What is the logic behind the simple systems approach to studying the neurobiological mechanisms of memory?

• To be able to say that synapses are modified by experience, you need to be find the synapses that support memory. 
• simple systems approach was 1 method they came up with to be able to do this
• the mammalian brain is to complicated to study because there are too many neurons to find synapses.
• believed that you need to find a less complex creature to study to fix this problem
• basically you simplify a preperation to get a handle on a problem
  

What special features of the sea slug Aplysia made it an attractive model for studying the biological basis of memory?

• it has a behavior called the gill withdrawal reflex that can be modified by experience
• it has a relativly simple brain located in its abdomen called the abdomical ganglion, which has far fewer neurons than any vertebrate brain
• the cell bodies of these neurons are very large, almost visible with no magnification
• the location of individual neurons is consistent from one animal to the next (for this reason they were able to use electrophysiology)
  

what is the gill withdrawal reflex?

A defensive behavior the animal displays when its skin is stimulated.

When the siphon is touched, the gill contracts

habituation

spontanious recovery

how can you sensitize the gill withdrawal reflex?

• sensitize: un- habituate it
• spontanious recovery: with the passing of time the behavior will return
• reactivate it with sensitization procedure:  strongly stimulate another part of the body and then re-present the stimulus

Draw the diagram of the neural circuit in Aplysia that supports habituation and sensitization of the gill withdrawal reflex

describe the anatomical organization of the hippocampus trisynaptic circuit

• neurons in the entorhinal cortex connect to a region in the hippocampus called the dentate gyrus by what is called the perforant pathway
• neurons in the dentate gyrus connect to the CA3 region by what are called Mossy fibers
• Neurons in the CA3 connect to neurons in the CA1 region by what is called Schafer collaterals
• information flows down from the entorhinal cortex to the CA1 pyramidal cells
  

what are the primary recording sites in the hippocampus used in the study of LTP?

Where did Bliss and Lomo stimulate and record?

Describe the 3 distinct phases of the basic LTP experiment. What was the experiment measuring? What did they discover from the outcome of this experiment?

In order to measure synaptic activity, in a live rabbit they implanted 2 electrodes- 1 which allowed them to stimulate the perforant pathway and the other to record synaptic activity in the extracellular space around the granule cells in the dentate gyrus

1. they applied a weak stimulus which generated very little synaptic activity- this recording is called the baseline
2. they then take the same electrode but give a much stronger stimulus
3. then you come back and test the same set of synapses with a weak stimulus

they discovered that by comparing the baseline response of the weak stimulus with the synaptic activity generated by the weak stimulus that followed the strong stimulus that you got enhanced synaptic activity that lasted a long time.

• Activating synapses changed the strength of connections between the pathway and cells
• this change in strength is LTP (long term potentiation)

describe the basic structure of a neuron

1. cell body (contains nucleus)
2. dendrites
3. axon

Dendrites

• the input of the neuron
• recieve information from other neurons or the external environment and transfer to the cell body or axons
  
• Are numerous, relatively short, and branch extensively in a tree-like fashion 
  
• have numerous spines on them, and provide a greater surface area for other neurons to synapse on, i.e., attach to
• they are postsynaptic since they recieve information

Axon

• the output of the neuron
• Transfer information to other neurons
• Are usually long (some reaching several feet)
• presynaptic since they send information to other cells

Neurons have several functions. What are they?

1. works as an input device that recieves chemical and electrical messages from other neurons
2. an integrative device that combine messages recieved from multiple inputs
3. a conductive- output device that sends information to other neurons, muscles and organs
4. a representation device that stores information about past experiences as changes in synaptic strength
• the synapse makes it into a representation device because when you modify it it gives the neuron the ability to represent information

What are the basic components of a synapse?

how is the basic way that a synapse works?

1. presynaptic terminal
2. synaptic cleft
3. post synaptic spine

when an action potential arrives in the presynaptic axon terminal, neurotransmitter molecules are released from the synaptic vessicles into the synaptic cleft where they bind to specific receptors, causing a chemical or electrical signal in the postsynaptic cell

Why is the synapse considered the basic unit of information storage in the brain?

