Lecture 4-6 Flashcards
What does the neurobiological approach to learning and memory aim to determine? (3)
The specific brain/CNS regions that support memory storage and recall.
How synapses and cellular activity are involved in memory in these regions are affected functionally during these activities.
The molecules/protein that mediate these changes at a synaptic level.
What is the definition of an engram?
The hypothetical physical manifestation of memory that is associated with those brain areas involved in the memory systems.
What are two alternative terms for engram?
Memory trace and cell assemblies.
Outline the details of the Aplysia californica as an animal.
Reddish Brown Mollusc
15-30cm in Length.
Lives in tide pools along the Californian coast.
Give three reasons why Aplysia has been useful in studies of learning.
It’s defensive withdrawal reflexes are alterable through experience.
The neural circuits for those reflexes are reasonably well understood.
It’s neurons are easily accessible for intracellular recording (due to being well defined).
Provide the basic structure of Aplysia as it is relevant to reflex behaviour.
Aplysia breathes through a delicate respiratory organ; the gill.
This gill is on the animals back and is covered by an overhanging ‘mantle shelf’.
The ‘parapodia’ - a wide and moveable external membrane - protects the gill by wrapping over the animals dorsal surface.
How does breathing occur in Aplysia?
Drawing water across the gill from the front (between the parapodia) and then ejecting water through the rear-facing exhalant siphon.
Why does Aplysia have the gill withdrawal reflex?
The gill is unprotected so this reflex serves to protect it in case of an animal attempting to attack it.
Define Habituation in Aplysia.
Progressive loss of gill reflex responsiveness to repeated weak tactile stimulation - gentle touch of the siphon (non-noxious).
Define Sensitisation in Aplysia.
Enhancement of gill reflex responsiveness following strong simulation - electrical shock to the tail (noxious).
What are habituation and sensitisation both examples of?
Non-associative learning; it’s responding to ONE stimulus, not associating two.
What are two example of non-associative learning?
Habituation and sensitisation.
Briefly describe how conditioning of the gill withdrawal reflex can occur in Aplysia.
Electric shock (noxious) to the tail is used as the unconditioned stimulus (US).
This produces the unconditioned response (UR) of the gill withdrawal.
A weak tactile stimulus (non-noxious) is used as the conditioned stimulus (CS) - with little gill withdrawal.
By pairing these two, the animal should learn (by association) that the CS predicts the occurrence of the US.
Thus, we should start to see the gill withdrawal (UR) in response to the CS, thus the UR becomes the CR.
Outline the study into timing of CS and US in Aplysia.
METHODS:
- Three groups of animals were tested:
1. Paired (received the CS rapidly followed by the US)
2. Unpaired (received the CS and US but with large CS-US interval)
3. US alone (received US only; used as sensitisation control).
- Aplysia received 30 training trials with a 5 minute interval.
- Then tested the CS alone to see the effect.
RESULTS:
- Post-training, the PAIRED group responded more to the CS alone than the US group of Unpaired group.
- This type of learning (associative) lasted for 4 days after a single training session (2.5 hours).
Outline the relationship between the CS-US interval and conditioning in Aplysia.
Optimal learning occurs if the CS precedes the US by 0.5 secs - forward pairing.
Learning DOES NOT occur if there is backward pairing where the US precedes the CS (CS-US interval is negative).
Summarise 5 main overview points about conditioning in Aplysia.
Conditioning can be produced after only a single training session or trial.
Better learning can be produced with repeated trials (5+).
However, learning persists for at least 24 hours even after a single trial.
Aplysia, like other animals, can learn in both non-associative and associative experimental paradigms.
These memories are long-lasting. Their duration increases as the number and spacing of trials increases (distributed learning).
Describe the basic neural architecture in Aplysia.
Aplysia has a ‘standard’ invertebrate nervous system.
CNS composed of ~20,000 (relatively few) but large neurons, many of which are identifiable in each individual Aplysia.
This is organised into a series of ganglia that communicate with each other via anatomical pathways called connectives.
What are connectives in Aplysia?
Bundles of nerve fibres that link different ganglia, allowing neural signals to travel between them.
How many major ganglia are there in Aplysia and what are their category names?
10 Major Ganglia - 5 Pairs - each containing ~2000 neurons.
From Top to Bottom:
- Buccal Ganglion
- Cerebral Ganglion
- Pleural Ganglion
- Pedal Ganglion
- Abdominal Ganglion
Which ganglion mediates the gill withdrawal reflex in Aplysia?
