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Describe the external nerve supply to the heart?
•Sympathetic nerves from
- - upper thoracic spinal cord, through sympathetic chain to cardiac nerves
- - directly to ventricular myocardium
- - can raise heart rate to 230 bpm
- - right vagus nerve to SA node
- - left vagus nerve to AV node
- - vagal tone – normally slows heart rate to 70 - 80 bpm
- - No vagal innervation to ventricular myocardium
Describe the cardiac conduction system?
•myogenic - heartbeat originates within heart
•autorhythmic – regular, spontaneous depolarization
•Sinoatrial (SA) node: pacemaker, modified cells, initiates heartbeat, sets heart rate, located in right atrium
•fibrous skeleton insulates atria from ventricles
•AV node: electrical gateway to ventricles in right atrium
•AV bundle: pathway for signals from AV node into ventricles
•Right and left bundle branches: divisions of AV bundle that enter interventricular septum
•Purkinje fibers: upward from apex spread throughout ventricular myocardium to excite cardiomyocytes
Relate the structure of cardiac muscle to its function?
•Short, branched cells, one central nucleus
- •Less Sarcoplasmic reticulum, large T-tubules
- - admit more Ca2+ from ECF
- •Intercalated discs join myocytes end to end
- - interdigitating folds - increase surface area
- - Desmosomes - hold cells together, structural support
- - electrical junctions - gap junctions allow ions to flow between cardiomyocytes
•Aerobic respiration, rich in myoglobin and glycogen, large mitochondria
•Organic fuels: fatty acids, glucose, ketones and lactic acid
. Explain how the SA node fires spontaneously and rhythmically?
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