CONDUCTION

The Cardiac Cycle Autonomic Control of Heart Rate The pacemaker sets the heart rate but can be altered Impulses from the autonomic nervous system modify the pacemaker activity Nerves associated with the ANS innervate the: SA node AV node Cardiac cells Smooth muscles in the cardiac blood vessels

The Cardiac Cycle Autonomic Control of Heart Rate (continued) Norepinephrine from the ANS causes: An increase in the heart rate An increase in the force of contractions Acetylcholine from the ANS causes: A decrease in the heart rate A decrease in the force of contractions

Figure 21.13 The Autonomic Innervation of the Heart Vagal nucleus Cardioinhibitory center Cardioacceleratory center Medulla oblongata Vagus (N X) Spinal cord Sympathetic Parasympathetic Parasympathetic preganglionic fiber Sympathetic preganglionic fiber Synapses in cardiac plexus Sympathetic ganglia (cervical ganglia and superior thoracic ganglia [T1–T4]) Parasympathetic postganglionic fibers Sympathetic postganglionic fiber Cardiac nerve 4

The Cardiac Cycle The cardiac cycle consists of alternate periods of contraction and relaxation Contraction is systole Blood is ejected into the ventricles Blood is ejected into the pulmonary trunk and the ascending aorta Relaxation is diastole Chambers are filling with blood

The Cardiac Cycle Cardiac contractions are coordinated by conducting cells There are two kinds of conducting cells Nodal cells Establish the rate of contractions Cell membranes automatically depolarize Conducting fibers Distribute the contractile stimulus to the myocardium

The Cardiac Cycle The Sinoatrial and Atrioventricular Nodes Sinoatrial node (SA node) Cardiac pacemaker Embedded in the posterior wall of the right atrium Determines the rate of heart contractions Bradycardia: slow heart rate Tachycardia: rapid heart rate Impulse travels to the AV node via the internodal pathways

The Cardiac Cycle The Sinoatrial and Atrioventricular Nodes (continued) Atrioventricular node (AV node) Sits within the floor of the right atrium Impulse travels from the AV node to the AV bundle The AV bundle travels along the interventricular septum and then divides to form the right and left bundle branches The bundle branches send impulses to the Purkinje fibers Impulses are then conveyed to the ventricular myocardium

Figure 21.12a The Conducting System of the Heart Sinoatrial (SA) node Internodal pathways Atrioventricular (AV) node AV bundle Left bundle branch Right bundle branch Moderator band Purkinje fibers The stimulus for contraction is generated by pacemaker cells at the SA node. From there, impulses follow three different paths through the atrial walls to reach the AV node. After a brief delay, the impulses are conducted to the bundle of His (AV bundle), and then on to the bundle branches, the Purkinje fibers, and the ventricular myocardial cells. 9

Figure 21.12b The Conducting System of the Heart SA node activity and atrial activation begin. SA node Time  0 Stimulus spreads across the atrial surfaces and reaches the AV node. AV node Elapsed time  50 msec There is a 100 msec delay at the AV node. Atrial contraction begins. AV bundle Bundle branches Elapsed time  150 msec The impulse travels along the interventricular septum within the AV bundle and the bundle branches to the Purkinje fibers and, via the moderator band, to the papillary muscles of the right ventricle. Moderator band Elapsed time  175 msec The impulse is distributed by Purkinje fibers and relayed throughout the ventricular myocardium. Atrial contraction is completed, and ventricular contraction begins. Elapsed time  225 msec Purkinje fibers The movement of the contractile stimulus through the heart is shown in STEPS 1–5. 10

Figure 21.12b The Conducting System of the Heart (Part 1 of 2) SA node activity and atrial activation begin. SA node Time  0 Stimulus spreads across the atrial surfaces and reaches the AV node. AV node Elapsed time  50 msec There is a 100 msec delay at the AV node. Atrial contraction begins. AV bundle Bundle branches Elapsed time  150 msec The movement of the contractile stimulus through the heart is shown in STEPS 1–5. 11

Figure 21.12b The Conducting System of the Heart (Part 2 of 2) The impulse travels along the interventricular septum within the AV bundle and the bundle branches to the Purkinje fibers and, via the moderator band, to the papillary muscles of the right ventricle. Moderator band Elapsed time  175 msec The impulse is distributed by Purkinje fibers and relayed throughout the ventricular myocardium. Atrial contraction is completed, and ventricular contraction begins. Elapsed time  225 msec Purkinje fibers The movement of the contractile stimulus through the heart is shown in STEPS 1–5. 12

Conducting System Process SA Sinoatrial Node- pacemaker of heart (generates action potentials greater frequency) .04 faster than to surrounding muscle AV Atrioventricular Node- Slow to transmit action potential and allows completion of atrial contraction AV Bundle of His Fast Bundle branches- right and left IV Septum Rt Ventrical apex Lt Ventrical apex Perkinje Fibers- large diameter cardiac muscle Transmits signal to apex of ventricles A lot of intercalated disks --- Myocardium in apex contracts in a wringing action

Figure 21.11 The Cardiac Cycle Start Atrial systole begins: Atrial contraction forces a small amount of additional blood into relaxed ventricles. Atrial systole ends, atrial diastole begins 0 msec 800 msec 100 msec Atr ial sy sto le Ven tri cul ar dia sto le Ven tri cul ar sy sto le Cardiac cycle Ventricular systole— first phase: Ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves. Ventricular diastole—late: All chambers are relaxed. Ventricles fill passively. Atr ial dia sto le 370 msec Ventricular systole— second phase: As ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected. Ventricular diastole—early: As ventricles relax, pressure in ventricles drops; blood flows back against cusps of semilunar valves and forces them closed. Blood flows into the relaxed atria. 14

Autorhymicity of Cardiac Muscle Action potentials in heart without external stimuli After each action potential the membrane potential returns to its resting membrane potential Unstable slow ion channels open and cause depolarization This causes fast channels to open and increase depolarization When depolarization reaches threshold--action potential happens more often in SA Node because more slow channels Plateau Phase- Prolonged period of depolarization, separates contractions in the heart Heart has long action potential so the heart will rest between contractions and not tetanic contractions Absolute refractory period – CM insensitive to further stimuli

EKG Electrokardiogram- device to record the action potential of cardiac muscle summation Cannot detect force of contraction Cannot detect Blood Pressure Can detect abnormal heart rates/ rhythems Can detect abnormal conduction pathways Can detect hypertrophy and atrophy and relative position of damage P Wave- Action potential depolarization of atrial myocardium Causes Atrial contraction QRS Complex- Ventricular depolarization Causes onset of ventricular contraction Also atrial repolarization masked by QRS signal T Wave- repolarization before ventricular relaxation U wave – repolarization of the purkinje fibers PQ/PR Interval- .16 sec- atria contract and relax QT Interval- .3 sec- ventricles contract and relax 1 Cardiac Cycle- from onset muscle contraction to next

The Cardiac Cycle ANIMATION The Heart: Cardiac Cycle ANIMATION The Cardiac Cycle: Part 1 ANIMATION The Cardiac Cycle: Part 2 ANIMATION The Cardiac Cycle: Part 3 ANIMATION The Cardiac Cycle: Part 4 ANIMATION The Cardiac Cycle: Part 5 18