Cardiac AP and ECG Flashcards
What is the heart’s self-contracting ability called?
Autorhythmicity: The heart contracts rhythmically as a result of APs it generates itself.
What are the types of cardiac cells?
Describe their % in the heart and function
There are two specialised types of cardiac cell:
- Contractile cells
- 99% of mass of heart/of all cells
- contract in response to APs (need stimulation) - normally do not initiate APs
- Autorhythmicc ells
- 1% of mass of heart
- do not contract - initiate and conduct APs
What is the activity of cardiac autorhythmic cells called?
Describe how its membrane potential functions
Cardiac autorhythmic cells display pacemaker activity:
- They do not have a resting membrane potential (vs. nerve or skeletal muscle cells)
- Instead, their cell membrane displays pacemaker potential: slow drift to threshold
- They cyclically initiate APs which then spread through the heart to trigger contraction without any nervous stimulation
Where are autorhythmic cells located in the heart?
What do they comprise?
Autorhythmic cells are loated at 4 specific sites:
1) The sinoatrial node (SA node)
2) The atrioventricular node (AV node)
3) The bundle of His (atrioventricular bundle): originates at AV node and enters the septum between the ventricles. Then it splits into right and left branches
4) Purkinje fibres: small terminal fibers that extend out the bundle of His and spread throughout the walls on the left and right side
They comprise the heart’s conducting system: for generating APs and conducting them throughout the atrial and ventricular muscles
Label the diagram
NB.
Interatrial pathway
Internodal pathway
What controls the rate of the heart?
What makes the transfer of potential within cardiac cells possible physiologically?
What is the implication?
The SA node dictates the rate of atrial and ventricular contraction
This is made possible because cardiac cells are linked electrically via gap junctions. Small molecules and ions can pass in between neighboring cells
The implication is that the rate of the fastest (the SA node) will be the rate of all the cells
What is the normal pacemaker of the heart? What is its firing rate?
What influences it?
What is the backup? What is its rate?
What is the slowest autorhythmic rate?
What is the firing rate of contractile cells?
The SA node acts as the normal pacemaker of the heart. It fires 70 - 80 APs per minute.
SA node discharge frequency can be altered by parasympathetic (slower) and sympathetic (faster) stimulation - ANS.
If the SA node fails the AV node could take over as the next pacemaker for the heart. It fires 40 - 60 APs a minute.
The firing rate of the Bundle of His and the Purkinje fibers is 20 - 40 APs per minute.
Atrial and ventricular myocardium (contractile cells) = 0 APs per min (normally).
How do APs spread from the SA node?
Initially theAPs spread across both atria through 2 pathways:
- The interatrial pathway: links the LHS and RHS of the heart
- The internodal pathway: conducts the AP down towards the AV node - the only point where an AP can spread from the atria down towards the ventricles
- There is a delay at the AV node: After atrial muscle contracts there is a c.100 ms delay before the contraction of the ventricular muscle - this allows time for blood to move from the atria to the ventricles (allows ventricular filling prior to ventricular contraction)
What is a special property of the AV node?
AV node cells have well developed latent powers of rhythmicity: can take over pacemaking if impulses from the SA node fail to reach them.
What are the membrane potential figures for pacemaker cells?
- Membrane potential starts at about -60 mV
- Pacemaker potential: spontaneous slow depolarization to about -40 mV (threshold potential)
- At threshold potential self-induced AP generated which peaks at about 0 mV
What are the ion concentrations inside/outside a pacemaker cell?
Describe how ion channels open/close during the pacemaker AP cycle
Concentration of Na+ and Ca2+ ions is greater OUTSIDE the cell
Concentration of K+ ions is greater INSIDE the cell
The autorhythmic cell AP cycle:
Pacemaker potential: Na+ in then Ca2+ in / Threshold: Ca2+ in / Peak: K+ out
- At -60 mV membrane potential: Funny channels open
- Na+ ions enter the cell
- Causes the first half of the pacemaker potential (slow depolarization to threshold)
-
Transient Ca2+ channels open/Funny channels close
- Ca2+ ions to enter the cell
- Causes the second half of the pacemaker potential (reaches threshold)
- At threshold: -40 mV
- Transient Ca2+ channels close
- Membrane depolarized enough for long-lasting Ca2+ channels to open: Ca2+ ions entering causes AP to peak to 0mV
- At peak AP: 0mV
- Long-lasting Ca2+ channels close
- K+ channels open, causing K+ ions to leave the cell, repolarizing the cell membrane back to -60 mV
What ensures that heart cells cannot contract continusously?
Contractile cells have a refractory period:
they cannot begin to contract until after the AP has subsided (they begin to contract in the shoulder phase of the contractile cell AP).
This ensures heart cells cannot be stimulated to contract continuously, since contraction has to wait for the AP to subside.
What does the ECG record?
The ECG is a record of the overall spread of electrical activity through the heart
What are the ECG waves?
What do they represent?
What do the segments between the waves represent?
- P wave: Atrial depolarization (AP)
- QRS segment: Ventricular depolarization (AP)/Atria repolarization simulateneously
- T wave: Ventricular repolarization
- PR segment: AV node delay
- ST segment: time during which ventricles are contracting and emptying
- TP interval: time during which ventricles are relaxing and filling again
NB: the segments/intervals relate to ventricles only, not to atria
What is the voltage at the peak of the QRS complex?
Voltage at the peak of the QRS complex is about 1 mV
