Origin and Conductance of Cardiac Impulse Flashcards
What is autorythmicity?
When 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)
What are pacemaker cells?
Cluster of specialised cells that initiate the heart beat
Where is the SA node locates?
in the upper RIGHT ATRIUM close to where the Superior Vena Cava enters the right atrium
What does the SA node do?
Drives the pace for the entire heart
What is sinus rhythm?
A heart controlled by the sino-atrial node ie normal rhythm
Do pacemaker cells have a stable resting potential?
No, they exhibit spontaneous pacemaker potential
What is the function of the spontaneous pacemaker potential?
It allows the membrane potential to reach a threshold to generate an action potential in the SA nodal cells.
What is the role of potassium in the spontaneous membrane potential?
Potassium leaving th cell will hyperpolarise the cell. Potassium remaining in the cell will depolarise the cell.
What other molecule affects pacemaker potential?
transient Ca++ influx
What happens when the threshold is reached?
- Once threshold is reached action potential will be fired
- Activation of long lasting calcium channels
- Calcium influx to pacemaker cells
- Causing depolarisation
How does repolarisation occur?
- Activation of potassium channels making calcium leave the cell causing repolarization.
- Closing of calcium channels occurs when potassium channels open.
What is the bundle of His and what is its function?
Specific conductance pathway across the septum. It divides into two pathways to get to each ventricle.
What are the Purkinje fibres?
End point of the electrical signal that cause contraction of the ventricles
what is cell to cell conductance?
Action potentials can spread from one cell to another via gap junctions located in intercalated disks which contain a specialised function.
What is the Av node?
- small bundle of specialised cardiac cells
- They are small in diameter and have slow conduction velocity
Where is the AV node located?
at the base of the right atrium; just above the junction of atria and ventricles
The AV node is the ONLY point of electrical contact between atria and ventricles
True or false
True
How do action potentials spread?
- cell to cell conductance across the atria
- cell to cell conduction from SA node to AV node however there are also some internodal pathways
What is the main function of the AV node?
Delay the conductance of the action potential. Delay is important as atrium needs to have time to contract before the ventricles contract.
The action potential in contractile cardiac muscle cells DIFFERS considerably from the action potential in PACEMAKER cells
true/false?
True
Describe the phases of ventricular muscle action potential
Phase 0- fast Na+ influx
Phase 1- closure of Na+ channels and transient K+ efflux
Phase 2- Mainly Ca++ influx
Phase 3- closure of Ca++ channels and K+ efflux (repolarisation stage)
Phase 4- resting membrane potentials (-90)
What is the Plateau phase of action potential?
- The action potential in contractile cardiac muscle cells DIFFERS considerably from the action potential in PACEMAKER cells
- unique to contractile cardiac muscle cells
- mainly due to influx of Ca++ through L type Ca++ channels
What mainly influences the heart rate?
Autonomic nervous system
