11-SystemsElectrophysiology

Declarative memory

conscious

• Episodic memory: memories of specific events (remember last birthday); involves temporal (when?) and spatial (where?) encoding

• Semantic memory: general knowledge and facts (Knowing capital of country); from accumulated episodic memories that lose their temporal and spatial details over time

Non-Declarative memory

implicit, does not require consciousness

often involves learned behaviors or responses

1. Encoding - converting sensory input into form that can be stored

   e.g Read sentence and associating with prior experience

2. Storage - retention (Speicherung) of encoded information over time (temporarily or permanently)

   • Short-term storage -> sec to minutes (working memory)

   • Long-term storage -> days, months, life-time

     Consolidation: process, STS stabilized to LTS

   e.g Memorize a phone number temporarily (STS) or recalling childhood memories (LTS)

3. Retrieval - accessing stored information to use it

   e.g Recall a friend's name when seeing after long time

underwent surgery for epilepsy -> removed hippocampus

—> could not form new long-term memories, but long-term memories before surgery was intact

hippocampus critical for making short-term memory in long-term memory

1. Encoding

   new memories rapidly stored in hippocampus

   different attributes (visual, auditory, spatial) linked together

2. Retrieval before Consolidation

   memory traces not fullly transferred (-> memory retrieval relies on hippocampus)

3. Retrieval after Consolidation

   memory consolidated in neocortex (-> retrieval does not depend on hippocampus)

—> Hippocampus is intermediate memory used to train neocortex

effortless memorization of extremly long lists, tables, …

mainly via mental travel and imagery (e.g walking route in hometown)

navigation strategy -> animals compute position relative to starting point by integrating directional and distance cues

e.g Ant : use path integration to naviagte btw nest and food source

Key Mechanisms:

1. Counting Steps to estimate distance

2. Optic Flow of surroundings to track speed + direction

3. Orientation to Light for directional cues

Limitations:

• imprecise measurements

• can only store 2 locations (goal, home)

uses external reference points to form a cognitive map

animals revisit same place multiple times and share experiences to build stable, allocentric map

Path Integration: navigation relies only on internal cues (steps, optic flow,…)

-> Episodic memory (Self-referenced)

Allocentric Navigation: combines external landmarks to form reliable cognitive map, enabling robust spatial understanding

-> Semantic memory (allocentric)

= trisynaptic circuit

1. Input from entorhinal cortex via Perforant Pathway

2. Dentated Gyrus

3. CA3 via Mossy Fibers

4. CA1 via Schaffer collaterals

Stimuli to Schaffer collateral Pathway, which can vary in intensity

• 1 Train: produces early-LTP (shorter-lasting potentiation)

• 4 Trains: produces late-LTP (longer-lasting potentiation)

—> measures EPSP in CA1 region

• EPSP slope (y-axis) = strength of synaptic response

• Early LTP = increase in synaptic strenght, but decays over time -> can induce short-term synaptic changes, insufficient for longer

• Late LTP = increase in synaptic strenght, remains stable over time -> repeated stimualtion induces stronger and long-lasting synaptic plasticity -> long-term memory formation

LTP depends on NMDA-receptors (act as coincidence detectors)

for LTP to occur, must be simultaneous activation of:

• Presynaptic neuron: Releases glutamate (excitatroy neurotransmitter)

• Postsynaptic neuron: must be depolarized at same time

-> synchronization curcial for strenghtening synaptic connections

increase Ca+ levels lead to recruitment of more AMPA receptors

—> synapse more responsive to future stimulation

• Morris Water Maze: long-term spatial memory

• Radial Arm Maze: integration of reference and working memory

• T-Maze: working memory and decision-making

High-Frequency band (>500 Hz)

represents spikes from local, single or small clusters of neurons

—> study single-unit or multi-unit activity

Low-Frequency band (<300 Hz)

represents Local Field Potentials (LFPs)

measure synchronous synaptic activity from upstream projections

—> study network-level activity

• High-pass filters isolate spikes by removing lower frequencies

• Low-pass filters capture LFPs by filtering out higher frequencies

cant distinguish spikes from multiple distinct neurons

small tip size -> smaller surface area -> high impedance

—> better isolation and recording

recording in freely moving animals via long cables requires preamplification: increases strength of weak signals, compensating for losses

represents how neurons respond to different values of specific feature (e.g orientation of bar) -> peak = neurons preferred feature

in primary sensory cortex

specialized neurons that fire when animal is in specific location

• no topography

• spatial tuning curve

• no locking to stimulus

• global synchronous oscillation

  
  

by analyzing activity of place cells

1. place cells fire when in specific location in certain ‘place fields’ (spatial location) -> recordings to detect firing patterns

2. each place cells associated with unique area in env -> forming spatial map

3. researchers measure neural activity -> find out where animal is

bc electrical field generated by neuronal activity propagates differnetly through extracellular space

• electrodes near soma -> higher amplitudes

• near dendrites/soma -> attenuated or inverted spikes

specialized electrode, composed of four thin wires twisted together

by detecting signals with varying amplitudes across channels

-> precise differentiation of neuronal signals through clustering on waveform features

• precise electrode size and spacing design

• high reproducability

• much higher electrode count than metal mircorwire

• smaller electrode array size

• precise location

• lienar and 3D arrays can be built

to precisely position silicon probes for chronic neural recordings in experiments

• Cambridge Nanotech Microdrive

• NeuroNexus eDrive -

• NeuroNexus nDrive

• total Travel Distance (3-6mm)

  -> positioning of probes across desired brain areas

• minimum step Distance (100-200μm)

  -> precise -> optimal targeting of neural structures

• smooth Movement of Shuttle

  -> delicate placement and minimizes tissue damage

• Mechanical Stability

ability of neural systems to stabilize into specific activity patterns (attractors) -> represent self-organizing and self-sustaining states

Key functions:

• Pattern Completion - system restores output from partial inputs

• Pattern Separation - differntiates similar patterns -> distinct outputs for similar inputs

Head-direction cells

neurons that fire based on animal’s head orientation -> firing rates accoriding to specific direction

-> enable animal to updates its position continously as it moves through environment

animal moves head -> ‚compass‘ looks in new direction

Place cells

represent animal’s position as stable firing patterns linked to specific spatial locations