Cardiac Properties of Rhythmicity and Conductivity (L3 Physiology) PDF

Cardiac properties Prof. Shereen Samir Objectives 1- Revision on action potential 2- Structure of cardiac muscle 3- to know the cardiac properties 4- discuss the factors affecting cardiac properties  BASIC CONCEPTS ACTION POTENTIAL (AP) It i...

Cardiac properties Prof. Shereen Samir Objectives 1- Revision on action potential 2- Structure of cardiac muscle 3- to know the cardiac properties 4- discuss the factors affecting cardiac properties  BASIC CONCEPTS ACTION POTENTIAL (AP) It is a transient change in RMP Occurs in excitable tissue; nerve & muscle caused by an effective stimulus Leading to changes in the cell membrane permeability to ions (Na, K) Composed of; Spike (depolarization +repolarization) After-hyperpolarization BASIC CONCEPTS DEPOLARIZATION BASIC CONCEPTS REPOLARIZATION The normal polarity is resumed The normal ionic distribution is rectified by Na-K pump  The Heart Is a hollow muscular organ Site: Left side of the thoracic cavity partly behind the sternum Size Atria Size of a man's fist About 320 gm in males and Ventricles 250 gm in females  Structure of Heart Its wall consists of 3 layers; Epicardium Endocardium Myocardium  Cardiac Ms Fibers 1. Nodal fibers 3. Contractile fibers Atria Ventricle 2.Conducting fibers  Nodal Ms Fibers 9 Sinoatrial node (SAN) Posterior wall of the right atrium immediately medial to the opening of the S.V.C. 10 mm long and 3 mm thick It is the region where cardiac impulse arises → pacemaker of the heart. Atrioventricular node (AVN) Right side of the interatrial septum at the junction of the atria and ventricles 5 mm long and 3 mm thick It conducts the impulse to the atrio- ventricular bundle (bundle of His)  Conducting Ms Fibers 1. AV Bundle of Hiss Continuous with the AV node and passes through the AV fibrous ring only muscular connection between the atria and ventricles. It breaks into right and left branches 2.Purkinje fibers Blend with the ordinary ventricular ms fibers  Contractile Ms Fibers A. 2 Atria (Rt and Lt) Functions of atria: 1. Blood reservoirs during ventricular systole. Atria 2. Pumping 30% of venous return into the ventricles during their diastole. 3. Atria contain SAN and AVN which are important for initiation and propagation of heart beat. 4. Right atrium contains receptors of many cardiac reflexes. 5. Atria secrete atrial natriuretic peptide (ANP): is important in regulation of ABP.  Contractile Ms Fibers B. 2 Ventricles (Rt and Lt) Functions of ventricles : Pump blood into aorta and pulmonary artery Ventricle  Fibrous Skeleton Is a fibrous ring which separates the atria from ventricles and contains 4 valves BASIC CONCEPTS STRUCTURE OF CARDIAC MUSCLE Cardiac myocytes are short branched striated muscle cells Connected with gap junctions These gap junctions or intercalated discs have low electric resistance So, action potential (AP) travels from one cardiac ms to another with slight resistance, so the cardiac ms responds as one cell i.e. functional syncytium. The heart is composed of 2 separate functional syncytia: atria and ventricles. AUTONOMIC INNERVATION OF CARDIAC MUSCLE; SYMPATHETIC & PARASYMPATHETIC Parasympathetic NS (vagus nerve) supplies the atria only Lt. vagus supplies AVN Rt. vagus supplies SAN Sympathetic NS: supply atria, ventricles & conducting system PROPERTIES OF CARDIAC MUSCLE 1. Rhythmicity 2. Conductivity 3. Excitability 4. Contractility  Properties of Cardiac Muscle 1. Rhythmicity: Ability of cardiac fibers to give regular impulses (action potentials) causing the heart to beat regularly. 2. Conductivity: Ability of cardiac fibers to conduct excitation wave from one part of the heart to another. 3. Excitability: Ability of cardiac fibers to respond to adequate stimulus (to generate action potential). 4. Contractility: Ability of the cardiac muscle to contract.  