L02-Generating the cardiac rhythm

Question 1

What is a syncytium? It describes how cardiac cells work

Answer 1

a cluster of cells allowing easy transmission between neighbours

Question 2

How does depolarisation spread between adjacent cardiac cells?

Answer 2

Through gap junctions

Question 3

What is the normal rate at which the SAN produces potentials and what causes the actual rate that is observed?

Answer 3

The normal SAN rate is around 100 action potentials per min however vagal tone of the autonomic nervous system predominates resulting in an actual rate of 60-70 bpm

Question 4

What transmitter and receptors are used by the parasympathetic innervation of the heart? and how does this work?

Answer 4

Ach at M2 receptors. Works by opening K+ channels which hyperpolerises the cell and reduces the slope of the pacemaker potential. (negative chronotropic effect)

Question 5

What transmitter and receptor are used in the sympathetic innervation of the heart? How does this work?

Answer 5

NorAd and Ad at Beta 1 receptors. This works by increasing Na+ and Ca+ conductance that increases the slope of the pacemaker potential.

Question 6

What is the function of the fibrocartilaginous skeleton in terms of conductance?

Answer 6

Acts as an electrical insulator to prevent the depolarisation from passing from the atria to ventricles without first being delayed at the AVN.

Question 7

What is the function of the AV delay?

Answer 7

AV delay ensures atrial depolarisation, contraction & ejection before ventricles depolarise.

Question 8

What is the pacemaker rate of the AVN?

Answer 8

Around 40 bpm

Question 9

What is the pacemaker rate of the ventricular muscle fibres?

Answer 9

20-40bpm

Question 10

What are the different stages of the ventricular myocyte action potential and which ions are responsible for each stage?

Answer 10

A rapid depolarisation followed by partial repolarisation, plateau and then repolarisation.
The rapid depolarisation is caused by inward Na+ movement by fast voltage gated channels,
The partial repolarisation is caused by inactivation of Na+ channels and opening of outward transient K+ channels.
The plateau is caused by inward Ca2+ movement through slow L-type channels which cancels the outward movement of K+ causing a plateau.

Question 11

how long does a ventricular myocyte depolarisation take?

Answer 11

250-300ms

Question 12

During the gradual depolarisation between action potentials in the SAN, what ions and channels cause this?

Answer 12

Initially from around -60mV the Na+ ion channels open allowing ‘funny’ currents to begin to depolarise the membrane.
At around -50mV transient T-type Ca2+ channels open which cause further depolarisation as calcium moves into the cell.
Finally at around -40mV long lasting L-type Ca2+ channels open.

Question 13

What channels are responsible for the depolarisation after threshold in SAN cells?

Answer 13

The L-type calcium channels that open towards the end of phase 4.

Question 14

How does repolarisation of the SAN cells occur?

Answer 14

K+ channels open allowing K+ to leave the cells causing repolarisation and L-type calcium channels inactivate.

Question 15

What is the advantage of the plateau in ventricular myocyte action potential?

Answer 15

It delays the next action potential preventing summation, this allows complete filling of the ventricles.

Question 16

What are tetanic contractions? why cant this happen in the heart?

Answer 16

A tetanic contraction is a sustained muscle contraction evoked when the motor nerve that innervates a skeletal muscle emits action potentials at a very high rate. Cant happen in heart because of the plateau in the action potential.