Chapter 9 Cardiac Physiology

Outline Comparison of types of muscle tissue Circulatory system overview Anatomy Electrical activity Mechanical events Cardiac output Coronary circulation

The next series of slides compares cardiac, skeletal, smooth muscle cells.

Contraction Uncovering of cross- bridge binding sites on actin in thin filament Phosphorylation of myosin cross bridges in thick filament Series of biochemical events Physical repositioning of troponin and tropomyosin Rise in cytosolic Ca 2+ (mostly from extracellular fluid) Rise in cytosolic Ca 2+ (entirely from intracellular sarcoplasmic reticulum) Muscle excitation Skeletal muscleSmooth muscle Contraction Binding of actin and myosin at cross bridges P i Fig. 8-31, p. 296 Comparison of the Role of Calcium In Bringing About Contraction in Smooth, Skeletal, and Cardiac Muscle Cardiac muscle

Skeletalsmoothcardiac

Cardiac Muscle Fibers Interconnected by intercalated discs and form functional syncytia Within intercalated discs – two kinds of membrane junctions –Desmosomes –Gap junctions Ap’s

fig 16-8b, pg 487 Myofibril Opening of Transverse tubule Intercalated disc Transverse tubule Longitudinal system Sacrolemma Basal lamina Myofibrils Mitochondria Nucleus Skeletal Muscle Fiber

fig 16-9a, pg 479 Nucleus Rough Endoplasmic reticulum Glycogen granules Mitochondria Thin filament Thick filament Dense bodies Plasma membrane Smooth Muscle Fiber

Outline Comparison of types of muscle tissue Circulatory system overview Anatomy Electrical activity Mechanical events Cardiac output Coronary circulation

Human heart Anatomy Heart Hollow, muscular organ about the size of a clenched fist Positioned between two bony structures – sternum and vertebrate

Circulatory System Three basic components –Heart Serves as pump that establishes the pressure gradient needed for blood to flow to tissues –Blood vessels Passageways through which blood is distributed from heart to all parts of body and back to heart –Blood Transport medium within which materials being transported are dissolved or suspended Pulmonary circulation –Closed loop of vessels carrying blood between heart and lungs Systemic circulation –Circuit of vessels carrying blood between heart and other body systems

Circulatory System Heart Arteries Carry blood away from ventricles to tissues Veins Vessels that return blood from tissues to the atria Septum –Continuous muscular partition that prevents mixture of blood from the two sides of heart Dual pump –Right and left sides of heart function as two separate pumps –Divided into right and left halves and has four chambers Atria –Upper chambers –Receive blood returning to heart and transfer it to lower chamber Ventricles –Lower chambers which pump blood from heart

Outline Internal Anatomy –Thoracic cavity, base, apex –AV and semilunar valves –endothelium, myocardium, epicardium –cardiac cells, intercalated disks –Comparison of cardiac cells to skeletal and smooth muscle cells –pericardium

Heart Valves Atrioventricular (AV) valves –Prevent backflow of blood from ventricles into atria during ventricular emptying –Right AV valve = tricuspid valve –Left AV valve = bicuspid valve or mitral valve –Chordae tendinae Fibrous cords which prevent valves from being everted Papillary muscles Semilunar valves –Aortic and pulmonary valves –Lie at juncture where major arteries leave –ventricles –Prevented from everting by anatomic structure –and positioning of cusps No valves between atria and veins –Reasons Atrial pressures usually are not much higher than venous pressures Sites where venae cavae enter atria are partially compressed during atrial contraction

fig. 18-9; pg: 563 Endocardium Myocardium Epicardium 3 layers Consists of three distinct layers –Endothelium Thin inner tissue Epithelial tissue which lines entire circulatory system –Myocardium Middle layer Composed of cardiac muscle Constitutes bulk of heart wall –Epicardium Thin external layer which covers the heart Pericardium –the fluid filled sac that surrounds the heart

