IMAT Biology Lesson 6.9 | Anatomy and Physiology | Nervous System II
Summary
TLDRIn this educational video, Andre from Med School EU delves into the intricacies of the nervous system, focusing on nerve pathways and action potentials. He explains the autonomic nervous system's structure, highlighting the roles of the sympathetic and parasympathetic divisions. The video clarifies the concept of synapses and details the process of action potential generation, including depolarization and repolarization. Andre also dispels common misconceptions about action potentials, emphasizing that their size and speed remain constant regardless of stimulus intensity, and that frequency is the key variable distinguishing strong from weak stimuli. The explanation of synapse function and neurotransmitter release provides a comprehensive understanding of neural signal transmission.
Takeaways
- 🧠 The nervous system's nerve pathways and action potentials are the focus of the video, discussing how signals are transmitted along the system.
- 🤔 The autonomic nervous system, which is involuntarily controlled by the brain, is divided into the parasympathetic and sympathetic divisions for controlling organ activity.
- 🌐 The preganglionic neuron originates from the central nervous system and synapses at the autonomic ganglion, while the postganglionic neuron transmits signals to the organs.
- 🔋 Myelinated preganglionic fibers have a myelin sheath composed of Schwann cells, which speed up signal transmission through nodes of Ranvier.
- ⚡ The action potential involves a rapid change in membrane potential from a resting state of -70 millivolts to a peak of +30 millivolts and back, facilitated by the opening and closing of sodium and potassium ion channels.
- 🚀 The speed of an action potential depends on the diameter of the axon and whether it is myelinated, with larger diameters and myelination increasing the speed of transmission.
- 🔗 The synapse is the connection between neurons, where neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron.
- 🔑 Neurotransmitters like acetylcholine and norepinephrine are chemical signals that transmit information across the synapse.
- 🔄 The sodium-potassium pump maintains the resting membrane potential and is essential for the action potential's depolarization and repolarization phases.
- 🌀 The action potential's size remains constant regardless of the stimulus intensity, but the frequency of action potentials can vary depending on the stimulus strength.
- 🏎️ Saltatory conduction, the process of signal jumping between nodes of Ranvier in myelinated axons, significantly increases the speed of signal transmission.
Q & A
What is the main topic of the video?
-The main topic of the video is the nerve pathway and action potentials of the nervous system.
What is the autonomic nervous system and how is it controlled?
-The autonomic nervous system is the part of the nervous system that is involuntarily controlled. It is completely controlled by the brain and operates automatically without conscious awareness.
What are the two divisions of the autonomic nervous system discussed in the video?
-The two divisions of the autonomic nervous system discussed are the parasympathetic nervous system (PNS) and the sympathetic nervous system (SNS).
What is the role of the preganglionic neuron in the nervous system?
-The preganglionic neuron is the part of the nervous system that comes off the spinal cord or the central nervous system. It is myelinated and plays a crucial role in transmitting signals to the autonomic ganglion.
Why are preganglionic fibers myelinated?
-Preganglionic fibers are myelinated because the myelin sheath, composed of Schwann cells, provides insulation for these fibers, allowing them to transmit signals much faster.
What are the neurotransmitters mentioned in the video that are used to transmit signals in the autonomic nervous system?
-The neurotransmitters mentioned in the video that are used to transmit signals in the autonomic nervous system are acetylcholine and norepinephrine.
How does the resting action potential of a neuron help in maintaining the membrane potential?
-The resting action potential is maintained by the sodium-potassium pump, which actively transports three sodium ions out of the cell and two potassium ions into the cell, keeping the membrane potential at around negative 70 millivolts.
What is the significance of the threshold potential in the generation of an action potential?
-The threshold potential is significant because it is the point at which an action potential is triggered. If the membrane potential reaches above negative 50 millivolts, it passes the threshold, leading to the opening of voltage-gated sodium channels and the initiation of an action potential.
What are the key steps in the process of an action potential?
-The key steps in an action potential are depolarization (influx of sodium ions), reaching the peak potential (positive 30 millivolts), repolarization (closing of sodium channels and opening of potassium channels), and hyperpolarization (overshoot below negative 70 millivolts due to potassium ion outflow).
How does the speed of an action potential transmission vary with the stimulus intensity?
