the cardiac action potential and ECG Flashcards
what is autorhythmicity
the heart contracting rhythmically as a result of APs it generates itself
what are the two types of specialized cardiac cell
Contractile cells (99%; normally do not initiate APs)
Autorhythmic cells (do not contract; initiate or conduct APs)
do cardiac autorhythmic cells have a resting membrane potential
no, instead they display pacemaker activity
what is pacemaker potential
an autorhythmic cell membranes’ slow drift to threshold
autorhythmic cells cyclically initiate what?
APs which then spread through the heart to trigger contraction without any nervous stimulation
special non-contractile cells that demonstrate autorhythmicity are located at 4 specific sites. What are they
1)The sinoatrial node
2)The atrioventricular node
3)The bundle of His (atrioventricular bundle)
4)Purkinje fibers
what is the normal pacemaker of the heart
sinoatrial node
the cells of the heart are linked electrically, what does this mean
the rate of the fastest will be the rate of all
how many APs per min does an atrial and ventricular myocardium normally have
0
how can SA node discharge frequency be altered
by parasympathetic and sympathetic stimulation
functions of the atrioventricular node
- AV node forms the only conducting pathway between the atrial muscle and Bundle of His and hence the ventricles
.* AV node introduces a considerable delay to the spread of excitation (~100 ms). This allows time for blood to move from the atria to the ventricles. - AV node cells have well-developed latent powers of rhythmicity and can take over pacemaking if impulses from the SA node fail to reach them
what goes in to increase the membrane potential and what goes out
Na+ and Ca2+
K+ goes out
The cycle of voltage change across cardiac myocytes occurs in 5 distinct phases
1) Depolarization
2) Early repolarisation
3) Plateau phase
4) Late repolarization
5) Resting potential
what happens in the plateau phase of the action potential
K+ out fast (slight dip on the graph)
Ca2+ in slow (flattens out a bit)
K+ out fast (drops all the way down on the graph)
at what rate do SA nodes fire APs
70-80 APs per min
The rate of firing is regulated by the autonomic nervous system
what links neighboring cardiac muscle cells
the presence of gap junctions
what do gap junctions do
they allow the rapid spread of APs from cell to cell, e.g. an AP originating in the SA spreads throughout both atria, stimulating the simultaneous contraction of R +L atrial muscle
what is an ECG (electrocardiogram) the record of
the overall spread of electrical activity throughout the heart
where the electrodes attach to and why does it work
it is attached to the skin and it is able to record the small parts of electrical activity that is able to reach the body surface
what are the 3 important points when considering what an ECG represents?
- An ECG is not a direct recording of the actual electrical activity of the heart.
- The ECG is a complex recording representing the overall spread of activity throughout the heart during depolarisation and repolarisation. It is not a recording of a single AP in a single cell at a single point in time.
- The recording represents comparisons in voltage detected by electrodes at two different points on the body surface, not the actual potential.
what is the specific arrangement of each pair of connections called
lead
how many leads are there on an ECG and what are the locations on which they record
there are 12 leads
6 different arrangments from the limbs
6 chest leads at various sites around the heart
what do the 6 limb leads include
1
2
3
aVR
aVL
aVF
leads 1,2,3,are what type of leads
bipolar leads because two recording electrodes are used
the electrode placed on the right leg serves as what
a ground and is not a recording electrode
aVR, aVL, and aVF leads are what type of leads
unipolar
what does the exploring electrode mainly record
the electrical potential of the cardiac musculature immediately beneath the electrode in six different locations surrounding the heart
Lead 1=?
Right arm and left arm
Lead 2=?
right arm and left leg
Lead 3=?
Left arm and left leg
explain ECG cycle
SA node fires
P wave = atrial depolarization
PR segment = AV nodal delay
QRS complex = ventricular depolarization (atria polarizing simultaneously)
ST segment = time during which ventricles are contracting and emptying
T wave = ventricular repolarization
TP interval = time during which ventricles are relaxing and filling
on a normal ecg is a separate wave for atrial repolarisation visible
no
the electrical activity associated with atrial repolarisation occurs simultaneously with ventricular depolarisation and is masked by the QRS complex
why is the p wave smaller than the QRS complex
because the atria have much smaller muscle mass than the ventricles
explain the p wave
atrial depolarisation moving towards the recording electrode
explain the Q wave
left to right depolarization of the interventricular septum moving slightly away from the electrode
explain the R wave
depolarisation of the main ventricular mass moving towards the recording electrode
explain the S wave
depolarization of ventricles at the base of the heart moving away from the recording electrode
explain the t wave
ventricular repolarisation moving in a direction opposite to that of the depolarization accounts for the usually observed upward deflection
the main thing to happen at the QRS complex???
ventricular depolarisation
