19.3 Cardiac Muscle and Cardiac Conduction System

cardiac conduction system

coordinates the heartbeat; consists of an internal pacemaker and nerve-like conduction pathways through myocardium

first step of cardiac conduction

signal originates in the sinoatrial node

sinoatrial node (SA node)

patch of modified cardiomyocytes in the right atrium; serves as the pacemaker that initiates each heartbeat and determines heart rate

second step of cardiac conduction

signal spreads from the sinoatrial node throughout the atria

third step of cardiac conduction

signal reaches the atrioventricular node

atrioventricular node (AV Node)

patch of modified cardiomyocytes that serves as the electrical gateway to the ventricles; fibrous skeleton prevents signals from passing to ventricles by any other route

fourth step of cardiac conduction

signal travels from atrioventricular node to the atrioventricular bundle

atrioventricular bundle (AV bundle)

pathway from which the signal leaves the AV node; forks into right and left bundle branches that travel within the interventricular septum

fifth step of cardiac conduction

signal travels through the right and left bundle branches to the subendocardial branches (purkinje fibers)

subendocardial branches (purkinje fibers)

processes arising from lower end of bundle branches; branch extensively through ventricular myocardium

signal reaches end

signal is eventually perpetuated by cardiomyocytes via gap junctions

myogenic

human heart’s signal originates within itself

autorhythmic

human heart has a built-in pacemaker and electrical system, so it does not rely on the nervous system for signal

cardiomyocytes

striated, short, thick, branched muscle cells; one central nucleus surrounded by light-staining mass of glycogen; sarcoplasmic reticulum lacks terminal cisterns; large T-tubules

intercalated disc

connections between cardiomyocytes; containing interdigitating folds, mechanical junctions, and electrical junctions

interdigitating folds

membranes of cells folded to lock cells together and increase surface area of contact

fascia adherens

broad band in which the actin of thin myofilaments is anchored to the plasma membrane; cells linked by transmembrane proteins; type of mechanical junction

desmosomes

mechanical linkages that prevent contracting cardiomyocytes from being pulled apart from each other

electrical junctions

gap junctions allow ions to flow between cells; cells stimulate neighbors and contract in unison

cardiac ATP generation

relies almost exclusively on aerobic respiration but can adapt to different organic fuels; more vulnerable to oxygen deficiency that lack of a specific fuel

cardiac fatigue resistance

makes little use of anaerobic fermentation or oxygen debt mechanisms allowing it to not ______ for a lifetime