-The speed of an action potential transmission does not vary with the stimulus intensity. The size and speed of action potentials remain constant regardless of the stimulus strength; it is the frequency of action potentials that increases with stronger stimuli.
What is the role of synapses in the nervous system?
-Synapses are the connections between neurons that allow for the transmission of signals from one neuron to another. They involve the release of neurotransmitters from the presynaptic neuron and the binding of these neurotransmitters to receptors on the postsynaptic neuron, potentially causing an action potential in the latter.
Outlines
🧠 Nervous System's Nerve Pathways and Synapses
This paragraph introduces the topic of nerve pathways and action potentials within the nervous system. It explains the autonomic nervous system's involuntary control by the brain, the division into sympathetic and parasympathetic systems, and the concept of synapses. The script discusses the structure of nerves, specifically the myelinated preganglionic neurons and unmyelinated postganglionic fibers, and the neurotransmitters involved in signal transmission across the autonomic ganglia. It also mentions the importance of this knowledge for medical exams like the 2021 IMAT.
🌿 Autonomic Nervous System's Structure and Function
The second paragraph delves into the structure and function of the autonomic nervous system (ANS), highlighting the differences between the sympathetic and parasympathetic nervous systems. It explains the origin of parasympathetic signals from the brainstem and pelvis, in contrast to sympathetic signals from the thoracic vertebrae. The paragraph emphasizes the long preganglionic fibers of the parasympathetic system and the short postganglionic fibers of the sympathetic system, which is crucial for understanding the ANS's role in bodily functions. It also advises memorizing these concepts for medical examinations.
🔋 Resting and Action Potentials in Neurons
This section discusses the resting membrane potential of neurons and the concept of action potentials. It describes the sodium-potassium pump's role in maintaining the resting potential at -70 millivolts and the process of depolarization during an action potential. The paragraph explains how an electrical stimulus can trigger an influx of sodium ions, leading to a change in membrane potential from -70 millivolts to a positive 30 millivolts. It also touches on the importance of reaching a threshold of -50 millivolts for an action potential to occur.
🚀 The Process of Depolarization and Repolarization
The fourth paragraph explains the process of depolarization and repolarization during an action potential. It details how the membrane potential reaches a threshold, causing voltage-gated sodium channels to open, leading to an influx of sodium ions and a rise in membrane potential to +30 millivolts. Following this, repolarization occurs as sodium channels close and potassium channels open, allowing potassium ions to leave the cell and returning the membrane potential to its resting state. The paragraph also describes the overshooting hyperpolarization phase and emphasizes the importance of these concepts for understanding neuronal signaling.
🔄 Understanding Action Potentials and Misconceptions
This paragraph clarifies misconceptions about action potentials, emphasizing that their size remains constant regardless of stimulus intensity. It explains that the distinguishing factor between strong and weak stimuli is the frequency of action potentials, not their size. The paragraph also discusses the speed of transmission of action potentials, which is influenced by the myelination and diameter of the axon, and introduces the concept of saltatory conduction in myelinated axons, which significantly increases the speed of signal transmission.
🤝 The Synaptic Connection Between Neurons
The final paragraph introduces the concept of the synapse, which is the connection between neurons. It describes the process of an action potential reaching the axon terminal and causing the opening of voltage-gated calcium channels. The influx of calcium triggers the release of neurotransmitters from synaptic vesicles, which then bind to receptors on the target cell, causing depolarization and potentially another action potential. The paragraph also differentiates between voltage-gated and ligand-gated channels and sets the stage for the discussion of muscle stimulation in the next video.
Mindmap
Keywords
💡Nerve Pathway
💡Synapse
💡Action Potential
💡Autonomic Nervous System
💡Myelin Sheath
💡Schwann Cells
💡Nodes of Ranvier
💡Neurotransmitters
💡Resting Action Potential
💡Depolarization
💡Repolarization
💡Saltatory Conduction
Highlights
Introduction to the topic of nerve pathways and action potentials in the nervous system.
Discussion on the autonomic nervous system, its involuntary control, and its division into parasympathetic and sympathetic branches.
Explanation of the synapse as the connection between neurons and its role in signal transmission.
Details on the structure of nerve pathways in the autonomic nervous system and their significance in the 2021 IMAT exam.
Description of preganglionic neurons, their myelination, and the role of Schwann cells in signal transmission speed.
