Exam 3 Flashcards
Lashley’s experiment
Lesioned different parts of the brain:
-learning/remembering maze was not impaired by local cortical legion anywhere
BUT: larger the lesion, greater the impairment (engram is everywhere)
Kandel’s initial research
made first intracellular recordings of APs from hippocampal neurons (in cats)
ended up working with aplysia later on with Tauc
characteristics of aplysia
- abdominal ganglion has only about 300 neurons
- neuron cell bodies are big
- identical organization of nervous system in all aplysia
Kandal and Tauc’s initial experiment: what did they do? initial conclusions?
intracellular recordings from neurons
found ones that got EPSP when they stimulated nerve 1, but not when stimulated nerve 2
conclusions: a cell w/ an axon in nerve 1 makes excitatory synapse on test nuron; no neurons w/ an axon in nerve 2 directly synapse onto test neuron
Kandal and Tauc’s major experiment: what did they do?
stimulate nerve 2 repetitively for a few seconds
test to see if amplitude of response to stimulating nerve 1 was larger or smaller (was larger)
homosynaptic facilitation and depression
repetitive stimulation of anueron leads to either transient increase (facilitation) or decrease (depression) in response amplitude
hetersynaptic facilitation
repetitive stimulation of one neuron changes the response to stimulation of another neuron
simple behaviors mediated by the aplysia abdominal ganglion
gill and siphon: respiratory organis
gentle tap to siphon causes gill to withdraw
repetitive taps causes less and less response (habituation)
sensitization in aplysia
behavioral event analogous to heterosynaptic facilitation
gentle tap to siphon causes gill to withdraw
give electrical shock to tail
responses to sphon tap is larger, more rapid, longer lasting gill withdrawal
cells involved in gill withdrawal
sensory neurons respond to siphon tap
motor neurons cause gill to withdraw
tail sensory neurons contact interneuron 5-HT
where is the locus of non-associative learning
habituation occurs at sensory motor synapses
how does habituation and facilitation work on molecular level?
habituation: decrease in release probability of NT release
sensitization: increase in release probability
short term and long term sensitization
single shock produces short term sensitization only
multiple shocks gives changes that are larger and persist for days
what do interneurons in aplysia release and what does it do
L29 facilitator interneuron releases serotonin
adding what 3 molecules can cause synaptic facilitation
serotonin (5-HT), cAMP, PKA
how does PKA increase transmitter release
PKA phosphorylates voltage independent “leak” K channel (gKs) and shuts it off, slowing spike repolarization
calcium channels stay open longer so more calcium enters
more transmitter is release, thus sensitization
whats the easiest way to study long term sensitization
motor neuron and one or more sensory neurons in a cell culture
directly apply serotonin with a pipette
long term sensitization requires what
new protein synthesis
protein synthesis inhibitor Anisomycin prevents long term sensitization to serotonin
Kelsey Martin’s long term sensitization experiment
single sensory neuron innervates 2 motor neurons
apply 5 pulses of 5-HT only near MN2
only SN-MN2 synapses facilitated (something beyond transcription/translation required for sensitization)
2 models to explain synapse specificity observed in Martin’s experiment
synpases that see 5x serotonin are marked:
1) RNA/protein synthesis are near nucleus, but the new protein is only incorportated into synapses that were marked by 5HT
2) RNA synthesis is in the nucleus, but protein synthesis occurs in marked-presynaptic terminals, not in cell body (CORRECT)
experiment by Martin to determine why synapse specificity occurs
put photoswitchable tag on newly translated proteins
initally glows green, turns red w/ UV light (all pre-existing protein glows red)
look for where the new green proteins are
RESULT: only branches that saw 5-HT make new protein, even though mRNA is in all branches
how did they find the first learning mutant drosophila (5 steps)
forward genetics
- feed EMS to male flies (random mutations)
- cross to WT females
- outcross offspring separately (offspring each carry different mutations)
- make inbred lines of new mutations
- test for any phenotype of interest
how did they measure defect in learning in drosophila mutants
- before odorant-shock pairing, flies have no preference for odor A or B
- associate odor A with electric shock
- WT flies choose odor A
fruit flies odor learning: performance index
-performance index = (flies avoiding odor A - flies avoiding odor B)/total flies
index = 1: learning; index = 0: no learning
what is the first learning mutant fly called, what does it do
- dunce*:
- do not learn well, forget rapidly (index closer to 0)
- dunce gene encodes phosphodiesterase enzyme that hydrolyzes cAMP
after dunce, what learning mutant was discovered
- rutabega:* encodes an enzymes that catalyzes cAMP synthesis
- decrease in performance index
how does the fly brain process odor cues
sensilla: have sensory neuron dendrites in them
odor molecules bind odor receptors on the dendrites of olfactory sensory neurons (OSNs)
axel and buck
nobel prize for discovering odor receptors and the organization of the olfactory system
in rats
axel and buck assumptions regarding olfactory receptor genes that led to discovery:
1) odor receptors should resemble rhodopsin receptors in eye
2) ORs belong to large family of related proteins
3) must be expressed only in rat’s olfactory epithelium
mammalian vs insect olfactory receptors (ORs)
mammalian ORs are GPCRs
insect ORs are ligand gated ion channels
both have 7 transmembrane domains
how is odor information relayed to the brain: first destination in brain
OSN axons go to antennal lobes in brain
each antennal lobe made up of 50 distinct glomeruli
olfactory sensory neurons (OSNs) of the same type….