• Because it is thought that memories are created by altering the strength of connections between the presynaptic element and the post synaptic element.
  
• It is where neurons communicate
  
• also it is where information is thought to be stored

Describe the basic steps in neuronal transmission at a chemical synapse

• transmitter is synthesized and then stored in vesicles
• an action potential invades the presynaptic terminal
• depolarization of presynaptic terminal causes the opening of the voltage gated calcium channels
• influx of calcium though channels
• calcium causes vesicles to fuse with presynaptic membrane
• the neurotransmitter is released into the synaptic cleft via exocytosis
• the transmitter binds to receptor molecules in postsynaptic membrane
• there is an opening or closing of postsynaptic channels
• the postsynaptic current causes excitatory or inhibitory postsynaptic potential that changes the excitability of the postsynaptic cell
• retrieveal of vesicular membrane from plasma membrane
  

What are neurotransmitters and how are they packaged?

• Neurotransmitters are substances released by synaptic terminals for the purpose of transmitting information from one neuron to another.

• they are packed in synaptic vessicles which are spherical membrane bound organelles in presynaptic terminals

what is the difference between the test stimulus and the inducing stimulus

These are the 2 stimuli used in an LTP experiment

1. test stimulus: relativly weak and only evokes a small fEPSP (field excitatory post synaptic potential)
2. inducing stimulus: much stronger and evokes a larger fEPSP

What are the 2 functions of the test stimulus in LTP methodology?

1. used to establish a baseline fEPSP (synaptic potential).
2. Used to determine if the inducing stimulus produced LTP

What is membrane potential?

• The difference in electrical charge inside the neuron's cell body compared to the charge outside of the cell body

1. intracellular fluid: fluid inside the neuron
2. extracellular fluid: fluid that surrounds the cell
3. they are seperated by a cell membrane
4. both fluids contain positivly charged molecules: ions
5. if the ionic composition of the fluids were exactly the same the membrane potential would = 0
6. there are more negativly charged ions in the intracellular fluid than in the extracellular fluid
7. the membrane potential in the inactive state aka resting membrane potential is negativley charged with respect to the extracellular fluid

what is the resting membrane potential and why is it negative?

• it is the membrane potential in its inactive state (when it is not generating action potentials).
• it is between -50 to -80 mV
• it is negative because naturally the intracellular fluid is more negative than the extracellular fluid

Depolarization

• the displacement of a cell's membrane potential toward more positive (less negative) value
• makes the intracellular fluid more like the extracellular fluid
• this process drives the neuron towards generating action potentials

Hyperpolarize

• the displacement of a cell's membrane potential toward more negative values-- the inside of the cell becomes more negative
• drives the neuron away from generating action potentials

what is the difference between and extracellular and intracellular recording?

In words what is the difference between an extracellular and intracellular recording?

Postsynaptic depolarization

In the context of the LTP experiment, how is synaptic strength measured?

• it is measured by how much postsynaptic depolarization is produced by the stimulus
• measured by how many positive ions flow into the neurons surrounding the extracellular recording electrode
• the extracellular recording electrode reads this as the flow of positive ions away from its tip
• thus the size of the fEPSP recorded by the extracellular recording electrode is assumed to indirectly measure the strength of the synaptic connections linking the presynaptic and postsynaptic neuron

Depending on whether the recording is intracellular or extracellular, stimulating the Schafer collaterals will produce depolarization or hyperpolarization.

Why?

• If you do an intracellular recording, it will detect depolarization because the electrode is inside the cell and positive ions are flowing into the cell making it more l positive like the charge of the extracellular fluid
• if you do an extracellular recording, it will detect hyperpolarization because the electrode is measuring the positive ions flow away from the electrode into the cell
• Extracellular: records positve ions flowing away
• Intracellular: recordings positive ions flowing in

what is the fiber volley?

