Description

This video covers the general physiology of a nerve, including potentials, polarizations, permeability, and excitability. Understanding these fundamental concepts is essential for grasping how nerves function and communicate through electrical and chemical signals.

Basic Physiological Rules of the Cell:

• More negative inside the cell than on the outside.
• Difference in extracellular and intracellular fluid composition.
• Nernst Equation calculates the charged ions across a membrane.

Transport Through the Cell Membrane:

Passive Transport:

• Simple diffusion (requires small, lipophilic, and uncharged molecules).
• Facilitated diffusion (via ion channels and gated channels).
• Filtration.
• Electrokinetic transport.
• Osmosis.

Active Transport:

• Primary Active Transport: Na/K Pump, Ca Pump.
• Secondary Active Transport: Na/Glucose, Na/Amino Acids, Na/H.

Nerve Structure and Function:

• Receptive Part: Dendrites.
• Integrating Part: Cell body and Axon Hillock.
• Transmitting Part: Axon.

Resting Potential:

• Typically -70 mV.
• Chemical gradient outside, Electrical Gradient inside = Electrochemical gradient outside.
• Sodium and potassium move through ion channels.
• Sodium-potassium pump: 3 sodium out, 2 potassium in.

Action Potential:

• Voltage-Gated Channels and Excitability.
• Two gates on VG-Channels: Activation gate and inactivation gate.
• Excitability changes during an action potential.

Permeability of the Cell Membrane During Action Potential:

1. Resting state: Sodium and potassium cannot pass through gated channels. Inside the cell: Negative.
2. Depolarization: Sodium enters the cell, making the inside positive.
3. Repolarization: Sodium channels close, potassium channels open, leading to potassium outflux. This results in hyperpolarization (refractory period).

Conduction of Action Potential Through a Nerve Axon:

• Continuous Conduction (Unmyelinated fibers).
• Saltatory Conduction (Myelinated fibers).

All-Or-None Law:

• An action potential either occurs completely or not at all.

Chemical Synapse:

• Ligand-gated channels or chemically gated channels.
• Excitatory or inhibitory responses.