(express same odor receptor) project their axons to the same glomerulus in the antennal lobe
OSNs connect w/ olfactory projection neurons in antennal lobe
each olfactory projection neuron
carries information from a single odor receptor
overview: odor to behavior steps
odor → olfactory sensory neuron → olfactory projection neuron → kenyon cells in mushroom body → mushroom body output neurons → learned behaviors
mushroom body
learning and memory center of drosophila
projection neuron axons terminate in the
mushroom body and the lateral horn
connections between projection neurons and kenyon cell
each kenyon cell gets input from 3-10 projection neurons
kenyon cells combine information about multiple olfactory cues
how can projection neuron inputs to kenyon cells be mapped out
1) label a single kenyon cell (using photoactivable-GFP)
2) label the presynaptic partners of that kenyon cell
3) identify what projection neuron type is labeled
5) repeat this for all the KC dendrites
5) repeat this for all KCs
___ is the key to plasticity in the mushroom body
dopmaine
kenyon cell to behavior?
kenyon cells transmit odor information to mushroom body output neuron (MBON), which transmits this information to other brain areas
dopaminergic neuron (DAN) also transmits information about the context/experience to MBON (DA only released during learning)
facilitation, augmentation, potentiation, long term potentiation
facilitation: very short lasting form of enhancement (few ms)
augmentation: enhancement lasting a few seconds
potentiation: enhancement lasting longer than a few seconds
LTP: lasting at least half an hour
basic hippocampal circuitry: 3 excitatory connections
synapse 1: perforant path (from entorhinal cortex) to granule cells of dentate gyrus
synapse 2: granule cell axons (mossy fibers) to CA3 cells
synapse 3: CA3 cell axons (Schaffer collaterals) to CA1 cells
different ways of studying hippocampus
intact animals
hippocampal slice preparations
hippocampus and dissociated cell culture
hippocampal neurons survive well in dissociated cultures:
- need secreted materials from astrocytes
- during dissociation, dendrites/axon torn off, but eventually grow back in culture
- allows for good access to synapses
problem of studying hippocampal neurons in culture
don’t know where hippocampal neurons originated from
mechanism of LTP is not the same at all synpases of the hippocampus
Neher and Sakmann
discovered how to record electrical activity of single channels in isolation with a patch clamp
invented whole cell recordings
patch clamp recording method used to study hippocampus:
cell attached recording
glass pipette mounted on micromanipulator, bring in contact w/ plasma membrane of a cell, form tight seal, feedback amplifier controls potential across membrane
patch clamp recording method used to study hippocampus:
whole cell recording
can apply pulse to destroy membrane at the tip but leave seal intact and do whole cell recording
samples activity of all channels, not single channels
patch clamp recording method used to study hippocampus:
excised patch recording
pull pipette off cell w/ seal intact
can be outside out or inside out
matters bc changing solution is easier on the face exposed to the outside
measuring synaptic responses in hippocampus: stimulus is usually ___ and you control ______
extracellular (gives info about large # of synapses)
control amplitude and stimulus frequency (as amplitude goes up, recruit more and more axons, target cell response increases)
whole cell recording: downward deflections means what
voltage clamp cell to cause inward current, depolarization, EPSP
paired whole cell recordings
initiate AP in one cell, record from nearby cell, see if there is EPSP (rapid inward current)
Bliss and Lomo
discovered LTP in rabbits
Bliss and Lomo’s experiment
extracellular stimulating electrode onto perforant path, extracellar recording electrode onto dentate gyrus
brief high frequency stimulus causes increased amplitude of EPSP, eventually LTP lasted few hours
differences in LTP in 3 major excitatory synapses in hippocampus
synapse 1 (entorhinal cortex to dentate gyrus) and synapse 3 (CA3 to CA1): use classic mechanism
synapse 2 uses alternative mechanism (granule neurons to CA3)
what is hippocampal excitator NT
glutamate
properties of classical LTP in CA1 and dentate gyrus
cooperative: need minimum number of activated synapses to get LTP
