7- Electric Activity Of The Heart Flashcards
What does the resting membrane potential depend on
And how can it be predicted
The membrane potential depends on the flow of K out of cells
•We can predict what a potential will be across a semi-permeable membrane using the Nernst equation
Why does the GHK equation give a better value of membrane potential
Takes into account relative permeabilities of ions
Why is the cardiac ap long
Compared with nerve the cardiac action potential is long (several hundred milliseconds vs 2 – 3 msec)
•Duration of action potential controls the duration of contraction of the heart
•Long, slow contraction is required to produce an effective pump
Why does refraction period occur in cardiac muscle
Occur as a result of Na channel inactivation. Na channels recover from inactivation when the membrane is repolarized
What are the phases of a cardiac ap
Phase 0 = upstroke •Phase 1 = early repolarisation •Phase 2 = plateau •Phase 3 = repolarization •Phase 4 = resting membrane potential (diastole)
What causes the upstroke
large increase in permeability of membrane to Na
What happens during early repolarisation
Ca influx required to trigger Ca release from intracellular stores
•Essential for contraction
•Ca current (ICa) activates rapidly (within a few milliseconds) but upstroke dependent more on INa
What happens during repolarisation
Gradual activation of K currents (K moving outward) that balance, then overcome, inward flow of Ca
•Large K current (IK1) that is inactive during the plateau starts to flow once the cells have partially repolarised
•IK1 is responsible for fully repolarising the cell
•IK1 is large and flows during diastole. It stabilizes the resting membrane potential reducing the risk of arrhythmias by requiring a large stimulus to excite the cells
Why do Different parts of the heart have different action potential shapes
•
Due to different ionic currents flowing
•Different degrees of expression of ionic channels
Describe the channeled in the SAN
Most channels exist in SA node – to some extent
•Exception is IK1 – no IK1 in SA node
•Very little Na influx – upstroke produced by Ca influx
•Also T-type Ca channels that activate at more negative potentials than L-type
How is intrinsic heart rate modulated
Vagus nerve and sympathetic nerves
Where is the SAN located
SA node lies lies just below the epicardial surface at the boundary between the right atrium and the superior vena cava.
What are the 4 components of the heart conduction system
SA node
(2) inter-nodal fibre bundles (stimulate the atria)
(3) atrioventricular node (AV node) and His bundle
(4) ventricular bundles (left and right bundle branches and Purkinje fibres)
Why is the resistance between cells low
The passive spread of current excites the neighbouring cells easily because the membrane resistance between cells is low due to gap junctions. Gap junction resistance determines extent of spread of excitatory current.
1: When a wave of depolarisation is moving TOWARDS the positive electrode
2: depolarising away from positive electrode
3 repolarising current away from positive electrode
When a wave of depolarisation is moving TOWARDS the positive electrode it causes an UPWARD deflection.
When it is moving AWAY from the positive electrode it causes a DOWNWARD deflection.
When a wave of repolarising current is moving AWAY the positive electrode it causes an UPWARD deflection.