The Abdominal Ganglion.
What are the 3 types of neurons involved in the gill withdrawal reflex and briefly describe the circuit.
Excitatory/Inhibitory Interneurons, Sensory neurons, Motor Neurons.
40 sensory neurons are associated with the siphon.
These drive 6 motor neurons for the contraction and withdrawal of the the gill.
Both excitatory and inhibitory interneurons modulate their function.
Draw a simplified version of the habituation circuit in Aplysia
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State the basic pathway of the habituation circuit in Aplysia (only the main unmodulated line).
Siphon -> sensory nerve endings -> sensory neuron -> sensory neuron terminal -> ganglion synapse -> motor neuron -> motor neuron terminal -> neuromuscular junction -> gill.
As we know, habituation is the decrease of the size of the EPSP generated from a stimulus.
Outline the findings for each of the Aplysia reflex pathway components (4) to be involved in this process.
Siphon: sensitivity to stimulus remains unchanged.
Sensory neuron axon: Induced by activity in this pathway “homosynaptic”.
Sensory neuron terminal: AP’s still reach the terminal BUT neurotransmitter release is decreased.
Motor neuron: EPSP size decreased during induction BUT sensitivity to NT (L-glu) unchanged.
What is the neural change underlying habituation in Aplysia?
The decrease of EPSP size at the SN-MN synapses.
Smaller EPSPs are less likely to generate action potentials (APs) in the MNs.
As habituation continues, MNs eventually produce no APs even though SNs continue to fire in response to tactile stimulus.
Which point in the reflex pathway is effected in short-term habituation to decrease the size of EPSP at the SN-MN synapses? Explain.
Presynaptic SN terminal.
Similar number of APs are seen in response to the stimulus in the SNs after habituation.
Postsynaptic MNs show a similar sensitivity to exogenous neurotransmitter - no change in postsynaptic L-glutamate receptor numbers after habituation.
Thus is must be a change in the NT release as same APs and same post synaptic sensitivity.
What is the cause of depression of transmission in short-term habituation in Aplysia?
It’s due to a decrease in the number of synaptic vesicles participating in release for each AP.
The overall number of synaptic vesicles remains unchanged, they are just less likely to become docked and release neurotransmitter - L-glutamate.
SHORT TERM HABITUATION ONLY
In SHORT term habituation, what was ORIGINALLY thought to cause the decrease in vesicles taking part in release for each AP?
Thought to be caused by voltage-gated Ca2+ channels becoming progressively and persistently down-regulated (inactivated) so that less Ca2+ enters the pre-synaptic terminal.
In SHORT term habituation, what is NOW thought to cause the decrease in vesicles taking part in release for each AP?
Thought to be caused by “silencing of release” - a direct switching off of release machinery that prevents vesicle fusion.
(Molecular mechanism still unclear: GTP binding protein, Arf maintains synaptic release and is inhibited by Ca2+).
What if Arf?
ADP ribosylation factor: regulates vesicle trafficking.
Outline the potential role of Arf in SHORT term habituation.
Arf seems to be a GTP binding protein that maintains the ability of the release machinery to occur but the ability of Arf is sensitive to the calcium levels in the presynaptic terminal.
When the presynaptic terminal is repeatedly stimulated, Arf is switched off and it doesn’t maintain the ability for release machinery to be in an active state.
*This doesn’t explain how habituation persists over the following weeks as this process occurs over the seconds/minutes time frame.
Draw the sensitisation circuit in Aplysia.
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What is the additional pathway added to the habituation circuit to make it the sensitisation circuit?
Sensory neurons that innervate the tail synapse both the MN directly and a modulatory interneuron that modifies the pre-synaptic terminal of the siphon SN.
This adds the ability to adapt the habituation pathway that is needed to bring about sensitisation.
What is the neural change underlying sensitisation?
Increased EPSP size at the SN-MN synapses.
Larger EPSPs are more likely to generate action potentials (APs) in the MNs.
Which point in the reflex pathway is affected to bring about SHORT term sensitisation?
Presynaptic terminal.
Postsynaptic MNs show a similar sensitivity to exogenous neurotransmitter - no change in post-synaptic L-glutamate receptor numbers.
Thus, appears to be the presynaptic SN terminal.
What type of transmitter do the modulatory interneurons in sensitisation use?
