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Topic 10 Membrane Potential 2021 Edition

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[Music] n

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this diagram shows how the membrane

potential is measured as you can see

there's a probe that is inserted in the

lipid by

layer which in which the tip sticks

inside so what that actually measures is

the charge of the inner surface of the

membrane as shown here it is 80

so that means the inner surface of the

cell membrane is negatively

charged all right so what is a membrane

potential so membrane potential are

changes in electrical charge of the cell

membrane particularly on the inner

surface as already

mentioned this also pertains to the

change and the polarity of the

membrane so what are the types of

potential so we have the resting

membrane potential and the action potential

potential

within the cell and then there are also

post synaptic potentials between two

neurons which are either excitatory or

inhibitory technically speaking they are

uh synaptic potentials of cells that are

produced between two cells okay we'll

discuss that in detail more uh in more detail

detail

later so let's start with the resting

membrane potential so the resting

membrane potential is the potential of

the cell

it is not performing a

task it is uh normally

negative and this means that a cell is

membrane is polar at rest so as you can

recall in the diagram previously it is

ne80 M so that means the resting

membrane of that cell is 80

M so this negativity of the inside of

the cell is maintained due to the

activity of the sodium pottassium pump

sodium pottassium pump as you may recall

uh pumps charges uh asymmetrically

meaning it uh pumps out more positive

charges than it uh pumps in uh positive

charges so that means as time passes by

then inner Sur inner surface of the

membrane becomes

negative now we have also the action

potential so what is the action

potential is the potential of the

membrane when it is performing a work or

task it is usually less negative or in

some cases zero or in some rare cases it

even becomes positive so remember we are

referring to the membrane potential of

the inner surface of the membrane so it

is um negative at rest so it becomes

less negative or we can say that it

becomes more positive or it approaches

zero or become zero itself or even

overshoots zero and becomes positive in

some cases so this

means that the membrane atress is

polarized and becomes depolarized during

action potential while it is performing

its work

okay so here are the here's a diagram

that shows the faces of the action

potential the first phase is the resting

membrane potential in this diagram it is negative

negative

70 so the number two is the threshold

potential and we can estimate it at

around uh -50 probably so the threshold

potential uh refers to the um charge

that when it is when it reach when it is

depolarized and the poiz depolarization

Peaks and even reaches up to a positive

30 in this case Okay so number three is the

the

depolarization number four is when uh

the polarization becomes positive it's

actually termed as overshoot but in some

cases uh the polarization does not reach

positivity so there's no

overshoot okay then the uh after

overshooting or after the

polarization the curve goes back so that

means the membrane becomes repolarized

again so it becomes polarized again

so as it uh is repolarized

sometimes it becomes more polarized than

the resting membrane potential so this

is what we call the hyper

polarization but after a while it goes

back to it resting uh level which is in

this case [Music]

[Music]