Differentiation between myelinated preganglionic fibers and unmyelinated postganglionic fibers in the autonomic nervous system.
Localization and function of the sympathetic chain of ganglia and collateral ganglia in the nervous system.
Contrast between the long pre-ganglionic fibers of the parasympathetic nervous system and the short post-ganglionic fibers.
Importance of memorizing the structure and function of the autonomic nervous system for medical exams.
Overview of the resting action potential, the sodium-potassium pump, and the maintenance of membrane potential.
Mechanism of action potential generation, including depolarization and repolarization processes.
Role of voltage-gated channels in the initiation and propagation of action potentials.
Clarification of misconceptions regarding action potential size and its relation to stimulus intensity.
Differentiation between the frequency of action potentials and the strength of the stimulus.
Influence of myelination and axon diameter on the speed of action potential transmission.
Explanation of saltatory conduction and its significance in increasing the speed of nerve signal transmission.
Description of the synapse formation, neurotransmitter release, and the process of signal transmission between neurons.
Function of ligand-gated channels in neurotransmission and the generation of action potentials in the receiving neuron.
Transcripts
[Music]
hi everyone my name is andre welcome to
med school eu
and the topic of today's video is going
to be
the nerve pathway and action potentials
of the nervous system so
today we're going to talk about the
second part of the nervous system unit
and we're going to discuss the
connection between neurons which is
called the synapse and we're going to
talk about action potentials
and overall we're just going to discuss
how signals are actually transmitted
along the nervous system so first of all
we're going to discuss the nerve
pathways how nerves are structured in
our autonomic nervous system so the one
that is
involuntarily controlled is completely
controlled
by our brain so
the signals that are being sent out by
the brain are not conscious signals
we're not aware of them and this happens
automatically we don't need to think
about it so the way our organs are
activated based on their activity in
order to increase their activity or
decrease their activity is activated by
the autonomic
nervous system
which would stem to to be the
parasympathetic and the sympathetic
division first of all this is called the
nerve pathway we are going to discuss
certain details about this because this
actually came up on the 2021 imat
exam you actually had to know uh some of
this information in order to answer one
of the questions
so this uh the one that comes off the
spinal cord or the central nervous
system
is uh called the preganglionic
neuron so
pre-gang glionic and it's going to make
more sense why it's called
pre-ganglionic
because
this
little bundle right here is called the
autonomic ganglion
and this preganglionic fiber is
myelinated
myelinated
and we talked in our previous lecture
about
the myelin sheath that is composed of
schwann cells so the the schwann cells
basically they provide
insulation for these fibers these nerves
so they are able to transmit their
signal much faster so the preganglionic
fibers are myelinated meaning that they
are going to have these schwann cells
wrapped around them as you can see here
the schwann cell schwann cell schwann
cell and they're going to have nodes of
ranvier
going right in between them so the
signal can just jump between one two the
other and transmit much faster now the
post ganglionic
fiber
does not have
the myelinated sheath it is unmyelinated
also the acetylcholine right here and
the norepinephrine these are
neurotransmitters
that are used
to transmit the signal these are
chemical signals that are going to be
able to be absorbed
by
the organ and then the activation of
further
action potential and here at the
autonomic ganglion the signal will be
transmitted only by the acetylcholine
not
nor epinephrine so now looking at both
sides we've got the sympathetic
on this side on the left
and the
parasympathetic so pns and we got sns
sympathetic nervous system
parasympathetic nervous system we
discussed these in great detail in our
last lecture so the sympathetic nervous
system is going to come off from the
thoracic
vertebrae from from the spinal cord t1
to t12 is standing stands for the
thoracic vertebrae and these nerves
these pathways are going to come out of
there and they're going to synapse at
the sympathetic chain of ganglia so this
is called sympathetic chain of ganglia
that will be located close
to the
start of the preganglionic fiber so as
you can see the preganglionic
neurons are going to be quite short
compared to the parasympathetic nervous
system
because these are going to go directly
to the organs or possibly go very close
to the organs we're going to synapse
however in sympathetic
nervous system these sympathetic chain
ganglia are going to be much closer
which which will make the preganglionic
fibers much shorter so they're going to
be short but they will have long
post ganglionic
fibers now these ones are called
collateral ganglia
where the preganglionic fibers are going
to go past the
sympathetic chain of ganglia and they're
going to synapse at collateral ganglia
now as you can see the preganglionic