synapse specific: inactive synapse is not potentiated
associative: LTP occurs when activity in weak input is paired w/ activity in strong one
manipulating postsynaptic membrane potential: results
- pairing weak presynaptic stimulus w/ properly timed postsynaptic AP (elicited by passing current) is sufficient to produce LTP
- strong presynaptic stimulus won’t produce LTP if postsynaptic cell is hyperpolarized with current
voltage clamp of AChR responses
change voltage, measure current
slope of IV relation is linear
ACh increases conductance, I follows conductance (g) bc driving force is constant
conductance of cys-loop receptor (ACh receptor) is voltage independent
voltage clamp of glutamate responses
voltage clamp (vary postsynaptic current) postsynaptic, stimulate presynaptic
slove of IV curve is J shaped
response of synapse using glutamate receptors is voltage dependent
4 classes of glutamate receptors
ionotropic: AMPA, Kainate, NMDA
metabotropic: mGluR
expression cloning approach: steps
- Demonstrate that Xenopus oocytes don’t respond to glutamate
- Demonstrate that injecting polyA+ RNA for glutamate receptors results in responses to glutamate in oocytes
- Make cDNA library of clones
- Find a single responsive clone
- Sequence DNA and study gene receptor
CNQX
antagonist of AMPA and kainate receptors
kainate agonist for what receptors
activate kainate receptors at low concentrations
activate AMPA receptors at high concentrations
NMDA agonist
needs 2 agonists: glutamate + glycine
or NMDA + glycine
ionotropic glutamate receptor structure
- 4 subunits (can be homo or heterotetramers)
- each unit has 3 transmembrane helices
- 4 agonist binding sites
- LBD: binds agonists, works like pacman
- ATD: binds multiple modulators
NMDA receptors are inhibited by
APV
activated NMDA receptors
produce prolonged responses to brief application of glutamate
have J shaped IV relation
highly calcium permeable
AMPA receptors produce ____ and are inhibited by ____
-brief responses
inhibted by CNQX
AMPA: IV curve and calcium
in majority of CNS neurons
linear IV curve
calcium impermeable
what does outward rectification mean
there is more outward current than inward current
inhibitory interneuron AMPA receptors
inwardly rectifying (unlike non-rectifying AMPA receptors)
highly calcium permeable
CA1 neurons express:
both NMDA and AMPA recptors
AMPA receptors: inward rectification in those that rectify, but most don’t rectify
NMDA receptors: outward rectification
different types of AMPA receptor genes
GluA1, GluA2, GluA3, GluA4
homotetramers: A1, A2, A4 large current w/ strong inward rectification; A2 doesn’t conduct current
heterotetramers: 2+1, 2+3, 2+4 gives large glutamate activated current; all combos of 1,3,4 give large inward rectifying currents
A2 alone doesn’t allow rectification
in all AMPA receptor genes, what is identical and what do ion substitution experiments show
in M2 regions, AA sequence is identical except Q/R site
mutate R to Q in GluA2, mutate Q to R in GluA3
→ QR site is necessary position to control rectification
→ linear receptors impermeable to Ca, inward rectifying receptors, Ca permeable
what is outward rectification in NMDA receptors (J shaped IV curve) caused by
extracellular Mg2+ getting stuck in the pore
what causes inward rectification in AMPA receptors
intracellular polyamines (such as spermine) cause inward rectification only in AMPA-R with Q in all four subunits
get stuck in pore
R electrostatically repels polyamines, prevents them from getting into pore
what happens to AMPA and NMDA receptors when glutamate concentration is low or high
low: few AMPA and NMDA receptors open gates
- current flows through AMPAR receptors, cell slightly depolarized
- in NMDAR, pore opens, Mg tries to enter cell, but gets stuck (no Ca can enter)
high: many gates open
- large currents flow through AMPAR
- at this more positive potential, Mg doesn’t try to enter cell via NMDAR (Na or Ca can enter, K can exit)
traditional method of inducing LTP is to…
give high frequency stimulation to the presynaptic neurons
APV experiment
APV (antagonist of NMDA) blocks induction but not maintanence of LTP in CA1
in CA1, induction of LTP is postsynaptic
role of calcium in LTP
LTP requires a rise in postsynaptic intracellular calcium
evidence that elevated postsynaptic calcium is necessary and sufficient for LTP
