The intrinsic conduction system The intrinsic conduction system consists of noncontractile cardiac cells specialized to initiate and distribute impulses throughout the heart in a sequential manner. Autorhythmic cells have an unstable resting potential that continuously depolarizes.
Pacemaker potentials are spontaneouslychanging membrane potentials that initiateaction potentials in order to trigger rhythmiccontractions.Fast Ca+2 channels cause autorhythmic cellsto develop an action potential using calciuminstead of sodium.
Sequence of excitation Impulses travel across the heart in the order of the following autorhythmic cardiac cells:1) Sinoatrial node2) Atrioventricular node3) Atrioventricular bundle4) Right and left bundle branches5) Purkinje fibers
Defects in the intrinsic conduction systemcan cause irregular heart rhythms known asarrhythmias.Fibrillation is a condition of rapid andirregular or out-of-phase contractions.
The cardioacceleratory center in themedulla oblongata increases both the rateand force of heartbeat.The cardioinhibitory center, also in themedulla oblongata, slows the heart throughparasympathetic inervation.
Electrical currents generated andtransmitted through the heart also spreadthrough the body and can be monitored withan instrument called an electrocardiograph.The graphic recording produced by anelectrocardiograph is called an ECG.
The cardiac cycle Systole refers to the contraction period of the heart. Diastole refers to the relaxation period of the heart. The cardiac cycle includes all events associated with the flow of blood through the heart during one complete heartbeat.
Events of the cardiac cycle1) Ventricular filling2) Ventricular systole (isovolumetric contraction phase)3) Isovolumetric relaxation The length of the cardiac cycle is about 0.8 seconds, half of which is a period of total relaxation called the quiescent period.
Heart sounds Two distinguishable sounds can be heard, called heart sounds, when the thorax is auscultated. Abnormal or unusual heart sounds are called murmurs.
Cardiac output Cardiac output is the amount of blood pumped out by each ventricle in 1 minute. Stroke volume is the volume of blood pumped out by a ventricle with each beat. The difference between resting and maximal CO is referred to as cardiac reserve.
End diastolic volume represents the amountof blood that collects in a ventricle duringdiastole.End systolic volume represents the volumeof blood remaining in a ventricle after it hascontracted.
The Frank-Starling law of the heart statesthat the critical factor controlling strokevolume is the preload, or the degree ofstretch of the cardiac muscle cells justbefore they contract.
Contractility is an increase in contractilestrength that is independent of musclestretch and the EDV.Afterload is the pressure that must beovercome for the ventricles to eject bloodfrom the heart (the back pressure exerted onthe aortic and pulmonary valves by arterialblood).
Regulation of heart rate The heart exhibits vagal tone due to the dominant influence of inhibitory autonomic inervation. The atrial reflex is a sympathetic reflex initiated by an increase in venous return and blood congestion in the atria.
Hormones and ions may influence heartrate regulation.
Tachycardia is an abnormally fast heart rate(>100 beats/minute).Bradycardia is an abnormally slow heartrate (<60 beats/minute).