Generating The Cardiac Rhythm Flashcards
Heart muscles depolarise and contract rhythmically without nerves how is this described?
Myogenic rhythmicity or autorhythmicty. It requires no stimulus to contract
Cardiac action potentials have three broad types what are they and how do they differ?
- SA and AV nodes
- Atrial muscle
- Purkinje fibre and ventricular muscle
Differ to each other by duration, shape and ionic basis
Why are SA nodes good pacemakers?
They don’t have a resting membrane potential, it sits at -60mV which is closer to the threshold than neurones at -70mV. Therefore, it doesn’t take a lot to produce an action potential. It generates signals faster than any of the others so it sets the pace for the heart beat.
What is an SA node?
A group of cardiac muscles that don’t contract. They generate impulses and signals can spread throughout the heart causing it to beat. It acts as a pacemakers because it generates signals faster than any of the others so it sets the pace for the heart beat.
How does the SA node cells produce and action potential?
The cells are permeable to Na+ so there is a Na+ influx this causes depolarisation which then reaches a threshold. When this happens the Na+ is rushing in and there is decreased conductance of K+, L-type voltage gated Ca+ open and Ca+ rushes into the cell causing rapid depolarisation and an action potential. The K+ channels open and K+ rushes out of the cell and repolarises the membrane.
SA nodes are under what control?
The autonomic control (antagonisitic)
Neurones making up the sympathetic innervation occurs at?
T1-L2
If the intrinsic rate is 100 action potentials/min what is the resting HR?
60-70 beats/min
Increase in sympathetic activity causes?
Increase in HR. Sympathetic stimulation stimulates NorAd acts on B1 receptors. Increases the slope of the pacemaker potential (increases Na+ and Ca+ conductance).
Increase in parasympathetic activity causes?
Decrease in HR. ACh acts on M2 receptors. It hyperpolarises the cell (opens K+ channels) and reduces the slope of the pacemaker potentials. Negative chronotropic effect.
Gap junction purpose
Current flows to adjoining cells though jap junctions to depolarise adjacent cells.
Where does the AP from the SA node spread to?
From the SA node through the atrial muscle.
Order of depolarisation through the heart
SA node, atrial muscle, AV node, Bundle of His, Bundle of branches, Purkinje fibres and ventricular muscle then ventricles from endocardium to epicardium.
Conduction through atrial node
0.5m/s. Rapid depolarisation
What is the AV delay?
Ensures atrial depolarisation, contraction and ejection before ventricles depolarise.
Why isn’t the AV node the pacemaker?
Because it produces action potentials much slower than SA nodes. Conductance rate falls around 0.05m/s
Time taken for depolarisation of bundle of His, bundle of branches, Purkinje fibres and ventricular muscle and ventricles from endocardium to epicardium?
Bundle of His = 2m/s
Bundle of branches = 2m/s
Purkinje fibres and ventricular muscle = 4m/s
Ventricles from endocardium to epicardium = 0.5m/s
Eptopic beats are?
Where parts of the ventricle contract out of rhythm. This can be very dangerous in an empty ventricle. These faulty spots can be burnt off.
Phases of ventricular myocyte action potential
Rapid depolarisation, partial repolarisation, plateau, repolarisation.
What ions are responsible for depolarisation of ventricular myocytes?
Inward movement of Na+ (fast voltage gated channels)
What ions are responsible for the partial repolarisation of ventricular myocytes?
Inactivation of Na+ channels
What ions are responsible for the plateau of ventricular myocytes?
Inward Ca2+ movement and outward K+ movement (the number of ions coming in is equal to the number of ions going out, therefore it plateaus.)
What ions are responsible for repolarisation of ventricular myocytes?
Outward movement of K+.
Ventricular absolute refectory period
Absolute refractory periods lasts around 200ms, here you cannot generate another action potential because Na_ channels are inactivated and unresponsive.
Ventricular relative refractory period
Relative refractory period lasts around 50ms. You may get an action potential but requires a stronger stimulus.
What is the temporal summation process?
Determines whether or not and action potential will be triggered.
What is incomplete tetanus?
Each stimulus causes a contraction to be initiated when the muscle has only partly relaxed from the previous contraction.
What is complete tetanus
Maintained contraction of the muscle
What is a twitch?
A single stimulus causing a muscle to contract and relax
What is the difference between complete and incomplete tetanus?
Incomplete tetanus has a relaxation period during contraction and complete tetanus has no relaxation period during contractions
