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
