Origin and Conduction of Impulse Flashcards
where are the electrical signals controlling the heart generated?
within the heart
what is the heart
an electrically controlled muscular pump
what is autorhythmicity
the heart is capable of beating rhythmically in the absence of external stimuli
where does excitation normally originate in the heart
the pacemaker cells of the SA node
what do pacemaker cells in the SA do
initiate the heart beat
where is the SA node located
in the upper right atrium, close to where the SVC enters right atrium
what does SA node do
drives the heart beat for the entire heart, controls sinus rhythm
do the cells of the SA node have a stable resting membrane potential
NO, they exhibit spontaneous pacemaker potential
what does the spontaneous pacemaker potential do
it takes the membrane potential to a threshold to generate an action potential
what is the typical value for threshold membrane potential for pacemaker cells
-40mV
what is the pacemaker potential
the slow depolarisation of membrane potential to a threshold
why does pacemaker potential occur
- decrease in K+ efflux
- the funny current (Na+ and K+ influx)
- Transient Ca++ influx (T type Ca++ channels open)
what is the rising phase of the action potential in pacemaker cells
it is depolarisation caused by activation of long lasting (L type) Ca++ channels, this then results in Ca++ influx
what is the falling phase of the action potential in pacemaker cells
repolarisation, caused by inactivation of the L type Ca++ channels and activation of the K+ channels causing K+ efflux
describe spread of cardiac excitation
originates in SA node, spreads by cell to cell conduction throughout atria, reaches AV node, AV nodal delay occurs, excitation continues into Bundle of His, down right and left purkinje fibres by cell to cell conduction, impulse has spread across whole heart
how does cell to cell excitation occur
gap junctions
what are gap junctions
junctions (low resistant protein channels) in the intercalated disc between two adjacent myocyte cells
what is the AV node
small bundle of specialised cardiac cells, small in diameter with slow conduction velocity
where is AV node
at the base of the right atrium, just above atrioventricular junction
are there other points of electrical contact between atria and ventricles besides the AV node?
NO
how is excitation spread across the atria
cell to cell conduction
how is excitation spread from SA node to AV node
mainly cell to cell conduction via gap junctions, but are some intermodal pathways
why is conduction delayed in AV node
to allow atrial systole (contraction) to finish before ventricular systole occurs
what is systole
contraction
what is diastole
relaxation
where does excitation go after AV node
Bundle of His and Purkinje fibres
what is Bundle of His
fibres travelling down septum which split into left and right bundles
what are Purkinje fibres
terminal conduction fibres from the Bundle of His
what do the Purkinje fibres allow
rapid spread of action potential throughout the ventricles
what must the heart do to the action potentials
translate them into chemical impulses (contractions)
is the action potential in contractile cardiac muscle cells different from that in pacemaker cells
YES
what is resting membrane potential for contractile cardiac muscle cells
-90mV
what is the rising phase of the muscle cell action potential
depolarisation, this is caused by FAST Na+ INFLUX which rapidly reverses membrane potential to about 20mV
what is the rising phase of the muscle cell known as
PHASE 0
what happens in phase 0 (cardiac muscle cell)
fast Na+ influx
what happens in phase 1 (cardiac muscle cell)
closure of Na+ channels and transient K+ efflux
what happens in phase 2 (cardiac muscle cell)
mainly Ca++ influx (through L type channels)
what happens in phase 3 (cardiac muscle cell)
closure of Ca++ channels and K+ efflux
what happens in phase 4 (cardiac muscle cell)
resting membrane potential returns, -90mV
what is the difference between action potential phases of pacemaker cells and cardiac muscle cells
cardiac muscle cells have phases 0-4, pacemaker cells don’t have phases 1 or 2 (only phases 0, 3, 4)