fibers are never going to reach any of
the organs on their own they're going to
have to synapse at some sort of ganglia
in order to reach
the the final
destination
however the parasympathetic nervous
system is going to be coming off
the
brainstem so these are actually going to
be
the cranial nerve so this would be
cranial nerve number three
uh cranial nerve number 10 which is the
vagus nerve very important for our body
and they will also be coming from the
pelvis as well
so that's the two locations where
the
parasympathetic nervous system is coming
from that's the pelvis and the brain
stem whereas sympathetic are coming off
from the thoracic vertebrae now what's
important to know is that the
parasympathetic nervous system has long
pre-ganglionic fibers that will likely
not be attaching to any ganglia however
the sympathetic will have long post
ganglionic fibers in comparison as you
can see the postganglionic fibers here
from the parasympathetic nervous system
are extremely short if at all because
over here they will be bonded very close
to
the organ so that's important to note
that that's the distinction that you
needed to know to answer one of the
questions on the 2021 imad exam so i
would suggest you memorize this diagram
and you memorize these concepts by heart
so you can avoid mistakes on this year's
exam so we are going to discuss a little
bit about the resting action potential
so basically we are going to have a
membrane this is the membrane
and we are going to have our pumps so
this is going to be this blue one right
here is going to be the pump that pumps
ions in and out of the membrane which
keeps the membrane either positive or
negatively charged and we're going to
discover
what actually happens here so what we're
what we will have is the sodium will be
pumped out of the cell so if if we're
going to label this is the extra
cellular fluid and this is the cytoplasm
which
would mean that this is the inside of
the cell the sodium and a plus will be
pumped out of the cell
into the extracellular fluid
and the potassium from outside the cell
is going to be brought into
the cell
so
and it's going to be done at the rate of
three
to two so three sodium will be pumped
out of the cell and two potassium will
be pumped into
the cell and this is the mechanism that
will be used the sodium potassium pump
protein that will be used in order to
keep the resting potential at
negative 70
millivolts so the membrane potential of
before any action begins before any
electrical impulse could be transmitted
across a cell
they are going to have to be kept at
this negative 70 millivolts
because more as you can see three sodium
more so more positive charges are
flowing out of the cell
than they are coming into the cell so
it's going to maintain
this sort of electrochemical
gradient in addition this gradient of
negative 70 millivolts will be
maintained by the potassium
leaving or leaking through the membrane
at a much faster rate than the sodium
coming back in because the sodium
concentration inside the cell is low
and the sodium concentration outside the
cell is high
so
this uh action pump this sodium
potassium pump protein
is active transport because it's pumping
against its chemical
gradient so it's going against the
electrochemical gradient and same with
the potassium here because the potassium
is coming in to a high concentration
from a low concentration on the outside
so it's naturally going to want to
diffuse out
into the extracellular fluid and because
the membrane is leaky meaning that it
leaks our potassium
slowly and
at a certain pace it leaks more positive
charges
but it's not as permeable to sodium
for having it leak back inside the cell
so therefore it kind of maintains this
whole structure works on maintaining
this negative 70
resting active potential so generally
the cell is more negative on the inside
than it is on the outside and this will
be the typical driving force
of the action potential why it even is
a phenomenon so now we are going to talk
a little bit about the action potential
so we know that our membrane potential
is minus 70 millivolts and if we take a
look at a chart or or a graph
of this and
try to kind of depict what really is
going on in terms of the potential
difference so here we're going to have
millivolts on this side and here the
time frame
and we would begin with the minus 70
because that's where membrane potential
is and here's the key with the plus
30. so typically the cell nicely resides
in this minus 70. however when a
stimulus occurs when electrical stimulus
comes through the cell onto its cell
membrane it's going to cause an influx
of positive energy which is going to
grow grow grow
and then once it
once it crosses this minus 50 threshold
it is going to cause an action potential
but it has to reach this threshold
because if the
action potential is not strong enough
and it reaches just underneath it
it does not reach this threshold and
action potential is not going to occur
so these these little stimulus
stimuli are not going to cause an action
potential they're not going to cause the
cell to do anything
about its environment however if we want
the cell to activate through the
electrical
signal and pass on the stimulus it's
going to go above the 50 mark and then
it's going to cause an influx of
positive energy
which will then
cause a slow decline
and then overshoot meaning it's going to
go below -70 and then it will realign