Transcript

Introduction
0:00
hello and welcome to another video here I’m gonna talk about the physiology of neuron so first I’m
0:06
going to talk about the membrane potentials which is a differences in voltage between the inside and
0:10
outside of a cell and there are basically three types of potentials and neuron can be in it can
0:16
be in something we call resting potentials and the name doesn’t come randomly it is basically
0:21
when the neuron is at rest meaning it’s not doing any work and if you look at this chart a neuron
0:27
is at this resting potential when the voltage inside the cell is at negative 70 millivolts then
0:33
we also have something called graded potentials so when we receive a signal we call this stimuli
0:39
and that stimuli is going to stimulate which is the changes in membrane potentials and we’ll talk
0:44
more about this later it can be a small or a big change as you see here but as soon as we hit that
0:50
threshold of negative 55 millivolts and that’s gonna trigger the neuron is sending the signal
0:56
further away and this process we call this an action potential and that’s basically what this
1:02
long line is going to represent I’m also going to talk about the mechanism of impulse conduction
1:08
in nerve fibers we’ve got this slow continuous conduction in unmonitored nerve fibers and it’s
1:16
quite slow compared to the saltatory conduction in an myelinated nerve fiber so before I go detailed
1:23
into this topics I want to go through the basic physiological rules to kind of get a better grasp
Basic Physiological Rules of a Cell
1:29
of what’s typically happens in a cell so if you feel like you’re comfortable with the physiology
1:33
of a cell you can just skip this part and go right over to make membrane potentials so there
1:40
are a couple of things you need to keep in mind when it comes to the cell so let’s imagine this
1:46
is a cell where you got the cell membrane outside and the nucleus on the inside and when it comes
1:52
to membrane potentials you need to keep in mind that there’s a greater number of negative charges
1:58
inside the cell than on the outside so if you look into the chemical composition of the intracellular
2:05
fluid and the extracellular fluid you will see that we got a huge amount of sodium without
2:12
of the cell band on the inside and that alone gives us a very huge difference in charge we’ve
2:19
also got a lot of calcium and outside of the cell that on the inside but we have more potassium at
2:25
the inside and this is a very important factor when it comes to a membrane potentials in the
2:31
neuron because it helps a trigger that action potential like I’ll get more into that later in
2:37
this video but we also get a greater amount of calcium and bar carbonate outside of the cell
2:42
than on the inside and you will also find more proteins within the cell and they’re typically
2:49
negative charged now in the late 1800s Walter Nance developed an equation especially for the
2:56
membrane potentials that’s the so called Nernst equation he used this formula to calculate the
3:02
potentials of a charged ion across the membrane so here we can put in the concentration of a
3:09
certain ion both outside and inside of a cell and we’ll get the equilibrium potential of a certain
3:16
ion in millivolts and this is very big because it helps us understand the membrane potentials even
3:23
better so now what are the rules in transporting things through the membrane so there are two ways
Rules of Transport Across the Membrane
3:30
transportation through the membrane can happen either it requires energy or not so first we
Passive Transport
3:36
get the passive transport and since it’s passive it does not require any source of energy because
3:42
passive transport always goes towards the chemical gradient and there are a couple
3:48
of ways this can happen so let’s imagine this is the cell membrane oxygen and carbon dioxide
3:54
can easily diffuse through and this will call simple diffusion and there are some criterias
4:01
for this the molecules has to be small enough to fit through the phospholipid bilayer and they also
4:07
need to be lipophilic as they dissolve in the lipid bilayer they also have to have no charge
4:13
or also just get repelled we also have something we called faciliate diffusion it is exactly like
4:21
simple diffusion except that it needs some help proteins because polar molecules or ions can get
4:28
through that easily like nargis potassium iron right here for this to be able to go
4:33
through we need the so called channel protein so that it can go through that channel we also have
4:40
something called gated channels on cell membrane and they’re gated so they’re closed but they can
4:46
be opened when a chemical like a neurotransmitter for example binds to the protein and that will
4:53
cause the gate to open and ions will just flow in so gated channel can also be opened by a stimuli
5:00
so if there’s a difference in charge across the membrane it will also open so examples of gated
5:08
channels would be voltage-gated channels like the sodium potassium pump we also have ligand
5:14
or at chemically gated channels which will open when a substance binds to the channel
5:20
protein like this example right here we got the information gated channels which will open the
5:26
two attention on the Meccano sensory proteins and we also got temperature gated channels
5:32
which will open due to a temperature increases or decreases so that’s these now other types
5:40
of passive transport would be an a filtration or the blue Meadows capsule of the nephron and the
5:47
kidneys would be an example of that we also get an electro kinetic transport remember inside the
5:54
cell is way more negative than on the outside and since we got more positive charge outside
5:59
of the cell than on the inside the direction of the electrical gradient would be like this hence
6:07
positive positive charged ions will diffuse in so that’s electro kinetic transport then we got
6:14
something as simple as osmosis where water travels towards the concentration gradient
6:21
so that’s passive transport now the other type of transport is active meaning it requires energy so
Active Transport
6:29
let’s just remove the passive transport so that we don’t get confused so here we see the cell
6:34
active transport is moving molecules against a concentration gradient and this process burns
6:41
ATP and we got two types of active transport first one is primary active transport and a
6:48
famous example of the primary active transport is the sodium potassium pump so if you add the same
6:55
number in again again there’s going to be more potassium inside the cell than on the outside
7:01
and the opposite goes to sodium now I’ll talk more about this later because the sodium potassium pump
7:08
is very important for the membrane potentials but basically if we add the pump right here with the
7:16
help of ATP the sodium potassium pump is gonna pump out three sodium ions outside of the cell
7:24
