0:28 [Music] n
0:36 [Music]
0:39 this diagram shows how the membrane
0:42 potential is measured as you can see
0:44 there's a probe that is inserted in the
0:45 lipid by
0:49 layer which in which the tip sticks
0:53 inside so what that actually measures is
0:56 the charge of the inner surface of the
1:00 membrane as shown here it is 80
1:03 so that means the inner surface of the
1:07 cell membrane is negatively
1:09 charged all right so what is a membrane
1:11 potential so membrane potential are
1:13 changes in electrical charge of the cell
1:15 membrane particularly on the inner
1:17 surface as already
1:19 mentioned this also pertains to the
1:22 change and the polarity of the
1:25 membrane so what are the types of
1:27 potential so we have the resting
1:29 membrane potential and the action potential
1:30 potential
1:33 within the cell and then there are also
1:35 post synaptic potentials between two
1:38 neurons which are either excitatory or
1:42 inhibitory technically speaking they are
1:45 uh synaptic potentials of cells that are
1:49 produced between two cells okay we'll
1:52 discuss that in detail more uh in more detail
1:53 detail
1:56 later so let's start with the resting
1:57 membrane potential so the resting
1:59 membrane potential is the potential of
2:00 the cell
2:02 it is not performing a
2:05 task it is uh normally
2:09 negative and this means that a cell is
2:13 membrane is polar at rest so as you can
2:15 recall in the diagram previously it is
2:19 ne80 M so that means the resting
2:23 membrane of that cell is 80
2:26 M so this negativity of the inside of
2:28 the cell is maintained due to the
2:31 activity of the sodium pottassium pump
2:34 sodium pottassium pump as you may recall
2:37 uh pumps charges uh asymmetrically
2:40 meaning it uh pumps out more positive
2:43 charges than it uh pumps in uh positive
2:47 charges so that means as time passes by
2:49 then inner Sur inner surface of the
2:52 membrane becomes
2:54 negative now we have also the action
2:56 potential so what is the action
2:57 potential is the potential of the
3:00 membrane when it is performing a work or
3:04 task it is usually less negative or in
3:07 some cases zero or in some rare cases it
3:10 even becomes positive so remember we are
3:14 referring to the membrane potential of
3:17 the inner surface of the membrane so it
3:20 is um negative at rest so it becomes
3:22 less negative or we can say that it
3:25 becomes more positive or it approaches
3:28 zero or become zero itself or even
3:32 overshoots zero and becomes positive in
3:34 some cases so this
3:37 means that the membrane atress is
3:40 polarized and becomes depolarized during
3:42 action potential while it is performing
3:45 its work
3:49 okay so here are the here's a diagram
3:53 that shows the faces of the action
3:56 potential the first phase is the resting
3:59 membrane potential in this diagram it is negative
4:00 negative
4:05 70 so the number two is the threshold
4:08 potential and we can estimate it at
4:13 around uh -50 probably so the threshold
4:17 potential uh refers to the um charge
4:21 that when it is when it reach when it is
4:31 depolarized and the poiz depolarization
4:35 Peaks and even reaches up to a positive
4:39 30 in this case Okay so number three is the
4:40 the
4:43 depolarization number four is when uh
4:46 the polarization becomes positive it's
4:49 actually termed as overshoot but in some
4:52 cases uh the polarization does not reach
4:54 positivity so there's no
4:58 overshoot okay then the uh after
5:00 overshooting or after the
5:03 polarization the curve goes back so that
5:07 means the membrane becomes repolarized
5:12 again so it becomes polarized again
5:16 so as it uh is repolarized
5:20 sometimes it becomes more polarized than
5:24 the resting membrane potential so this
5:26 is what we call the hyper
5:29 polarization but after a while it goes
5:32 back to it resting uh level which is in
5:34 this case [Music]
5:35 [Music]
5:41 --7 Okay so shown below are some of the
5:43 Gated sodium
5:46 ions uh channel that are involved in the
5:49 different phes of the action potential
5:55 as shown here at the level of
5:58 the uh uh
6:01 depolarization uh you can see that a
6:05 sodium gated channel is open so what uh
6:07 triggers its opening is
6:10 actually reaching the threshold