--7 Okay so shown below are some of the

Gated sodium

ions uh channel that are involved in the

different phes of the action potential

as shown here at the level of

the uh uh

depolarization uh you can see that a

sodium gated channel is open so what uh

triggers its opening is

actually reaching the threshold

potential so threshold potential is can

be considered like a switch after it's

it has been reached it's like flipping a

switch then uh action potential will

occur okay but if it is not reached no

action potential can UR okay so we will

discuss uh that a little

later then for the initial

repolarization you can see that the

sodium uh gated sodium channel is closed

and there are potassium channels open up

part uh at the hyperpolarization level

so that

means uh the reason why

hyperpolarization occurs is that the potassium

potassium

gated potassium channels remains

open after a while so it keeps on uh

um removing potassium ions from the cell

so the cell membrane becomes more

negative than the resting level but

after it

closes then the resting membrane

potential is then again

reached okay so that's it for Action

potentials so what do we have to

remember about action

potentials uh we should understand the

all or none principle of action

potential as

mentioned uh there's a threshold

potential this threshold potential

determines if an action potential will

occur or not so that's the all or nonone

principle if the threshold is reach

there's action potential if it is not

then there's no action

potential then um at this different uh

stages uh there is what we call

refractoriness of the cell membrane that

means it will not respond to any

stimulation however strong it is

so going back to the

curve uh phase one to phase five or

halfway through phase five is

actually the absolute refractory period

okay you have to remember that it is not

Tre here it is the absolute refractory

period that

means as the cell is undergoing these phases

phases

and no matter how strong a stimulus is

applied during the time during this

point to this

point the membrane or the cell will not

respond to it no matter how strong the

stimulus is but from this

point up to uh this point of course

it's what we call the relative

refractory period wherein uh the cell

could respond to a stronger

stimulus okay not the normal stimulus

that elicited this first action

potential what do we mean why is that the

the

case as you can

see at this stage for

instance the threshold uh the the the

membrane potential of the cell is not

yet below the threshold

right or not even in the level of the

threshold so to be able to elicit

another Action Potential from this

point you have to have a stronger

stimulus that is above this threshold

threshold level so at least about at

this level so we can estimate it about

negative uh 30 or -2 so that means

for relative refractory period the

threshold is actually increased for the

threshold uh potential is actually

increased okay so we need a stimulus

that can reach that threshold potential

that that new threshold

potential okay so that's it for refractor

refractor period

period

then uh APS are forward moving that's

one thing that we should remember and it

has something to do with refractor

just imagine a me a the membrane of the

neuron specifically the neuron

axon so the action potentials can only

move from one point to one point and in

Only One Direction

because the portion which is previously

stimulated is

refractory to stimulus right so that's

why it has to move

forward and then there's one uh

condition which is called saltator

conduction which is I'm sure you're very

familiar with it has something to do

with the presence of myelin

sheet and the nodes of run Veer so

saltatory conduction basically is just

an increase in the speed of action

potentials as it goes along the

neurons wherein it jumps from one node

of R gear to another

due to the presence of myin sheet which

acts as

a an an insulator okay so that those are

some important principles or some

important things to understand with

regards to action

potentials at this point I want to uh

emphasize or I want to mention already

that action potentials are called

impulses when we are discussing the neurology

so now let's see what the syapse is or

the synapse is so this diagram shows a

interneuron synapse wherein we have the

ation terminal of one neuron and the the

D right of another neuron so this shows

that um there are neurotransmitters

which are stored in the synaptic

vesicle and are released through

exocytosis but their release is actually

regulated by uh voltage gated calcium

channels so this allows entry of calcium

and as calcium enters this uh action

terminal the inner bul becomes more

positive and triggering the release of

neur neurotransmitters from the synaptic

vesicle then these

neurotransmitters uh goes to the go to

the synaptic C or the space between the

two neurons and then binds it to its

receptor in the post synaptic neuron or the

the

dendrite which causes or initiates the

dep polarization or changes in this post

synaptic neuron okay so that's how

simple it is you can even look for uh

animations of this um neurotransmitter

but in the next slide you will see one

very simple

or uh very simple video of that uh newm

enlarged then we have the Press synaptic

cell and the post synaptic

cell and we have synaptic vesicles

neurotransmitters and there are ducking proteins

proteins

actually that

allows um synaptic vesicles to fuse with

the plasma

membrane then that's the exocytotic [Music]

[Music]

process and then the new

receptors causing changes in the potion AB

AB [Music]

