Physiology Properties of the Heart Flashcards
Describe the first phase (4) of atrial/ventricular depolarisation
Stable resing membrane potential where the oermeability for potassium exceeds sodium by 50:1
Describe the second phase (0) of atrial/ventricular depolarisation
0 - Rapid depolarisation due to increase in Na permeability as Na channels open.
Describe the third phase (1) of atrial/ventricular depolarisation
Attempt/start repolarisation as fast sodium channels close
Describe the fourth (2) phase of atrial/ventricular depolarisation
The plateau after phase one is due to the depolarisation of the cell causing the opening of L type channels (Dihydrapiridine receptors) allowing for an influx of calcium into the cell. So this plateau is due to calcium
Describe what happens in fifth (3) phase of atrial/ventricular depolarisation
There is rapid repolarisation as the influx of calcium stimulates potassium channels to open and the permeability for potassium increases. The calcium L-type channels close and the membrane repolarises.
Describe the functions of F-type sodium channels
These work in the opposite way to the sodium channels in skeletal muscles. So as the membrane potential reaches closer to the resting potential for potassium (more negative), the more likely it is for these channels to open. The more positive the membrane then more likely for these channels to close.
Describe the first stage in sinoatrial node depolarisation
There is a gradual increase in resting membrane potential due to an increased permeability to sodium as F-type sodium channels open and a decrease in permeability for K+ as K+ channels close. The f-type channels close when membrane becomes more positive which stops sodium from triggering action potential but it activates transient calcium channels.
Describe the second stage in sinoatrial node depolarisation
There is a moderatly rapid depolarisation due to calcium entry via slow L-type channels.
Describe the third phase of sinoatrial depolarisation
There is rapid repolarisation as elevated internal calcium levels stimulate the opening of postassium channels and so therefore an increase in the permeability of potassium.
Describe the sympathetic stimulation of pacemaker activity (positive chronotropic effect)
Noradrenaline acts on beta one receptors to increase cAMP production. This increases the rate of SAN phase 1 depolarisation. So it increases permeability to calcium and sodium.
Describe the parasympathetic stimulation on pacemaker activity (negative chronotropic effect)
Acetylcholine acts on M2 receptors which decrease cAMP production and reduces the rate of SAN phase 1 depolarisation. This hyperpolarises membrane potential to a lower starting level. It increases permeability to potassium by increasing the extent and duration of opening K+ channels.
Describe the electrical conduction of the heart
Begins at SAN, electrical signal then moves via internodal tracts to the AVN where it is held for 0.09s. It then gets transported to the bundle of His where it splits into right and left bundles. Finally ends by traveling through purkinje fibres in ventricles.
What occurs if the conduction from the SAN is blocked?
All tissues have their own intrinsic pacemaker but it is suppressed by the activity of SAN (90/min), if conduction is blocked then the tissue will assume their intrinsic rate with the next tissue in the conductive pathway assuming role as SAN. If that tissue gets blocked then it will move on again but HR gets slower.
Briefly describe the function of the electrocardiogram (ECG)
Measures electrical activity of the heart over time using electrodes. Four on limbs; One is an ‘earth’ to remove background noise and three are used to create virtual leads between pairs of electrodes. Electrodes on chest give more specific info.
What is the function of the limb leads
Measure the sum of electrical activity of the heart and the direction the electrical activity is moving in. One end of lead is ‘positive’ so if depolarisation moves towards positive end then the tracing will go up and if it goes away from positive end then the tracing will go down.