Serotonin/5-HT
Thus they are serotonergic.
Outline the structure of the sensory neuron presynaptic membrane that allows sensitisation from serotonergic interneurons to occur.
5-HT receptors are connected via a G(s) protein to adenylyl cyclase, causing formation of cAMP to activate protein kinase A (PKA).
*Using a PKA inhibitor during sensitisation stops it from occurring.
5-HT -> 5-HT receptor -> G(s) activates -> activates adenylyl cyclase -> formation of cAMP -> activates PKA -> (1) phosphorylates K+ channels (reducing activity), (2) enhances Ca2+ channels, (3) modulates vesicle release -> increased NT release from sensory neuron -> stronger gill withdrawal.
In Aplysia sensitisation, what is the complete signalling pathway from 5-HT receptor to neurotransmitter release via the PKC pathway?
5-HT → Receptor → G(o/q) protein → PLC → PIP₂ splits into DAG + IP₃ → DAG activates PKC while IP₃ releases Ca²⁺ from ER → increased NT release
In the context of neuronal signalling, what specific molecular reaction does Phospholipase C (PLC) catalyse?
PLC cleaves phosphatidylinositol-4,5-biphosphate (PIP₂) into two second messengers: diacylglycerol (DAG) and inositol-1,4,5-triphosphate (IP₃)
How does IP₃ (inositol-1,4,5-triphosphate) cause calcium release in neurons?
IP₃ binds to IP₃ receptors on the endoplasmic reticulum membrane, causing calcium channels to open and release Ca²⁺ into the cytoplasm
What two molecular components are required for Protein Kinase C (PKC) activation in neurons?
1) Increased intracellular calcium (iCa²⁺) and 2) Diacylglycerol (DAG).
What does the effect of H7 (a PKC inhibitor) tell us about the mechanism of sensitisation in Aplysia?
The fact that H7 blocks sensitisation demonstrates that PKC activity is necessary for the sensitisation process to occur.
In neuronal signalling, what is the relationship between PKC activation and neurotransmitter release?
PKC activation, together with increased intracellular calcium, leads to enhanced neurotransmitter release from the presynaptic terminal
In the context of neurotransmitter signalling, what is the primary function of a G(o/q) protein?
G(o/q) proteins activate Phospholipase C (PLC), initiating the PKC signalling pathway that leads to increased neurotransmitter release
Explain the complete Short Term Sensitisation pathway and it’s effects in Aplysia neurons (from interneuron to increased receptor activation).
Activation of the interneuron → Release of 5HT (serotonin) → Activates G(s) and G(o/q) linked GPCRs →
- G(s) activates adenylyl cyclase → increased cAMP → activation of PKA
- G(o) activates PLC → increased DAG → activation of PKC →
- K+ channels phosphorylated (PKA) → decreased K+ conductance → broadens AP
- Ca2+ channels phosphorylated (PKA & PKC) → increased Ca2+ conductance
Increased Ca2+ entry via Ca2+ channels → increased presynaptic Ca2+ → increased NT release → increased MN activity → more/longer receptor activation.
Explain how kinase modification of the properties of ion channels augments the responses from neurons in SHORT term sensitisation.
Kinases can modify the properties of ion channels, particularly switch off K+ channels, and are critical for augmenting and maintaining Ca+ channel function.
The balance between the effect of K+ channels and Ca+ channels on presynaptic excitability is modified.
So AP’s will be broader; Ca+ channel function is enhance, while K+ channel function is reduced, so AP repolarises less quickly.
This means the depolarisation maintained last longer, more Ca+ channel activation and if their function has been upregulated, you get augmented Ca+ ion entry.
At that point, the intracellular Ca+ concentration will be higher during each AP, so likely to facilitate more vesicular fusion (or likelihood of it occurring), generating larger post-synaptic events.
Outline how the conditioning paradigm in Aplysia occurs.
A noxious electric shock to the tail is used as the unconditioned stimulus (US), producing the unconditioned response (UR) of gill withdrawal.
A weak tactile stimulus (non-noxious) to the siphon is used as the conditioned stimulus (CS) - with little gill withdrawal.
Pairing the CS with the US the animal should learn (by association) that the CS predicts the occurrence of the US.
Thus, a gill withdrawal (UR) should be elicited to the CS - the UR becomes the conditioned response (CR).
Draw a picture of the basic conditioning circuitry in Aplysia.
