cardiovascular control 1

Question 1

what is IK1 responsible for?

Answer 1

responsible for fully repolarising the cell
IK1 is large and flows during diastole
It stabilises the resting membrane potential reducing the risk of arrhythmias by requiring a large stimulus to excite the cells

Question 2

sinoatrial node depolarisation and repolarisation

Answer 2

Most of the types of channels exist in the SA node cells with the exception of IK1.
o The function of IK1 is to maintain a stable membrane potential so SA node cells do not have a stable membrane potential.
§ There is very little sodium influx into SA node cells, instead, the upstroke is caused by Ca2+ influx.
§ The SA node cells contain T-type Ca2+ Channels (TTCC) which activate at a more negative potential than LTCC.
§ The ITO (transient outward) current is very small.
§ The pacemaker current If is present.

Question 3

cardiac muscle cell depolarisation and repolarisation

Answer 3

Phase 1 – Early Repolarisation:
§ Inactivation of sodium channels (no further depolarisation) and the transient outward (TO) potassium current starting. Phase 2 – Plateau:
§ Large intracellular sodium causes calcium influx which triggers CICR from intracellular stores.
§ Potassium efflux and calcium influx stabilise the membrane potential around 0mV.
§ Calcium influx can be blocked by dihydropyridine calcium channel antagonists (bind to LTCC, blocking the channel) such as Nifedipine, Nitrendipine and Nisoldipine. Phase 3 – Repolarisation:
§ Small potassium current starts to activate towards the end of the plateau which begins repolarisation.
§ As the membrane becomes repolarised (due to efflux of potassium via normal potassium channels), the IK1 potassium channel switches back on (it turns off during depolarisation).
§ The IK1 current is large and flows during diastole à it acts to stabilise the resting membrane potential and reduce risk of arrhythmia.

Question 4

modulating the intrinsic heart rate

Answer 4

PNS innervation slows heart rate

PARASYMPATHETIC NERVE: (uses vagus nerve)Cardioregulatory centre and vasomotor centres in medulla

SYMPHATHETIC NERVES: SNS innervation increases heart rate (chronotropy) and contractility (inotropy)

Question 5

explain the cardiac conduction system

Answer 5

Basic Parts:
§ Sinoatrial Node.
o Located in the right atrium.
§ Inter-nodal Fibre Bundles.
o Conduct AP to AV-node at a greater velocity than ordinary atrial muscle.
§ Atrioventricular Node
o Produces a delay of ~100ms.
§ Ventricular Bundles (bundle branches, purkinje fibres).
o Bundle of His descends from AV-node and splits into two bundle branches made of Purkinje fibres
o Purkinje Fibres - conduct AP at around 6x the velocity ordinary cardiac muscle and penetrate 1/3rd distance into myocardial wall. Impulses are propagated between cells by the use of gap junctions (cluster at intercalated discs) which have a low resistance. The effect of depolarisation gradually decays.

Question 6

what is the propagation of the cardiac action potential due to?

Answer 6

due to a combination of passive spread of current and the existence of a threshold which, once reached, causes the cell to generate its own action potential

Question 7

intercellular communication and impulse conduction from one cell to the next relies on?

Answer 7

gap junctions

- gap junctions form at intercalated discs