ECG, Electrical AP and Chemical AP Flashcards
What are some distinguishing features of the SA Node Current?
- The AP in the SA Node is responsible for the pace-making/rhythm regulation of the heart’s contraction
- Involves less Na+ ion channels (leaking); ONLY an influx of Ca2+ to depolarise and an K+ ion channels to repolarise.
- Significantly more rapid process than the Ventricular cell AP
Outline the stages involved in a Ventricular Cell AP.
- Na+ ion channels open and a rapid influx of Na+ enters the myocyte to reach threshold and increase membrane potential then become inactivated.
- Ca+2 ion channels slowly open while K+ ion channels ‘leak’ and a plateau occurs due to extend depolarisation in the AP.
- There is an increase in the number of K+ ion channels open and diffuse out of the cell, while Ca+2 channels close, which causes a more rapid repolarisation of the membrane potential, simultaneously the inactivated Na+ channels are reprimed.
- A Refractory period occurs whereby the K+ and Na+ settle to the correct concentrations across the membrane through Na/K ATPase in preparation for the subsequent AP.
What is the relevance of a refractory period in Cardiac Muscle Cells? How does it compare to Skeletal Muscle Cell refractory?
- Prevents tetanus and summation from occurring
- Allows for adequate time for heart to refill the ventricles before the next contraction.
- ## Refractory period is significantly longer in cardiac muscle than skeletal muscle
Outline the stages involved in the SA Node AP.
- Gradual rise in resting phase of Ca2+ influx from T type channels (unstable resting membrane current (if)) –> help from leaking Na+ channels
- When threshold is reached, rapid Ca2+ influx from added L type channels –> voltage sensitive channels (certain channels become active as depolarisation occurs.)
- Repolarisation occurs as K+ channels open
Explain the difference between Absolute and Relative Refractory Periods.
Absolute Refractory Period
What are the sequence of mechanical events in one cardiac cycle relative to an ECG?
P wave = atrial depolarisation (contraction)
QRS Complex = ventricular depolarisation (contraction) and simultaneously atrial repolarisation (relaxation)
T wave = ventricular depolarisation (relaxation)
What are the chemical and electrical currents in an ECG?
Electrical
Compare the duration and other characteristics of nerve cell AP and cardiac cell AP and how that impacts cardiac contraction.
Feedback:
(i)
- Different action potential duration. Nerve cell AP ~1ms. Cardiac myocyte AP ~200-400ms.
- In cardiac myocytes slow Ca2+ influx through L-type Ca2+ channels prolongs the duration of the AP and produces a plateau phase.
(ii)
- Influx of calcium is essential for initiating contraction of cardiomyocytes.
- Prolonged refractory period in cardiac myocytes is important in preventing irregular heartbeat (arrhythmia) caused by summation of force and tetanus.
What is it essential that the L-type calcium channels remain open during a cardiac action potential essential?
Having the L-type calcium channels remain open during a cardiac action potential maintains a depolarised state so that Na+ channels remain in the inactivated state during the contractile response. This creates a refractory period which prevents further excitation of the cardiomyocyte while the cell is contracting. This prevents a summation of force and tetanus which could lead to arrhythmias. Influx of calcium is essential for starting the series of events that leads to the contractile response of the heart.
Describe the P-wave and QRS complex, in respect to ventricular contraction and relaxation.
The electrical wave moves from the SA node via the internodal tracts, causing a small increase (P) on the ECG. When this occurs the atria depolarise, followed by atrial contraction. The QRS complex represents ventricular depolarisation, which is complex due to the movement of action potentials from the AV node down the ventricular septum to the apex and via the Purkinje fibres spreading throughout the ventricles. Ventricular contraction follows and can be identified in the period between QRS complex and the T wave. The T waves is ventricular repolarisation and is associated with ventricular relaxation or diastole.
What causes heart sounds? When are these sounds detected relative to the ECG?
The heart sounds are associated with closure of the heart valves. There are two main sounds. The first heart sound results from the closing of the mitral and tricuspid valves. The second heart sound is produced by the closure of the aortic and pulmonic valves. The first occurs when the mitral/tricuspid valves close around the QRS complex and depolarisation of the ventricles. The second occurs when the semilunar valves close towards the end T wave as ventricles relax.
Describe the movement of the electrical signal from the pacemaker to the Purkinje fibres, referencing how this affects the cardiac cycle and what is seen on the ECG graph.
The electrical wave moves from the SA node to the internodal tracts, causing a small increase (P) on the ECG. Arriving at the AV node there is a small delay on the ECG (PR interval). The wave then travels to the bundle of HIS causing a small decrease on the ECG (Q). After this the wave travels through the Purkinje fibres, causing an increase in ECG voltage (R), followed by a decrease as the Purkinje fibres depolarise the top of the ventricle (S).
The Sinoatrial node is the pace maker of the heart and the unstable funny current is responsible for initiating the nerve AP in the heart, whereby a sudden influx of Ca2+ from L-type channels enters the membrane and increases the membrane potential to 70-80mV. This generates the P-wave on the ECG as depolarisation as taken place. The signal runs from the SA node and runs along both atrial chambers to generate mechanical contraction. It then crosses to the internodal pathway to reach the Atrioventricular Node. There is a 0.2ms delay that occurs in the AV to prevent tetanus and summation of signalling and contraction, this is reflective of the PQ interval on an ECG. From the AV Node, the signal enters the region of the ventricular chambers when reaching the Bundle of His, this then branches into the Left and Right Branches which extend through both sides of the heart, which then disperse into the Purkinje Fibres to increase the surface area covered. This signal causes a depolarisation of the ventricular chambers which are representative of the QRS Complex in an ECG, it is a large representation as it is a significant electrical event to trigger the contraction of both chambers. The Atria, once allowing the signal to pass through begin to repolarise simultaneously in the QRS, however it is not distinguished on the ECG. The signal then reaches the end and once the ST interval is completed the repolarisation of the ventricles occur which generates the a repolarising event on the ECG, the T wave.
Describe the movement of the electrical signal from the pacemaker to the Purkinje fibres, referencing how this affects the cardiac cycle and what is seen on the ECG graph.
The electrical wave moves from the SA node to the internodal tracts, causing a small increase (P) on the ECG. Arriving at the AV node there is a small delay on the ECG (PR interval). The wave then travels to the bundle of HIS causing a small decrease on the ECG (Q). After this the wave travels through the Purkinje fibres, causing an increase in ECG voltage (R), followed by a decrease as the Purkinji fibes depolarise the top of the ventricle (S).