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Explain conditioning in Aplysia via the circuitry.
US Pathway:
* Tail sensory neuron activated by a noxious shock
* Synapses with facilitatory interneurons and motor neuron that drive the gill motor neuron → withdrawal reflex
CS Pathway:
* Siphon sensory neuron activated by gentle touch
* Also synapses with the same facilitatory interneuron/gill motor neuron circuit
Conditioning:
Pairing the CS (siphon touch) with the US (tail shock) causes the strong depolarisation from the US to enhance the CS synapses, so the siphon touch alone eventually produces the withdrawal reflex.
What is forward pairing?
CS precedes the US (i.e. the CS-US interval is positive).
Provide an analysis of each of the potential effected components in the Aplysia tail/siphon conditioning pathway.
Siphon:
Sensitivity to stimulus unchanged.
Sensory Neuron Terminal:
Same number of AP’s reach the terminal but increased NT release.
Motor Neuron:
EPSP size is increased by the sensitivity to NT (L-glu) is unchanged.
What occurs during weak-tactile stimulation to the siphon (Aplysia) in forward pairing?
(Molecular level priming).
Weak tactile stimulation to the siphon causes depolarisation of presynaptic sensory neuron in the SN-MN synapse.
This causes Ca+ influx, causing activation of calcium calmodulin (a protein able to activate and inhibit a number of signal transduction molecules).
Calcium calmodulin upregulates catalytic activity of adenylate cyclase, leading to it having a more profound biochemical action.
What occurs, once weak-tactile stimulation has occurred, during noxious shock to the tail (Aplysia) in forward pairing?
(Molecular level priming).
5-HT is released from that facilitatory serotonergic neuron, activating the GPCR and adenylyl cyclase (which has been primed by calcium calmodulin in response to CS).
This results in more cAMP to be generated than would occur in sensitisation, ultimately leading to increased NT transmitter release in subsequent pairing of the stimuli.
Over trials, more and more NT will be released so you get a bigger response to tactile stimuli.
Summarise the key point as to why the presence of CS in conditioning leads to a bigger and bigger response over trials, allowing the associative learning to take place?
CS leads to upregulated catalytic activity of adenylate cyclase, so when the noxious shock comes in and 5HT activates the GPCR and thus adenylate cyclase to produce cAMP, it leads to higher NT release.
What does the blocking of NMDA receptors with D-AP5 during conditioning paradigms result in? What does this suggest?
It can prevent the enhancement of motor neuron EPSPs.
This suggests that there is a post-synaptic component to the mechanism that supports long-term conditioning.
May also suggest that there has to be some post synaptic activation of NMDA receptors that maintains the upregulation of presynaptic function so that NT’s release is maintained after the trial that induces the condition has stopped (maintain the learned association).
What is an important property of NMDA receptors that supports their function?
What is that function?
At negative membrane potentials the open channel is blocked by Mg2+ ions meaning that there is an electrostatic attraction to the negativity inside the neuron.
If membrane potential is depolarised, this attraction decreases and the Mg2+ no longer block the channel, so cations can flow through.
They act as detectors of depolarisation and if they are involved then it shows the neurons they are on are depolarised.
How does the presynaptic mechanism function in Aplysia conditioning?
Adenylyl cyclase acts as a coincidence detector in the presynaptic neuron.
The CS (tactile stimulus) causes Ca²⁺ influx, while the US (tail shock) causes serotonin (5-HT) release.
Both Ca²⁺ and 5-HT, via its receptor, activate adenylyl cyclase synergistically, leading to a greater increase in cAMP and PKA activity than either stimulus alone.
This enhances L-glutamate release, modifying synaptic function.
How does the postsynaptic mechanism function in Aplysia conditioning?
The US depolarises the postsynaptic motor neuron.
The increased glutamate release (due to presynaptic CS/US pairing) provides the ligand for NMDA receptors.
The combination of depolarization and glutamate binding opens NMDA receptors, allowing Ca²⁺ influx.
This Ca²⁺ influx triggers a retrograde signal that travels back to the presynaptic terminal, further enhancing adenylyl cyclase activity and contributing to long-term synaptic facilitation.
This associative mechanism, similar to Hebbian LTP, strengthens the synapse through the temporal coordination of CS and US.
Outline the Presynaptic and Postsynaptic Mechanisms in Aplysia Conditioning.
