Physiology: Electrical Properties of the Heart Flashcards
What is different about the RMP of Pacemaker cells?
RMP is less negative
RMP is unstable - generates APs spontaneously and rhythmically at varying rates
Name the 3 phases of the SA node action potential.
Phase 4: pre-potential or pacemaker potential
Phase 0: upstroke
Phase 3: repolarization
What is the initial potential of the SA node?
-60 to -70mV and it declines spontaneously to become less negative
What causes the initial decay of the pacemaker potential?
Pacemaker current = inward Na+ current
Declining potassium conductance
Small inward current of Ca ions at -50mV
Why is the upstroke of the AP slow rising?
Cells lack the functional fast sodium channels
What mainly causes the 0 phases?
Slow inward current of Calcium
Opposed by outward K+ current
What causes Phase 3: Late Repolarization?
K+ exits bringing the membrane back to threshold
What controls the HR?
Frequency of SA pacemaker APs
Autonomic inputs control rate of SA node discharge
Name the fibres innervating the SA node.
Parasympathetic fibres –> vagus nerve
Sympathetic fibres –> cardiac nerves
What does sympathetic and parasympathetic stimulation do?
Sympathetic stimulation increases HR
Parasympathetic stimulation reduces HR
What does sympathetic innervation do to the curve?
Slope of Phase 0 is increased (pacemaker potential) = spontaneous rate of SA node depolarization is increased leading to an increased HR
What does parasympathetic stimulation do to the curve?
Reduces the slope of the pacemaker potential and hyperpolarizes the SA node –> HR slows
Sympathetic
NA binding to B1 adrenergic receptors
Rise in cAMP
PKA - phosphorylates the Ca channel (increase Ca) and K channel (increasing repolarization = shorten duration of AP)
Parasympathetic
ACh binds to muscarinic (M2) receptors Decreases cAMP Reduces sympathetic stimulation effects Hyperpolarization - activation of ACh sensitive potassium channel - potassium permeability is increased = hyperpolarization (RMP more negative) Longer to get to threshold = HR slows
Which system dominates at rest?
PSNS - vagal tone/vagal breaking
What does bradycardia/tachycardia do?
Bradycardia - increase PSNS
Tachycardia - increase SNS/decrease PSNS
What is the dominant pacemaker?
SA node
Greater rate of SA node suppresses others (AV node and conducting system)
Name the structures involved in the excitation conduction system.
SA node AV node Bundle of His Left and right bundle branches Purkinje fibres
What must the AP pass through to get to the ventricles?
Atrioventricular ring
What join myocardial cells?
Joined by structures called intercalated discs which contain gap junctions
Gap junctions permit cell-to-cell conduction of excitation = functional syncytium
ALL or NONE contraction
What is the function of the AV node in the conduction system?
Permits a delay which allows full depolarization and contraction of the atria before the ventricles are depolarized
What are Purkinje cells?
Modified cardiac cells
What ensures that cardiac muscle does not produce tetany?
The electrical and mechanical activity overlap considerably in time - cardiac contractile force cannot summate.