6:14 potential so threshold potential is can
6:17 be considered like a switch after it's
6:20 it has been reached it's like flipping a
6:23 switch then uh action potential will
6:27 occur okay but if it is not reached no
6:31 action potential can UR okay so we will
6:34 discuss uh that a little
6:37 later then for the initial
6:39 repolarization you can see that the
6:42 sodium uh gated sodium channel is closed
6:45 and there are potassium channels open up
6:52 part uh at the hyperpolarization level
6:53 so that
6:55 means uh the reason why
6:58 hyperpolarization occurs is that the potassium
7:00 potassium
7:03 gated potassium channels remains
7:07 open after a while so it keeps on uh
7:09 uh
7:15 um removing potassium ions from the cell
7:17 so the cell membrane becomes more
7:21 negative than the resting level but
7:22 after it
7:25 closes then the resting membrane
7:28 potential is then again
7:31 reached okay so that's it for Action
7:32 potentials so what do we have to
7:36 remember about action
7:38 potentials uh we should understand the
7:40 all or none principle of action
7:41 potential as
7:43 mentioned uh there's a threshold
7:45 potential this threshold potential
7:47 determines if an action potential will
7:50 occur or not so that's the all or nonone
7:51 principle if the threshold is reach
7:54 there's action potential if it is not
7:56 then there's no action
8:00 potential then um at this different uh
8:02 stages uh there is what we call
8:05 refractoriness of the cell membrane that
8:07 means it will not respond to any
8:12 stimulation however strong it is
8:16 so going back to the
8:20 curve uh phase one to phase five or
8:22 halfway through phase five is
8:27 actually the absolute refractory period
8:28 okay you have to remember that it is not
8:31 Tre here it is the absolute refractory
8:33 period that
8:37 means as the cell is undergoing these phases
8:39 phases
8:42 and no matter how strong a stimulus is
8:44 applied during the time during this
8:46 point to this
8:49 point the membrane or the cell will not
8:51 respond to it no matter how strong the
8:54 stimulus is but from this
9:00 point up to uh this point of course
9:01 it's what we call the relative
9:05 refractory period wherein uh the cell
9:08 could respond to a stronger
9:11 stimulus okay not the normal stimulus
9:13 that elicited this first action
9:16 potential what do we mean why is that the
9:17 the
9:19 case as you can
9:22 see at this stage for
9:25 instance the threshold uh the the the
9:27 membrane potential of the cell is not
9:30 yet below the threshold
9:33 right or not even in the level of the
9:36 threshold so to be able to elicit
9:38 another Action Potential from this
9:44 point you have to have a stronger
9:47 stimulus that is above this threshold
9:50 threshold level so at least about at
9:52 this level so we can estimate it about
9:59 negative uh 30 or -2 so that means
10:02 for relative refractory period the
10:05 threshold is actually increased for the
10:07 threshold uh potential is actually
10:10 increased okay so we need a stimulus
10:13 that can reach that threshold potential
10:15 that that new threshold
10:17 potential okay so that's it for refractor
10:19 refractor period
10:21 period
10:25 then uh APS are forward moving that's
10:27 one thing that we should remember and it
10:30 has something to do with refractor
10:34 just imagine a me a the membrane of the
10:37 neuron specifically the neuron
10:41 axon so the action potentials can only
10:45 move from one point to one point and in
10:47 Only One Direction
10:51 because the portion which is previously
10:53 stimulated is
10:57 refractory to stimulus right so that's
11:00 why it has to move
11:05 forward and then there's one uh
11:07 condition which is called saltator
11:10 conduction which is I'm sure you're very
11:12 familiar with it has something to do
11:15 with the presence of myelin
11:18 sheet and the nodes of run Veer so
11:20 saltatory conduction basically is just
11:22 an increase in the speed of action
11:25 potentials as it goes along the
11:28 neurons wherein it jumps from one node
11:29 of R gear to another
11:32 due to the presence of myin sheet which
11:35 acts as
11:40 a an an insulator okay so that those are
11:42 some important principles or some