- preventing rise of intracellular calcium postsynaptically prevents LTP (Ca is necessary)
- elevating postsynaptic calcium w/o presynaptic activity causes LTP (Ca is sufficient): caged calcium introduced to cells, UV pulse uncages it
cooperativity explained using properties of NMDA receptors
small stimulus only opens calcium impermeable AMPA receptors
large stimulus activates sufficient AMPA receptors to depolarize the cell enough to relieve Mg block from NMDAR, allowing large calcium influx that triggers LTP
synapse specificity explained using properties of NMDA receptors
at inactive synapses, there is no glutamate, so even a large depolarization produces no calcium entry through NMDAR at these specific synapses
associativity explained using properties of NMDA receptors
when weak stimulus paired with strong, cooperative mechanism (NMDAR w/o Mg in pore) is activated at both weak and strong synapses
what is the target of postsynaptic calcium
CaMKII protein highly expressed in CA1 neurons
CaMKII inactive when [Cain] is low
when [Cain] rises, calcium binds to calmodulin (CAM), complex activates CaMKII
when [Cain] is sustained, CaMKII autophosphorylates and stays active
evidence for role of CaMKII in LTP
- selective inhibitors of CaMKII prevents LTP induction (CaMKII is necessary for LTP)
- T286A mutation prevents autophosphorylation and prevents LTP
- injecting active CaMKII postsynaptically produces LTP (CaMKII is sufficient)
if previous LTP has occured and you add CaMKII, what happens?
previous LTP occludes the effect of CaMKII (very little addition of LTP)
potential mechanisms for expression of LTP: presynaptic changes
1) increase release probability
2) increase number of release sites
3) increase number of vesicles
potential mechanisms for expression of LTP: postsynaptic changes
1) increase receptor sensitivity
2) increase number of functional receptors
3) add more synapses
why did Stevens and Tsien think LTP expression locus was presynaptic
whole cell recordings, found that the number of responses that are failures goes down greatly after LTP: matches presynaptic theory
what causes decrease in the number of failures after LTP
- *silent synapse hypothesis:**
- failures are due to inability to respond to glutamate, not due to failure to release NT
-LTP adds AMPA receptors onto dendritic spines that initially only have NMDA receptors
according to silent synapse hypothesis, what should happen when postsynaptic potential is shifted from -60 to +30
at -60mV, NMDA receptors are blocked by Mg
at +30mV, postsynaptic neuron will show synaptic potentials bc Mg block is relieved
according to silent synapse hypothesis, what happens when shift postsynaptic potential from -60 to +50 mV
the number of failures before and after LTP should be similar
this is observed, so the synapses that show LTP are not NMDAR silent
final prediction of the silent synapse hypothesis
if LTP expression is postsynaptic and based on number of AMPA receptors, there should be no change in amplitude of NMDA component of synaptic responses after LTP
conclusion: there is no increase in presynaptic glutamate release by LTP
how can we watch new AMPA receptors appear
label extraceullar surface of receptor (N terminal) w/ synaptophlourin
synaptophlourin: isn’t fluorescent at acidic vesicle pH (filled w/ AMPAR), fluoresces when vesicles introduced to spine membrane (non-acidic)
conclusion about mechanism of induction of LTP in CA1 pyramidal neurons
induction is postsynaptic: 2 things needed:
- sufficient AMPA receptor activation to depolarize cell enough to allow Mg to leave NMDA receptors
- Calcium influx through NMDA receptors, to bind to calmodulin and the complex activate CaMKII
conclusion about mechanism of initial expression of LTP in CA1 pyramidal neurons
active CaMKII leads to:
- enhanced delivery of AMPA receptors to cell surface (transform silent synapses to functional ones)
- enhancement of responsiveness of individual AMPA receptors
morris water maze (hidden platform test)
training: opaque liquid w/ invisible platform at fixed location; pictures on wall of room so animal can orient themselves
success in this task depends on intact hippocampus
morris water maze: testing:
acquisition, retention, extinction
acquisition: how long does it take until the animal climbs onto the hidden platform (normal mice learn quickly)