[Music] set

okay then actually uh the last part

shows the how the

neurotransmitters or at least the

elements of the

neurotransmitters are recycled by the

cell member of the post preoptic

NE all right just now uh instead of just

imagining I want to show this but this

is the typical interconnection

between uh cell

or neurons in the brain so this shows one

one

neuron connected to multiple other

neurons as you can see here there are

many neurons connected to one

neuron so what would determine that uh

what would trigger this neuron to

undergo action potential or to transmit

syapse or to to depolarize the cell so

what does it take

okay now we go to the principle of post

synaptic potentials okay so as uh still

I I want you to still picture the DI

previous diagram so the post synaptic

potentials are in real life actually sub

threshold potentials meaning potentials

that are below the threshold level so by

itself or alone this postoptic potential

cannot Trigger action potentials in

their postoptic cell because it is sub

threshold so the AP generated in the

post synaptic neuron is actually

determined by uh the net uh the net potential

potential

generated by the PO synaptic potentials

of the pratic neurons okay so it is the

sum or summation of the poptic

potentials

so we have uh actually two types of

poptic potentials one is excitatory

which are positive potentials which of

course makes the poptic neuron less

negative then we have the inhibitory or

the negative po synaptic potentials

which makes the post synaptic potential

more negative instead of making it more

positive so this ex excitatory postoptic

potential is

depolarizes while inhibitory postoptic

potentials may actually

hyperpolarize the poptic

neuron okay shown here is the previous

diagrams of the action potential and the

typical synapsis present in the brain

neurons you can see here that the

excitatory synapses are depicted by

green and the inhibitory ones are red

right so if we still have time to count

this you can estimate that the reds are

as many as the green ones

okay so we mentioned that the post

synaptic potentials can either be

negative or positive and but are both uh

sub threshold okay so with that in mind

we estimated the threshold of this

diagram as about about

-50 so that means we uh need a charge of

Plus or

positive uh 20 to reach that threshold

okay so in this case we are talking

about the net positive or net negative

charge if we have more executory

synapses than there are inhibitory

synapses then we can say that uh if it

reaches plus 20

then we can elicit a a an action

potential in this neuron but if the exit

synapse is just as many as the

inhibitory synapse and they have the

same amount of charge then we will not

reach the threshold thus we cannot

elicit an action

potential so that is uh what we call the

summation of the post synaptic synapsis

so that determines

the um if an action potential can be

generated in this post synaptic neuron

okay so imagine this is a typical neuron

in the brain then these are information

from other parts of the brain so this

actually determines what action this

neuron would take in response to

information coming from other neurons

okay it will either elicit an action

okay now let's go to our last um topic

which is the neurotransmitters so there

are three major mechanisms or me

mechanism of action or effects of

neurotransmitters one is the ionotropic

effect within the neurotransmitters are

lians for ION channels so that means if

they bind to their receptors they will

open an ion

Channel then we have the metabotropic

effect or the second messenger system

which effects is through a series of metabolic

metabolic

reactions okay so if you can still

recall the second messenger system now

is the time to do it so that's the

metabotropic Tropic effect then we have

the neurom modulation it is the

intermediate between ionotropic and

metabotropic and it is intermediate

between the

neurotransmitter and hormone it

influences the effects of other

neurotransmitters so neurom modulation

is uh basically the effects of one NE

transmitter to another NE

transmitter so for uh this table shows

uh some of the

neurotransmitters their receptors their

second messenger and their net Channel

effects as you can see acetal Coline has

nicotinic and the M1 M2 M3 and M4 up to

M5 uh receptors so the nenic we can say

it is an

ionotropic uh has ionotropic mechanism

while for the M series receptor acet

choline we have the metabotrophic and

also the ionotrophic okay for M1 and M2

okay so that's simply how we uh read

this table so for dopamine we have

dopamine receptors 1 to 5 and as you can

see only uh only

um receptor D2 has both the metabotropic

and the ionotropic so it is involved in

neurom modulation it's said okay so for

norepinephrine we also have the uh a uh

or Alpha this is actually

alpha alpha receptors and beta receptors

as you can see Alpha

receptors um

miates um neur modulation while beta

receptors are

metabotropic all of them increase cyclic

amp so you might be wondering why bother

with the beta 1 2 and three so these are

different slightly different uh

receptors and probably located in

different organs or tissues okay

although their effects are the

same then we have glutamate

and Gaba actually for these examples

glutamate and Gaba are inhibitory

neurotransmitters so as you can see um

it has uh the glutamate

has ionotropic only ionotropic effect

ionotropic okay so that's it for now

thank you very much for list listening

our next topic will be a discussion a

[Music]

brain based on what you learned which

aspect of this whole topic on membrane

potential is evidence that homeostasis

is not totally synonymous to [Music]

[Music]

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YouTube Transcript:
Topic 10 Membrane Potential 2021 Edition