the cardiac action potential and ECG Flashcards
what is autorhythmicity
the rhythmical contraction of the heart as a result of the action potentials it generates itself
what are the two specialised types of cardiac cells
- contractile cells
- autorhythmic cells
what are contractile cells
99% of cardiac cells
normally do not initiate action potentials
what are autorhythmic cells
do not contract
initiate or conduct action potentials
what are cardiac autorhythmic cells
display pacemaker activity
do not have a resting membrane potential
cyclically initiate action potentials which then spread through the heart to trigger contraction without any nervous stimulation
what is the pacemaker potential
an autorhythmic cell membranes slow drift to threshold
where are autorhythmic cells located
- the sinoatrial node
- the atrioventricular node
- the bundle of His (atrioventricular bundle)
- purkinje fibres
what is the pacemaker of the heart?
the sinoatrial node
pacemaker controls heart rate
the cells of the heart are linked electrically therefore, the rate of the fastest will determine the rate of them all
what are the number of action potentials per minute - SA node
70-80
what are the number of action potentials per minute - AV node
40-60
what are the number of action potentials per minute - bundle of His and purkinje fibres
20-40
what are the number of action potentials per minute - atrial and ventricular myocardium
0
how can the frequency of discharge of the SA node be altered
by parasympathetic and sympathetic stimulation
what is the spread of cardiac excitation
from the sinoatrial node through the right atrium, the interatrial pathway into the left atrium and down the internodal pathway to the AV node
through the AV node down the bundle of His up through the purkinje fibres
what are the functions of the atrioventricular node
forms the only conducting pathway between the atrial muscles and the bundle of His, and therefore the ventricles
introduces considerable delay to the spread of excitation (≈100ms) - this allows time for blood to move from the atria to the ventricle
have developed latent powers of rhythmicity and can take over pacemaking if impulses from the SA node fail to reach them
what is the cycle of voltage in ventricular muscles
changes across cardiac myocytes
occurs in 5 distinct phases:
1. depolarisation
2. early repolarisation
3. plateau phase
4. late repolarisation
5. resting potential
how do cardiac muscles contract
cardiac muscle fibres must contract and relax in a coordinated manner during the cardiac cycle
contraction is triggered by action potentials normally initiated by the SA node and conducted across the heart via a specialised conduction system;
- SA node fires action potentials at a rate of 70-80 per minute (at rest)
- rate of firing is regulated by the autonomic nervous system
how are neighbouring cardiac muscle cells linked.
by the presence of gap junctions
what is a gap junction
gap junctions allow the rapid spread of action potentials from cell to cell, e.g. an action potential originating in the SA node first spreads throughout both atria, stimulating the simultaneous contraction of left and right atrial muscle
what is an ECG (basic)
the ECG is a record of the overall spread of electric activity through the heart
electrical currents generated by cardiac muscles are conducted through the body fluids
a small part of this electrical activity reaches the body surface where it can be detected using recording electrodes
the record produced is an electrocardiogram
what are the important points about an ECG
- 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 action potential 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 are the ECG electrodes
the pattern of activity recorded depends upon the orientation of the recording electrodes
whether an upward or downward deflection is recorded is determines by the way the electrodes are orientated with respect to the current flow in the heart
different waveforms representing the same electrical activity, results when this activity is recorded by electrodes at different points on the body.
what are ECG leads
ECG record routinely consist of 12 conventional electrode systems, or leads
the specific arrangement of each pair of connections is called a lead
the 12 different leads each record electrical activity in the heart from different locations;
- 6 different arrangements from the limbs
- 6 chest leads at various sites around the heart
what are the six limb leads
include; I, II, III, aVR, aVL, and aVF
leads I, II, III are bipolar leads because two recording electrodes are used
the tracing records the difference in potential between the two electrodes
aVR, aVL, aVF leads are unipolar leads, even through two electrodes are used, only the actual potential under one electrode, the exploring electrode, is recorded. the other electrode is set at zero potential and serves as a neutral reference point
what are the six chest leads
V1 to V6
are unipolar leads
the exploring electrode mainly records the electrical potential of the cardia musculature immediately beneath the electrode in six different locations surrounding the heart
what is the connection within ECG lead I
right arm to left arm
what is the connection within ECG lead II
right arm to left leg
what is the connection within ECG lead III
left arm to left leg
what is the sequence of events of ECG recording (basic)
SA node fires > Pwave = atrial depolarisation
> PR segment = AV nodal delay
> QRS complex = ventricular depolarisation (atria repolarise)
> ST segment = ventricles contract and empty
> T wave = ventricular repolarisation
> TP interval = ventricles relax and fill
what is the ECG record
different parts of the ECG record can be correlated to specific cardiac events
a normal ECG has 3 distinct waveforms - P wave, QRS complex, T wave
these shifting waves of de- and repolarising bring about alternating contraction and relaxation of the heart, respectively, the mechanical events of the heart lag slightly behind the rhythmic changes in electrical activity
what are the important points for the ECG record
- no wave is recorded for SA nodal depolarisation
- in a normal ECG, no separate wave for atrial repolarisation is visible, the electrical activity associated with atrial repolarisation occurs simultaneously with ventricular depolarisation and is masked by the QRS complex
- the P wave is smaller than the QRS complex because the atria have a much smaller muscle mass than the ventricles
- the ECG remains at baseline when;
a) during AV nodal delay (PR segment)
b) when the ventricles are completely depolarised and the cardiac contractile cells are undergoing the plateau phase of their action potential (ST segment)
c) when the heart muscle is completely repolarised and at rest and ventricular filling is taking place (TP interval)
what is the P wave
atrial depolarisation moving towards the recording electrode
what is the PR segment
AV nodal delay
what is the Q wave
left to right depolarisation of the interventricular septum moving away from the recording electrode
what is the R wave
depolarisation of the main ventricular mass moving towards the recording electrode
what is the S wave
depolarisation of ventricles at the base of the heart moving away from the recording electrode
what is the QRS complex
ventricular depolarisation, atria repolarise simultaneously
what is the ST segment
time during which ventricles are contracting and emptying
what is the T wave
ventricular repolarisation moving in a direction opposite to that of depolarisation, accounts for the usually observed upward deflection
what is the TP interval
time during which ventricles are relaxing and filling
what is the electrocardiogram
ECG is much smaller than intracellular action potentials (1mV > 100mV)
waves only recorded when the potential is changing across cell membranes - ECG flat during plateau phase of the action potential and diastole