itself back
to its original
membrane potential
now the key here is that it's going to
revert from -70 millivolts to positive
30 so it's going to be influx of
positive energy now all of this is
caused by the changes in the
permeability of the
membrane and
that is going to cause the sodium ions
and the potassium ions to be going
across the cell membrane at a very fast
pace
so when the cell is at its resting
potential these pumps and and these
channels between
so these sodium
and potassium channels
that are associated in in the membrane
they are going to be closed so they're
closed when the membrane potential is
minus 70 they're
closed off
they're not going to allow the sodium
potassium ions to leak quickly through
from one side to another the potential
is going to be maintained again by the
sodium potassium pump and these channels
are going to be completely closed
so what happens during an action
potential first the electric current
used to stimulate the axon causes the
opening of the voltage-gated channels in
the cell surface membrane
which is going to allow this sodium ions
to pass through so we get an influx of
sodium
ions now obviously sodium ions
are going to make
the membrane
or the inside of the cell the cytoplasm
more positive so it's going to go from
negative 70 to more positive side
negative 50 negative 30
and so on and because there's a much
greater concentration of sodium ions
outside the axon than the on the inside
the sodium ions enter through these open
channels that were previously closed now
they're open because they were
stimulated by voltage remember these are
voltage-gated
channels and so therefore the sodium
potassium pumps are not going to be as
effective
because these channels are going to be
open they're going to be leaking ions
all over the place so the sodium ions
will
leak through from the outside to the
inside of the cell causing the cell to
become more positive
compared to its environment
and this part is called depolarization
so this this aspect right here is called
d
polarization and it is done by the
sodium influx
inside the cell and so there is this
threshold as i've mentioned
before if this threshold is passed so
the depolarization originally happens a
little bit slower but as soon as it is
passed it caused a huge influx because
more and more
are becoming
open and if the potential difference
reaches about 50 negative 50 millivolts
then
all of these channels are going to open
and the potential will then quickly
appear to be plus 30 millivolts compared
to the outside so after about one
millisecond so after this would be about
one
millisecond where this action potential
becomes
uh plus 30 millivolts that's the
potential difference between the inside
of the cell and the outside the sodium
ion voltage-gated channels
close so the sodium ions stop diffusing
into
the axon and at the same time
the
potassium
ion channels open so at this point the
potassium ion channels are closed
meaning that the positive charge that's
already inside the cell is not going to
be leaking out to the outside only the
sodium will be leaking into the cell as
you can see right here by this and
that's part of depolarization
and that's what we highlighted here
however once it reaches the plus 30 and
the action potential has already
occurred
what we're going to get is something
called re
polarized
and during repolarization
we are going to have the sodium
ion
gate voltage-gated channels are going to
be close so less sodium will be coming
into the cell
and more potassium will
come out of the cell so the positive
charges will be leaving the cell and
less positive charges will be coming
into
the cell because now the potassium ion
channels are open
and the
sodium channels are closed
so as you can imagine here we're going
to have
more an influx of positive charge that's
going to be coming out of the cell it's
going to be leaving the cell
meaning that the cell is going to become
more and more progressively more and
more
negative because it removes the positive
charge from inside of the axon to the
outside therefore
it's going to return the potential
difference to -70 and this is that's the
whole process of repolarization
however the potential difference across
the cell briefly causes an influx
of this positive
of this positive charge leaking out and
therefore it's going to cause a
hypershoot which is going to be called
hyper polarization
because the positive charge will be
leaking out at such a fast pace
that it's going to cause an overshoot
and as you can see it's going to go
below
minus 70 millivolts so again a couple of
concepts you need to remember i'm going
to summarize this because this is
something that's extremely important for
the test
and it's something that you should know
by heart
so in order for an action potential to
occur the axon
membrane potential must be depolarized
they must go from negative 70 to plus
30. however in order for that to happen
the stimulus the electrical stimulus
must be strong enough
that it reaches above
negative 50 millivolts once it reaches
above negative 50 it is past the
threshold so this is the
threshold
potential and once it's passed then the
potassium will
the sodium will leak in
more and more and more positive charge
will be inside of the axon and this all
occurs this depolarization occurs why
because the sodium ion voltage-gated
channels
are open while the potassium
voltage-gated channels are closed