I. Rhythmicity Definition: Tissue Rhythmicity Ability of cardiac fibers to give regular without impulses (action potentials) causing vagus the heart to beat regularly. SAN 120 / min Origin: Myogenic (not neurogenic). AVN 90 / min Nerves do not initiate it but control it. Evidence: Transplanted heart (no Bundle 45 / min nerve supply) continues to beat. tissues SAN has the fastest rhythm (so, it is the Purkinje 35 / min pace maker of the heart). fibers Self-excitation of SAN fibers is due to: Ventricles 25 / min Natural leakiness of the membrane to Na+ … this makes RMP not stable  Mechanism of rhythmicity of the SAN (SAN action potential): 1. Na influx through funny (If) slow Na channels + Ca influx through T (transient) Ca channels + decreased K+ efflux → membrane potential changes gradually from - 55 mV (resting) to - 40 mV. This is called Phase 4 = Pace-maker potential = pre- potential = diastolic depolarization (DD).  Mechanism of rhythmicity of the SAN (SAN action potential): 1. Ca influx through L (long lasting ICa) Ca channels → membrane potential changes from -40 mV (firing or threshold level) to + 10 mV → Phase 0 = Upstroke phase. 2. K efflux (IK) → membrane potential returns to - 55 mV (resting) → Phase 3 = Repolarization. 3. Then, the process is repeated throughout life. FACTORS AFFECTING RHYTHMICITY AUTONOMIC MODULATION OF SAN RHYTHM: PARASYMPATHETIC NS Parasympathetic stimulation (vagal innervation) →slowing of SAN rhythm Via: increasing K+ efflux → hyperpolarization → decreasing heart rate (negative chronotropic effect) Normally, parasympathetic through vagus nerve decreases SAN rhythm from 120 to 70 (vagal tone) vagus nerve supply only the atria it is called vagus escape phenomena.  FACTORS AFFECTING RHYTHMICITY AUTONOMIC MODULATION OF SAN RHYTHM: SYMPATHETIC NS Sympathetic NS stimulates SAN rhythm (positive chronotropic effect)→ increases heart rate By increasing Ca++ influx →stimulating depolarization  II. CONDUCTIVITY Definition: Ability of cardiac fibers to conduct excitation wave from one part of the heart to another. Cell-to-cell conduction is facilitated by the presence of gap junctions that transmit electrical currents PROPERTIES OF CARDIAC MS.; II- CONDUCTIVITY THE CONDUCTING SYSTEM Responsible for transmission of SAN action potential to the rest of the cardiac muscle From SAN→ atrial muscle & atrioventricular node (AVN) From AVN → atrioventricular (AV) bundle (bundle of His) →left & right bundles →purkinje fibres  Atrial conduction: velocity = 0.4 m/sec Action potential travels from the SAN into the atria and to the AVN through: Atrial mass & Internodal bands (Anterior, middle, and posterior).  AV nodal conduction (very slow): velocity = 0.04 m/sec. Causes of slow conductivity in AVN: Small size of fibers & few gap junctions. Significance of AVN delay: Gives time for atria to empty blood into ventricles Protects ventricles from high pathological atrial rhythms.  Purkinje fibers conduction: velocity = 4 m/sec Cause of high velocity: Large size of fibers & more gap junctions. Significance: immediate transmission of impulse in ventricles  Question In the sinoatrial (SA) node, phase 4 depolarization (pacemaker potential) is attributed to A. an increase in K+ conductance B. an increase in Na+ conductance C. a decrease in Cl− conductance D. a decrease in Ca2+ conductance E. simultaneous increases in K+ and Cl−  Sympathetic stimulation cause which one of the following in the heart: a)It decreases the heart rate. b) It increases the permeability of SAN to K+. c)It elevates the slope of the prepotential. d) It increases the conduction time in the AVN. e)It decreases the permeability of SAN to calcium. About the cardiac muscle which of the following are true a) Low electric resistance of the membrane at the intercalated discs b) They form true syncytium c) They are unstriated muscles d) There is a multiple muscle connections between atria and ventricles e) Ventricles contain SAN which as a pacemaker