retention: if well trained animal gets a break of a few hours/days, how well does it perform the task
extinction: if platform moved, how long does it take to stop looking at the old location
contextual fear conditioning
if given an auditory, visual, or olfactory cue, rodents remember the location where they received a mild shock and act different there (freeze more)
this behavior requires intact hippocampus and amygdala
does LTP occur when an animal is learning? experiment to test this
- implant electrodes into CA1 of mice
- record activity to single stimuli before and after passive avoidance traning
- some synapses show LTP as mice learn
manipulations that impair classical LTP on morris water maze
- NMDA antagonist prevents learning
- NMDA receptor KO alters learning
- CaMKII mutation that prevents autophosphorylation alters learning
manipulation that increases classical LTP, improves learning on multiple tasks
overexpression of one of NMDAR subunits (GluN2) enhances learning
created transgenic mice called Doogie mice
how do we know that LTP is different at the mossy fiber synapse onto CA3 neurons
NMDA antagonists don’t inhibit LTP
postsynaptic calcium chelators don’t inhibit LTP
some synapses on CA3 neurons…
express classic NMDA receptor dependent LTP (at associational-commissural synapses on CA3 neurons)
different mechanism at mossy fiber synapses
how is LTP different at the mossy fiber synapse onto CA3 neurons
presynaptic locus for induction and expression
mossy fiber LTP produces an increase in glutamate release
experiment that indicates that glutmate release is increased in mossy fiber LTP
- express receptors at high density in some cell
- form an outside out patch with many receptors
- place patch close to the expected source of transmitter release
result show more glutamate after LTP inducing stimulus
experiment to determine why more glutamate is released in mossy fiber LTP
- introduce calcium chelator into presynaptic neuron
- BAPTA: fast calcium chelator, abolishes all NT release
- EGTA: slow calcium chelator, permits nearly normal amount of release
- Find that baseline synaptic transmission is unaffected, but LTP not produced
- presynaptic Ca required to activate mossy fiber LTP
what does and does not prevent LTP at mossy fibers
- inhibitor of CaMKII doesn’t prevent LTP
- inhibitors of PKA prevents LTP
common pathway that activates PKA
- calcium activates adenylyl cyclase
- adenylyl cyclase makes cAMP
- cAMP activates PKA
target of active PKA
RIM1α and Rab3a are known targets
both proteins play role in mobilizing vesicles
KOs of both are deficient in LTP at mossy fibers
example of chemogenetics with fruit flies
fruit flies have no receptors for ATP, but mammals do
- P2X receptors are non-selective cation channels that excite cells, respond to ATP
- Make UAS-P2X2 transgenic flies, cross w/ line that has promoter that drives GAL4
- only cells w/ promoter of interest will be activated when ATP is applied
chemogenetics example with DREADDS (designer receptors exclusively activated by designer drugs)
- making minor mutations to muscaranic ACh receptors (metabotropic) can make them sensitive to a drug
- excitatory DREADD: modified human M3 receptor (hM3Di)
- inhibitory DREADD: modified human M4 receptor (hM4Di)
- transgenic animals w/ floxed allele crossed w/ cre line to express designer receptor
- feed drug to animal to chronically excite or inhibit neurons of interest
- first agonist was CNO (can be metabolized to clozapine, acts on serotonin and DA receptors
birth of optogenetics: flies
a lab introduced drosophila rhodopsin and its downstream molecules into mammalian neurons to trigger activity
green algae has two opsins that are ion channels:
experiment
ChR1: proton selective
ChR2: channel rhodopsin that is nonselective cation channel (jackpot)
express fusion protein of ChR2-EYFP (fluorescent protein) in hippocampal neurons in culture, goes to plasma membrane of neurons
can excite ChR2 w/ blue light, which depolarizes cells
what maintains LTP? CaMKII hypothesis + experiment
ongoing expression of high CaMKII activity in the absence of elevated calcium (self-sustaining autophosphorylation)
add caged glutamate, shine UV light to release glutamate, measure CaMKII activity
CaMKII activation at first, but returns to baseline soon after
what is the actual way that learning is maintaied for weeks+?
