Heart Flashcards
Where does the electrical activity start and travel
Initiated in the SA node (pacemaker)
Conducted to the atria and AV node.
Then to the atrioventricular ring and through the bundle of his
Then to the purkinje fibres and ventricular muscles.
Desmosomes
Connect the cells in the heart and form electrical synapses
Very fast transfer of electrical activity
Faster than neurons synapses.
What happens if the SA node isn’t working.
The AV node can take over
SA node size and conduction and where it is
Where the pulse spreads.
Conduction from the right to left atrium
15mm x 5 x 2
Conduction 0.05ms (slow)
Found in posterior aspect of the heart
Electrical activity spreads to the Av node and left atrium.
Conduction is 1ms via atrial myocardium or Bachman’s bundle.
It is a lot faster in the myocytes in the atria than it is in the nodes.
AV node size, where it is and conduction.
Zones
Why is there the conduction that is has
AV refractoriness
22mm x 10 x 3
Found in the posterior part of the heart in the interatrial septum.
Slow conduction from AN to N.
The zones are the atrial nodal AN, nodal N and nodal ventricular NV.
The slow conduction is the AV delay which allows atrial contraction to finish before stimulating the ventricles.
AV refractoriness- the AV node has a refractory period after firing to stop the over stimulation of ventricles. To slow down the heart rate you increase the AV refractoriness.
Sub nodal conduction and speed
What does the timing cause
Fast conduction via bundle of his 1ms
Septal activation is where the electrical activity travels down the septum separating the ventricles.
Purkinje fibre speed is 4ms.
Ventricular muscle is 1ms.
The timing of how the ventricular muscles contract causes the heart to undergo a spiral muscle contraction.
Cardiac action potentials
Cardiac vs neuron
There are two types. Nodal or contractile/myocyte.
Both the nodes have nodal action potentials
Atrial muscle, purkinje fibres, ventricular muscle, have contractile action potentials.
Nodal action potentials can fire without other signals coming in. Autorythmicity.
Contractile ones need signals from pacemakers to fire.
Cardiac action potentials are longer than neural ones.
Pacemaker cells
What is the pre potential
How does it repolarise
Both the nodes and the purkinje fibres can be pacemakers.
They don’t have a resting membrane potential because as soon as it reaches the lowest potential of -60 it starts to increase again and slowly depolarise.
Slow Na influx until -40 is reached and then there is fast Ca influx to reach the action potential.
Fast K outflow
Ion channel activity graph
IK and IF
The further the outward and inward current moves from 0 the bigger the function of ion channels.
0 means there is no current
IK is the K leaving the cell.
IF is the funny current which mediates the slow Na influx.
Look at picture
When is the IF current active and inactive
The IF current is an ion cation channel and is activated by hyperpolarisation.
When the membrane potential is positive the IF current is inactive.
Control of pacemaker cells
Para NS- hyperpolarises the cell so the potential is moved further away from the threshold and the action potential takes longer to fire.
The pre potential will take longer and start from a more negative potential. And this will slow heart rate.
Symp NS- they will increase the heart rate by increasing the action potential frequency. The prepotential will happen quicker and easier.
What does noradrenaline and ach do for heart rate
The sympathetic noradrenaline activates beta 1 and 2 receptors which increase CAMP levels.
This activates the IF current and will accelerate depolarisation.
The parasympathetic ach activates M2 receptors which will decrease CAMP levels and inhibit the IF current and slow depolarisation
Action potential in the cardiac muscle fire when
What is a consequence of the long action potential
Absolute refractory period and then what
A signal tells them to and are not automatic.
They can’t do summation which is good because we don’t want to make too much of a large response. We only want one and then it turns off.
Action potentials can’t fire.
Then there is the relative refractory period where it is very hard but possible to fire one.
What’s the process of making an action potential
Depolarisation is initiated by opening of Na channels.
This triggers the opening of more Na channels.
They close when the voltage gets to +30 and an action potential fires.
Ca channels open and Ca enters slower than Na. This will prolong the depolarisation and create a plateau on the graph.
Ca channels then close and K open. And this causes repolarisation.
Picture. Between blue and pink is absolute refractory period.
What are the channels used in making the action potential.
What inhibits them
Na channel is the NaV1.5 channel that is found in cardiac myocytes and is inhibited by TTX and local anaesthetic.
Ca channel is CaV1.2 which is inhibited by nifedipine and verapamil.
K channels are IKS AND IKR. KV11.1 and Kv7.1.
If one of the many types of channel are mutated it’s okay because you have more.
IF current is most permeable to Na the channel is called HCN4 and is blocked by Cs.
