Origin and the Heartbeat

Question 1

What are the 2 major cell types found in the heart muscle?

Answer 1

• Contractile cells (the majority)
• Autorhythmic cells (non-contractile cells with unstable membrane potentials)

Question 2

How does the membrane of non-contractile cells generate an action potential

Answer 2

• The membrane depolarises slowly until it reaches threshold value
• At this point, an action potential is generated

Question 3

What is the primary pacemaker region in the heart?

Answer 3

Sinoatrial node

Question 4

Complete the sentence…
The …(1)… of depolarisation sets the pace of the heart

Answer 4

Fastest rate

Question 5

How does the conduction system work?

Answer 5

• The sinoatrial node spontaneously depolarises
• The autorhythmic cells then form a specialised conduction system
• Provides a faster conduction system than possible through gap junctions

Question 6

What is the order of conduction through the heart?

Answer 6

• SAN
• Atrial muscle
• Atrioventricular node
• Bundle of His
• Purkinje fibres
• Ventricular muscle

Question 7

Describe a pacemaker potential…

Answer 7

• 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)

Question 8

What type of membrane potential do most mammalian cells have?

Answer 8

Most have negative membrane potential (inside of the cell is -ve compared to the outside)

Question 9

How does the movement of Na+, Ca2+ and K+ affect membrane potentials?

Answer 9

• Na+ moving in depolarizes the membrane
• Ca2+ moving in depolarizes the membrane
• K+ moving out repolarizes the membrane

Question 10

Explain what is happening during each of the 5 steps of polarization/ repolarization in the SAN cells

Answer 10

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

Question 11

Where in the heart has the 2nd highest rate of depolarization?

Answer 11

Atrioventricular node

Question 12

Where are the other autonomic foci?

Answer 12

• Atrial foci (60-80 bpm)
• Junctional foci (40-60 bpm)
• Ventricular foci (20-40 bpm)
  (these can be important in cases of arrhythmia)

Question 13

What are the key features of cardiac muscle?

Answer 13

• Striated
• Cells have one central nucleus, many nitochondria and a good blood supply.

Question 14

How are myocytes connected?

Answer 14

Myocytes connected via intercalated disks
-> Electrically by gap junctions
-> Mechanically by desmosomes

Question 15

How is an action potential formed in cardiac muscle?

Answer 15

There is no slow depolarisation of myocytes, they must wait for an action potential to reach them via a gap junction

Question 16

How does Ca2+ enter myocytes during conduction?

Answer 16

• 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)

Question 17

How is Ca2+ moved during relaxation of the cardio-myocytes?

Answer 17

• 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.

Question 18

What is the importance of the refractory period in cardiac muscle?

Answer 18

Allows time for the atria and ventricles to empty and refill before the next contraction

Question 19

When does coronary circulation flow?

Answer 19

Coronary circulation only flows between heart beats.
When the heart is contracting, the coronary arteries are constricted and blood flow is halted.

Question 20

What causes the heart sounds you can hear through a stethoscope?

Answer 20

The sound of heart valves closing.

Question 21

What is an ECG?

Answer 21

An electrocardiogram used to measure the electrical activity of the heart.

Question 22

Explain the 7 stages of the cardiac cycle.

Answer 22

1) Atrial contraction
2) Isovolumetric contraction
3) Rapid ejection
4) Reduced ejection
5) Isovolumetric relaxation
6) Rapid filling
7) Reduced filling

Question 23

What are the 4 normal heart sounds we can hear?

Answer 23

S1) (lub) = closure of AV valves at ventricular systole
S2) (dub) = closure of semilunar valves at ventricular diastole
S3 & S4 = only heard in horses

Question 24

What is the difference in the heart sounds between horses, dogs, and cats?

Answer 24

Horses have all 4
Dogs and cats = only S1 & S2

Question 25

What is a murmur?

Answer 25

An abnormal heart sound that is often caused by leaky valves

Question 26

What is the P wave on an ECG?

Answer 26

P wave shows atrial depolarisation

Question 27

What is the QRS complex on an ECG?

Answer 27

Ventricular depolarisation

Question 28

What is the T wave on an ECG?

Answer 28

Ventricular repolarisation

Question 29

What is the PQ interval on an ECG?

Answer 29

Atrioventricular conduction time

Question 30

What can an ECG provide information on?`

Answer 30

• Heart rate, rhythm and origin of excitation
• Spread of decay and excitation
• Anatomical orientation of the heart
• Relative chamber size