3 lecture 5 Flashcards
How does the brain work?
stimulus –> processing –> behaviour
what is Stimulus
this is any input
light, sound, touch, taste, gravity, movement
what is Processing
what you do with the information
This can be very simple (like a reflex when you touch a hot pan or your need to breathe harder when running) It can be complex
(taking an exam or determining when and whether to swing at a baseball pitch)
what is Behavior
this is anything you do in response Removing your hand from the pan, filling in the circle for answer ‘A’, etc
what is the definition of action potential
an electrochemical wave, driven by the movement of ions along their electrical and concentraFon gradients
what are neurons
cells
they contain the same complement of organelles, cytoskeleton, DNA, etc. as other cells
SSthey also have features that make them distinct
what are dendrites
Processes near the cell body Receive input from other neurons
(branches out from neutron body)
what is an axon
Main conducting unit of the neuron
Can convey information great distances by propagating an electrical signal (the action poten’al)
what is the synapse
Used to communicate that signal to other neurons
Where the terminal of one cell connects to the dendrites of another cell is the synapse
what is concentration gradient
Molecules move from areas of high concentration to low concentration
until they are evenly distributed
can diffusion still happen even when there is a barrier with a hole in it
yes, it will just happen more slowly
what is there a high concentration of outside of a neuron cell
High concentration sodium ions (Na+) OUTSIDE of the cell
Low concentration sodium ions INSIDE
This means we have a concentration gradient for sodium Na+ to go IN
sodium has a positive charge, what does this mean for the neuron
the charge of the neuron will change
charge particles can either be ____ or ____
positive or negative
what does it mean for a particle to be charged
they experience a force when they are around other charged parFcles
what is voltage
it is a force that moves oppositely charged particles toward each other
how do you create voltage
to create voltage, you need to separate two (or more) opposite charges
When you do this, when you separate charge, you create a voltage difference or potential
what is voltage difference
potential
That difference is a force, so that if you release the charges they will move toward one another
how does lightning relate to voltage
If you separate charge, you create a voltage difference or poten9al
Clouds moving over the surface of the earth produce a difference in charge, a potenFal, between the clouds and the surface
When that potenFal is released, i.e. the posiFve and negaFve charges connect, it’s lightning
current: the electricity or flow of charge that happens when there is a voltage
how can you relate gravity to voltage
If I separate a ball from the ground, I’ve created potential energy
When I release the ball, it should fall to the ground, pulled by a force (gravity)
This is how a battery works, the positive and negative charges are separated (creating a voltage)
Current:
If you connect a wire to them, completing the circuit, the electricity or current will move through the wire
how does electrical gradient work
If you separate the charge, for example with a membrane, you’ve created a voltage difference or potetial
If we add a channel to the membrane, our charged parFcles will flow through, down the electrical gradient
what are the 2 types of gradients
concentration gradient
electrical gradient
what is Concentration Gradient
Molecules move from areas of high
concentraFon to low concentration
what is Electrical Gradient
Opposite charges move toward each other
what are the 2 necessary parts to action potential
concentration and electrical gradient
what is prominent inside the neuron cell before sodium moves in
inside the cell there are a lot of negatively charged proteins and some negative ions
what is the change inside the cell
-70 mV
how do e know it is -70 inside the cell
We measure it with ELECTRODES
thin pieces of metal that we place inside or outside the cell
let us monitor the potential
describe how the sodium uses the electrical gradient to move inside the cell
High concentraFon of sodium ions Na+ OUTSIDE of the cell (therefore a LOW concentraFon INSIDE)
So, concentra’on gradient for sodium Na+ is to go IN
Meanwhile, inside the cell there are a lot of negaFvely charged Na+ proteins and some negative ions Na+
This gives us a voltage or poten9al of <70mV or so
Sodium, Na+, is a posiFvely charged ion
Thus, there is also an electrical gradient for Na+ to go IN
what are channels a key part of
how neurons work
how do channels open up
they open in response to
different changes in the cell
what are ion channels
There are some channels just for Na+ (nothing else can flow through them)
They only let Na+ go one way: IN
Whether a channel is open or closed can depend on voltage
Na+ channels are normally closed at very low voltage
-70mV INSIDE = CLOSED
if then inside id slightly LESS negative, what happens to the channels
they will open
(an be anything less than -70)
Once the channels open, Na+ will rush in
Pushed by both the Concentration gradient and The voltage difference or electrical gradient
The charge on the inside goes up eventually reaching +40 mV
once the voltage gets really high (e.g 40mV), what happens
Then, once the voltage gets really high (e.g. +40 mV) the Na+ channels close
This is known as the refractory period
The channels go offline for a little while because they
need to reset
During that time, you can’t Na+ get them to open
what happens in the refractory period
the cell closes the channels
what is the resting potential
where the cell starts (the voltage it starts at)
while there is sodium on the outside of the cell, what is on the inside
potassium
what is the gradient for potassium
to go OUT
how do we get from the refractory period to the start again
If the inside gets a liUle posiFve, it opens the Na+ channels, Na+ rushes in
This makes the inside of the cell gets less negaFve (now 0 mV for example)
This opens up K+ channels
K+ flows OUT
Na+ keeps flowing in until 40 mV inside
Sodium channels close
But K+ channels are still open
Concentration gradient and electric driving force still pushing K+ out
K+ continues to flow out until the voltage is below the
resting potential
(we’re not going to go into why it goes BELOW the resting potential, it just does)
K+ channels then close
That brings us back to -70 mV
But we’re not quite back to normal
Because now, All the K+ is now outside and all the Na+ is inside
the concentrations are reversed!
