Origin of the heartbeat Flashcards
What are the 2 major cell types found in the heart muscle?
- Contractile cells (the majority)
- Autorhythmic cells (non-contractile cells with unstable membrane potentials)
How does the membrane of non-contractile cells generate an action potential
- The membrane depolarises slowly until it reaches threshold value
- At this point, an action potential is generated
What is the primary pacemaker region in the heart?
Sinoatrial node
Complete the sentence…
The …(1)… of depolarisation sets the pace of the heart
Fastest rate
How does the conduction system work?
- The sinoatrial node spontaneously depolarises
- The autorhythmic cells then form a specialised conduction system
- Provides a faster conduction system than possible through gap junctions
What is the order of conduction through the heart?
- SAN
- Atrial muscle
- Atrioventricular node
- Bundle of His
- Purkinje fibres
- Ventricular muscle
Describe a pacemaker potential…
- Gradual and slow depolarisation of the membrane of SAN cells
- This then reaches threshold and triggers an action potential
- AP in SAN is terminated
- Gradual depolarization
(this then repeats)
What type of membrane potential do most mammalian cells have?
Most have negative membrane potential (inside of the cell is -ve compared to the outside)
How does the movement of Na+, Ca2+ and K+ affect membrane potentials?
- Na+ moving in depolarizes the membrane
- Ca2+ moving in depolarizes the membrane
- K+ moving out repolarizes the membrane
Explain what is happening during each of the 5 steps of polarization/ repolarization in the SAN cells
1) Leaky F channels allow Na+ to diffuse down its conc gradient into the cell, causing slow depolarization
2) As the cell depolarizes, F channels close but Ca2+ channels open, continuing depolarization
3) When membrane potential reaches threshold, another type of Ca2+ channel opens, allowing rapid influx. This causes steep depolarization
4) At the peak action potential, Ca2+ channels close and K+ channels open to allow for slow repolarization
5) K+ channels shut and F channels re-open
Where in the heart has the 2nd highest rate of depolarization?
Atrioventricular node
Where are the other autonomic foci?
- Atrial foci (60-80 bpm)
- Junctional foci (40-60 bpm)
- Ventricular foci (20-40 bpm)
(these can be important in cases of arrhythmia)
What are the key features of cardiac muscle?
- Striated
- Cells have one central nucleus, many nitochondria and a good blood supply.
How are myocytes connected?
Myocytes connected via intercalated disks
-> Electrically by gap junctions
-> Mechanically by desmosomes
How is an action potential formed in cardiac muscle?
There is no slow depolarisation of myocytes, they must wait for an action potential to reach them via a gap junction
How does Ca2+ enter myocytes during conduction?
- An action potential arrives
- There is then a plateau phase, Ca2+ in the cytosol increases and more Ca2+ is released from the cytoplasmic reticulum
- Calcium binds to troponin, changes the shape of tropomyosin, revealing myosin binding sites on the actin.
(This process is required for systole contraction)
How is Ca2+ moved during relaxation of the cardio-myocytes?
- Ca2+ pumped out of teh cell using ATPase pumps
- Remaining Ca2+ recycled back into sarcoplasmic reticulum
- Facilitated transport of Ca2+ and Na2+ using an antiporter.
What is the importance of the refractory period in cardiac muscle?
Allows time for the atria and ventricles to empty and refill before the next contraction
When does coronary circulation flow?
Coronary circulation only flows between heart beats.
When the heart is contracting, the coronary arteries are constricted and blood flow is halted.
What causes the heart sounds you can hear through a stethoscope?
The sound of heart valves closing.
What is an ECG?
An electrocardiogram used to measure the electrical activity of the heart.
Explain the 7 stages of the cardiac cycle.
1) Atrial contraction
2) Isovolumetric contraction
3) Rapid ejection
4) Reduced ejection
5) Isovolumetric relaxation
6) Rapid filling
7) Reduced filling
What are the 4 normal heart sounds we can hear?
S1) (lub) = closure of AV valves at ventricular systole
S2) (dub) = closure of semilunar valves at ventricular diastole
S3 & S4 = only heard in horses
What is the difference in the heart sounds between horses, dogs, and cats?
Horses have all 4
Dogs and cats = only S1 & S2
What is a murmur?
An abnormal heart sound that is often caused by leaky valves
What is the P wave on an ECG?
P wave shows atrial depolarisation
What is the QRS complex on an ECG?
Ventricular depolarisation
What is the T wave on an ECG?
Ventricular repolarisation
What is the PQ interval on an ECG?
Atrioventricular conduction time
What can an ECG provide information on?`
- Heart rate, rhythm and origin of excitation
- Spread of decay and excitation
- Anatomical orientation of the heart
- Relative chamber size