• a measure of action potentials arriving at dendrites in the region of the recording site of an LTP experiment
• the action potentials generated by the electrical stimulus
• it represents the fact that the electrical stimulus applied to the fibers generated action potentials that arrived at the recording site

What is a field EPSP (fEPSP)

• a measurement of the change in the ion composition of extracellular fluid as positive ions flow away from the extracellular recording into the surrounding neurons.
• it is the dependent variable in an LTP experiment
• it is an indirect measure based on assumptions
• it is a measure of a population of synapses not individual synapses: you stick an electrode into the region you think axons are terminated
• it records as the movement of positive ions away from the extracellular electrode recording synaptic potentials indirectly
• it is recorded as a change in slope

Why is the slope of the fEPSP negative?

• because when synapses depolarize, positive ions move away from the tip of the electrode into the neuron
• the electrical potential between the extracellular fluid and ground electrode become negative leading to the negative slope

What is the reasoning behind the following statement, " the slope of the field potential measures the number of synapses that have been depolarized"

Theoretically, as more synapses contribute to depolarization, the negative potential will increase because more positive ions will flow away from the extracellular recording electrode. this will be reflected in the slope of the field potential.

the steeper the slope (the greater fEPSP), the more synaptic depolarization there is

Describe the basic methodology for studying LTP in detail

• basic methodology for studying LTP is with an invitro preperation
• dissect a very thin slice of tissue from the hippocampus and place it into a chamber that contains a bunch of chemicals that keep the slice of tissue viable for hours
• then a stimulating electrode is positioned to deliver electrical current to a chosen set of fibers and a recording electrode is placed in the region where the fibers terminate

how would you choose the value of the test stimulus?

you do some tests varying the intensity of the stimulus to find the test stimulus which should evoke the fEPSP that is about 30%-50% of the maximum field reponse

how would you chose the value of the inducing stimulus?

• it is typically above the 50% mark of maximum field response
• if a weak induction protocall is desired the stimulus might be set to evoke at least half the maximal fEPSP: in this protocol the stimulus would be presented at a high frequency for 1 second
• if a stronger protocol is desired, then the intensity of the inducing stimulus might be set to evoke the near maximal response. in this protocol the stimulus would be presented 3 times at 100 hz and the 3 presentations would be seperated by 20 seconds

How do you calculate the dependent variable in a standard LTP experiment? (DV= field EPSP or % of baseline)

fEPSP (% of baseline)=T2/T1 X 100

in order to conclude that the inducing stimulus strengthened the synaptic connections between axon fibers stimulated by the test stimulus and the postsynaptic neurons, that is that it induced LTP, this value must exceed 100%

What is the primary neurotransmitter responsible for inducing LTP?

glutamate

What is an ionotropic receptor?

• also called ion gated- receptors
• located in the plasma membrane
• they expand the membrane and respond to molecules in extracellular space
• they have channels that can be in closed or open state
• when they are in an opened by neurotransmitters binding to receptors, ions can travel into the intracellular compartment
• examples: AMPA receptors and NMDA receptors- respond mostly to glutamate

What are the similarities and differences between AMPA and NMDA receptors?

• they both bind to the neurotransmitter glutamate and conduct sodium into the cell
• AMPA receptors are the major contributor to whether or not the sending neuron will depolarize the recieving neuron-this is because the gated AMPA receptors open and allow positivly charged sodium ions to enter the cell
• NMDA receptor is ion gated and can conduct sodium into the cell and under some conditions it also allows calcium into the cell and the influx of calcium turns out to be very critical for the induction of LTP

What are the basic differences between AMPA and NMDA receptors?

• AMPA only requires the binding of glutamate to allow an influx of sodium
• NMDA receptors require 2 events in order to allow the influx of calcium and sodium (this is because it needs to get rid of the magnesium plug ontop of binding to the glutamate)
• the binding of glutamate
• depolarization of the neuron
• AMPA receptors open before NMDA receptors and provide the depolarizing event nessicary for the magnisium plug to release from the NMDA and allow for an ion influx

What are the special properties of NMDA receptors?

• magnesium plug removal and glutamate binding allows for calcium influx and calcium influx allows for the neurotransmitter to release to the next neuron

What are the requirements for opening the NMDA calcium channel?

