Cardiovascular system composed of
- Heart – the pump
- Blood vessels – conducting channels
- Blood – transport medium
- Transport O2 and other substances to tissues
- Transport CO2 metabolic waste products from tissue for excration
- Regulation of body temperature
- Distribute hormones and other signaling molecules to regulate cellular function
Pace maker cells
- Rhythmically discharging cells
- Show prepotentials ( pacemaker potentials)
- Membrane potential declines to firing level after each impulse .triggers the next impuls
- At peak of each impulse, K+ efflux causes repolarization
- K+ efflux decreases , so membrane becomes depolarized , forming first part of prepotential
- Ca+2 channels then open
- Two types
- T- Transient – complete the prepotentials
- L- Long Lasting – produces the impulse
Other features
- RMP varies from -50 to -60mV
- Prepotential are only prominent in SA and AV node
- Latent pacemaker discharge only when SA or AV nodes are depressed or blocked
Effect of autonomic nerves system
Parasympathetic
- Membrane becomes hyperpolarized
- Slop of prepotentials reduce
- Due to increased K+ conductance
- Reduce firing and heart rate
- Membrane potential falls rapidly
- Slop of prepotential increases
- Due to increased opening of Ca+2 channels
- Increased rate of discharge
Rate of pacemaker cell discharge
Increased by
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Decreased by
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Drugs
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Drugs ( digitalis )
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Temperature
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Conducting system of heart
Tissue
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Location
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Rate of discharge
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Importance
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SA node
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Junction of SVC and right Atrium
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60- 100 per minute
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Referred to as the pacemaker
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AV node
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Right posteror prtion of I.A septum
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40- 60 per minute
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Bundle of His
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I.V septum ( subendocardium )
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20- 40 per minute
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Solo connection between atrial and ventricular syncytia
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Purkinje fibers
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Ventricular myocardium
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20 -40 per minute
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Electrical
Activity of a contractile cell
Initial rapid
depolarization and overshoot
- Due to opening of voltage gated Na+ channel
Initial rapid
repolarization
- Closure of voltage gated Na+ channels
Plateau
- Slower opening of voltage gated Ca+2 channels
Final repolarization
- Due to closure of voltage gated Ca+2 channels and efflux of K+ ions through K+ channels
Resting
membrane potential
- -90 mV
Other features
- Depolarization (duration 2 mS) is followed by a prolonged plateau (200 mS) (contraction time 250 mS)
- Importance – cell is refractory to excitation during a contraction. Prevent fatal tetany
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