Conducting system

Question 1

Where does the sino atrial node lie, what is it comprised of and what is its role?

Answer 1

Below epicardial surface at the boundary between the right atrium and superior vena cava
Comprised of specialised cells that start a conduction pathway
Spontaneously depolarises to allow autorhythmic contraction

Question 2

How does an AP spread through the heart?

Answer 2

Sino atrial node
Internodal fibres
Atrio ventricular node
bundle of His
Ventricular fibres - purkinje fibres

Question 3

Where are the internodal fibres located and what is their purpose?

Answer 3

Within atrial myocardium

Rapid conduction tracts that stimulate the atrial myocardium

Question 4

What is the atrioventricular node?

Answer 4

Specialsed cells that delay the wave of excitation and insulate the superior ventricular myocardium, to allow ventricular filling via separation of atrial and ventricular contraction

Question 5

What is the bundle of His?

Answer 5

Rapid conducting fibres, slightly insulated

Allow conduction to apex and the ventricular fibres

Question 6

What are the ventricular fibres?

Answer 6

Spread up the ventricular myocardium
Allows upwards spread from apex
Produces ventricular excitation for increased pressure

Question 7

How can we estimate the membrane potential?

Answer 7

Goldman-Hodgkin-Katz equation

takes into account relative permeabilities of K+, Na+ and Cl-

Question 8

What does the resting membrane potential depend on?

Answer 8

Flow of K+ out of cell

Concentration of K+ maintained by Na+/K+ ATPase pumps

Question 9

Explain the stages of the changes in ionic permeability of the membrane.

Answer 9

0 - upstroke
-Na+ permeability increases, allowing influx of Na+
1 - early repolarisation
-Transient outward current due to brief K+ efflux
2 - Plateau
-Ca2+ permeability increases, allowing Ca2+ influx to prolong the AP
3 - Repolarisation
-K+ permeability slowly increases to partially depolarise
-When potential low enough, Ik1 opens significantly to efflux large amount of K+ and returns cell to
4 - Resting membrane potential
-Ik1 open to allow flow in diastole to stabilise RMP

Question 10

Explain the differences between the ventricular action potential and the sinoatrial node action potential.

Answer 10

SA node is always oscillating
SA node has no Ik1 current and ∴ no RMP
Na+ channels open in SA node diastole to produce small depolarisation but the upstroke is provided by Ca2+ influx

Question 11

How is the length of the SA node action potential modified to alter heart rate?

Answer 11

Increased sympathetic stimulation = shorter AP caused by noradrenaline - depolarises and reaches threshold more quickly to increase HR
Increased parasympathetic stimulation = Longer AP caused by acetylcholine - depolarises and reaches threshold more slowly to decrease HR

Question 12

How is intrinsic heart rate modulated?

Answer 12

Parasympathetic vagus nerve from cardioregulatory/vasomotor centres in medulla to SLOW HR
Sympathetic innervation increases HR and contractility (chronotropy and inotropy)

Question 13

What is the main difference between a cardiac action potential and a nervous action potential and what does this mean for the heart?

Answer 13

Cardiac AP much longer
The duration of AP controls the duration of contraction
Longer and slower contraction needed for effective pumping

Question 14

What is a refractory period and what is it caused by?

Answer 14

Absolute refractory period - when no AP can be initiated regardless of stimulus intensity
Relative refractory period - larger than normal stimulus can produce AP
Caused by Na+ channel inactivation allowing recovery of repolarisation (more -ve membrane potential = more reactivated channels)