CA1 long term LTP requires new mRNA and protein synthesis (aplysia long term sensitization requires this)
-new proteins cause permanent changes in synaptic structure: spines grow larger quickly w/ LTP
homeostatic scaling
global upward or downward adjustments across all synapses (strengthen or weaken)
long term depression
synapse specific decrease in synaptic strength
how does homeostatic scaling work
- neurons have set point for spike activity
- if spiking deviates significantly for a sustained time period, global changes in synaptic strength occur
long term depression experiment
at slow enough stimulus frequency, synapses show long term depression
all synapses that are capable of LTP are capable of LTD
metaplasticity
the stimulus frequency at which there is no long-term change varies w/ previous history of the synapses
what is cellular mechanism of induction of LTD at schaffer collateral-CA1 synapses
similar to LTP:
- requires NMDA postsynaptic receptor activation
- requires rise in postsynaptic intracellular calcium, but not enough to effectively activate CaMKII
low calcium activates calcium dependent phosphatases (causes fewer AMPA receptors)
potential paradox of how LTP works
after large calcium influx that produces LTP, there is a period of time that calcium remains elevated at a level that would produce LTD
LTP isn’t cancelled out though because high intracellular calcium that triggers LTP also activates pathway that prevents LTD
O’Keefe, Moser, and Moser
made recordings from hippocampus and entorhinal cortex of free moving rodents
Mosers discovered entorhinal cortex grid cells
O’keefe discovered hippocampal place neurons
Tonegawa experiment overview
developed method to erase memories or create false memories in mice:
- introduce mice to two boxes w/ different attributes, figure out which cells respond to Box A but not Box B (potential engram cells)
- Shock in Box B while activating Box A engram cells
- if animal freezes in Box A and B, not C, have created false memory
engram
physical location or set of locations at which a memory is stored
marking and controlling engram cells: trangenic mice portion
- intersectional genetics: transgenic and viral expression approach, using tet-off strategy
- recently active neurons make c-fos mRNA
- make transgenic mice that have c-fos promoter driving tTA (only make tTA if neurons recently active)
- if DOX present, tTA can’t bind to TRE and expression of target gene is off
- if DOX taken away, tTA drives downstream target gene
making and controlling engram cells: virus expression portion
- AAV (virus) infects all neurons, replace most of viral genes w/ sequence of interest
- trangenic mice receive virus w/ TRE driving fusion of ChR2 (channelrhodopsin) and mCherry sequences
- ChR2 causes depolarization when excited by blue light
- mCherry fluoresces red to green light
- sequences that were inserted in the virus are only expressed when tTA is present and DOX is absent
experimental design of creating false memories
- feed transgenic cfos-tTA animals DOX
- inject virus into dentate gyrus, implant optic fibers to deliver blue light
- remove DOX when ready to test behavior
pupil + lens + macula definiton
pupil limits amount of light that gets to retina
lens focuses light onto retina
macula part of retina that is specialized for fine form vision (includes fovea_
experimental conclusion of photon experiment
abosroption of a single photon can lead to visual sensation
range fractionation
multiple receptors respond to different amplitude levels, eg rods highly sensitive, cones less sensitive
Hartline
used limulus eye to characterize mechanisms of visual encoding and contrast detection (lateral inhibition)
Wald
characterized the biochemistry of light absorption
limulus horseshoe crab eye structure
each facet (ommatidium) has a single large neuron (eccentric cell) with an axon projecting to the CNS
eccentric cell depolarizes and fires AP in response to light
rate coding of light intensity in the limulus eye
shine light on one ommatidium and record from its axon extraceullarly:
the more intense the light, the higher the spike frequency, but takes 100x increase in intensity to get 2x increase in firing rate
Weber-Fechner law
one’s ability to detect a change in stimulus is related to a constant fraction of the stimulus, not the absolute amount of change
relationship b/w actual intensity and perceived intensity is logarithmic (larger stimuli require bigger changes to detect difference)
relationship b/w amplitude of stimulus and AP frequency is:
logarithmic
