7. Electrical Properties of the Heart Flashcards
What is meant when the ion is in equilibrium?
The point where the electrical gradient is equal to the concentration gradient
Describe the movement of in a particular ion in equilibrium?
Ions can move back and forth randomly but there is no net movement of ions
How can the resting membrane potential be predicted?
Using the Nernst equation with potassium. The Goldman-Hodgkin-Katz
How is resting membrane potential established?
Through the movement of potassium through channels. It does NOT have anything to do with the sodium-potassium pump.
How will the membrane potential change? What dictates the value of the membrane potential?
Depends on the relative permeabilities to different ions
How long does a nerve action potential last?
2ms
How long does an action potential in the heart last?
200-400ms
What is the upstroke caused by?
This is caused by the opening of sodium channels
What does the influx of sodium mean?
The cell drives towards the equilibrium potential of sodium
After the upstroke what happens?
The sodium channels inactivate, so the membrane potential starts to recover
As the sodium channels inactivate what occurs?
There is brief increase in the permeability to potassium which repolarises the membrane.
What term is used to describe the sodium channels not opening for a long period of time?
Absolute refractory period
Why will cardiac muscle not tetanise?
It has a long absolute refractory period
What is the point of increasing the permeability to calcium?
The influx of calcium balances the efflux of potassium?
What channels does calcium move through?
L type Calcium channels (L = long lasting)
When does total repolorisation occur?
The eventual inactivation of the L-type calcium channels and the opening of a another subtype of potassium channel
What type of contraction is required to produce an effective pump?
Long slow contraction?
Define absolute refractory period
Time during which no action potential can be initiated regardless of stimulus intensity
Define relative refractory period
Period after absolute refractory period where an action potential can only be elicited with stimulus strength larger than normal
Define full recovery time
The time at which a normal action potential can be elicited with normal stimulus
How is the internal electrical activity of the heart modulated?
It is modulated by sympathetic and parasympathetic nerves
What are the phases of the action potential?
Phase 0 = upstroke Phase 1 = early repolarisation Phase 2 = plateau Phase 3 = repolarization Phase 4 = resting membrane potential (diastole)
What is Phase 1 caused by?
Early repolarisation is caused by the inactivation of sodium channels and the transient outward potassium current starting
What is phase 2?
The plateau stage caused by the calcium influx which keeps the membrane potential more positive
What does the influx of calcium cause?
The influx of calcium triggers calcium to be released from intracellular stores used for contraction
What can be used for antihypertensive therapy?
Calcium permeability inhibiting drugs: Nifedipine Nitrendipine Nisoldipine These work by blocking the L-type calcium channels in the smooth muscle cells preventing contraction
Which current is responsible for fully repolarising the cell?
IK1 switches off during depolarisation but as the membrane gradually becomes more repolarised the IK1 channel switches on
Describe the IK1 current?
This IK1 current is large and flows during diastole
What does IK1 do?
Stabilises the resting membrane potential and reduces the risk of arhythmia
Where are there no IK1 channels?
SA node cells
How much Na influx are there in the SA node cells?
very little Na influx
How is the upstroke produced in SA node cells?
It is produced by Ca influx
What Ca channels are there in SA node cells?
T-type Ca channels. These channels activate at a more negative potential than L-type Ca channels
What does sympathetic stimulation of the heart cause?
A steeper pacemaker which means it reaches the threshold potential more quickly therefore increasing heart rate
What does parasympathetic stimulation of the heart cause?
A decrease in the gradient of the pacemaker potential which means the it takes longer for the membrane potential to be reached thus decreasing heart rate
What are the four basic components of the conduction system of the heart?
- Sinoatrial node
- Inter-nodal Fibre Bundles
- Atrioventricular Node
- Ventricular bundles
What is the sinoatrial node?
Small mass of specialised cells situated in the superior aspect of the right ventricle
Where the sinoatrial node located?
On the anterolateral margin between the orifice of the superior vena cava and atrium
What is the function of the inter-nodal fibre bundles?
They conduct the action potential to the AV node at a greater velocity than ordinary atrial muscle
What is the function of the AV node?
Electrically connects the conduction systems between atrial and ventricular chambers
How long of a delay does the AV node produce?
0.1 seconds
Where does the bundle of His descend from?
The AV node
How many branches does the bundle of His divide into?
Two bundle branches
What makes up the bundle branches?
Purkinje fibres
Where do the terminal Purkinje fibres extend?
Beneath the endocardium and penetrate approximately one-third of the distance into the myocardium
What is essential for impulse propagation?
Low resistance
How is low resistance established?
Gap junctions
Describe the excitation sequence
- Initially the SA node fires and the action potential propagates across the atria
- The depolarisation moves towards the electrode so you get a small upward deflection
- The depolarisation then moves away from the electrode giving a downstroke
- It moves towards the electrode as it moves down the bundle branches
- It moves away from the electrode as it goes up the Purkinje fibres
