IMAT Biology Lesson 6.9 | Anatomy and Physiology | Nervous System II

Summary

In 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

00:00

🧠 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.

05:00

🌿 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.

10:03

🔋 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.

15:05

🚀 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.

20:06

🔄 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.

25:07

🤝 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.

Keywords

💡Nerve Pathway

A nerve pathway refers to the route that nerve impulses travel within the nervous system. It is central to the video's theme as it discusses how signals are transmitted along the nervous system. In the script, the autonomic nervous system's structure, which includes the parasympathetic and sympathetic divisions, is highlighted to explain nerve pathways.

💡Synapse

A synapse is the junction between two neurons where information is passed from one to another. The video discusses the synapse in the context of signal transmission, emphasizing its role in the communication between neurons. The script explains the process at the autonomic ganglion where the signal is transmitted only by acetylcholine.

💡Action Potential

An action potential is an electrical signal that travels along a neuron's membrane. The video's main theme revolves around explaining how action potentials function within the nervous system. The script describes the process of depolarization and repolarization, which are stages of an action potential, using the sodium-potassium pump as an example.

💡Autonomic Nervous System

The autonomic nervous system is the part of the nervous system that controls involuntary actions. It is integral to the video's narrative as it explains how this system controls organs without conscious thought. The script details the sympathetic and parasympathetic divisions, which are key components of the autonomic nervous system.

💡Myelin Sheath

The myelin sheath is a protective covering around nerve fibers that aids in faster signal transmission. In the script, it is mentioned in relation to the preganglionic fibers, which are myelinated to allow for quicker signal transmission, a concept crucial for understanding nerve pathway efficiency.

💡Schwann Cells

Schwann cells are glial cells that produce the myelin sheath in the peripheral nervous system. They are mentioned in the script as providing insulation for nerve fibers, which is essential for the rapid transmission of signals along the nerve pathways.

💡Nodes of Ranvier

Nodes of Ranvier are gaps in the myelin sheath where the nerve impulse jumps from one node to the next, increasing the speed of signal transmission. The script explains this concept in the context of myelinated preganglionic fibers, illustrating how the nervous system optimizes signal speed.

💡Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across a synapse. The video discusses acetylcholine and norepinephrine as neurotransmitters used in the autonomic nervous system. The script explains their role in signal transmission at the synapse, which is vital for neuron-to-neuron communication.

💡Resting Action Potential

The resting action potential is the electrical charge across a neuron's membrane when it is not transmitting signals. The script describes the sodium-potassium pump maintaining this potential at negative 70 millivolts, which is a fundamental concept in understanding how neurons prepare for action potential generation.

💡Depolarization

Depolarization is the process where the membrane potential of a cell changes from negative to positive, initiating an action potential. The script explains this process through the influx of sodium ions, which is a critical step in the action potential and nerve signal transmission.

💡Repolarization

Repolarization is the return of the neuron's membrane potential to its resting state after an action potential. The script describes this as the closing of sodium channels and the opening of potassium channels, which helps reset the neuron for another action potential.

💡Saltatory Conduction

Saltatory conduction is the process by which an electrical signal jumps from one node of Ranvier to the next in a myelinated axon, increasing the speed of the signal. The script mentions this as a significant mechanism for rapid signal transmission in the nervous system.

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]

you