1. the binding of glutamate
2. depolarization of the neuron (produced by AMPA)

When glutamate binds to AMPA receptors, why does the neuron depolarize?

What are the respective roles of NMDA and AMPA receptors in the initiation of LTP?

• induction: processes that result in the inital strengthening of connections
• AMPA is needed for both induction and expression. without it you cannot do either.
• without depolarization the NMDA receptor will not let in calcium its primary ion
• NMDA is only used for induction not expression

What is the difference between an agonist, competitive antagonist and a noncompetitve antagonist?

agonist: has the ability to activate a receptor in the same way as a natural compound

competitive antagonist: acts as a sheild. it competes with the natural ligand for the receptor site and binds to the receptor site

noncompetitive antagonist: binds to a different site as the natural ligand and changes the shape of the receptor site so the receptor we need can no longer bind

What is the difference between induction and expression of LTP?

Induction: the initial development of LTP (strengthening of connections)

Expression: LTP after it is established

why do researchers believe that NMDA receptors play a critical role in the induction of LTP but not in the expression of LTP?

why are AMPA receptors necessary for both the induction and expression of LTP?

• AMPA receptors allow for the opening of NMDA receptors which are responsible for the strengthening of the synapse and thus the induction of LTP
• after the induction of LTP is present, AMPA receptors are necessary for glutamate binding and depolarizing of the cell

A drug called CNQX is an AMPA receptor antagonist. if this drug is applied to the slice preperation what effect should it have on the induction of LTP? why?

What is the potential relationship between associative learning and the NMDA receptor?

• Associative learning: brains ability to capture things that are concurrently experienced
• NMDA receptor: ability for 2 separate sets of neurons participating in the acquisition of the memory
• representation of CS is initially weakly connected to the US representation
• the US strongly connected to the US representation
• if you only activate the CS alone it wont activate the neurons representing the US enough to change anything
• when you follow the CS with the US it will strongly activate the synapses and depolarize them.
• then you have the potential to strenghten the connection between the CS representation and the US representation
• at a later time you can present the CS and the pathway you created will be activated so you see the representation of the US
• you need various combinations to get changes in synaptic connections and for this you need enough synaptic input to produce synaptic depolarization and lead to the strenghtening of connections active at that time

What is the difference between a first and second messenger?

first messenger:It carries information from one neuron to another neuron. example: neurotransmitter like glutamate

Second messenger: Molecules that relay the signal received by receptors located in the plasma membrane, such as NMDA and AMPA receptors, in the cell. They do this by initiating biochemical processes within the cell. example: calcium

what is long term depression (LTD)? How is it produced?

• used to represent the case in which synaptic activity weakens the strength of synaptic connections
• its the polar opposite of LTP
• 2 ways to produce LTD

1. apply hundereds of low frequency stimulus for about 15 minutes. this reduces the fEPSP evoked by the test stimulus for at least an hour
2. apply the NMDA antagonist APV can block the induction of LTP

what were the respective contributions of Bliss, Lomo and Collingridge to the study of LTP?

Bliss and Lomo

• discovered LTP by stimulating the perforant pathway and recording the activity in the dentate gyrus.
• They first applied a weak stimulus to the perforant path and measured synaptic activity
• then they applied a strong stimulus to the perforant path. this evoked more synaptic activity than the initial weak stimulus
• they then repeadedly presented the weak stimulus and found that it now evoked a bigger response
• The strong stimulus potentiated the response to the weak stimulus and the potentiated response lasted a long time
• this enhanced response is called LTP
• this was big because it provided a way to study how synaptic strength can be modified by experience

Collingridge

• discovered NMDA receptor as the gateway to induction of LTP (processes that result in the strengthening of synaptic connections)
• By using the NMDA antagonist APV before the induction stimulus it prevented the induction of LTP because NMDA requires glutamate and synaptic depolarization for induction
• if you applied the induction stimulus first and then wait 20-30 minutes before applying APV it had no effect on the expression of LTP which proved that NMDA is not needed for the expression of LTP and something else is responsible for the change in fEPSP