adaptation
the range a cell responds to changes
change sensitivity of receptors
lateral inhibition in limulus
shining light on one ommatidium, then shine on nearby ones (frequency goes down)
lateral inhibition is reciprocal (every ommatidium inhibits its neighbors), and graded (more intense surround stimulus, stronger the inhibition), and logarithmic
eccentric cell center surround organization
on center, off surround
what is the point of lateral inhibition
enhances edges/contrast
photoreceptors in arthropods vs vertebrates
arthropods: photoreceptors depolarize to light
vertebrate: rod and cones hyperpolarize to light
melanopsin positive retinal ganglion cells
directly respond to light as well as receiving synaptic input indirectly from rods and cones
important for circadian regulation and pupil reflexes
retinal and light
11-cis-retinal –light–> all-trans-retinal
retinal is bound to an opsin protein to form either rhodopsin or cone opsin
retinal pigment epithelium cells (RPE)
take up all-trans-retinal from photoreceptors and reform 11-cis-retinal
resting potential in retinal cells
resting potential in many retinal cell types is not as negative as other neurons
photoreceptors hyperpolarize to light
what produces hyperpolarization in response to light in rods and cones
non-selective cation channels are open in dark, close in response to light, causing decrease in conductance
when these channels close, membrane potential is closer to Ek
second messenger in rods
non-selective cation channels are in plasma membrane, rhodopsin is in intracellular disks
second message b/w these two is cGMP
metabolism of cGMP: enzymes
guanylate cyclase: GTP → cGMP
phosphodiesterase (PDE): cGMP → GMP
dark and light: cGMP and enzyme levels
dark: low PDE activity, high cGMP concentration
light: high PDE activity, low cGMP concentration
evidence that second messenger in photoreceptors is cGMP: what did they use
inside out patches used to test whether cGMP activates channels in plasma membrane of photoreceptors from intracellular side
cascade for activation of PDE
- rhodopsin absorbs light
- G protein binds GTP, releases α subunit
- α subunit interacts with PDE
- PDE catalyzes cGMP → GMP
phosphodiesterase is activated by
transducins
cGMP gated channels
- do not select well b/w Na, K, Ca
- not voltage dependent
- have extra C terminal region that binds cGMP, which causes opening of channel
in the dark, what is happening
- cGMP concentration high
- cGMP-gated channels are open
- glutamate released continuously
in light, what is happening
- PDE activated
- cGMP destroyed
- cGMP channels close
- cell hyperpolarizes, voltage gated Ca channels close
- glutamate release stops
mechanisms of rod adaptation in constant light
rod responses begin to decline after 1 second:
- in darkness, Ca entry through cGMP-gated channels inhibits guanylate cyclase by binding to GCAP
- in light, low calcium increases guanylate cyclase activity, increasing cGMP, rod response decreases
GCAP KO mice
have larger light response
show less adapatation
GCAP is important for rod adaptation
mechanism of recovery after rod adaptation
RGS9 is a GAP, which accelerates GTP hydrolysis
arrestin phosphorylates rhodopsin, making it inactive
compared to rods, cone responses are:
faster and more transient
require more photons
depend on wavelength: S blue, M green, L red
in humans, mutations in which cones are common
red and green cone opsins
receptive fields of retinal ganglion cells
most have center surround antagonism
some on-center/off surround, some off-center/on-surround
properties of bipolar cells
don’t fire action potentials
have center-surround organization (50% off-center, 50% on-center)
how do photoreceptors communicate w/ bipolar cells
photoreceptors release glutamate, change membrane potential in postsynaptic bipolar cells
rod bipolar cells receive information from rods, cone bipolar cells from cones
glutmate receptors of bipolar cells
off center: have AMPA receptors w/ GluA2 + another AMPA subunit (light hyperpolarizes these)
on center: have metabotropic glutamate receptors (light depolarizes these) that close TRP-M channels when active
how do bipolar cells communicate w/ ganglion cells
- bipolar cells don’t spike, release glutamate when depolarized
- ganglion cells spike: have ionotropic glutamate receptors that depolarize the cell when active
- on-center bipolar cells connect to on-center ganglion cells and vice versa
horizontal cells characteristics
- build the surround
- do not fire APs
- express AMPA receptors (glutamate released by photoreceptors depolarizes)
