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Lecture

Lecture #3 Notes (Non-associative learning) Topics: the process of neural firing, action potentials, the myelin sheath, spatial summation, temporal summation, process in the synapse, "Myshkin", facilitation, potentiation, titanic stimulation, LTP/LTD and,


Department
Psychology
Course Code
PSY260H1
Professor
Martin Ralph

Page:
of 11
Lecture Three
The question we will focus on today is: What is the system for the mechanistic
change of our information?
The neuroscience will not be on the test (that much). The sections on LTP and
LTD are really important however and we need to understand more about these
two than was mentioned in the textbook. So refer to slide notes and lecture
notes to understand these two processes the way the professor wants.
What is the mechanism that allows for learning and memory to take place?
Anatomy of the neuron should give us more information about the way we learn
and memorize.
In general, higher organisms (with a very developed central brain) tend to
encode information not based on just the frequency of signals. It is less so in
more simple systems where there are fewer neurons. The opportunity to do
what the mammalian brain does not exist in any other specie (or computer, to a
degree). The systems we use to learn, to encode, to do everything, relies more
so on code than frequency of signals.
The process of neural firing
We must first understand that a single neuron passes an all or nothing response.
A working neuron has a potential difference across the membrane that is
maintained down the axon. It uses this difference within the axon (which
involves the separation of charges) and by changing the permeability of the
membrane (usually this is done in a coordinated fashion), the neuron has the
capacity to “do work” (learn, encode, etc). This is the mechanism of the
propagation of signals.
But what is the mechanism for receiving and responding to input?
The axon is a tube with the inside separated from outside. It has holes in it
(channels) that can be controlled (this allows ions to cross, which allow changes
in membrane potential, which allows for changes down the axon to occur and
an action potential to be “fired”).
The separation of charge . This separation really only means that different types
of ions are concentrated on one side of the membrane and one on the other
side of the membrane. This function is mostly performed by Sodium and
Potassium ions. By separating charges you create diffusion potential in both
directions. The membrane potential is based on the fact that you have equal
positive charges on the outside and inside of this membrane but on both sides
you have a particular type of ion that wants to diffuse across. There is a
tendency or probability that potassium, given the opportunity, will cross form
the inside to the outside and there is a probability that sodium, which is
concentrated on the outside, will push its way in. It’s the electrostatic charge
that holds those ions back (that we measure as the potential to do work, the
potential difference across the membrane). In the case of the neuron in general,
the pressure, the probability that these positive charges will move out is
positive because of the higher concentration and the fact that they can move
more easily across the membrane (more easily than the sodium ions can move
back in). They are carrying and providing most of that potential difference. It
takes more electrostatic charge to get potassium in than to keep sodium out
(when everything is at rest). This leaves us with a tendency to be negative
inside versus the outside. We tend to see a negative internal cytoplasm and a
positive outside. This can change.
Propagation of a signal down a neuron changes the permeability of the
membrane to one or the other of these ions. The membrane separates charges,
the charges have to be kept in place by electrostatic charges which produce the
potential to do work, which is to move ions.
The formula (on the slide) represents the difference in concentrations of each
ion on the inside and outside and what it takes to keep those ions where they
are. The voltage (overall) that one measures is the function of the difference
inside and outside (mainly potassium). Understand what is going on in this
equation. How the membrane potential is produced. How does it transmit a
signal?
The difference between the neuron and its membrane and just a plain wire is
that when you put a voltage at one end of a wire you can detect it almost
instantly at another end. But it takes a while for a neuron to charge up and
that’s because the membrane is a capacitor. It doesn’t allow all charges to go
across. It then takes a while to switch of the voltage. When the voltage reaches
a certain threshold something else happens in the membrane.
The permeability of the membrane changes so that different ions can move
through with different probabilities. In the beginning of an action potential we
see that it starts of with a positive feedback effect. You make the inside of the
cell more positive and then at some point the sodium channels open up
because of that and more sodium can potentially move in. This creates an even
more positive charge. This causes the rising charge for an action potential. Very
quickly after that the sodium channels are deactivated and potassium channels
open up, bringing the membrane potential back down. There is then a
refractory period. It is there to allow the channels to reset so that they may be
open again.
Why does the action potential tend to move in one direction once it is
propagated? It can be that an action potential goes both ways but usually, once it
is initiated it tends to move in one direction and not spread out. As the action
potential brings in positive charge to the axon, making the potential more positive
inside and opening sodium channels so that sodium comes in and raises this
potential even further. As this AP moves, this positive charge can be distributed
along the membrane. It is moved because the next region of the membrane has
become more positive. The region we leave is becoming more negative because
the sodium channels are being deactivated and also because the potassium
channels are being activated to bring the membrane potential down. The AP won’t
reverse since there are deactivated sodium and highly negative region behind it.
The positive charge brought in by sodium ions opens up the potassium
channels. As they open up it increases potassium influx. Driven by sodium, the
AP moves through the axon.
Can’t have action potentials right on top of the other. We have to wait for the
channels to reset before a second AP can be propagated down the axon.
The myelin sheath directs the flow of the charge down the axon instead allowing
the charge to contribute to the capacitants. So basically, the AP does not charge
the membrane (the capacitant). You have positive nodes between two Schwann
cells (these are wrapped around the axon). Sodium can come in and potassium can
come out along the axon… except the channels are concentrated (in one area) and
the movement of charge and change of potential happens in these nodes. As the
node depolarizes the inside of the axon becomes positive and that positivity
spreads down the axon instead of to the membrane. This will influence sodium
channels in the next node. The previous node has potassium channels which open
up and repolarise that part of the axon and just like before the sodium channels
are deactivated. This bounces down the axon. This form of firing has an increased
speed at which the signal moves through the whole cell. Hence … the myelin
sheath is like an insulator (it prevents the membrane from absorbing the charge,
thus propagating a signal faster).
Why is the myelin sheath important for healthy brain function?
There are some cells that require very fast transmission. Any cell that is
involved in cortical activity, where information has to be sent and received from
a variety of difference places over distances through the brain requires fast
transmission in order to keep information coordinated. If it the signal firing is
slow, information will be delayed and reaction to events or situations will be
slow.
This process (explained above) is what gets information from one place to
another.
We need to remember that the function of one cell is simply to take a piece of
information, something that contributes to all the information, and take it to
another place in an all or nothing fashion.
There is a way to control if a cell is to do this at any particular time.
Cells aren’t always active or there would be confusion in the brain.