and at the same time is gonna pump into sodium ions so we’re basically moving stuff against the
7:31
concentration gradients so this is basically the primary active transport directly caused
7:38
by ATP so the second type is a secondary active transport so if you add the cell again and add
7:45
sodium now after the sodium potassium pump has pumped out a lot of that sodium we’re gonna have
7:53
an extremely difference in concentration when it comes to sodium so it’s not really enough going
8:00
to diffuse in towards the concentration gradient slowly so if we add glucose for example now as
8:09
sodium goes in glucose comes along goes in with it it could be glucose could be another charge
8:16
molecules so that’s the thing is the secondary active transport is the form of transporting down
8:23
a concentration gradient because of the extremes between those which was generated by the primary
8:31
active transport so it’s still a form of active transport as it ultimately resolved from the use
8:37
of ATP by the primary active transport so now that we understand a little bit more of what’s
Membrane Potentials
8:42
going on we can finally go ahead and look the membrane potentials in neuron so here you see
8:49
the basic types for nerve cell functionally we divided into three parts we got the dendrites
8:56
which is a the receptive part which receives information from outer environments or inner
9:03
environments meaning other nerve cells next the integrating part which is the cell body and at
9:11
the beginning of the axon the axon hillock so the cell receives a signal and then it integrates it
9:20
either to cause a response or not and last part is the transmitting part which is the axon because it
9:29
transmits impulses from the cell body and so in this video we’re going to look at potentials we
9:35
got the resting potentials that’s when the neuron is at rest it’s not with at rest I’ll show you why
9:41
later and then we’ll get the graded potentials based on the stimulus of that neuron and then
9:47
we may or may not have an action potentials I’m gonna go through them in the following order and
9:53
first we’re gonna look at what a resting potential is and how the sodium potassium pump and how the
9:59
permeability of different ions in the membrane contribute to the resting potential we’ll then
10:06
skip forward to the reaction potentials and the importance of voltage-gated channels and
10:12
then we’ll look at how a ligand gated channels contribute to either firing an action potential
10:18
or not so what is potential potential is just a separation of charge but we can measure that
10:29
voltage it’s typically negative 70 millivolts in an typically human neuron and so when you’re
10:35
seeing pictures of an action potential like this on either side of that action potential we can
10:41
see that the neuron is at rest now it doesn’t mean that it doesn’t require energy to do that so let’s
10:48
zoom in into the surface of a neuron to really understand what’s going on now remember we’re
10:55
gonna have a higher amount of potassium inside the cell than on the outside I’m also gonna have
11:00
many negatively charged proteins inside the cell as well and then on the outside we’re gonna have
11:06
salt we’re gonna have a high amount of sodium ions and chloride ions on the outside now if
11:13
you look at this there’s still no potentials there are still no separation in charges and to really
11:21
figure out how that works let’s just remove a lot of it and only focus on the potassium
11:26
itself because it’s really the potassium and the permeability of the potassium that establish that
11:34
number of potential and so remember I said that we’ve got channels on the cell membrane this is
11:39
what we call ion channel or simply a potassium leak channel and it’s a leak channel that allows
11:46
potassium to move in and out and so if you think about where these potassium is going to go there’s
11:54
a chemical gradient from the inside to the outside in other words there’s a higher concentration of
12:01
potassium on the inside than on the outside and therefore naturally potassium is going
12:07
to go towards it the concentration gradients but there’s actually one more thing we need
12:13
to think about and that is since we got all that positive charge on the outside of the cell these
12:20
ions are gonna repel each other and make an electrical gradient to the inside so the more
12:27
potassium that leaks through you can see that the electrical gradient is increasing and so actually
12:33
what could happen is that potassium can start flowing back in now we got a chemical gradient
12:39
outside and an electrical gradient inside but we’ve got an electrochemical gradient outside
12:45
electrochemical gradient outside now why doesn’t all the molecules flow out well that’s because
12:54
these are leak channels they don’t really allow for many ions to go through they’re fast now if
13:00
we would add a sodium ion a channel as well we will see that ions are starting to move in and
13:07
out of the cell through those ion channels but if we would let this go eventually we’ll lose
13:15
that potential because all that sodium will just come in so to solve this problem we need to add
13:21
this sodium potassium pump or we cash in a little bit of ATP to move that sodium out three sodium
13:29
ions for every two potassium ions in and so as all of this is moving the sodium potassium pumps
13:36
gonna keep up making sure that the potassium stays inside the cell and the sodium outside
13:41
and so that is our resting potential minus 70 millivolts is established because we will have
13:47
that separation of those ions now it requires energy to do that and keep that in mind so what
Action Potentials
13:55
can resting potential eventually to action potential so here you see our ion channels
14:01
again but we’re going to have other types of channels here as well and those are the
14:07
voltage-gated channels remember I mentioned them earlier we’re going to have a sodium water channel
14:12
and a potassium voltage-gated channel now these are gated meaning they need a certain amount of
14:18
stimuli to get excited to open basically and one important word to remember here is excitability
14:27
meaning the ability to respond to a stimuli so if you look at the sodium voltage-gated channel as
14:33
an example there are two gates on the sodium voltage-gated channel one of them is called
14:40
activation gate located on the outside of the cell and an inactivation gate on the inside so
14:48
if we go back to this diagram again at the resting potential of negative 70 millivolts the activation
14:55
gate is closed and the inactivation gate is open as you see right here and what’s special
15:01
with these gates is that they’re both have their own properties activation gates for reason for
15:09
instance is sensitive to stimulus meaning it opens due stimuli applied in that evasion gate in other
15:18
hand is not sensitive to stimuli but it closes when the membrane potential becomes more positive
