Organisation of the Body
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What is the heart | show 🗑
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What is cardiac output and how is it regulated | show 🗑
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Gross structure of the heart | show 🗑
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show | Ventricular - brick like appearance with striations, mono-nucleated to provide the contraction force
Atrial - smaller as less force is needed, still striated
SAN - very thin, no sarcomeric structures, key cells in the pacemaker
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Ultrastructure of myocardium | show 🗑
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show | Make of connexin 43, 6 monomers form a connexon tunnel which spans the membrane projecting into the extracellular space where it connects to connexons of adjacent cells
This allows movement of ions and hormones between myocytes
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Organisation of contractile filaments | show 🗑
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show | The greater the filling of the cardiac chamber the greater individual myocardial fibres are stretched, hence the greater the subsequent force of contraction
Small changes in size of a myocyte will therefor produce large changes in force generated
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show | Generated in the SAN, cannot go directly to the ventricles so must pass through the AVN. This ensures the atria contract before the ventricles
Action potential is prolonged, so the muscle fully relaxes before another AP can be generated
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Different Action Potential shapes | show 🗑
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show | Pacemaker function - myogenic rhythm
Fast/slow conduction to synchronise contraction
Long plateau to maintain refractoriness and avoid high frequency activation
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Phases of the Action potential in the SAN | show 🗑
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What are HCN channels | show 🗑
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What sets the pacemaker current | show 🗑
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Phases of the action potential in ventricular myocytes | show 🗑
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Excitation contraction coupling | show 🗑
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show | This is used to exclude calcium from myocytes following contraction
This is an electrogenic pump as per Ca extruded 3 Na move in leading to a positive inwards flow
The Na/K pump helps to maintain the Na gradient and offset this
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Action of Ouabain | show 🗑
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show | Adding a Ca2+ dye to a cardiac myocyte reveals a Ca2+ wave on excitation, which moves through the cell allowing for synchronous contraction
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show | The rise in [Ca2+] in normal cardiomyocytes during a beat is only large enough to produce a fraction of the intrinsically available strength (50%)
The strength of contraction can be increased by increments of [Ca2+] attained via ionotropic interventions
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show | Cardiac output = stroke volume x heart rate
Can change frequency of contraction - chronotropy
Can change force of contraction - inotropy
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Regulation of Chronotropy - Increased heart rate | show 🗑
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Regulation of Chronotropy - Decreased heart rate | show 🗑
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Regulation of inotropy - hormones | show 🗑
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show | Starling's law states that as length of the myocyte increases, force generated increases
Steep change in tension depends on: change in double actin overlap and length dependant change in TpnC Ca affinity
More blood in chambers more forceful contraction
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