Parasympathetic Stimulation z Vagus nerve z Primarily innervates

Parasympathetic Stimulation z. Vagus nerve z. Primarily innervates atria, but some fibers to ventricles also z. Chemical mediator: acethycholine z. Effect: slows heart rate and AV conduction z. Methods of stimulation: Valsalva maneuver, carotid sinus pressure Ken

Sympathetic Stimulation z. Nerves arising in thoracic and lumbar ganglia z. Innervate both atria and ventricles z. Chemical mediator: norepinephrine z. Receptor sites: alpha, beta Ken

Effect of alpha Stimulation: z. No effect on heart z. Peripheral vasoconstriction Ken

Effect of beta Stimulation: z Increased rate and conduction z Increased contractility z Bronchodilation z Peripheral vasodilation Ken

Role of Electrolytes z. Cardiac function, electrical and mechanical, influenced by electrolyte imbalances z. Major electrolytes influencing cardiac function y. Na+ Sodium y. Ca++ Calcium y. K+ Potassium Ken

Role of Electrolytes z. Sodium (Na +): major role in depolarization phase of myocardial cells z. Calcium (Ca ++): major role in depolarization phase of myocardial pacemaker cells and in myocardial contractility x. Hypercalcemia: increased myocardial contractility x. Hypocalcemia: decreased myocardial contractility and increased electrical irritability Ken

Role of Electrolytes z. Potassium (K +): major role in repolarization phase z. Hyperkalemia: decreased automaticity and conduction z. Hypokalemia: increased irritability z. Potassium levels are critical to life z. Hyperkalemia = Tall peaked T waves Ken

Electrophysiology z. Electrical properties of the heart y. Automaticity: ability to generate an electrical impulse without stimulation from another source ‑ property of pacemaker cells y. Excitability: ability to respond to an electrical stimulus ‑property of all myocardial cells y. Conductivity: ability to propagate an impulse from cell to cell Ken

Electrical Conduction System z. Allows electrical impulses to spread through the heart six times faster than through muscle alone z Sequence of normal electrical conduction y. SA node y. Internodal and interatrial tracts y. AV node y. Bundle of His y. Bundle branches y. Purkinje fibers Ken

Function of electrical conduction structures z Sinoatrial (SA) node z Located in right atrium near entrance of superior vena cava z Usually heart's dominant pacemaker sa Ken

Internodal and interatrial tracts z Pathways that carry impulse between SA node and AV node and spread it across atrial muscle z Impulse travel time: 0. 08 seconds Ken

Atrioventricular (AV) node: z. Part of area called the "AV junctional tissue" along with some surrounding tissue and the non-branching portion of the Bundle of His z. Responsible for creating slight delay in conduction before sending impulse to ventricles z. Impulse travel time: 0. 08‑ 0. 16 seconds z. No pacemaking properties in node itself Ken

Bundle of His z. Bundle of fibers coming off AV node, located at top of interventricular septum z. Considered part of the AV junction z. Makes electrical connection between atria and ventricles Ken

Bundle branches z. Created by bifurcation of Bundle of His into right and left branches z. Carry electrical impulse at high velocity to interventricular septum and each ventricle simultaneously Ken

Purkinje fibers z. Terminal ends of bundle branches z. Network of fibers helping to spread impulse throughout ventricular walls z. Rapid impulse spread through ventricles: 0. 08 -0. 09 seconds Ken

Depolarization z. Process by which muscle fibers are stimulated to contract by the alteration of electrical charge of the cell accomplished by changes in electrolyte concentrations across the cell membrane Ken

Depolarization at The Cellular Level z. Chemical pumps in cell wall maintain certain concentrations of electrolytes within and outside the cell z. Resting (polarized) cell normally more electrically negative inside cell wall than outside ( -90 millivolts (mv) in working cells) Ken

Depolarization at The Cellular Level z Electrical stimulation of cell wall changes its permeability to sodium (Na+) z Na+ rushes into cell, causing inside to become more positive z Slower influx of calcium (Ca++) also causes cell to become positive z Muscle contraction is response to depolarization z Depolarization wave is passed from cell to cell along the conduction pathway to reach the muscle cells Ken

Spontaneous diastolic depolarization of pacemaker cells z. Pacemaker cells capable of self-initiated depolarization (automaticity) z. Found throughout conduction system except in AV node z. During diastole, become less and less negative until a certain threshold reached, then rapidly and fully depolarize Ken

Pacemaker Capabilities & Rates z. SA node: 60 -100/minute intrinsic rate z. AV junctional tissue: 40 -60/minute intrinsic rate z. Ventricles (bundle branches and Purkinje fibers): 20 -40/minute intrinsic rate z. SA node usual pacemaker because it discharges the fastest; pacemaker cells below SA node normally suppressed by it Ken

Repolarization z Process by which cells re-establish internal negativity and are readied for stimulation return to resting or polarized state z Caused by rapid escape of potassium (K+) from the cell z Proper distribution of electrolytes re-established by cell wall pumps (Na+ pumped out of cell, potassium pumped back into cell) z Cell returns to -90 mv. internal chargerepolarized Ken

Relationship of ECG to electrical activity z. ECG is record of electrical activity of heart as sensed by electrodes on body surface z. Gives information only about electrical activity tells us nothing about pump function z. Isoelectric line: a flat line on the ECG indicating absence of net electrical activity Ken

P wave z Rounded wave preceding QRS; usually upright (positive) in Lead II z Indicates depolarization of atrial muscle Ken

QRS complex z. Collective term for three deflections following the P wave Ken

QRS complex z Wave-first negative deflection after P wave z R wave-first positive deflection after P wave z S wave-first negative deflection after R wave Ken

QRS complex z All three waves not always present - QRS has many shapes z Indicates depolarization of the ventricular muscle Ken

T wave z Rounded wave following QRS complex; usually in same direction as QRS z Indicates repolarization of ventricles z Atrial T wave (atrial repolarization) usually not visible buried within QRS complex Ken

P-R interval z Distance between beginning of P wave and the beginning of QRS complex z Indicates length of time it takes depolarizatin wave to go from atria to ventricles Ken

S-T segment: z Distance between the S wave of the QRS complex and the beginning of the T -wave usually in isoelectric line Ken

Refractory period z Period of time when cells have been depolarized and not yet returned to polarized state z Heart unable to be stimulated again z On ECG, includes, QRS complex and T wave Ken

Absolute refractory period z. Time when stimulation will produce no depolarization whatsoever y. From beginning of QRS complex to apex of T wave z. Relative refractory period: time when a sufficiently strong stimulus may produce depolarization y. Corresponds to down slope of T wave Ken

Nervous control of electrical activity z. Sympathetic (adrenergic) control y. Effects of alpha stimulation: no direct effect on heart y. Effects of beta stimulation: increased rate, increased conduction velocity in atria and ventricles, increased irritability, (increased contractility mechanical effect) Ken

Parasympathetic (cholinergic) control z Effects of parasympathetic (vagal) stimulation z Decreased firing rate of SA node, decreased AV conduction, little effect on ventricles Ken

? Ken