Fortunately, there is a pump It uses energy (ATP) to move K+ in and Na+ out until everything is back to normal
why does K+ flow out
Concentration gradient
Less voltage holding K+ in
what are the steps to action potential
Na+ channels open, Na+ begins to enter cell
K+ channels open, K+ begins to leave cell
Na+ channels become refractory, no more Na+ enters cell
K+ continues to leave cell, causes membrane potential to return to resting level
Extra K+ outside diffuses away
how does action potential happen all the way down the axon
like dominos
At the start, the membrane is completely polarized
SSmore negaFve inside
Small increases in posiFve charge result in the opening of voltageSgated sodium channels and an influx of Na+
This Na+ influx further depolarizes the membrane (makes the inside more posiFve) The positive charge spreads passively, opens adjacent Na+ channels This repeats for the length of the axon
Meanwhile, voltage gated potassium (K+) channels open, and Na+ channels close K+ flows out and the inside become negaFve again
Why does it only go one direc’on? Why doesn’t it go backwards?
Aler Na+ flows in and makes the inside posiFve, the Na+ channels in that patch close
As I menFoned, they won’t open again for a liUle while, they have to reset.
This means that even though Na+ will diffuse in both direcFons when it enters, it can only open channels that haven’t recently opened
Action Potentials were first studied in what
giant squid; Loligo
which scientists did work on the action potential
Hodgkin and Huxley working in Woods Hole MassachuseUs did a lot of measurements to figure out how the acFon potenFal works
why was the big axon important to the scientists
A big axon was important to them because it was easy to work with (electrodes!)
A big axon is important to the squid because it conducts the action potential faster than an axon of a smaller diameter
In the squid, these giant axons convey signals for things like critical reflexes–for times when the information needs to get to the muscles quickly
why is a small axon more efficient
This is more efficient: there are fewer channels to open and reset, there are fewer pumps that need to be involved, etc. Basically, it makes the Na+ that you let in more effective
But, there’s only so efficient we can make the system this way
you can only fit so many GIANT axons into a brain before it becomes unwieldy and expensive
that would limit the number of cells and connections that you could have
So evoluFon determined a way to get the same increase in the ability of the axon to hold and spread charge but in a small axons– what is it
myelination
what is myelination
In many vertebrates, axons are wrapped by other cells (known as Schwann cells) this insulates the axon, preventing Na+ and K+ from leaking out or in
enables the cell to hold the charge for longer
i.e. the positive charge from the influx of Na+ can travel farther down the axon
adding myelin is equivalent to doing what to the diameter
increasing
they both make the action potential faster and more efficient
does myelin coat the entire axon
no
Rather, there are many Schwann cells, that wrap the axon
In between those cells are what are called “nodes of ranvier”
what are at the nodes of ranvier
there are lots and lots (and lots) of channels and pumps
how do nodes or ranvier help
This means that instead of having to open channels along every micron of the axon
The acFon potenFal can “skip” rapidly from one node to the next, regeneraFng just at the nodes instead of at all of the areas in between
what is Multiple Sclerosis
Multiple Sclerosis is an autoimmune disease that results in damage to and breakdown of the myelin
As a consequence, paFents exhibit impaired movements and other deficits because signals are no longer travelling quickly or efficiently
give a summary of action potential thus far
the action potential is an electrochemical wave, driven by the movement of ions along their electrical and concentraFon gradients
this means that we can measure it by measuring the voltage and manipulate it either by altering the chemistry or by altering the voltage
The acFon potenFal is an electrochemical wave, driven by the movement of ions along their electrical and concentraFon gradients SScan measure it (with electrodes) SSmanipulate it either by altering the chemistry or by altering the voltage
It is an “allJorJnone” signal
Once it’s started, once you nudge the first domino, it will conFnue down the length of the axon without decreasing in size or strength This is because it gets regenerated over and over again along the way
It takes energy
Aler an acFon potenFal occurs, energy is used to put all the pieces back into the right spot (move K+ back in, Na+ back out)
as code, what is the action potential represented as
This means that as a code, we basically have 0s and 1s, the acFon potenFal is on or off
why is the 1 and 0’s a problem
But there are also many instances where you need more complexity