Outline Electrical activity of the heart –Autorhymicity –Pacemaker (function, ions) –Conductive system (SA, AV, bundle of His, Purkinje fibers) –Abnormal rhythms –Spread of cardiac excitation –Cardiac cell action potentials Characteristics vary by location

Electrical Activity of Heart Heart beats rhythmically as result of action potentials it generates by itself (autorhythmicity) Two specialized types of cardiac muscle cells –Contractile cells 99% of cardiac muscle cells, do mechanical work of pumping,normally do not initiate own action potentials –Autorhythmic cells Do not contract but send electrical signals to the contractile cells, specialized for initiating and conducting action potentials responsible for contraction of working cells

Electrical Activity of Heart Locations of noncontractile cells capable of autorhymicity –Sinoatrial node (SA node) Specialized region in right atrial wall near opening of superior vena cava Pacemaker of the heart –Atrioventricular node (AV node) Small bundle of specialized cardiac cells located at base of right atrium near septum –Bundle of His (atrioventricular bundle) Cells originate at AV node and enters interventricular septum Divides to form right and left bundle branches which travel down septum, curve around tip of ventricular chambers, travel back toward atria along outer walls –Purkinje fibers Small, terminal fibers that extend from bundle of His and spread throughout ventricular myocardium

(a) Specialized conduction system of the heart Atrioventricular (AV) node Right atrium Left atrium Interatrial pathway Left ventricle Right branch of bundle of His Internodal pathway Sinoatrial (SA) node Right ventricle Purkinje fibers Left branch of bundle of His Fig. 9-8a, p. 312

Electrical Activity of Heart Cardiac impulse originates at SA node Action potential spreads throughout right and left atria Impulse passes from atria into ventricles through AV node (only point of electrical contact between chambers) Action potential briefly delayed at AV node (ensures atrial contraction precedes ventricular contraction to allow complete ventricular filling) Impulse travels rapidly down interventricular septum by means of bundle of His Impulse rapidly disperses throughout myocardium by means of Purkinje fibers Rest of ventricular cells activated by cell-to-cell spread of impulse through gap junctions

Pacemaker potential and Action potential in autorhythmic cells Pacemaker activity - slow depolarization or drift towards threshold Increased inward Na movement Decreased outward K movement Inward Ca movement

Fig. 9-7, p. 311 Physiology of pacemaker cells End of repol opens funny channels Na channels that open upon hyperpolarization

Conduction between atria and ventricles AV nodal delay 100 msec allows ventricular filling Purkinje cell transmission in 30 msec Some spreading thru gap junctions

Action potential of cardiac contractile cell Different than nodal pacemaker cell (conductive cell) Rp = -90mv K channel subtypes –Leak channel keeps -90 mv –Peak potential efflux channels (brief repol)(fast transient); Cessation of efflux plus Ca influx creates plateau –Ordinary k channels repolarize

Electrical Activity of Heart Atria contract as single unit followed after brief delay by a synchronized ventricular contraction Action potentials of cardiac contractile cells exhibit prolonged positive phase (plateau) accompanied by prolonged period of contraction –Ensures adequate ejection time –Plateau primarily due to activation of slow L- type Ca 2+ channels

Electrical Activity of Heart Ca 2+ entry through L-type channels in T tubules triggers larger release of Ca 2+ from sarcoplasmic reticulum –Ca 2+ induced Ca 2+ release leads to cross-bridge cycling and contraction Because long refractory period occurs in conjunction with prolonged plateau phase, summation and tetanus of cardiac muscle is impossible –Ensures alternate periods of contraction and relaxation which are essential for pumping blood –Refractory= unresponsive to stimulus

Relationship of an Action Potential and the Refractory Period to the Duration of the Contractile Response in Cardiac Muscle Physiology of contractile cell

(b) Spread of cardiac excitation Bundle of His Interatrial pathway SA node Right atrium Right ventricle Electrically nonconductive fibrous tissue Interatrial pathway Left atrium Left ventricle AV node Purkinje fibers Fig. 9-8b, p. 312

fig ; pg: 568 Milliseconds SA node pacemaker Atrial muscle Atrioventricular Bundle branch Purkinje fibers Ventricular muscle Coordination of noncontractile and contractile cells

