the ventricles and works its way up.
What cell structures allows the spread of excitation by cell-to cell current flow?
Gap junctions
What is the atrial ventricular node and what is its purpose?
It is a small bundle of specialised cardiac cells
It is the only point of electrical contact between the atria and the ventricles
Where exactly is the atrioventricular node?
Located at the base of the right atrium, just above the junction of the atria and the ventricles
Describe the AV node cells
They are small in diameter and have slow conductance velocity
What is the importance of only having one point of electrical contact between the atria and the ventricles?
Important for coordination
The AV acts as a gateway for electrical transmission
How is excitation spread?
Mainly through gap junctions but also through internodal pathways
What is the purpose of delayed conductance in the AV node?
Allows atrial systole to precede ventricular systole
What structures allow rapid spread of action potential to the ventricles?
The Bundle of His and Purkinje fibres
In ventricular muscle cells, what is the value of the resting membrane potential before the cell is excited?
-90
In ventricular muscle cells, what causes the rise of action potential i.e. depolarisation?
Fast influx of Na+
In ventricular muscle cells, what is the peak value that the action potential reaches due to depolarisation?
+30
What phase is a unique characteristic of contractile cardiac muscle cells?
Plateau phase of action potential
What causes the plateau phase in contractile cardiac muscle cells?
It is mainly due to the influx of Ca++ through voltage gated Ca++ channels
What is the parasympathetic nerve supply to the heart?
The vagus nerve
What is the affect of vagal tone?
Vagal tone dominates under normal resting conditions.
It slows the intrinsic heart rate of 100bpm to the normal resting heart rate of 70bpm
What is normal resting heart rate?
60-100bpm
What is it called when resting heart rate is below 60bpm?
Bradycardia
Define trachycardia
When resting heart rate is above 100bpm
What structures in the heart does the vagal nerve supply?
The sinoatrial node and the artrioventricular node
What does stimulation of the vagal nerve do?
Slows down the heart rate and increases AV nodal delay
What neurotransmitter and receptor are involved in parasympathetic stimulation?
Acetylcholine
M2 muscarinic receptor
What drug is given to treat extreme cases of bradycardia and how does it work?
Atropine
It is a a competitive inhibitor of acetlycholine and therefore speeds up the heart
What do cardiac sympathetic nerves supply?
The SA node, AV node and myocardium
What is the neurotransmitter and receptor for sympathetic nerves?
Noradrenaline
B1 adrenoceptors
What is an ECG?
Electrocardiogram
A record of the depolarisation and repolarisation cycle of cardiac muscle obtained from the skin surface