Presynaptic:
The CS (tactile stimulus) triggers Ca²⁺ influx. The US (tail shock) triggers serotonin (5-HT) release. Both Ca²⁺ and 5-HT activate adenylyl cyclase, increasing cAMP and PKA, leading to enhanced glutamate release.
Postsynaptic:
The US depolarizes the postsynaptic motor neuron. Increased presynaptic glutamate binds to postsynaptic NMDA receptors. This binding, coupled with depolarization, opens NMDA receptors, allowing Ca²⁺ influx. This triggers a retrograde signal, further enhancing presynaptic adenylyl cyclase activity and contributing to long-term synaptic facilitation. This associative mechanism resembles Hebbian LTP.
Summarise Habituation within the following categories:
- Type of Learning
- Synaptic Route
- Pre-synaptic Induction Mechanism
- Post-synaptic Induction Mechanism
- Type of Learning: Non-Associative
- Synaptic Route: Homosynaptic (occurs at SN-MN synapse)
- Pre-synaptic Induction Mechanism: Ca2+ mediated silencing of release (potentially)
- Post-synaptic Induction Mechanism: No Change
Summarise Sensitisation within the following categories:
- Type of Learning
- Synaptic Route
- Pre-synaptic Induction Mechanism
- Post-synaptic Induction Mechanism
- Type of Learning: Non-Associative
- Synaptic Route: Heterosynaptic (Modulatory Serotoninergic Interneuron)
- Pre-synaptic Induction Mechanism: 5HT activation -> Increased cAMP -> activated PKA -> decreased K+ channel activation & increased Ca2+ channel activation -> Increased NT release.
- Post-synaptic Induction Mechanism: No Change
What does PKA activation do to K+ and Ca2+ Channels? What net effect does this have?
K⁺ channels:
Reduced K⁺ current prolongs the falling phase of the action potential. Normally, K⁺ channels open to repolarize the membrane quickly. By inhibiting these channels, the action potential stays depolarized for a longer period.
Ca²⁺ channels:
Increased Ca²⁺ current enhances and prolongs the action potential. More Ca²⁺ enters the presynaptic terminal during the prolonged depolarization. This increased Ca²⁺ concentration is crucial for triggering neurotransmitter release.
Synaptic vesicles:
PKA facilitates vesicle mobilization to the active zone, priming them for release.
Net effect:
Broader, longer-lasting action potential -> more Ca²⁺ influx -> increased neurotransmitter release.
Summarise Conditioning within the following categories:
- Type of Learning
- Synaptic Route
- Pre-synaptic Induction Mechanism
- Post-synaptic Induction Mechanism
- Type of Learning: Associative
- Synaptic Route: Heterosynaptic (primarily presynaptic, with facilitating interneuron involvement)
- Presynaptic Induction Mechanism: CS (Ca²⁺ influx) + US (5-HT release) -> Synergistic activation of adenylyl cyclase -> Increased cAMP -> PKA activation -> Decreased K⁺ channel activity & Increased Ca²⁺ channel activity -> Enhanced neurotransmitter release. Requires precise CS-US timing.
- Postsynaptic Induction Mechanism: Increased neurotransmitter + US-induced depolarisation -> NMDA receptor activation -> Postsynaptic Ca²⁺ influx -> Retrograde messenger -> Enhanced presynaptic mechanisms (facilitates future cAMP production).
What does the long-term duration of LTM suggest?
There is permanent change in our nervous systems to support these memories.
Why are biochemical processes likely inefficient for supporting LTM?
You wouldn’t be able to maintain the regulation of the relevant biological processes over long time periods and it could become saturated so that learning would cease.
What is massed learning?
Repeated exposure in a single learning session
What is distributed learning?
Repeated exposure over several learning sessions
Outline the study on the differences between massed and distributed learning of LTM for **habituation **in Aplysia.
METHODS:
- Tactile stimulation of 40 trials in total
- Distributed Learning: 10 trials given on 4 training days.
- Massed Learning: 40 trials given in 1 session - day 4 only)
- Retention tested after 1 (R1) and then either 7 (R2) or 21 days (R3).
RESULTS:
- Distributed learning: retained habituation after 21 days
- Massed learning: far less retention.
Shows the difference between these two.
Outline the study on the differences between massed and distributed learning of LTM for sensitisation in Aplysia.
METHODS:
- Inducing stimulus was electric shock for 1s and were provided as single shocks or trains.