however once it reaches this one
millisecond
time
and it has caused this action potential
the sodium channels are going to close
and potassium channels
are now going to open meaning that the
positive charge will begin leaking out
because potassium
potassium has
the uh chemical difference
electrochemical gradient going from the
inside of cell towards the outside so it
wants to leave because that's where
there's less potassium so it's going to
go from higher concentration to lower
concentration by just general diffusion
so potassium is going to leave the cell
the axon
because these channels are now open it
allows them to just leave
and the sodium
will then not be able to leak into the
cell from the outside since the outside
is more concentrated than the inside and
therefore you're going to get less
positive coming in and more positive
leaving the cell and this part is called
the re-polarization
now i wanted to revisit this concept
again to understand why depolarization
and repolarization really occurs so if
we have our
membrane here we have the cytoplasm and
we have our extracellular fluid
what we have is
this
enzyme
which is just going to be a protein it's
it's going to act as a pump of sodium
and potassium
there is a higher concentration of
sodium
outside of the cell
keep that in mind and there's a lower
concentration of sodium
on the inside of the cell now because
this pump is an active pump it's active
transport it's going to go against its
electrochemical
gradient and it's going to pump three
sodium outside the cell even though
there's already low amount of sodium on
the inside of the cell
now
potassium is the opposite we got low
concentration of potassium on the
outside and high concentration of
potassium
on the inside and the way it's going to
work is again it's going to pump against
its concentration gradient and bring the
positive inside
now what happens during depolarization
during depolarization we're going to get
these other channels they're called
voltage gated channels because they
respond to
voltages they respond to electrical
stimuli
and these channels are going to open the
sodium channels will open once
electrical stimulus goes through and
they're going to open and the sodium
is now going to go down its
concentration gradient from higher
concentration
down to the lower concentration which is
inside the cell so it's going to cause
the cell to be more positive
so from minus 70 it's going to begin to
rise towards -50 and then once it passes
that threshold it's going to go to plus
30 millivolts
because this
sodium ion voltage gated channel is open
however
once we reach the peak at plus 30 we're
going to get repolarization
and during repolarization the opposite
occurs sodium ion voltage-gated channels
are going to close so right here
they're just going to close these will
be closed
and
these
channels are going to open so the
potassium
channels are going to
now open
and the potassium channels will force
the potassium to go down its
concentration gradient
going from
the inside of the cell to the outside of
the cell so more and more potassium more
positive charge will be leaving and then
we are going to have our membrane
potential go right back to negative 70
and do an overshoot which is going to be
hyper repolarization below -70 because
of this crazy crazy
outflow of positive charge and no no
positive charge is coming in the sodium
is not coming in because these these are
inhibited these are blocked
and so therefore the the potassium is
going to leave and that's how an action
potential occurs this is how it occurs
down the axon where it goes from one
node of ranvier to the next node of
render and that's how the membrane is
going to be depolarized and how it's
going to activate the signal and it's
going to be transmitted from one part of
the cell to the next part now i also
wanted to address several misconceptions
about action
potentials because i i believe it's
extremely important
in the way that you're going to
understand what an actual potential is
and how a signal is actually transmitted
and how it carries
information how we're able to perceive
that something is bright light versus
dim light or something is a strong
stimuli
versus a weak stimuli
and it's not going to be
perceived by the
action potential size so the size
of the action potential is not going to
change it will remain constant
it is not going to be changing
throughout the axon
nor do they change in size according to
the intensity of the stimulus so
intensity is another variable that is
also going to be constant
the action potential will continue to
reach the peak value of plus 30
millivolts
inside all the way along the axon so if
it's traveling from the spinal cord all
the way down to the toe it is
continuously going to produce plus 30
millivolts throughout the entire length
of the axon
now if you have a very strong light
shining in your eyes it's going to
produce action pretend potentials of
precisely the same size as dim light so
again size is not going to be a variable
because the size is always going to be
the same in terms of the action
potential even if it's bright light or
it's dim light it's still going to have
the same stimuli or if it's
hot
stove or if it's cold stove
you're going to perceive the same size
of action potentials they're not going
to be
the difference
that distinguishes these two stimuli
and the speed at which action potentials
