SA node - located in the right atrial wall, just inferior to the entrance of the superior vena cava. Original Impulses from S-A Node The electrical impulses are normally generated by a group of specialized pacemaker cells at sinoatrial (SA) node.
Conduction of Electrical Impulses in the Heart
Conduction of Action Potentials from Cell to Cell
through gap junctions in intercalated discs
Conduction in Atria The electrical impulses from SA node spread through the entire right and left atrial muscle mass, triggering contraction of the right and left atrium.
Delay at A-V Node - The impulses from S-A node travel to atrioventricular (A-V) node . - A-V node is located in lower end of the interatrial septum near the tricuspid valve. A-V node
Delay at A-V Node - A-V node is the only normal route that impulses from SA node are transmitted into ventricles. - Conduction speed in A-V node is slow (delay). - This delay allows time for the atria to finish contraction and empty their contents into the ventricles before ventricles start to contract.
After the delay at A-V node, the impulses rapidly spread to the ventricles via specialized fibers, Purkinje fibers . Rapid Conduction in Ventricles
Rapid conduction in the ventricles simultaneous excitation of the ventricles functional syncytium
NNote : - Each electrical impulse can trigger cardiac muscle contraction normally only once. - A normal heart generates 60 to 100 impulses in 1 minute at resting state. 1 1
Excitation Contraction [ Ca ++ ] i (Action Potentials) (shortening) Properties of Cardiac Muscle Excitation of the heart is triggered by electrical impulse rather than neural transmitters. Contraction of the heart is triggered by elevation of intracellular calcium influx.
Properties of Cardiac Muscle - Myocytes depend heavily on oxygen and blood supply. - Not fatigue - Excitability Cycle The myocytes have Long refractory period during which they do not respond to any electrical impulses.
RRole of a Long Refractory Period – 1 prevent ventricles from contracting at too high rates so that enough time is allowed for refill of the ventricles
Role of Long refractory period - 2 Prevent retrograde excitation
Disorders of the Cardiac Conduction System ---- Arrhythmias - refers to abnormal initiation or conduction of electrical impulses in the heart. - caused by ischemia, fibrosis, inflammation, or drugs.
occurs when the atrioventricular (AV) valves close at the beginning of ventricular contraction.
generated by the vibration of the blood and the ventricular wall
is louder, longer, more resonant than the second heart sound.
- occurs when aortic and pulmonary semilunar valves close at the beginning of ventricular dilation - generated by the vibration of the blood and the aorta - Aortic valve closes slightly before pulmonary valve. Second heart sound
Heart Murmur - abnormal heart sound - occur in valvular diseases and septal defects
Two Basic Types of Valvular Diseases 1) valvular stenosis , a narrowing of the valve 2) valvular insufficiency (incompetence). A valve is unable to close fully; so there is some backflow (regurgitation) of blood.
the force that stretches the muscle before contraction.
the force that stretches muscle during contraction.
Preload to ventricles = ventricular end diastolic pressure - the degree of stretch of the ventricular muscle cells just before they contract. - determined by ventricular filling.
Afterload to left ventricle: aortic arterial pressure Afterload to right ventricle: pulmonary arterial pressure Afterload to the left ventricle is greater than that to the right ventricle. Aortic arterial pressure
Contractility - the intrinsic strength of cardiac muscles.
1) Baroreceptor Reflex - stimulated by increase in arterial pressure (stretch) - Effect: negative chronotropic and inotropic - regulate the heart when BP increases or drops - involved in short term regulation of BP
dull white and slightly elevated fibrous plaque ( atheroma ) on coronary arterial lumen. Typical lesion of Coronary Atherosclerosis
composed of lipid, smooth muscle, macrophages, and connective tissues. cause stenosis of coronary arteries Histology of the plaque occlude arterial lumen when combined with internal hemorrhage, thrombosis, and arterial spasm