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
How is the rising phase of action potential in myocytes caused and what does this cause?
Caused by fast Na+ influx and this rapidly reverses the membrane potential to about +20mV (phase 0)
What are 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
Phase 4- Resting membrane potential
What is the plateau phase of the action potential and what causes it?
The membrane potential is maintained near the peak of action potential for few hundred milliseconds and this is mainly due to influx of Ca++ through L-type Ca++ channels
What is the heart rate influenced by mainly?
Autonomic nervous system
What are the roles of both the sympathetic and parasympathetic?
- SYMPATHETIC STIMULATION INCREASES the HEART RATE and decreased AV nodal delay
- PARASYMPATHETIC STIMULATION DECREASES the HEART RATE
What role does the vagus nerve (parasympathetic) play ?
It exerts a continuous influence on the SA node under resting conditions
What is Vagal tone?
Slows the intrinsic heart rate from 100bmp to produce normal resting heart rate of 70 ppm
What is a normal resting heart rate?
Between 60 and 100 ppm
What is the name if the heart rate is less than 60bpm?
Bradycardia
What is the name if the heart rate is more than 100bpm?
Tachycardia
What does vagal stimulation do?
Slows heart rate and increase AV node delay
What is atropine and when is it used?
It is a competitive inhibitor of acetylcholine and is used in extreme bradycardia to speed up the heart
How does vagal stimulation low down heart rate?
The cell hyperpolarizes and so takes longer to reach threshold and so has a negative chronotropic effect
What do the cardiac sympathetic nerves supply?
SA node
AV node
myocardium
What is the neurotransmitter in parasympathetic nerves?
Acetyl choline
What is the neurotransmitter in sympathetic nerves?
Noradrenaline
What is the effect of noradrenaline on pacemaker cells?
Pacemaker potential reaches threshold quicker and frequency of action potentials increase - positive chronotropic effect
What is an ECG?
Uses surface electrodes to record the wave of depolarisation and repolarisation of cardiac muscle obtained from skin surface
What do the P, QRS and T waves represent in an EGC?
P- atrial depolarisation
QRS- Ventricular depolarisation (masks atrial repolairzation)
T- ventricular repolarisation
What do the PR, ST and TP phases on the ECG represent?
PR interval- largely AV node delay
ST- ventricular systole
TP- diastole
