lecture 21 and 22 - anya hurlbert Flashcards
learning is…
the acquisition of new knowledge
memory is…
the retention of learned knowledge
memory is stored as a…
neural trace (memory must involve changes in connections between neurones)
recall is…
reactivation of the trace
forms of memory
declarative and procedural
declarative
- records of facts and events
- accessible to consciousness, easy to forget
- describe what we know and what we remember about things that have happened to us
procedural
- learned skills and behaviours
- involved learning a motor response in association with a sensory input
- inaccessible to consciousness, but never forget
- cant verbalise is e.g riding a bike
time scale of memory can be
short term (STM) - verbal or visuospatial
ling term (LTM)
example of verbal and visuospatial STM
verbal - memory of someones phone number
visuospatial - jigsaw puzzle (remembering what piece you need and where it is)
example of LTM
visual - people faces
auditory - music
non associative learning in procedural memory:
habituation=
learn to ignore
e.g people hear planes going over head who live near to airport and they learn to ignore the sound
sensitisation=
learn to intensify
stimuli that normally wouldnt evoke a response suddenly becomes more powerful and causes you to react e.g home alone and you hear a noise
associative learning in procedural memory:
classical conditioning=
process by which a neural stimulus becomes associated with a meaningful stimulus
e.g. bell –> food –> salivation
bell –> salivation
instrumental conditioning=
process by which a response is gradually learned via reinforcement or punishment
e.g we learn that a behavior produces a particular consequence
if positive –> repeat behavior
if negative –> do no repeat behavior
why are invertebrates used in procedural learning experiments
- simple learning circuits
- big cells - easy to identify + isolate
- small systems
- simple genetics
- easy to keep alive
which marine invertebrate is mainly used in procedural memory experiments
sea slug aplysia californica
why are aplysia used
- simple nervous system
- gill withdrawal reflex
habituation in aplysia
decrease over time in response to repeated stimulus that lacks meaning
sensitisation in aplysia
exaggeration in response to normal stimuli after strong stimulus
the gill withdrawal reflex
when the water is squirted onto the siphon and detects touch the siphon and gill contract
neuronal circuitry of the gill withdrawal reflex
siphon skin –> sensory neurone (24) –> synapse –> motor neurone (6) –> gill muscle
how does the gill withdrawal reflex show habituation
repeated stimulation of the siphon skin leads to progressively less contraction of the gill
where is the locus if the habituation
the SN - MN synapse.
EPSPs decline as the SN is repeatedly stimulated
this is a form of…
homosynaptic depression
is it a pre or post synaptic effect
pre as there is fewer quanta (of glutamate) release per AP because there is less calcium entry per AP
how does the gill withdrawal reflex show sensitisation
an electrical shock to the head or tail sensitises the aplysia and causes a squirt of water to cause a much bigger response (withdrawal of the gill)
how does the sensitisation occur
when the shock arrives at the head it activates the L29 interneuron that synapses on to the sensory neuron
what is released from L29
serotonin which causes more calcium to enter per AP and thus more quanta of glutamate per AP
this causes an increases response
what is this called
heterosynaptic facilitation (as there is an extra synapse)
presynaptic mechanisms involved in sensitisation:
5-HT receptor -
is G protein coupled and metabotropic which activates cAMP as second messenger
cAMP activates two kinases…
PKA and PKC which act to enhance release of transmitter
how does PKA work
phosphorylates and closes K+ channel
this stops potassium leaving the cell whih will keep positive charges in the cell and keep is depolarised for longer broadening the AP, this causes more calcium to enter and more quanta of glutamate per AP
how to PKC work
acts directly on vesicle release mechanism and causes enhances transmitter release
metabotropic receptor (G protein coupled) examples
ACh muscarinic
glutamate metabotropic
GABAb
5HT
DA
NE
structure of metabotropic receptors
7 membrane spanning alpha helices
Gs and Gi do what
Gs - stimulates effector protein
Gi - inhibits effector protein
two effector systems of metabotropic receptors
G protein gates ion channels (e.g GABAb couples to K+ channels)
G protein activates enzymes (2nd messenger cascade) (e.g NEbeta receptor activates Gs which increases cAMP, activating PKA)
alzheimers - loss and damage to synapses
amyloid beta reduces number and plasticity of synapses
modifies tau proteins impair synaptic vesicle release and postsynaptic receptor insertion
associative learning =
the forming of associations between two events
classical conditioning =
learning of association between unconditional stimulus (US) (which evokes a response without training) which is predicted by a conditional stimulus (CS) (which does not normally evoke a response)
before conditioning:
CS (bell) —-> no response (no salivation)
US (meat) —-> response (salivation)
conditioning:
CS + US (bell, then meat) —-> response (dribble)
after conditioning?
