Generating the cardiac rhythm Flashcards
What is heart muscle able to do without the input of nerves?
Depolarise and contract rhythmically
What are the three types of cardiac action potential?
SA and AV nodes
Atrial muscle
Purkinje fibres and ventricular muscle
In what three ways do cardiac action potentials differ from each other and also from action potentials in muscles or nerves?
Duration
Shape
Ionic basis of depolarisation
Where is the sino atrial node located?
Wall of right atrium where superior vena cava enters
Does the sino atrial action potential have a resting membrane potential?
No
How many action potentials does the SA node have per minute?
100
Is this rate of 100 AP/min changing?
It constantly changes as the SA node is a pacemaker
100 beats per minute is the intrinsic rate
Describe what is happening in the rising limb of the SA node action potential
Inward movement of Na+
Decreased conductance of K+
Inward movement of Ca+
What is the cardiac equivalent of a resting membrane potential?
Pacemaker potential
Describe what is happening when the action potential fires
Inward movement of Ca+ depolarises the cell
Decrease of conductance of K+ continues
Cell repolarised by outward movement of K+
If the intrinsic rate of the SA node is 100 beats per minute, why is resting heart rate closer to 70 beats per minutes?
Vagal tone
Parasympathetic vagal nerve innervates the atria
Where do sympathetic nerves that innervate the heart exit the spinal cord?
T1-L2
How is heart rate controlled?
Autonomic nervous system
By altering pacemaker potential
How does increased parasympathetic vagal activity lower the heart rate?
Hyperpolarising cells
Opens K+ channels
Reduces gradient of pacemaker potential
Takes longer to reach threshold
What does negative chronotropic mean?
Slows heart rate
How is heart rate neuronally increased at SA node?
Sympathetic nervous system
Noradrenaline
Beta1 receptors
Increased Na+ and Ca+ conductance
How is heart rate hormonally increased at the SA node?
Adrenaline acting on beta1 receptors
Increased Na+ and Ca+ conductance
What does functional synctium mean?
All cells are structurally and electrically coupled together and so their behaviour reflects that of one muscle cell
What allows current to flow between adjacent cardiac cells?
Gap junctions
Why is the SA node the pacemaker as opposed to the AV node?
It is faster and so dictates pace
How quickly does conduction move through the atria?
0.5m/s
How long does it take an impulse to travel from the SA node to the AV node?
0.09s
What is the pacemaker rate of the AV node?
40 beats per minute
What is the conduction rate at the AV node?
0.05m/s
What does AV delay ensure?
Atrial depolarisation, contraction and ejection before the ventricles depolarise
Where does the current flow after the AV node?
Through the bundle of His and the the Purkinje fibres
At what speed does current flow through the bundle of His?
2m/s
At what speed does current flow through the Purkinje fibres?
4m/s
Is AV delay constant?
No it changes in response to heart rate, e.g. with exercise
What is an ectopic beat?
Random ventricular contraction with no stimulation
In what order does the ventricle depolarise?
From the endocardium to the epicardium,
What is the resting potential of a ventricular myocyte?
-80 to -90
What are the phases of ventricular myocyte action potentials?
0 rapid deoplarisation 1 partial repolarisation 2 plateau 3 repolarisation 4 resting potential
What is responsible for 0?
Inward movement of Na+
Through fast voltage gated channels
What is responsible for 1?
Inactivation of Na+ channels
What is responsible for 2?
Inward Ca+ movement and outward movement of K+
Momentarily in equilibrium
What is responsible for 3?
Outward movement of K+
How long is the absolute refractory period of a ventricular myocyte?
200ms
What is different between skeletal muscle and ventricular muscle twitch and the delay after an action potential?
Ventricular muscle contracts while the action potential is firing
Skeletal muscle contracts after the action potential has occurred
Is tetanic contraction possible in cardiac muscle?
No