15:25
than negative 40 millivolts now let’s go ahead and animate it a little bit and to see how it works
15:31
let’s say a stimuli is applied the first step is here at negative 70 millivolts at this point if
15:40
we apply more stimulus to the cell the activation gate will open therefore we say that the cell is
15:47
excitable then when the membrane potential gets close to the threshold the sodium gated channel
15:54
is still in the same position remember because we haven’t broken the threshold yet but since
16:01
the membrane potential is close to the threshold will say that the cell is more excitable meaning
16:08
less stimuli is required to open that activation gates so now as soon as it reaches the threshold
16:16
of negative 55 millivolts the activation gate will open up and a lot of sodium ions will flow
16:24
into the cell and this process of going towards a positive potential we call the depolarization
16:30
and now since both gates are open the next stimulus the cell gets can’t do anything to
16:38
that voltage-gated channel because both gates are already open and so therefore we say that the cell
16:45
is not excitable and it is already responding to with a previous stimulus then at the top here at
16:52
plus 30 millivolts until the threshold level the potassium gate will open up and the inactivation
16:59
gate of the sodium gated channel will close and that will lead to a huge out flocks of potassium
17:08
ions and this process of going back to the resting potential this is called repolarization so since
17:17
the inactivation gate is closed we say that the cell is still not excitable then it reaches the
17:24
stage of hyperpolarization where the member potential is lower than the actual resting
17:31
potential as you see right here in the graph and during this time the voltage-gated channels are
17:36
gradually brought back to its initial States at the beginning the cell is not so very excitable
17:44
but becomes more excitable as the ion leak channels established at resting potential
17:52
again so that is the action potential so so far we’ve only looked at the segments of an axon how
Impulse Conduction
18:00
does the message move all over to the end of the axon what really happens is that we have
18:05
an a depolarization of a segment in an axon what really happens is that we have a depolarization of
18:12
a segment in axon which triggers depolarization on the next segment and then to the next one
18:20
and so on so let’s zoom in and look at these voltage-gated channels and then see what’s going
18:27
on so if we look at this should be a near the axon hillock at the beginning of the axon itself regard
18:33
voltage-gated sodium and potassium channels and so when we hit that threshold those sodium gated
18:40
channels are gonna open up and we’re gonna have an influx of sodium ions and they’re gonna change
18:46
your voltage of the next sodium voltage-gated channel now why is it that they’re only moving
18:51
in this direction and not other not to the left well that’s because to the left we’ve got that
18:57
repolarization of segments where the outflux of potassium ions happening we’re not even gonna be
19:03
close to being excitable at that segment and so it’s gonna travel all the way through that
19:10
axon and so this is a similar that signal looks like as it moves down the axon but we have some
19:17
really long accessory body we have the sky at eclair which goes from the base of the spinal
19:23
cord all the way down to the big toe of each foot so this needs to go faster and so most
19:30
of these neurons are myelinated which isolates that nerve fiber this process of leading that
19:37
signal is called an impulse conduction and in an unmonitored in durafiber we call it a continuous
Saltatory Conduction
19:44
conduction while in an myelinated nerve fiber we call it as saltatory conduction where the area of
19:52
excitability lies inside the node of ranvier between the myelinated cells and since these
20:00
myelinated nerve fibers are covered with myelin sheath impulses jump to the next node from here
20:08
so impulses are isolated by the myelin which makes a signal faster and so the problem with
20:15
non myelinated nerve fiber is that some current are lost on its way as the impulses are slower
20:22
and there’s no isolation now action potentials are followed the so called all-or-none law what that
All-or-None Law
20:30
means is that an action potential will occur or not it doesn’t depend on how much stimuli it gets
20:36
it only depends on if you’re able to hit that threshold or not we can get really close to it
20:43
as possible without anything happening but once we hit that threshold an action potential will
20:49
happen so so far we’ve looked at what happens in action potential and at the resting potential but
20:57
what really happens here in between so when a signal is sent to another cell as you see
21:03
right here what really happens between those two neurons so let’s zoom into that to understand it
21:10
better so the neuron that sends the signal is called a presynaptic neuron while the neuron
21:16
that receives a signal is called the postsynaptic neuron this type of synapse is called a chemical
21:22
synapse now I’m not going to go into much detail into this I’m just going to go through the basic
21:26
so that you really understand what’s going on so so far we looked at ion channels and
21:31
voltage-gated channels but there’s another very important channel when we’re talking about graded
21:38
potentials and these are they like and gated channels or simply chemically gated channels
21:44
remember we mentioned those earlier as well so when an action potential reaches the end of an
21:49
son it depolarizes those calcium voltage-gated channels which will open the way for a calcium
21:57
to flow in and that calcium ion will dock to those to the surface of those vesicles
22:04
containing neurotransmitters and it will allow these to connect to the surface of the presynaptic
22:10
neuron and then release the neurotransmitters through exocytosis so these neurotransmitters
22:20
will talk to the likeand gated channels and that will open them up and what does that do well it
22:27
all depends on what kind of neurotransmitter it is and what kind of receptor it is some are going
22:33
to be excitatory and if you look back at their graded potentials excitatory means that they’re
22:39
gonna move us towards the thresholds towards that all or non action potential now what’s the easiest
22:47
way to do that well that’s by moving a positively charged ions in there’s also some that’s going to
22:53
be inhibitory meaning if you move us away from that threshold and this is done by either let
23:00
a chloride flow in or simply potassium flow out and so the all-or-nothing principle happens at
23:07
the axon we either have an action potential or we don’t well the graded potentials happens at the
23:13
cell body so that was all I had for the physiology of a neuron for now and I hope that was helpful