CS (bell) —-> response (drool)
associative learning in aplysia (conditioning)
before:
- CS (siphon stim) - no response
- US (shock to tail) - response
during:
- CS + US -> response
after:
- CS -> response
presynaptic facilitation after classical conditioning
the US (tail shock) activates interneurons (e.g L29) which release 5-HT onto the SN
activity dependent facilitation
if the CS (siphon touch) precedes the US, the 5-HT release elicits even stronger presynaptic facilitation
coincidence detection in presynaptic mechanisms (classical conditioning)
1) weak CS does not initiate response from MN on its own, but it does elicit APs which cause calcium to enter presynaptic terminal of SN
2) calcium activates adenylyl cyclase which acts as a coincidence detector which detects the coincidence of the weak activation of the presynaptic terminal and then the strong activation from the US.
3) strong US causes L29 to release 5-HT onto presynaptic terminal of SN, activation the G protein associated with the 5-HT receptor
4) the G protein further activates the primed adenylyl cyclase
5) the super activated adenylyl cyclase activates cAMP more strongly which activates PKA and PKC
6) kinases have the same effects as in the sensitisation mechanism
post synaptic mechanisms in classical conditioning:
1) pairing of the CS and the US leads to strong depolarisation of the…
motor neuron
the SN releases ________ into the MN
glutamate
the postsynaptic NMDA receptor in the presence of strong depolarisation is…
unblocked (Mg ion is removed) and calcium enters the postsynaptic MN
calcium activates a retrograde messenger system that signals the pre synaptic cell to release more transmitter
calcium also activates CAMK II that enhances non-NMDA receptor
how can classical conditioning phenomenon be mimicked
by applying 5-HT to SN (representing the strong US) just after the SN fires APs (representing the weak activity of the CS)
after training the weak CS will now elicit a response greater and longer term than from
sensitisation
this is due to
modification of pre synaptic proteins and increased protein synthesis and modification of post synaptic proteins
differences between sensitisation and conditioning
- conditioning causes longer lasting and stronger
enhancement of withdrawal reflex (via the SN-MN) than short term sensitisation does. - conditioning requires sequential activation of the SN by the CS and then the US
- conditioning requires activity of both the SN and the MN
- both conditioning and sensitisation utilise the same route to increase transmitter release (cAMP activated phosphorylation)
unpaired stimuli
- siphon touch and tail shock may be close in time but the former does not reliably predict the other
- not only is there no sensitisation but depression may occur (EPSP declines over time)
paired stimuli
- siphon touch and tail shock are close in time, and the former reliably predicts the latter
- EPSP response is enhanced
time scale of habituation (short term)
decrease in response following ~10 stimuli delivered every minute
response recovers after a rest period of several minutes to 1 hour
time scale of habituation (long term)
four sessions of stimulation, at intervals of several hours to 1 day, leads to habituation response lasting up to 3 weeks
can lead to a decrease in number of synaptic contacts between SN and MN (from 90% of SNs with detectable connections to 30%)
mechanisms of short term sensitisation
occurs after a single brief training session
- activity dependent presynaptic facilitation which is mediated by the action of PKA
- serotonin binding to its receptor activates G protein which activates cAMP which causes activation of PKA which phosphorylates the potassium channel and theres an increase in depolarization of the membrane which causes an increase in calcium into the cell and an activation of PKC which causes enhanced neurotransmission
intermediate term sensitisation
same set of mechanisms that happen in classical conditioning
longer application of 5-HT or paired stimulation as in conditioning you can have effects that last for hours
requires both presynaptic and postsynaptic protein modifications
PKA acts on increasing calcium which activates CAMK and PKC
those activations then cause an increase depolarisation on the post synaptic side which causes unblocking of the NMDA receptor and an increase of calcium into the postsynaptic cell which activates the non-NMDA receptor and strengthens the effectiveness of these receptors
also leads to insertion of new postsynaptic receptors
long term sensitisation
multiple training sessions spaces over several days
PKA migrates to the nucleus and activates a cAMP response element binding protein
this activates a gene transcription pathway leading growth of the synapse
starts with persistent PKA which activates transcriptional factors, creating proteins which cause increase insertion of receptors in the postsynaptic membrane and increase response to an incoming stimulus in the presynaptic neurone
if you block protein synthesis in aplysia you don’t get long term sensitisation but still get short term sensitisation
