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
What supplies the parasympathetic signals to the heart? And what does it supply?
Vagus nerve, the AV AND SA nodes
How does the parasympathetic affect the heart rate?
- exerts a continuous slowing effect on the SA node by slowing the spontaneous firing of the node
- ie produces a normal resting heart rate
- increases AV nodal delay
What is the neurotransmitter involved in parasympathetic supply of the heart?
Acetyl choline acting through muscarinic M2 receptors
Give an example of a drug that can reverse the effects of the parasympathetic supply of the heart
ATROPINE is a competitive inhibitor of acetylcholine - used in extreme bradycardia to speed-up the heart
what are the effects of vagal stimulation on pacemaker potentials?
- time to reach the threshold increases
- slope of pacemaker potential decreases
- negative chronotropic effect?
What is negative chronotropic effect?
Something that slows the heart rate
What do the cardiac sympathetic nerves supply?
SA node, AV node and myocardium
What are the effects of the sympathetic nerves on the heart?
- increases heart rate
- decreases AV nodal delay
- increases the force of contraction
What is the neurotransmitter involved in sympathetic supply of the heart?
noradrenaline acting through B1 adrenoceptors
what are the effects of sympathetic stimulation on pacemaker potentials?
- increases pacemaker potential slope
- causes pacemaker potential to reach threshold quicker
- positive chronotropic effect
What is Positive chronotropic effect
-speeds up heart rate
What is an ECG?
The ECG is a record of depolarisation and repolarisation cycle of cardiac muscle obtained from skin surface
