Physiology 3120 Lecture Notes - Lecture 14: Purkinje Fibers, Pr Interval, Vagus Nerve

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Stavraky Lecture 14
NOTE: time is in sec for this whereas in neuronal AP time was in milliseconds
-nodal AP is called slow response AP because the rising phase after threshold is slow as
opposed to ventricular AP which is fast response AP as the rising portion is much steeper
-the resting membrane potential (lowest part) is much lower in ventricular AP than nodal AP
-they use different ion channels at different times and places to produce distinct APs
-the slow rise or depolarization up to threshold gives the SA node the ability to self-excite
-list of channels in heart (do not memorize)
-appreciate the range of channels available for use
-so SA node might use second fourth fifth channels while ventricular muscle fiber may use the
same three in different order or in different spots
RMR standard conc gradients
-Na K Ca predominantly when looking at SA nodal AP
-Cl comes in when looking at ventricular muscle AP
-the movement of these ions sets up the currents which set up the AP
-slow depolarization from -60 up to threshold and the period during which this occurs is called
the pre-potential
-two things cause this in terms of membrane potential
-have Na and Ca leaking in thus more positive ie: depolarization
-do not want K to leave either since trying to get more positive thus K becomes LESS permeable
so they remain inside during diastole ie: relaxation phase
-SA nodal cells do not have stable RMP vs. neuronal RMP was at -70mV and it would stay there
as long as there is no excitation from another neuron, NTs, etc.
-also note once again how short (in ms) neuronal AP is
-pacemaker potential caused by ion movements seen in previous slide  more Na and Ca
coming in, less K going out
-membrane potential normally varies between -60mV to 20mV during the length of the SA
nodal AP (will see later that it can drop below -60mV)
-diastole = heart is at rest thus not contracting
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-Na channels are called funny channels because they open when membrane is at -60mV and
close when it reaches -40mV ie: threshold
-Ca channels are called transient channels because they open very briefly at certain times
-first T-type Ca channels open followed by SOME slow L-type Ca channels open  both let in
Ca slowly
-so now have reached threshold and now at depolarizing phase
-for neuronal AP, depolarization was caused by VG Na channels opening whereas here
depolarization phase has VG Na funny channels closing and slow L-type VG Ca channels opening
-called slow because they open slowly thus Ca flows in slowly = slow response AP until
membrane potential reaches 15-20mV
-so now T-type Ca channels close while the REST of slow L-type Ca channels open
depolarizing phase where MP goes from threshold to +20mV
-now it is time to repolarize
-neuronal repolarization is similar to SA nodal AP
-slow L-type VG Ca channels start to close at the same time that VG K channels start to open 
get rounded peak
-Ca stop coming in and K start leaving cell making the inside more negative ie: repolarization
to about -60mV
-and the cycle continues so VG K channels start to close (less K leave cell) and Na and Ca start to
open (more Na and Ca enter cell) to get depolarization to threshold once again to fire the next
SA nodal AP
-another way of looking at SA nodal AP based on ion conductance (here it goes up to only 0
when it should go up to 15-20mV)
-VG K channel shows that less K is moving out during pre-potential and then when they start to
open at the peak during repolarization, more K goes out
-Na channel shows that more Na is moving in during pre-potential and then when they close at
the threshold, less Na comes in
-T-type Ca channel shows that more Ca is moving in during pre-potential and then slow L-type
VG Ca channels open at the threshold, more Ca comes in until reaches 15-20mV where less Ca
comes in and then more Ca comes in through T-type Ca channel during the pre-potential of the
next AP
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