11:45 important things to understand with
11:47 regards to action
11:50 potentials at this point I want to uh
11:53 emphasize or I want to mention already
11:54 that action potentials are called
11:58 impulses when we are discussing the neurology
12:03 so now let's see what the syapse is or
12:09 the synapse is so this diagram shows a
12:12 interneuron synapse wherein we have the
12:15 ation terminal of one neuron and the the
12:18 D right of another neuron so this shows
12:21 that um there are neurotransmitters
12:24 which are stored in the synaptic
12:27 vesicle and are released through
12:29 exocytosis but their release is actually
12:32 regulated by uh voltage gated calcium
12:35 channels so this allows entry of calcium
12:39 and as calcium enters this uh action
12:42 terminal the inner bul becomes more
12:45 positive and triggering the release of
12:47 neur neurotransmitters from the synaptic
12:50 vesicle then these
12:52 neurotransmitters uh goes to the go to
12:56 the synaptic C or the space between the
12:59 two neurons and then binds it to its
13:01 receptor in the post synaptic neuron or the
13:03 the
13:06 dendrite which causes or initiates the
13:08 dep polarization or changes in this post
13:11 synaptic neuron okay so that's how
13:15 simple it is you can even look for uh
13:19 animations of this um neurotransmitter
13:21 but in the next slide you will see one
13:23 very simple
13:32 or uh very simple video of that uh newm
13:40 enlarged then we have the Press synaptic
13:42 cell and the post synaptic
13:46 cell and we have synaptic vesicles
13:52 neurotransmitters and there are ducking proteins
13:53 proteins
13:55 actually that
13:58 allows um synaptic vesicles to fuse with
14:01 the plasma
14:04 membrane then that's the exocytotic [Music]
14:06 [Music]
14:11 process and then the new
14:20 receptors causing changes in the potion AB
14:22 AB [Music]
14:23 [Music] set
14:30 okay then actually uh the last part
14:34 shows the how the
14:36 neurotransmitters or at least the
14:37 elements of the
14:41 neurotransmitters are recycled by the
14:45 cell member of the post preoptic
14:49 NE all right just now uh instead of just
14:52 imagining I want to show this but this
14:56 is the typical interconnection
14:59 between uh cell
15:02 or neurons in the brain so this shows one
15:03 one
15:06 neuron connected to multiple other
15:10 neurons as you can see here there are
15:12 many neurons connected to one
15:17 neuron so what would determine that uh
15:21 what would trigger this neuron to
15:24 undergo action potential or to transmit
15:27 syapse or to to depolarize the cell so
15:29 what does it take
15:32 okay now we go to the principle of post
15:37 synaptic potentials okay so as uh still
15:39 I I want you to still picture the DI
15:42 previous diagram so the post synaptic
15:45 potentials are in real life actually sub
15:47 threshold potentials meaning potentials
15:50 that are below the threshold level so by
15:54 itself or alone this postoptic potential
15:57 cannot Trigger action potentials in
16:00 their postoptic cell because it is sub
16:03 threshold so the AP generated in the
16:05 post synaptic neuron is actually
16:10 determined by uh the net uh the net potential
16:11 potential
16:15 generated by the PO synaptic potentials
16:18 of the pratic neurons okay so it is the
16:21 sum or summation of the poptic
16:26 potentials
16:29 so we have uh actually two types of
16:32 poptic potentials one is excitatory
16:35 which are positive potentials which of
16:37 course makes the poptic neuron less
16:40 negative then we have the inhibitory or
16:42 the negative po synaptic potentials
16:44 which makes the post synaptic potential
16:48 more negative instead of making it more
16:52 positive so this ex excitatory postoptic
16:54 potential is
16:56 depolarizes while inhibitory postoptic
16:59 potentials may actually
17:03 hyperpolarize the poptic
17:07 neuron okay shown here is the previous
17:10 diagrams of the action potential and the
17:14 typical synapsis present in the brain
17:17 neurons you can see here that the
17:21 excitatory synapses are depicted by
17:26 green and the inhibitory ones are red
17:29 right so if we still have time to count
17:33 this you can estimate that the reds are
17:36 as many as the green ones