Electrocardiogram (ECG) Record of overall spread of electrical activity through heart Represents –Recording part of electrical activity induced in body fluids by cardiac impulse that reaches body surface Not direct recording of actual electrical activity of heart –Recording of overall spread of activity throughout heart during depolarization and repolarization Not a recording of a single action potential in a single cell at a single point in time –Comparisons in voltage detected by electrodes at two different points on body surface, not the actual potential Does not record potential at all when ventricular muscle is either completely depolarized or completely repolarized

ST segment = Time during which ventricles are contracting and emptying T wave = Ventricular repolarization TP interval = Time during which ventricles are relaxing and filling Recorded potential P PR segment = AV nodal delay QRS complex = Ventricular depolarization atria repolarizing simultaneously) P wave = Atrial depolarization SA node fires TP interval ST segment PR segment Q P S 200 msec R T Fig. 9-14, p. 320

Normal ECG. Credit: © Mediscan/Visuals Unlimited 3202

Abnormalities in Rhythm and rate Rhythm –Regularity or spacing of ECG waves Arrhythmia –Variation from normal rhythm and sequence of excitation of the heart Atrial flutter ( BPM) Atrial fibrillation Ventricular fibrillation Heart block Tachycardia >100 beats per minute Bradycardia < 60 beats per minute Damage of the heart muscle –Myocardial ischemia Inadequate delivery of oxygenated blood to heart tissue –Necrosis Death of heart muscle cells –Acute myocardial infarction (heart attack) Occurs when blood vessel supplying area of heart becomes blocked or ruptured

Outline Mechanical events –Systole, diastole –animation (volumes, pressures, sounds and EKG)

Specialized Conduction System of Heart

Cardiac Output Volume of blood ejected by each ventricle each minute Determined by heart rate times stroke volume

Heart rate is varied by altering balance of parasympathetic and sympathetic influence on SA node: –Parasympathetic stimulation slows heart rate –Sympathetic stimulation speeds it up Fig. 9-20a, p. 329

Cardiac Output Stroke volume –Determined by extent of venous return and by sympathetic activity –Influenced by two types of controls Intrinsic control Extrinsic control –Both factors increase stroke volume by increasing strength of heart contraction

Frank-Starling Law of the Heart States that heart normally pumps out during systole the volume of blood returned to it during diastole

(c) Stroke volume with combination of sympathetic stimulation and increased end- diastolic volume End-systolic volume 65 ml Stroke volume 140 ml End-diastolic volume 135 ml (a) Normal stroke volume (b) Stroke volume during sympathetic stimulation End-systolic volume 35 ml End-systolic volume 35 ml End-diastolic volume 175 ml End-diastolic volume 135 ml Stroke volume 70 ml Stroke volume 100 ml Fig. 9-23, p. 331

Coronary circulation Nourishing the Heart Muscle Muscle is supplied with oxygen and nutrients by blood delivered to it by coronary circulation, not from blood within heart chambers Heart receives most of its own blood supply that occurs during diastole –During systole, coronary vessels are compressed by contracting heart muscle Coronary blood flow normally varies to keep pace with cardiac oxygen needs

Coronary Artery Disease (CAD) Pathological changes within coronary artery walls that diminish blood flow through the vessels Leading cause of death in United States Can cause myocardial ischemia and possibly lead to acute myocardial infarction –Three mechanisms Profound vascular spasm of coronary arteries Formation of atherosclerotic plaques Thromboembolism

Collagen-rich smooth muscle cap of plaque Normal blood vessel wall Lipid-rich core of plaque Endothelium Plaque Fig. 9-29, p. 328

Fig. 9-30, p. 330

Left coronary artery B A Right coronary artery Area of cardiac muscle deprived of blood supply if coronary vessel is blocked at point A: Area of cardiac muscle deprived of blood supply if coronary vessel is blocked at point B: Left ventricle Right ventricle Fig. 9-31, p. 339

Table 9-4 p339