L3 Flashcards
what are membranes highly permeable to
K+
what is the membranes permeability for Na+ and Ca+
it has a low permeability to Na+ (about 10%) and a very low permeability to Ca+
what is another name for conduction
depolerisation wave
name the structures in the conduction system
sinoatrial node (SA) - origin of the action potential
atrioventricular node (AV)
bundle of hiss
purkinje fibers
cardiomyocytes
where is the AV node located
it boundaries the atria and the ventricles
this is where the fibrocartilaginous structure are in the heart (AV valves)
what is the role of the AV node
it acts as an insulator
describe the conduction pathway
it starts in the SA node. it travels through the atria to the AV node
after it passes through the AV node it passes through the bundle of hiss into the left and right bundle branches
it then travels down the septum, into the purkinje fibers
the purkinje fibers wrap around the heart and going individual cardiomyocytes which all communicate with each other via gap junctions
what is special about cardiomyocytes that allow them to conduct APs
they are interwoven and have branched ends.
the ends contain intercalated discs which allow for cell to cell communication (via gap junctions)
what is a functional syncytium
cells that are very tightly bound together
this means that cardiomyocytes have an all or nothing contraction
at what rate does the SA node generate action potentials
100 min-1
what is the speed of conduction through the atria
0.5m/s
what os the speed of conduction through the AV node
0.05m/s
why is conduction through the AV node slower than it is through the atria
because the delay permits full depolerisation and contraction of the atria before depolerisation and contraction of the ventricles
it lets the ventricles fill with blood before it contracts
what does the contraction of the atria represent
atrial top up
what is the speed of conduction in the bundle of hiss, bundle branches and purkinje fibers
5m/s
this is VERY quick for the distance that it is traveling meaning that the speed of ventricular contraction is mostly dependent on the contraction of the myocardium
what is the speed of conduction in the ventricular myocardium
0.5m/s
what do all the different speeds of conduction allow for
synchronous depolerisation and contraction of all ventricular regions
the speed of conduction spreads slowest in the……
A) SA node
B) Atria
C) AV node
D) Bundle of Hiss
C
what is the difference in action potentials in pacemaker cells compeered to ventricular cells
pacemaker cells are the origin of action potentials. they are located in the SA node and are known as the leaders
ventricular cells (cardiomyocytes) are known as the flowers
what are the 3 phases of the pacemaker cell
phase 4
phase 0
phase 3
explain phase 4 of a pacemaker cell
phase 4 is the equivalent of the cell being at RMP (-60 to -70mV) however the membrane is not resting as it is always becoming more up and down
this is because pacemaker cells have funny Na+ channels which cause a slow influx of Na+
the slow influx of Na into the cell causes T-type Ca2+ channels (TTCC) to open in the later end of phase 4 causing Ca2+ to enter the cell
the cell then reaches threshold which is about -50 to -40 mV
what happens when the pacemaker cell reaches threshold
L-type Ca2+ channels (LTCC) open
what is phase 0 of a pacemaker cell
this phase is also known as the upstroke
L-type Ca2+ channels (LTCC) open
this causes a rapid influx of Ca2+ into the cell
what is phase 3 of a pacemaker cell
this is when the Ca2+ channels close (and the Na+ channels as much as they can) and repolarisation starts to happen SLOWLY because of K+ diffusing through the membrane
why do pacemaker cells have weird phase names
because they are based of what the ventricular cells are doing
how many phases do ventricular cells have
5
what phase is at RMPin a ventricular cell
4
describe phase 4 of a ventricular cell
unlike the pacemaker cells this phase is very stable with a very negative RMP of -90mV
this is because the membrane is VERY permeable to K+
describe phase 0 of a ventricular cell
it is the fast and rapid depolerisation of the cell
this happens because of the opening of fast Na+ channels
what is the threshold of ventricular cells
-65mV
what is another name for phase 1 in a ventricular cell
early repolarization
describe early repolarization of a ventricular cell
this is when the fast Na+ channels close because they become inactivated but the membrane is still very permeable to K+ therefore the cell get slightly more -ive
what is another name for the plateau phase of a ventricular cell
phase 2
what happens in phase 2 of a ventricular cell
this is when L-type or voltage operated Ca2+ channels open
this causes the plateau because of sustained depolerisation as the Ca2+ coming in balances the K+ Efflux
