Origin and Conduction of Cardiac Impulse Flashcards
what is autorhythmicity?
Heart is capable of beating rhythmically in the ABSENCE of external stimuli e.g. nervous stimulation
where does the excitation normally originate?
in the specialised pacemaker cells of the sino-atrial node
where is the SA node located?
in the upper RIGHT ATRIUM close to where the Superior Vena Cava enters the right atrium
what is a heart controlled by sino atrial node called?
said to be in sins rhythm
how does the excitation arise?
the cells in the SA node do not have a stable resting membrane potential and therefore exhibit spontaneous pacemaker potential
what must happen for an action potential be generated in the SA nodal cells ?
the threshold
what generates the pacemaker potential?
decrease in K efflux superimposed on a slow Na influx
what is the rising phase of the AP (i.e. depolarisation) caused by?
activation of voltage gates Ca channels which results in Ca influx
what is the falling phase of AP (i.e. repolarisation) caused by?
activation o K channels resulting in K efflux
what is the chain of electricity in the heart?
SA node - AV node - Bundle of His - left and right branches - purkinje fibres
what is the pacemaker potential?
gradual drift of the membrane towards depolarisation
why is there such good cell to cell flow of excitation?
because the signals are passed through gap junctions
what anatomical feature is the only point of electrical contact between the atrium and the ventricle?
the AV node
what are gap junctions composed of?
low resistance protein channels
what is the AV node?
a small bundle of specialised cardiac cells
where is the AV node located?
at the base of the right atrium; just above the junction of atria and ventricles
how are the AV node cells different from other node cells?
they are smaller in diameter and have slow conduction velocity
how are electrical signals passed from SA node to AV node?
via gap junctions but there is also some internodal pathways
what does the delay between conduction from the SA to AV allow for?
atrial systole to precede ventricular systole
what allows for the rapid spread of action potential to the ventricles?
the bundle of His and its branches and the network of purkinje fibres
are the APs different in ventricular muscle than nodal cells?
YES
what is the resting membrane potential for myocytes?
-90
what results in the extremely rapid depolarisation of the membrane potential of myocytes?
fast Na influx
what does the myocyte membrane potential then reach?
30 mV
what is this phase of rapid increase in MP known as?
phase 0
what is phase 0 of ventricular muscle action potential?
fast Na influx
what is phase 1 of ventricular muscle action potential?
closure of Na channels and transient K efflux
what is phase 2 of ventricular muscle action potential?
mainly Ca influx
what is phase 3 of ventricular muscle action potential?
closure of Ca channels and K efflux
what is phase 4 of ventricular muscle action potential?
resting membrane potential
what is the plateau phase ?
the MP of ventricular muscle cells is maintained near the peak of action potential for a few hundred milliseconds
what is the plateau phase mainly due to?
an influx of Ca through voltage gated Ca+ channels
what is the falling phase of the AP (repolarisation) caused by?
inactivation of Ca channels and activation of K channels
which nerve and nervous system exerts a continuous influence on the SA node under resting conditions
the vagus nerve - parasympathetic
what done the vagal tone do?
slows the intrinsic HR from 100bpm to produce a normal resting HR of of 70 bpm
what is a resting heart rate of less that 60 called?
bradycardia
what is a resting heart rate of more than 100 called??
tachycardia
which cells does the vagus nerve supply in the heatr?
SA node and AV node
what two things does vagal stimulation do?
slows the HR down and increases AV nodal delay
what is the neurotransmitter and its receptor for the parasympathetic innervation?
acetyl choline acting through M2 receptors
what does atropine do?
acts as a competitive inhibitor of acetylcholine - used in extreme bradycardia to speed-up the heart
how does vagal stimulation slow down the HR?
cell hyperpolarises for longer to reach threshold
the slope of pacemaker potential decreases
so the frequency of AP decreases
what is this vagal stimulatory effect called?
negative chronotrophic effect
which cells do the sympathetic nerves supply?
the SA node and the AV node and the myocardium
what 3 things does the sympathetic stimulation do?
increases heart rate and decreases AV nodal delay and increases the force of contraction
what is the neurotransmitter and its receptor for the sympathetic innervation?
noradrenaline acting on B1 adrenoceptors
why does the slope of the pacemaker potential in sympathetic innervation increase?
reaches threshold quicker
what is an ECG?
The ECG is a record of depolarisation and repolarisation cycle of cardiac muscle obtained from skin surface
