Cardiovascular Physiology Lecture 2 & 3: Origin and Conduction of Cardiac Impulse Flashcards
What is autorhymicity?
The heart is capable of beating rhythmically in the absence of external stimuli
Where does excitation of the heart normally originate?
In the pacemakers cells in the Sino-atrial node
Where is the SA node loacted and how does it innate the heart beat?
It is located in the upper right atrium close to where the superior vena cava enters the right atrium and the cluster of specialised pacemaker cells initiate the signal
When a heart is controlled by a Sino-atrial node, what is it called?
Sinus rhythm
Do the cells in the SA Node have a stable resting membrane potential?
No- they exhibit spontaneous pacemaker potential
How is an action potential generated in the SA nodal cells?
The spontaneous pacemaker potential takes the membrane potential to a threshold to generate the action potential
What causes the pacemaker potential ?(i.e slow depolarisation of membrane potential to threshold
- Decrease in K+ efflux
- Na+ and K+ influx (funny current)
- Transient Ca++ influx
What causes depolarisation?
when threshold is met, activation of long lasting (L-type Ca++ channels_ result in Ca++ influx
What is repolarisation caused by?
Inactivation of L-type Ca++ channels and activation of K+ channels (resulting in K+ efflux)
During cell- to-cell spread of excitation , how is excitation spread from the SA node to the AV node?
Via gap junctions that allows cell to cell conduction
What is the AV node and where is it located?
The av node is a small bundle of specialised cardiac cells and is located at the base of the right atrium, just above the junction of the atria and ventricles
What is important about the AV node?
The only point of electrical contact between atria and ventricles
What are 2 characteristics of AV node cells?
- small in diameter
- slow conduction velocity
Describe how the cardiac excitation normally spreads across the heart?
- The signal moves from SA node to AV node mainly cell-to-cell conduction via gap junctions; but there is also some internodal pathways
- The conduction is delayed in the AV node. This allows atrial systole (contraction) to precede ventricular systole
- The Bundle of His and its branches and the network of Purkinje fibers allow rapid spread of action potential to the ventricles
- Ventricular muscle: cell-to-cell conduction
How does the action potential differ between contractile cardiac muscle cells and pacemaker cells?
Contractile cardiac muscle cells (myocytes) do not have a drifting membrane potential but remains constant until the cell is excited