at the end of this phase the L-type channels close which causes phase 3
what is phase 3 of a ventricular cell
it is the late repolarisation
the LTCC channels close and K+ Efflux which causes a fast repolerisation
why does an ECG look the way it does
the ECG is the combination of all of the different action potentials in the conduction system that happen over the time leading up to the contraction
in ventricular cells, slow depolerisation occurs through funny Na channels BECAUSE the resting membrane potential of ventricular cells is stable around -90mV
the first is false and the second is true
what is the physiological basis for ECG
different APs at different locations in the hearts at different times
what is an ECG
a recording of potential changes at the skin surface that result from depolarisation and repolarisation of the heart
the scale of electrical recordings from inside the cells of the heart differ greatly from the scale of an ECG why is this
inside the cell is about 100mV and an ECG recording is about 1mv
this is because an ECG is taken from the body surface therefore most of the amplitude is lost
what does an ECG allow you to measure
the electrical activity of the heart
and
the direction of relative size of potential changes
where are the electrodes placed for an ECG
the right arm, left arm and left leg
where are the charges on lead 1
-ive on the right and +ive on the left arm
where are the charges on lead 2
-ive on right arm and + on left leg
where are the charges on lead 3
+ on left leg and - ive on left arm
the combination of 2 electrodes is called a what
bipolar lead
what are Einthoven’s standard limb leads
leads 1, 2, and 3
what is Einthoven’s triangle
leads 1, 2, and 3
the ECG that gives you the picture that you always think about is taken from what lead
lead 2
describe what causes the ECG to look the way it does when using lead 2
imagen a camera sitting at your left hip and it is looking down lead 2
when depolerisation is coming towards the camera which causes a positive deflection and when it is moving away from the camera it causes a negative deflection
repolarisation is the opposite
what causes you to see the P wave in lead 2
SA cells depolarise which causes the right and left atria to depolarise towards the AV node
towards AV node = towards + electrode therefore you get positive deflection (the P wave)
what causes you to see the Q wave in lead 2
in the ventricular septum the overall direction of the depolerisation is slightly away from the +ive electrode therefore you get a slight negative deflection
what causes you to see the R wave in lead 2
ventricular depolarisation from ‘inside to outside’
the bottom of the ventricles contract before the top
therefore these is a large depolerisation towards the +ive electrode causing a large positive deflection
what causes you to see the S wave in lead 2
this is late ventricular depolerisation. since now the top of the ventricles are contracting the depolerisation is away from the +ive electrode.
this is only small therefore it causes a small negative deflection
what causes you to see the T wave in lead 2
repolerisation of the ventricles goes from outside to inside
therefore repolerisation is happening away from the +ive electrode causing a positive deflection
why is atrial repolerisation not shown on the ECG
because atrial repolerisation is very small therefore it is hidden by the large ventricular depolerisation
what are augmented limb leads
when you create 3 more views without adding any more leads on you
what are the names of the augmented limb leads
aVR - augmented lead right
aVL - augmented ead left
aVF - augmented lead fount
what does aV in the augmented leads stand for
augmented voltage (thats measured)
are the augmented lead bipolar
no
they are unipolar
what is aVR
it is lead 3 with another lead coming from the middle connected to a +ive electrode on the right hand
what is aVL
it is lead 2 with another lead coming from the middle connected to a +ive electrode on the left hand
what is aVF
it is lead 1 with another lead coming from the middle connected to a +ive electrode on the left foot
the augmented leads plus the einthoven (standard) limb leads creates what kind of system
hexaxial reference system
what does the hexaxial reference system allow you do to
to see the position of your heart as there is a large variance between each person
the position can range from the apex pointing at your left arm to your right hip
it also allows you to see APs at different location s in the heart at different times
which is the only lead with a negative deflection
aVR
what are chest leads
active electrodes in one of 6 positions on the chest
V1, V2, V3, V4, V5, V6
what is a common characteristic between all of the chest leads
they all have large ECG deflections
what plane do chest leads examine the plane in
the horizontal plane
