Lecture 9 + Assignment 8 Flashcards
Plasticity
ability of brain to change aspects of itself
- response to experience or injury
Aplysia - sea slug
water enters gills - which extract oxygen - and exits siphon
head tentacles = tactile sensors
rhinospores = chemical sensors (like antennae)
- gastropod invertebrate; coastal waters in California and Mexico
release ink from below mantle
Aplysia gill withdrawal reflex circuit
When siphon touched + sensitization
Axo-axonic synapse
- sensory neuron touching siphon skin
- directly synapses onto motor neuron
- sensory neuron touching tail skin
- synapses onto modulatory interneuron
= serotonergic
- synapses onto siphon SN
glutamate onto motor neuron, ACh to move gill muscles and pull them in
Serotonin role in aplysia
makes SN release more glutamate than normal during APs
= greater motor neuron EPSP
= more voltage-gated Ca channels stay open
= longer AP falling phase
serotonin=5HT
Eric Kandel
- won Nobel prize for simple memory and learning in aplysia
Aplysia habituation
a progressive decrease in response to a repeated stimulus
stop responding to a harmless touch to siphon bc it’s a waste of energy
The scientific method
- Research question
- Hypotheses
- Predictions
- Data
then back to hypotheses
Sensitization in aplysia
a heightened response to an innocuous stimulus, caused by a previous noxious stimulus to the tail
shock then touch again
= sensitization
In-depth aplysia gill withdrawal reflex
Adenylyl cyclase mechanism too
- Tail shock activates the tail sensory neuron
- The modulatory interneuron releases serotonin onto the siphon sensory
neuron’s axon terminal (axo-axonic synapse) - Tail sensory neuron (glutamatergic) excites the modulatory (facilitatory)
interneuron - Inside the sensory axon terminal, the enzyme adenylyl cyclase converts ATP
into cAMP - cAMP activates the enzyme protein kinase A
- Protein kinase A phosphorylates (adds PO4 to) voltage-gated K+ channels
- This reduces the probability that the K+ channels will open during AP
- This prolongs the AP falling phase (depolarization of axon lasts longer)
Now siphon stimulation → prolonged action potential in siphon sensory axon terminal → more Ca++ entry than normal → more glutamate release
than normal → greater motor neuron response!
Tetanus
a high frequency (100 Hz) stimulation
can even have an effect hours after it’s done = long-term potentiation
LTP induction
When Ca enters the postsynaptic dendrite, LTP occurs at that location
2 conditions for Ca to enter:
1. The postsynaptic cell must be depolarized
2. Glutamate must be present
means Mg will pop out of the NMDA receptor channel and Ca can enter
First, glutamate is released by the presynaptic cell. When glutamate binds to the ionotropic NMDA receptor channels on the postsynaptic neuron’s dendritic spine it causes them to open. However, the channel is blocked by magnesium ions that are attracted to the negative cell interior but get stuck due to their size. All three of these mechanisms involve calcium ion entry into the postsynaptic dendrite which initiates LTP.
LTP specificity
Axon 1: strong stimulation
Axon 2: inactive
Ca++ enters the dendrite at synapse 1 only, because:
1. The postsynaptic cell is depolarized (everywhere)
2. Glutamate is present only at synapse 1
LTP associativity
Axon 1: strong stimulation
Axon 2: weak stimulation
Ca++ enters the dendrite at both synapses, because:
1. The postsynaptic cell is depolarized (everywhere), and
2. Glutamate is present at both synapses.
LTP cooperativity
Axon 1: weak stimulation
Axon 2: weak stimulation
Ca++ enters the dendrite at both synapses, because:
1. The postsynaptic cell is eventually depolarized (by the
joint action of both synapses), and
2. Glutamate is present at both synapses
2 possible mechanisms for LTP expression at dendritic spine
post-synaptic plasticity
ionotropic AMPA receptors (also respond to AMPA)
- permeable to K/Na
what calcium does once inside
1. Enhancement of existing AMPA receptor conductance
- activates enzyme Cam kinase II
- phos. AMPA receptors
- conformational change increases conductance
- Insertion of new AMPA receptors
- cam kinase II causes insertion of more AMPA receptors
= more sensitive to glutamate