- Single shocks 30-120 mins apart.
- Trains were 4 shocks 3 secs interval and 30 mins apart.
- Distributed:
- 4 trains of 4 tail shocks over 4 days.
- 1 train of 4 shocks per day.
- Massed:
- 4 trains of 4 tail shocks in a single day
- 4 single tail shocks (not a train) on one day.
RESULTS:
- Four days of training produces LTM for sensitisation, showing that distributed learning is better for memory retention.
Outline the sensitisation mimicked by 5HT application study and what it showed.
METHODS:
- Sensitisation mimicked by application of 5HT directly onto the SN-MN synapse.
- One application or Five applications spaced 1 every 22.5 minutes.
- Looked at resulting synaptic facilitation:
RESULTS:
- One application induces synaptic facilitation that lasts 15 mins (short-term).
- Five applications induces synaptic facilitation that lasts over 24 hours (long-term),
Indicates that 5-hT initiates both short and long term changes.
What are the two basic steps in which proteins are produced and what are the two types of blockers?
Produced:
- In the cell nucleus DNA is transcribed into messenger RNA (mRNA).
- In the cytoplasm, mRNA Is translated into chains of amino acids (forming the building blocks of proteins).
Blocked:
- Transcriptional (DNA to RNA) blockers (e.g., actinomycin D)
- Translational (RNA to protein) blockers (e.g., anisomycin)
What is an example of a transcriptional blocker of protein synthesis?
Actinomycin D
What is an example of a translational blocker of protein synthesis?
Anisomycin
What are the effects of translational and transcriptional blockers on long-term changes in aplysia?
They prevent long term changes induced by 5-HT receptor activation.
Hence, it suggests that DNA in SN is being targeted to support long-term sensitisation.
Summarise the role of protein synthesis in sensitisation.
Long term expression of 5-HT-induced facilitation of the SN-MN synapse and tail-shock sensitisation REQUIRE protein synthesis.
Protein synthesis inhibitors block both, but ONLY if delivered during training trials and not afterwards.
Protein synthesis inhibitors have no effect on short-term facilitation induced by repeated 5HT application (mediated by local signalling which is biochemical).
ONLY CRITICAL TO LONG-TERM EXPRESSION of these phenomena.
What did Bailey & Chen (1988) show in their serial reconstructions of control and sensitised neurons?
Reconstructed their morphology and looked at the extent of changes at their terminals.
In SN, there were an increase in the endings of axons, forming more functional connections - synaptogenesis occurring to support long term modifications.
Sensitisation increased varicosities - areas where the axon is forming a presynaptic terminals and vesicles are present.
Decreases in habituated neurons.
Summarise the results of Bailey and Chen’s sensitisation and habituation SN models for LTM in Aplysia.
Sensitised SN:
- Increase varicosities.
- Increased release sites.
Habituated SN:
- Decreased varicosities
- Decreased release sites.
What are varicosities?
They are swellings along the axon that act as distributed neurotransmitter release sites.
They allow for modulation by other neurons (e.g., serotonergic interneurons) and are crucial for synaptic plasticity and learning.
They contain vesicles with neurotransmitter and release it upon stimulation, influencing multiple postsynaptic targets.
What is the role of PKA in the formation of new synaptic proteins?
During sensitisation with tail shock or applied 5-HT, activation of 5-HT receptors lead to increased cAMP and activation of PKA.
After repeated 5-HT-induced sensitisation, activated PKA is translocated from the axon terminal to the nucleus of sensory neurons.
This suggests that PKA is the link to the genome and the control of protein synthesis.
How does PKA target the genome?
DNA contains sites called cAMP-response-elements (CRE).
CREs are found in the promoter/enhancer regions, upstream from the main transcription sites for so called “immediate early genes” (IEGs) - usually transcription factors.
When CREs are bound to by the cAMP-response element binding protein (CREB), the transcription of these IEGs is initiated.
One activation route for CRE/CREB interactions is via the phosphorylation of CREBs by PKA.
What are CRE and CREBs?
cAMP response elements
cAMP response element binding proteins
What are IEGs?
Immediate Early Genes.
How does persistent activation of PKAs lead to synapse building? What is the pathway?
Persistent activation of PKA, which gets translocated to the somatic region of neuron where DNA is held, leads to modification in gene expression that will generate proteins for synapse building.
What are the synaptic components needed for synapse building?
