Cardiovascular

Question 1

T/F - The normal resting membrane potential of a ventricular myocyte is -70mV

Answer 1

FALSE

The normal RMP of a ventricular myocyte is -90mV

The maximum diastolic potential for a pacemaker myocyte is -65mV

Question 2

T/F - At the resting membrane potential, the ventricular myocyte is more permeable to K+ than is it to either Na+ or Ca2+

Answer 2

TRUE

The increased permeability to K+ through the inwardly rectifying K+ current maintains the RMP close to the Nernst potential for K+ (-94mV)

It has low permeability to Ca2+ and Na+ through the Na+/Ca2+ ATPase exchanger and the Na+/K+ ATPase pumps, which allow some movement of these ions and thus the RMP does not quite reach the Nernst potential for K+.

Question 3

T/F - Initial depolarisation of the ventricular myocyte (phase 0) is mostly due to the opening of ligand gated Na+ channels

Answer 3

FALSE

Phase 0 is due to opening of VG Fast Na+ channels.
These channels open at MP -70mV, which is triggered by local currents from spread via gap junctions

Question 4

T/F - In fast-response action potentials, L- type calcium channels open as the myocyte membrane potential becomes less negative in phase 0

Answer 4

TRUE

The L-type Ca2+ channels start to open at -40mV however they are much slower than the VG Na+ channels.
Therefore, the effects of the ionic flux that occurs with opening of L-type Ca2+ channels is not seen until phase 2

Question 5

T/F - Voltage gated calcium channels maintain the relatively positive membrane potential during the plateau phase of the action potential (phase 2)

Answer 5

TRUE

Phase 2 of the FRAP is the plateau phase. There is a balance between positive Ca2+ ion influx through VG L-type Ca2+ channels, and positive K+ efflux through the delayed rectified current

Question 6

T/F - There are multiple different types of potassium channels in the ventricular myocyte

Answer 6

TRUE

Delayed rectifier current - slow VG K+ channels
Inwardly rectifying current - slow VG K+ channels
Transient outward K+ current - fast VG K+ channels

Question 7

T/F - Conductance for K+ ions leaving the myocyte is reduced during phase 2

Answer 7

TRUE

K+ conductance is greatest during phases 3 and 4

Question 8

T/F - Ventricular myocytes have an absolute (effective) refractory period (ERP), which prevents the development of cardiac tetany

Answer 8

TRUE

The absolute refractory period is during phase 0, 1, 2, and the first two thirds of phase 3. The Fast VG Na+ channels have an inner h gate that has time dependent closure and will not open until MP reaches -60mV. This facilitates time for myocyte relaxation and prevents tetany.
Between -60mV and -90mV, if a supramaximal stimulus is applied to the myocyte, a slow-response action potential is generated (relative refractory period)

Question 9

T/F - The absolute (effective) refractory period (ERP) terminates when the cardiac sodium channel h gates move from the closed to open position

Answer 9

TRUE

The absolute refractory period is during phase 0, 1, 2, and the first two thirds of phase 3. The Fast VG Na+ channels have an inner h gate that has time dependent closure (~0.1secs after activation) and will not re-open until MP reaches -60mV. This facilitates time for myocyte relaxation and prevents tetany.

Question 10

T/F - The upstroke of the slow response action potential (phase 0) is caused by the activation of voltage gated sodium channels

Answer 10

FALSE

Phase 0 of the SRAP is due to activation of slow VG L-type Ca2+ channels and subsequent Ca2+ influx

Question 11

T/F - The resting membrane potential (RMP) in pacemaker cells is less negative that that of cells which normal exhibit a fast response AP

Answer 11

TRUE

Pacemaker cells do not have a true “RMP”, but instead have a maximum diastolic potential of -65mV.
The RMP of FRAP myocytes is -90mV

Question 12

T/F - Calcium channels play an important role in phase 0 of the slow response AP

Answer 12

TRUE

VG L-type Ca2+ channels are responsible for phase 0 of the slow response AP

Question 13

T/F - In phase 4 of the slow response AP, the RMP slowly becomes more positive due to the the movement of both sodium and calcium into the cell

Answer 13

TRUE

The pacemaker current is determined by HCN channels which permit Na+ leak into the cell.
When MP reaches -50mV, fast BG T-type Ca2+ channels open, which permit Ca2+ leak into the cell.

