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Lecture 5

BIOB32H3 Lecture Notes - Lecture 5: Skeletal Muscle, Calcium Atpase, Mollusca


Department
Biological Sciences
Course Code
BIOB32H3
Professor
Kenneth Welch
Lecture
5

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Lec7
Vm=-92, whie lEk=-58
Review the goldman equation
o Combine all the ions together
Concentration and permeability
o In frog, Pk is extremely larger than Pna
Why?cuase the leak channel of K+ is open in resting axonmain reason of
the Vm ( leak channel only refers to the K+)
Pcl can be ignored m, since almost impermeable
o Compare with Nernst equation
N only gives you Ek , not thinking about permeability ,only concentration
It’s linear
While goldman, takes into account multiple ions
Not linear
More close to observed values
Take into account f permeability
CAP
o Mini stimulasu intensity
The intensity necessary to trigger at least one or a few axon to fire AP
Max stim: max # of axons fire max CAP
TM: The Hodgkin cycle
o Positive feedback loo
Current applied from outside ( electrode stimulus on the neuron), to trigger just
a few Na+ channel to open, but not enough to trigger a all-or non-AP
Then more Na+ channel( voltage-gated Na+ cchannel, activated by
depolarization open due to depolarization )
Lead to more depolarization
o So it’s the voltagegated ion (Na+) channel, that lead to AP happening
Compare the Na+ and K+ channel
o Na+ rapidly activated by depolarization, but K+ is slower
o Na+ become inactivated even if Vm is depolarized(leading to refractory period) , but K+
become inactivated slowly, and not completely if even Vm remains depolarized, (leading
to the hyperpolarization)
Equilibrium potential of the membtrnae potential
o Def: when no net movement of any specific ion
o Look t Na+ :
Start with Vm resting at -70
concentration gradient of Na+ always want Na+ to flow in ---so always inward
of Na+
while the electrical gradient changes direction of ion movement (Na+) as Vm
changes: Ena is +58, and Vm at resting is -70

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so when you are starting from resting state, you want to move more +
ions(Na+) inward to drag the Vm up to +58
once your Vm pass +58(wrong! Should be 0, since a voltage of 0 means
the outside and inside charge are the same, and hence no electrical
gradient/difference) , from their onward, your electrical gradient flip the
direction of ions to outward , but still smaller than the force of
concentration gradient
once you reach Ena, the two gradient becomes equal
o look at K+:
start with Vm at near Ena+(+58), but for K+ wants to drag it down to its Ek=-90
so concentration gradient is always out, since K+ inside is larger than outside
so electrical gradient FIRST is out( K+ positive ions outward), and then no at
Vm=0, and then flip to in after becomes negative
o Na+/K+ pump
Its effect is 3Na+ outward,with 2K+ inward, so a net negative charge inside the
cell, hencemaintain a negative Vm
Not directly cause the repolarization
Though it is activated by AP phenomenon(Na+ inward and K+ outward
due to AP), but needs many AP before the pumping activity becomes
noticeable
o What do you see in refractory period?exctitability of the AP/Na+ voltage gated
channels are reduced
This is talking about CAP, as you look at a population of Na?K+ channels
Threshold of AP is higher
Because not all the Na+ voltage gated channels are active during
refractory period, some of them are deactivated(rapid closing of voltage
gated Na+ channel) recall the comparision between Na+ and K+
channels
Ie , not enough ‘synchronization ‘ of the opening Na+ channels
o This can also be resulted from a slowly stimulation
Ie, the population ( each individual Na+ V gated channel) varies wrt to
open and inactivation time
Hodgkin cycle may not at all, or is harder to get started higher
threshold of AP
o Length constant(lambda)
Distance
Over which Vm decays by 63%
So the larger, the faster the conduction velocity
Squid giant to increase rm(numerator)
Myelination to decrease rl(total longtitudinal resistance)
High memebrane resistance + reduce membrane capacitance

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Lead to a very high threshold for AP whre myelin is present
So only NOR can generate APsaltatory conduction
o ELECTRICAL SYNAPSE
SIGNAL IN CELL B looks like in cell A graded potential
If it’s depolarization above threshold, then magnitude of AP is smaller in B, but
still above threshold
Rectifying ES
Due to one of the two connexon is only functional to respond to
depolarization to open so one way only
o Chemical synapse short answer-to compare
Fast:
ligand gated channels
cleft is small
NT is small
vesicles releaed at specialized active zone
NT recycling de: NT deactivated and removed from synapse
o where the NT is endocytosed
in the axon terminus ( ie the presynase)
eg: Ach bind to bicotinic synapseslead to post-synapstic
depolarization , GABA
o desensitization of the nicotinic channel
normal: Ach bind->channel open,
note open to both Na= and K+, so no difference
between Na+ and K+unlike the ones seen in
single cell (pre0synapse)
but desensitized: if Ach binds, then channel will close,
and stay close
hence needs the Acetylchoniesterase to break
down Ach in the cleft and recycle into the
axonal terminal
slow
NT is larger
Not released at specialized active site
Indirect post-synapse action
o The G protein channel
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