Cardiac conducting systemAnatomy and Physiology

QUIZ

a)

b)

f)

e)

d)

c)

1. Overview

Label the diagram.

2. Membrane potentials 1

The graph shows the changing membrane potentials for a sinus nodal fiber and a muscle fibre.Which is which?

a)

b)

2. Membrane potential timing – extra info

3. Membrane potentials 2

Where on the Y-axis is the zero millivolt level?

A

B

C

D

E

4. Pacemaker membrane potential

Identify the currents

IbBackground inward Na+ current

ICaLLong lasting Ca++ channels

ICaTTransient Ca++ channels

IfFunny current (inward K+/Na+, hyperpolarised voltage gated )

IKDelayed rectifier K+ current

a)

b)

c)

e)d)

5. Ectopic membrane potential

Identify the currents: ICaL, IK, IK1, INa, Ito

ICaLLong lasting Ca++ channels

IKDelayed rectifier K+ current

IK1Inwards rectifier K+ current

INaInward Na+ current

ItoTransient outward K+ current

a)

b)

c)

e)

d)

6. Histology

Match the labels a) Endocardiumb) Myocardium

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c) Bundle Branch fibresd) Purkinje fibres

H&E x150 H&E/Elastin x400

(a) shows the left branch bundle of conducting fibres in the interventricular septum beneath the endocardium.At this level the conducting fibres are separated from the myocardial fibres by a layer of fibrous tissue.The conducting fibres are specialised cardiac muscle fibres, and contain few myofibrils, mainly located beneath the cell membrane, but abundant glycogen granules and mitochondria. This makes these fibres paler staining than normal myocardial fibres by most stains.

(b) shows the Purkinje fibres beneath the thin endocardium.These fibres are larger than cardiac muscle fibres and have a pale staining central area with most of the red-staining myofibrils around the periphery of the cell. Purkinje and other conducting fibres have no T tubule system, and connect with each other by desmosomes and gap junctions rather than intercalated discs.

6. Histology explained

In what order does the impulse activate the labelled parts?Bonus points – give an expected time in seconds for the impulse to reach each part.

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7. Propagation of cardiac impulse

In a non-pathological heart, does a particular cross-section of the ventricle wall depolarize:

A. Outer to innerB. Inner to outerC. All at onceD. Middle to inner and outer

8. Easy multi-choice

The SA node is located near the juncture of the:

A. Left atrium and left pulmonary veinsB. Left atrium and right pulmonary veinsC. Right atrium and superior vena cavaD. Right atrium and inferior vena cava

9. Easy multi-choice

If cardiac impulses are completely blocked at the bundle of His, then the atricular beat will be:

A. NormalB. SlowC. FastD. ceased

10. Multi-choice

If cardiac impulses are completely blocked at the bundle of His, then the ventricular beat will be:

A. NormalB. SlowC. FastD. ceased

11. Multi-choice

Which of the following would be expected to result in a decrease in ventricular contractility?

A. hypercalcemiaB. hypocalcemiaC. hypokalemiaD. hypernatremia

12. Multi-choice

14. Nasty

During an acute MI, extracellular potassium rises rapidly to 10-20mM. This will produce:

A. Decreased Na+ channel inactivation

B. Decreased thresholdC. Membrane

hyperpolarizationD. Slowing of conduction

The decreased K+ concentration gradient will increase the resting potential. This mean some Na channels will stay inactivated, reducing the available Na current. This will slow or block conduction.

In a patient with atrial tachyarrhythima, giving a drug that increases the effective refractory period in the AV node would be expected to:

A. Abolish the arrhythmiaB. Increase the PR interval (the time to conduct

an impulse through the AV node)C. Slow the ventricular rateD. Induce a third degree AV block

15. Nasty

A rise in intracellular concentration of which ion can indirectly result in an increase in ventricular contractility?

A. Cl-B. K+D. Na+C. HCO3-

16. Nasty

Reducing the Na+ gradient makes the Na/Ca pump less effective, so less calcium is pumped out of the cell.The excess calcium is sucked into the sarcoplasmic reticulum, which then releases extra calcium on subsequent impulses, for more contractility.

The following condition can result in an increase in ectopic automaticity, resulting in an increased incidence of ventricular arrhythmias:

A. hypercalcemiaB. hyperkalemiaC. hypokalemiaD. hypermagnesemiaE. hyponatremia

17. Nasty

Don’t understand why. Something to do with the K1 current?