Electrical Activity of the Heart Lecture II PDF

ELECTRICAL ACTIVITY OF THE HEART LECTURE II Prof. Ahmed A Alsunni Dr. Nazish Rafique Department of Physiology College of Medicine, IAU  Objectives Describe The conducting system of the heart....

ELECTRICAL ACTIVITY OF THE HEART LECTURE II Prof. Ahmed A Alsunni Dr. Nazish Rafique Department of Physiology College of Medicine, IAU  Objectives Describe The conducting system of the heart. Understand The role of SA Node as a pacemaker. Describe SA Nodal action potential. Pathophysiological basis of Cardiac Undrstand arrhythmias. Tuesday, October 29, 2024 Presentation title  Cardiac Muscle Cells 1% Autorhythmic Contractile 99% Cells Cells Initiate Do the and conduct mechanical action work of potentials pumping  Types of cardiac muscle cells Contractile Cells 99% of Cardiac Muscle Do mechanical work of contraction Do not generate action Potential in normal conditions (Can generate Action Potential in abnormal conditions) Autorhythmic cells (1%) Initiate Action Potential Conduct Action Potential Do not contract Tuesday, October 29, 2024 Presentation title  Route/ pathway of impulse transmission from SA node to the ventricular contractile cells Impulse is generated in the Sinoatrial (SA) node by the process of autorhytmicity. Is transmitted through the Internodal pathway to the Atrioventricular (AV) node. Is transmitted from atria to ventricles via bundle of His( AV bundle) Is transmitted to the ventricles through the Purkinje fibers. The action potentials are than rapidly distributed to the cardiac contractile cells through the gap junctions.  Gap junction Cardiac cell 1 Cardiac cell 2 Gap junctions are the areas of low electrical resistance that allow the action potential to spread from cell to cell. Are there gap junctions between atria and ventricular contractile muscle cells? Why? Atria and ventricles are separate pumps There are no gap junctions between atria and ventricle They communicate only through the specialized junctional tissue  Spread of action potentials from the autorhythamic to the contractile cells Once the SA node generates action potential, it quickly passes to all parts of the heart as a wave of excitation. This spread of cardiac excitation occurs via junctional tissue (through AV Bundle) and gap junctions. Tuesday, October 29, 2024  Both the gap junctions and the fast conducting system between the atria and ventricles make the heart a: Functional syncytium  Generation of pacemaker potential AND Action potential in autorhythmic cells Tuesday, October 29, 2024 Presentation title  Pacemaker potential In contrast to the nerve and muscle cells, in which the membrane potential remains at constant resting membrane potential (RMP) unless the cell is stimulated, the cardiac autorhythmic cells do not have a resting membrane potential. Instead, Cardiac autorhytmic cells display pacemaker activity. Their membrane potential slowly depolarizes between the action potentials until the threshold is achieved, at which time the membrane fires. An autorhythmic cell’s drift to the threshold is called as pacemaker potential. Tuesday, October 29, 2024 Presentation title  SA nodal Pacemaker potential The first half of the pacemaker potential is caused by: 1: The opening of funny Na channels(which causes inward Na+ current). 2: The Closure of K+ channels ( which reduces outward K current). The second half of the pacemaker Potential is caused by: The opening of T-Type Ca+ channels( Which causes inward Calcium+ current). All these factors make the inside of the membrane more positive, which helps to drift the potential towards the threshold. Tuesday, October 29, 2024  ‘Fire’ DRIFT to threshold ‘f ’ channels: ‘ f ’ from funny T-Type Calcium Channels : “Transient” Calcium Channel L-Type Calcium Channel : Longer Lasting Calcium Channel P: Permeability Tuesday, October 29, 2024 Presentation title  SA nodal action potential The rising phase of SA nodal action potential is due to rapid influx of calcium. The falling phase of SA nodal action potential is due to the efflux of potassium. Tuesday, October 29, 2024  Autorhythmicity Action Potential is generated within the specialized heart cells(SA Node). This generation of the action potential is rhythmic (60- 100/Min). It spreads to the whole heart by specialized cardiac muscle cells. Which leads to Rhythmic Contraction of the Heart (Heart Beat). Tuesday, October 29, 2024 Presentation title  Draw, label, and Compare the “SA Nodal pacemaker potential” with the “Neuronal action potential”. Tuesday, October 29, 2024 Presentation title  Compare and contrast the action potentials of the neuron and autorhythmic cells. Tuesday, October 29, 2024 28th 4  Transmission of the cardiac impulse through the heart. Tuesday, October 29, 2024 Presentation title Rate of action potential discharge in the cardiac autorhytmic cells. SA Node= 70-80/Minute AV Node= 60-70/Minute Bundle of His= 20- 40/Minute Purkinje Fibers= 20-40/ Minute  Conduction velocity AV Node = 0.02-0.1 Bundle Of His= 0.3-1 Purkinje fibers=4  SA node as a pacemaker SA Node is the pacemaker of the heart because: The cells with the fastest action potential Are located in SA Node. These fastest cells drive the rest of the heart. Other autorhythmic cells, therefore, cannot generate their own action potentials, because before they can reach to the threshold of their own they are activated by the potentials originating in the SA node. Tuesday, October 29, 2024  Importance of AV Nodal delay AV NODE HAS THE SLOWEST CONDUCTION VELOCITY. WHICH CAUSES AV NODAL DELAY. It allows the atria to be excited and to contract completely before the impulse reaches His bundle. Meanwhile, the ventricles get enough time to be completely filled. PURKINJE FIBERS ARE THE FASTEST CONDUCTING CELLS. This fast transmission ensures rapid and simultaneous excitation and contraction of both the ventricles. Tuesday, October 29, 2024  Generation and transmission of the cardiac impulse through the heart What is the physiological importance of slow conduction of impulses through the AV node? Why SA node is the pacemaker of the heart? What is the physiological importance of fast conduction of impulses through the purkinje fibers? Tuesday, October 29, 2024  Abnormal cardiac rhythms (cardiac arrhythmias) Tuesday, October 29, 2024 Presentation title  ARRHYTHMIA (Any Variation from the normal rhythm) Major causes: Shift of the pacemaker from the sinoatrial node to other pacemaking regions. Abnormal impulse formation in the sinoatrial node. Blocking or delay of conduction of the impulse through the heart. Spontaneous generation of abnormal impulses in any region of the heart through ectopic foci (e.g. atrial fibrillation, atrial flutter ventricular fibrillation). Tuesday, October 29, 2024 Presentation title  Do you know what is the relationship between ECG and Action potentials? ECG is a composite (combinad) recording of all of the action potentials produced by the nodal and myocardial cells. Tuesday, October 29, 2024 Presentation title  ECG and action potentials Tuesday, October 29, 2024 Presentation title  The electrocardiogram (ECG): It is the record of the heart’s electrical activity, recorded from the surface of the body. Because the body is a very good conductor therefore the small currents generated in the heart can be detected at the body surface. The ECG is a recording of these small currents and reflects the depolarization and repolarization of different regions of the heart. Tuesday, October 29, 2024  Normal ECG = 3 Phases P WAVE = Atrial activation (atrial depolarization) QRS COMPLEX = Activation of the ventricles (ventricular depolarization) T WAVE = Recovery of the ventricles (ventricular repolarization) Tuesday, October 29, 2024 Reference to standard books Page numbers 309-311 9th edition Lauralee Sherwood. Human Physiology form cells to systems.