17:40 okay so we mentioned that the post
17:43 synaptic potentials can either be
17:48 negative or positive and but are both uh
17:54 sub threshold okay so with that in mind
17:56 we estimated the threshold of this
17:59 diagram as about about
18:05 -50 so that means we uh need a charge of
18:06 Plus or
18:10 positive uh 20 to reach that threshold
18:12 okay so in this case we are talking
18:15 about the net positive or net negative
18:18 charge if we have more executory
18:22 synapses than there are inhibitory
18:26 synapses then we can say that uh if it
18:29 reaches plus 20
18:32 then we can elicit a a an action
18:36 potential in this neuron but if the exit
18:38 synapse is just as many as the
18:40 inhibitory synapse and they have the
18:45 same amount of charge then we will not
18:47 reach the threshold thus we cannot
18:49 elicit an action
18:53 potential so that is uh what we call the
18:57 summation of the post synaptic synapsis
18:59 so that determines
19:03 the um if an action potential can be
19:06 generated in this post synaptic neuron
19:09 okay so imagine this is a typical neuron
19:12 in the brain then these are information
19:15 from other parts of the brain so this
19:17 actually determines what action this
19:19 neuron would take in response to
19:22 information coming from other neurons
19:25 okay it will either elicit an action
19:35 okay now let's go to our last um topic
19:37 which is the neurotransmitters so there
19:40 are three major mechanisms or me
19:42 mechanism of action or effects of
19:44 neurotransmitters one is the ionotropic
19:46 effect within the neurotransmitters are
19:49 lians for ION channels so that means if
19:51 they bind to their receptors they will
19:53 open an ion
19:55 Channel then we have the metabotropic
19:57 effect or the second messenger system
19:59 which effects is through a series of metabolic
20:00 metabolic
20:03 reactions okay so if you can still
20:05 recall the second messenger system now
20:08 is the time to do it so that's the
20:11 metabotropic Tropic effect then we have
20:13 the neurom modulation it is the
20:17 intermediate between ionotropic and
20:19 metabotropic and it is intermediate
20:21 between the
20:23 neurotransmitter and hormone it
20:25 influences the effects of other
20:28 neurotransmitters so neurom modulation
20:31 is uh basically the effects of one NE
20:34 transmitter to another NE
20:38 transmitter so for uh this table shows
20:40 uh some of the
20:42 neurotransmitters their receptors their
20:45 second messenger and their net Channel
20:48 effects as you can see acetal Coline has
20:52 nicotinic and the M1 M2 M3 and M4 up to
20:57 M5 uh receptors so the nenic we can say
20:59 it is an
21:03 ionotropic uh has ionotropic mechanism
21:07 while for the M series receptor acet
21:10 choline we have the metabotrophic and
21:15 also the ionotrophic okay for M1 and M2
21:20 okay so that's simply how we uh read
21:23 this table so for dopamine we have
21:26 dopamine receptors 1 to 5 and as you can
21:29 see only uh only
21:33 um receptor D2 has both the metabotropic
21:37 and the ionotropic so it is involved in
21:41 neurom modulation it's said okay so for
21:45 norepinephrine we also have the uh a uh
21:47 or Alpha this is actually
21:50 alpha alpha receptors and beta receptors
21:52 as you can see Alpha
21:56 receptors um
21:59 miates um neur modulation while beta
22:01 receptors are
22:05 metabotropic all of them increase cyclic
22:09 amp so you might be wondering why bother
22:14 with the beta 1 2 and three so these are
22:17 different slightly different uh
22:20 receptors and probably located in
22:23 different organs or tissues okay
22:25 although their effects are the
22:28 same then we have glutamate
22:31 and Gaba actually for these examples
22:35 glutamate and Gaba are inhibitory
22:39 neurotransmitters so as you can see um
22:42 it has uh the glutamate
22:46 has ionotropic only ionotropic effect
22:57 ionotropic okay so that's it for now
22:58 thank you very much for list listening
23:02 our next topic will be a discussion a
23:15 [Music]
23:18 brain based on what you learned which
23:20 aspect of this whole topic on membrane
23:22 potential is evidence that homeostasis
23:25 is not totally synonymous to [Music]
23:26 [Music]
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