Ion channels, receptors, intracellular signalling proteins, cytoskeletal proteins, synaptic vesicle proteins.
What is the 5 step process of initiation of de novo protein synthesis for synapse building in sensitisation?
Draw it.
1, PKA phosphorylates CREB proteins
2. CREB-1 binds with CRE.
3. IEG transcription activated (mRNA)
4. IEG-translated protein activates LRG
5. LRGs transcription activated and mRNAs translated to proteins.
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What two pieces of evidence support the role of CREB-1 in sensitisation?
Inhibiting CREB-1 prevents long-term sensitisation.
Injecting phosphorylated CREB-1 induces facilitation of motor neuron EPSP.
What did Bedecarrats et al. (2018) shows about RNA from sensitisation trained Aplysia?
RNA from trained Aplysia injected into naive (non-sensitised) recipients (in vivo) or applied to cultured SN (in vitro) had the following effect:
- Sensitised siphon withdrawal reflex after RNA injection from trained but not control donors (24 hours).
- More action potential firing in sensory but not motor neurons after RNA applied from trained but not control RNA donors (24 hours).
- This was prevented by blocking DNA methylation in recipient with RG-108, showing downstream epigenetic changes in genome.
What does Patient HM not being able to retain declarative information for more than 8 seconds suggest?
Suggests that his problem was being unable to encode the information from STM to LTM.
What does the delayed non-matching-to-sample task reveal about the difference between familiarity and episodic memory?
Animals are trained in a non-matching-to-sample task where they first see a sample object with food beneath it, then - after a delay - must choose a new object rather than the sample to receive a reward.
Over hundreds of trials using the same sample object, even animals with significant hippocampal lesions learn to choose the new object reliably.
This suggests that their improved performance is due to developing a sense of familiarity with the sample, rather than recalling a specific episodic memory of seeing it.
Thus, it highlights an important distinction: familiarity (recognition based on previous exposure) can operate independently of episodic memory (detailed recollection of past events).
What effect does lesions of the amygdala have on declarative/explicit memory?
No effect.
How do lesions in perirhinal, entorhinal, and parahippocampal regions affect different types of memory?
Perirhinal Cortex: Critical for recognising and discriminating objects (familiarity).
Entorhinal Cortex: Important for linking objects to places (object-place memory).
Parahippocampal Cortex: Lesions here, combined with entorhinal damage, worsen memory deficits.
Hippocampus: Primarily supports episodic (experience-based) memory; not essential for short-term object recognition.
Why is testing episodic research in animals hard?
You cannot ask them what they know.
Outline the smell-based DNMTS task.
METHODS:
- Plastic cups filled with sand scented with 1 of 9 common odours (e.g., cinnamon, coffee)
- On each trial, the cup was placed randomly at one of 9 fixed locations.
- Food reward was placed in the sand if the odour did NOT match that on the previous trial.
- Firing of hippocampal neurons was recorded as the rats approached the cup on each trial.
RESULTS:
- 65% of cells fired in association with one or more task variable.
- One third of these had spatial, one third non-spatial firing correlations.
- Others fired at a particular odour or match/mistmatch condition.
- Minority of cells fired to a particular cup position.
- Most cells fired to a combination of cup position, odour, match/mismatch and start of approach to the cup.
Explain Eichenbaum’s Memory Space Hypothesis.
The Memory Space Hypothesis suggests that the hippocampus creates a multidimensional “memory space” by integrating various aspects of an experience - such as spatial locations, timing, and contextual details - into a unified cognitive map. This process involves:
- Encoding Dimensions: Hippocampal neurons capture where events occur (spatial information), when they occur (temporal information) and the surrounding context.
- Integration: These separate streams of data are bound together, forming a cohesive representation of an experience.
- Flexible Retrieval: The integrated memory space allows for the flexible recall of complete episodes, even when only partial cues are available, by navigating through this cognitive map.
In essence, this hypothesis explains how the hippocampus supports the encoding and retrieval of episodic memories by linking diverse elements of an experience.
Why are smells more strongly linked to memories compared to other senses?
The olfactory bulb has synapses to hippocampus whereas other senses have to go through much more processing before it reaches the hippocampus.
Why do lesioned animals still perform above zero in spatial tasks, such as the cued version of the 8-arm radial maze?
Lesioned animals rely on non-declarative or cue-based strategies rather than recall.