Question 14

T/F - Sodium flux into the cell via the funny current is more pronounced when the cell is most depolarised

Answer 14

FALSE

HCN channels open when MP -60mV and close when MP -40mV.
When the cell is most depolarised (e.g. MP +20mV) the funny current is inactive due to closure of the HCN channels

Question 15

T/F - The ionic basis of automaticity has been found to be increasingly complex as the ability to sequence various genes has become more advanced

Answer 15

TRUE

Question 16

T/F - Electrical activity moving toward an ECG electrode will been seen as positive deflection

Answer 16

TRUE

The ECG isoelectic line occurs when there is no net electrical activity. A deflection represents the magnitude and direction of the electrical.
Positive deflection = electrical activity towards the electrode (relative to the indifferent electrode)
Negative deflection = electrical activity away from the electrode (relative to the indifferent electrode)

Question 17

T/F - The peak of the R wave corresponds to phase 0 in the ventricular muscle

Answer 17

FALSE

The QRS complex represents ventricular depolarisation.
Q wave = left to right septal depolarisation
R wave = septal and left ventricular depolarisation
S wave = late depolarisation of the ventricular walls closer to the AV junction

The peak of the R wave corresponds to phase 1 in the ventricular muscle

Question 18

T/F - Increased ventricular mass results in a larger R wave as there is a larger amount of current flow

Answer 18

TRUE

Ventricular hypertrophy can be seen as increased QRS amplitude due to greater net electrical activity

Question 19

T/F - The ST segment occurs during the plateau phase of the venticular action potential

Answer 19

TRUE

The QRS complex represents phases 0-1 of the ventricular AP. The ST segment represents phase 2 (plateau), The T wave represents phase 3 (repolarisation) of the ventricular myocytes.

Question 20

T/F - During the ST segment, the ventricle is depolarised so there is no potential difference between to enable current flow

Answer 20

TRUE

Although there is ionic movement during phase 2 (plateau of the cardiac AP, it is balanced Ca2+ influx and K+ efflux, resulting in no net potential difference. This is represented with an isoelectric line on the ST segment

Question 21

T/F - Ischaemic cardiac tissue is more permeable to K+ than healthy tissue, resulting in earlier repolarisation of that tissue compared to the surrounding tissue, which may contribute to the elevation of the ST segment

Answer 21

TRUE

During cardiac ischaemia:

1) Accelerated opening of K+ channels –> Abnormally rapid repolarisation –> ST elevation
2) K+ efflux –> Loss if intracellular [K+] –> Decreased RMP –> Infarcted fibres depolarise more slowly –> TQ segment depression (seen as ST elevation)

Question 22

T/F - Hyperkalaemia is associated with peaked T waves. This could be explained by a larger gradient for inward flux of potassium into the cell during repolarisation

Answer 22

TRUE

Increased plasma [K+] –> Increased K+ flux into the cell during repolarisation ==> Tall, peaked T waves

Question 23

T/F - Low potassium levels, may result in a more negative resting membrane potential, making normal initiation of both fast and slow action potentials more difficult

Answer 23

FALSE

Hypokalaemia –> RMP is closer to threshold potential –> Increases myocyte excitability

Question 24

T/F - With severe hyperkalaemia, the cardiac membrane becomes un-excitable the heart arrests in diastole

Answer 24

TRUE

Hyperkalaemia –> Hyperpolarisation of the myocyte membrane –> Increased stimulus required to generate and propagate AP

Question 25

T/F - Severe hypocalcaemia will reduce the ability of the heart to generate SRAPs, as calcium is essential for phase 0

Answer 25

FALSE Severe hypocalcaemia prolongs phase 2 (plateau) of the fast response action potential.

Question 26

T/F - Hypocalcaemia prolongs the ST segment as the FRAP plateau is lengthened

Answer 26

TRUE Hypocalcaemia causes QTc prolongation due to prolonged phase 2 (plateau)

Question 27

T/F - Lengthening of the cardiac action potential plateau will tend to prolong the QT interval

Answer 27

TRUE The QT interval is the time from ventricular depolarisation (phase 0) to repolarisation (phase 3) of the fast AP. Therefore, if phase 2 (the plateau) is increased, the QT interval will be prolonged

Question 28

T/F - Sodium channel blockade by local anaesthetics will have more effect on the FRAP than SRAP

Answer 28

TRUE VG Na+ channels are not involved in slow response action potentials, whereas they are vitally important for phase 0 of the fast response action potential.