- receive input from many more photoreceptors than do bipolar cells
- have larger receptive fields than bipolar cells
presynaptic terminals of horizontal cells communicate w/
photoreceptor presynaptic terminals, not bipolar cell postsynaptic dendrite
when horizontal cells are depolarized, they release signal that makes photoreceptor transmitter release mechanism less effective
what creates antagonistic surround for bipolar cells
lateral inhibition from horizontal cells:
Light → Center photoreceptor hyperpolarization → Horizontal cell hyperpolarization → Surround photoreceptor depolarization
proton hypothesis for horizontal cell signaling
dark: resting horizontal cells release protons, making calcium channels of photoreceptors less effective, less glutamate released
light: hyperpolarization of horizontal cells, less proton release, calcium channels open easier, more glutamate
amacrine cells
fire APs
release GABA or glycine and cause inhibition of ganglion and/or bipolar cells
major types of retinal ganglion cells
P-type: small RF, high resolution analysis of object shape, sustained response to light
M type: large RF, low spatial resolution, transient response, sensitive to movement
receptive fields of every retinal ganglion cells type ______ toward the periphery of the retina
RFs get progressively bigger
outside fovea and very dim environemnt
outside fovea: low acuity and poor color vision
dim environment: no color vision, blind spot at central fovea
color senstiive (P-type) bipolar and ganglion cells are:
color opponent
blue yellow: blue on center, yellow no response
red-green
blue-On bipolar cells
receive photoreceptor input from only S cones
have m-GluRs that result in hyperpolarization when active
midget bipolar cells
in the fovea, each red/green cone contacts a single midget bipolar cell
there are on and off midget bipolars
this circuity creates red-green color opponent receptive fields in the fovea
Sperry
discovered funcational specialization of cerebral hemispheres
Hubel
discoveries concerning information processing in the visual system
characterized V1
Wiesel
discoveries concerning information processing in the visual system
characterized V1
optic chiasm
axons from nasal retina cross, axons from temporal retina do not cross
LGN layer inputs
6 layers:
- Layers 1, 2: input from M-type RGCs
- Layers 3, 4, 5, 6: input from P-type RBCs
LGN layer eye inputs
Layers 1, 4, 6: info from contralateral eye
Layers 2, 3, 5: info from ipsilateral eye
cerebral cortex: LGN terminals end in:
layer 4
V1 layers input
in cortex:
Layer 4a: from parvocellular layers
Layer 4c-α: from magnocellular layers
Layer 4c-β: from parvocellar layers
simple cells found in what layers in V1
layers 1-3, 4b, 5-6
simple cells respond to what
respond to bars or edges
motion sensitive
orientation of bar matters
mechanism that helps for simple cell receptive fields
LGN neurons receptive fields (center surround) combine to form the receptive field of a single simple cell
complex cells of V1
respond to bars or edges, but bar can be at multiple locations and still activate
motion sensitive
orientation matters
binocular vision
cortex uses small differences in where singal hits the retina in two eyes to create a 3D image
monocular vs binocular cells
LGN cells monocular
V1 layer 4a and 4c monocular
most cells in other layers of V1 are binocular
monocular deprivation
if one eye is kept closed past critical period that end w/in months of birth, animal behaves as if blind in that eye
disuse leads to atrophy
binocular deprivation
animals can still see and use both eyes
monocular deprivation experiment
- inject radioactive AA into eye
- some are taken up into ganglion cells, transported to LGN
- at LGN, some of these proteins are broken down, made into new proteins, transported to V1
- injection into non deprived eye labels all of layer 4 in V1
- injection into deprived eye, only tiny area of layer 4 is labelled
- conclusion: patterno f terminations from the LGN is altered
what mechanism produces change in termination patterns during critical period for monocular deprivation
termination patterns start out mixed, then some pull back terminal branches, causing input elimination
ocular dominance column
cells with similar ocular dominance (cells in cortex driven by open eye), tend to be grouped together
strabismus
eyes don’t converge to a common point
humans with this have no stereo vision (get 3D image from 2D input)
shows that tightly synchronized activity from two eyes is required to maintain binocularity
