Generating the Cardiac Rhythm Flashcards
Which nerve(s) are the source of sympathetic innervation to the heart?
Spinal nerves T1-T4.
Which nerve(s) are the source of parasympathetic innervation to the heart?
The vagus nerve.
What is the heart rate of a heart that receives no external innervation?
100bpm.
What is myogenic rhythmicity / autorhythmicity?
The ability of cardiac muscles to depolarise and contract rhythmically without external innervation.
What are the three types of action potentials in the heart?
1 - Those arising from SA and AV nodes.
2 - Those arising in atrial muscle.
3 - Those arising in purkinje fibres and ventricular muscle.
Which structure is known as the pacemaker of the heart?
The sinoatrial node.
Are the cells that constitute the sinoatrial node contractile or non-contractile?
Non-contractile.
What electrical event must occur between action potentials in the sinoatrial node?
The pacemaker potential.
Which ions are responsible for the pacemaker potential in the SA node and how are they involved?
- Inward movement of Na+ (NOT voltage gated channels) increases voltage.
- Inward movement of Ca2+ through T-type channels (low voltage gated) increases voltage.
- A decrease in conductance of K+ decreases outward movement so increases voltage.
Which ions are responsible for the action potential in the SA node and how are they involved?
For the initial increase in voltage:
1 - Inward movement of Ca2+ through L-type channels (high voltage gated) for initial upwards slope.
2 - A further decrease in K+ conductance decreases outward movement so contributes to initial upwards slope.
For the repolarisation:
3 - Outward movement of K+.
How does parasympathetic activity decrease heart rate?
What this effect on the heart rate known as?
- By opening K+ channels, thereby hyperpolarising the cell and increasing K+ conductance, reducing the slope of the pacemaker potential.
- Known as a negative chronotropic effect.
How does sympathetic activity increase heart rate?
What is this effect on the heart rate known as?
- By increasing Na+ and Ca+ conductance, thereby increasing the slope of the pacemaker potential.
- Known as a positive chronotropic effect.
Why are heart muscle cells collectively known as a functional syncytium?
Because the contraction of cardiac muscle is coordinated along its entire length, as the current flows to adjoining cells through gap junctions at intercalated discs.
What is the rate of conduction through the atria?
0.5m/s.
Describe the profile of electrical activity of atrial tissue.
- Stable resting membrane potential of -80mV.
- Very steep depolarisation; almost vertical to +10mV.
- Initially steep repolarisation, then plateau.
- Repolarisation steepens until -80mV resting potential (no hyperpolarisation).
Where does the electrical current pass after depolarising the atria?
The atrioventricular node.
Describe the profile of electrical activity of the sinoatrial and atrioventricular nodes.
- A very shallow pacemaker potential, beginning at -60mV.
- A smooth increase at the beginning of the action potential (-40mV) to reach +10mV.
- An equally smooth decrease to -60mV, where the pacemaker potential begins again.
Why does the atrioventricular node have pacemaker capability if it is overridden by the SA node?
In order to take over pacemaker responsibility should the sinoatrial node cease to function.
What is the rate of conduction through the atrioventricular node?
0.05 m/s
What can affect the conduction rate through the atrioventricular node?
The sympathetic nervous system only (can act to increase the rate of conduction).
Why is the rate of conduction so much slower through the atrioventricular node than in the atria?
To ensure atrial depolarisation, contraction and ejection before depolarisation of the ventricles.
Where does the electrical current pass after depolarising the atrioventricular node?
The Bundle of his.
What is the rate of conduction through the Bundle of His?
2m/s.
Where does the electrical current pass after depolarising the Bundle of His?
Left and right bundle branches.
What is the rate of conduction through the left and right bundle branches?
2m/s.
Where does the electrical current pass after depolarising the left and right bundle branches?
Purkinje fibres.
What is the rate of conduction through Purkinje fibres?
4m/s.
What is the pacemaker rate of the atrioventricular node?
40 bpm.
List the areas of the heart that are supplied with parasympathetic innervation from the vagus nerve.
1 - The SA node.
2 - The AV node.
List the areas of the heart that are supplied with sympathetic innervation.
1 - The SA node.
2 - The AV node.
3 - The left and right bundle branches.
4 - Purkinje fibres.
How long does the atrial action potential last?
200ms.
How long does the ventricular action potential last?
250-300ms.
How long does the sinoatrial node action potential last?
200ms.
How long does the atrioventricular action potential last?
200ms.
Describe the profile of electrical activity of ventricular tissue.
- Stable resting membrane potential of -80mV.
- Very steep depolarisation; almost vertical to +25mV.
- Initially steep repolarisation (partial repolarisation), then plateau.
- Repolarisation steepens again until -80mV resting potential (no hyperpolarisation).
What causes the plateau during repolarisation for ventricular tissue?
Inward Ca2+ movement via L-type Ca2+ channels AND outward movement of K+ after inactivation of Na+ channels.
Why is it important that cardiac myocytes have a long absolute refractory period?
To prevent further contractions from being generated, as this would result in summation / tetanic contractions, and therefore an increase in muscle tension.