travel does not vary according to the
size of the stimulus well what is
different about action potentials
resulting from strong and a weak
stimulus is the frequency
so the frequency
of stimuli is the most important thing
that distinguishes between a strong
stimuli and a weak stimuli so something
like bright light will have the
frequency of
of like this it's going to have stimuli
stimuli stimuli
whereas
the dim light will have a frequency of
one one
one
one it will come at a lower frequency it
will not be as often stimulating the the
nerve but the nerve it will be
stimulated very often in terms of
the action potential so it's going to be
creating new action potentials over and
over and over again at a
greater frequency
if the stimulus is strong so
bright light versus versus a dim light
now another thing we must talk about is
the speed of transmission now the speed
of transmission i already mentioned it
depends on
the myelination so my
limitation
of the axons so if the axon is
unmyelinated it's going to travel much
slower than in a myelinated axon so if
they have these nodes of ranvier they're
going to the stimuli is going to travel
way faster
another thing is going to be diameter
of
the
axon diameter of the axon so of course
the larger the axon let's say it's this
wide it's going to have less
of constriction it's going to be more
dilated
so it's going to be bigger it's going to
have a bigger volume to fit
and therefore it's going to have lower
resistance so the stimuli will just
travel freely however if it is
a tiny
neuron or the tiny axon it's going to be
hard
for the stimuli to
push through so it's going to have a
higher resistance in this case so the
diameter of the axon is also
going to be of high importance now in
terms of myelination the way that the
signal is being conducted
so it's going to be going between these
nodes of ranvier
which are the nodes that are in between
these myelinations
and this is going to be called saltatory
conduction in the myelinated axon
selectatory conduction can increase the
speed of transmission of up to 50 times
than in an unmyelinated axon
of the same diameter so it's very
important that our nervous system has
these myelinations
from the schwann cells
because they are able to provide much
faster
signals and the transmission the speed
of transmission is going to be way way
faster now the final thing the final
concept that we're going to discuss
today
is going to be a synapse and what's a
synapse
it is basically the connection between
one neuron
and another neuron so the the terminal
ends and dendrites are going to be
connected to form a synapse so we're
going to zoom in here and see what's
going on and how
a synapse really occurs
so first we're going to have this action
potential that we just talked about the
action potential is going to reach the
end of the axon at the axon terminal
and depolarize its membrane so the
action potentials will be
going down this way and it's going to
depolarize this membrane
now as soon as the membrane is going to
be depolarized
the voltage gated remember the
voltage-gated are the ones that will
open
when a voltage passes through so an
action potential passes through we have
an action potential it arrives
and it's going to open the voltage-gated
calcium
channels
now now the calcium from the outside
from the synaptic cleft
is going to flow inside
over here it's going to flow inside
to
the
axon terminal now the calcium influx
triggers synaptic vesicles to release
neurotransmitters so these these circles
right here
they are gonna they're called synaptic
vesicles and these vesicles
are going to be holding neurotransmitter
and i've mentioned about
neurotransmitters
previously in our last lecture
how the preganglionic fibers are going
to attach to the ganglion and they're
going to release acetylcholine
however the postganglionic fibers are
going to release acetylcholine or
norepinephrine now these are going to be
typically acetylcholine so these are
neurotransmitters
and what's what's going to happen is
these neurotransmitters are now going to
fuse with the membrane
right here they're going to fuse with
the membrane they're going to do
exocytosis
and now there's going to be plenty of
these
neurotransmitters
and the neurotransmitter binds to
receptors so it's going to bind right
here to these receptors
on the
the target cell and in this case it's
going to cause positive ions to flow in
so we're going to have positive ions
plus here plus
plus and plus and all of that
is going to flow into the cell
now of course
this
influx of positive energy is going to
cause
depolarization
and it's more likely going to cause an
action potential and by the way these
are not voltage-gated channels and their
actual name is going to be
ligand-gated channels and these channels
are not going to be
exposed to voltage so they're not going
to be gated by voltage
they're only going to be influenced by
an a neurotransmitter which is going to
be acetylcholine here so the white parts
that are released here are acetylcholine
and what it's going to do
is it will cause the ligand gated
channels to open and cause an influx of
sodium ions which will cause
another action potential on the
receiving
neuron
now something very similar is going to
be happening with the muscle stimulation
and this is something that we're going
to talk about
in our next
video
[Music]
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