Since they focus on external cues and do not attempt to remember the food’s location through hippocampal-dependent spatial memory, their performance remains above chance despite the lesion.
How large are place fields and in what direction?
They are approximately 30cm wide in open space (for rats and mice) and fire omnidirectionally.
However, they tend to become directional in linear environments (e.g., corridors).
Where are grid cells located in the medial entorhinal cortex (mEC)?
Grid cells are found in all layers of the mEC, from Layer II through Layer VI.
Which layer of the mEC contains the highest concentration of grid cells, and what is the main cell type there?
Layer II has the largest concentration of grid cells, which are primarily stellate cells.
Do grid cells only form a 2D hexagonal lattice, or can they also map space in 3D?
While grid cells are best known for their 2D hexagonal firing patterns, studies suggest they also map*3D space, though the 3D lattice pattern is less clearly defined.
How do grid cells vary across different layers of the mEC?
- Layer II: Mainly stellate cells (highest grid cell density).
- Other Layers (III–VI): A mix of stellate and pyramidal cells, each contributing to the overall spatial representation in different ways.
Where are head direction cells found?
In all layers III, V and VI of mEC (and presubiculum).
What are conjunctive cells and where are they found?
They are Grid + Head Direction tuned cells. They fire at particular locations when travelling in a particular location.
They are likely key for path integration.
They are found in layers III, V and VI of mEC.
Outline the Kraus et al. (2015) study into grid cell encoding.
METHODS:
- Rats were put on treadmills with varied time and speed of treadmill.
- As the speed and duration are manipulated, distance can be measured.
- Grid cell activity was recorded.
RESULTS:
- Some cells are tuned for distance ONLY.
- Some cells are tuned to the elapsed time on the treadmill.
Fits into the idea of a path integration system within the EC.
What is the difference between classical and instrumental conditioning? What type of learning are they?
Both are examples of associative learning (procedural).
Classical conditioning the animal has NO control over the stimuli and learn the consequences of an external event.
Instrumental (operant) conditioning the animals have control over the events and learn the consequences of their behaviour instead.
What is another term for instrumental conditioning?
Operant.
What did Skinner suggest operant conditioning was - given that it wasn’t stimulus-response?
He argued that the basic association in operant conditioning was between the operant response and the reinforcer; a discriminative stimulus (e.g., light turning on) just signalled when the association should be acted upon.
Detail the differences between negative reinforcement and punishment in operant conditioning.
Negative Reinforcement:
- Perform operate to avoid footshock (operant freq increases).
Punishment:
- DO NOT perform the operant, as operant triggers footshock (operant freq decreases).
Draw a flow chart of the synapses within the rodent hippocampus.
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How does mEC grid field pattern emerge?
Integration of velocity input by continuous attractor networks built from interacting excitatory (E) and inhibitory (I) neurons can generate grid firing fields.
The tessellated triangular structure can be modelled by appropriate E-I network activity - but we have no direct evidence for this from experimental intervention.
What are the three main synaptic connections in the rodent hippocampus and which cells do they originate from?
The first synapse is the perforant path, where entorhinal cortex layer II neurons project to the dentate gyrus granule cells.
The second is the mossy fibre pathway in which dentate gyrus granule cells project to CA3 pyramidal cells.
The third is the Schaffer collateral pathway, where CA3 pyramidal cells project to CA1 pyramidal cells.
What are internal representations in the brain?
Internal representations are pieces of information that represent external stimuli or internal motor actions as internal or external events to the brain.
How are internal representations in the brain organised?
They are organised hierarchically, such as retina to LGN to V1, where each level becomes more efficient and conceptual by eliminating redundant information.
What happens to information as it moves through the hierarchical levels of internal representation?
As information moves through the levels, it becomes more efficient and conceptual, with redundant information eliminated, eventually entering consciousness.
How do grid fields change with exposure to 3D structures or movement?
Grid fields become sparser, larger, more variably sized and irregularly arranged with exposure to 3D structures or movement, even when considering fields closest to the lower surface.
What was observed about grid fields in rats when exposed to 3D environments?
In rats, grid fields in 3D environments were larger and more irregularly arranged compared to 2D environments, indicating adaptation to 3D space.
How do bat grid cells adapt to 3D environments compared to 2D environments?
In 3D environments, bat grid cells show local regular spacing but lack a typical 2D grid lattice.
They also include 3D head direction and border cells, indicating adaptation to 3D space.
