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degree of stretch of cardiac muscle cells before they contract (Frank-Starling law of the heart)
•Cardiac muscle exhibits a length-tension relationship
•At rest, cardiac muscle cells are shorter than optimal length
•Slow heartbeat and exercise increase venous return
•Increased venous return distends (stretches) the ventricles and increases contraction force
Increased Ca2+ influx due to sympathetic stimulation
Hormones (thyroxine, glucagon, and epinephrine)
Sympathetic nervous system
activated by emotional or physical stressors
Norepinephrine causes the pacemaker to fire more rapidly (and at the same time increases contractility)
Parasympathetic nervous system
opposes sympathetic effects
Acetylcholine hyperpolarizes pacemaker cells by opening K+ channels
The heart at rest exhibits vagal tone (parasympathetic)
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