5.14 - Synapses and Synaptic Models Flashcards
What is synaptotagmin?
Calcium (Ca2+) sensor, involved in vesicle fusion
What is a synapse?
A junction between two cells that permits for the transmission of electrical or chemical signals
What are the two main types of synapses?
Electrical and chemical synapses
What are the two principle ionotropic receptors for glutamate? And what are their dynamics?
AMPA and NMDA
AMPA current activates/deactivates faster.
NMDA current is slower to activate and it takes even longer to deactivate.
True or False: Transmission in an electrical synapse is unidirectional?
False
Which is faster, an electrical or a chemical synapse?
An electrical synapse is instantaneous, a chemical synapse takes longer
However: the electrical synapse is often ‘low pass filtered’
What is the cause of short-term synaptic depression?
Depletion of the synaptic vesicle in the presynaptic terminal.
What is the quantal hypothesis/quantal theory of synaptic transmission?
The hypothesis holds that discrete quanta, individual neurotransmitter-containing vesicles, are released at the presynaptic terminal probabilistically.
How can a facilitating synapse reduce noise in information transfer?
Single, isolated action potential (noise) in the presynaptic cell will not be well transmitted to the postsynaptic cell, while bursts of activities (signal) will.
In an electrical synapse, ____ flow between cells
ions
What is classical Hebbian plasticity?
If neuron A repeatedly or persistently takes part in firing neuron B, the efficiency with which neuron A excites neuron B will increase.
What are three typical mechanisms for LTP (long-term potentiation)?
Recruitment of AMPA receptors
Facilitation of synaptic release
Synthesis of new synapses
How can you model a protein with multiple binding sites using ODE (ordinary differential equations)?
The rate of change of each state can be described as a differential equation.
One specifies all the possible states (unbound, single bound, double bound, etc) as gating variables.
One needs time constants for rate of transition between different states.
What is the major molecule that forms an electrical synapse?
Gap junctions in an electrical synapse are formed by connexins.
One connexon (a hemichannel) is made up of six connexins.
How are the two types of short-term synaptic plasticity called?
Depression and facilitation.
What properties of the NMDA receptor help mediate long-term potentiation?
1) It has an extracellular magnesium block and thus only opens during large depolarizations.
2) It allows the flow of Ca2+ into the cell to act as a signal for metabolic processes.
Name and explain the three properties of LTP long-term potentiation.
- Cooperativity: the simultaneous activation of multiple weak inputs can lead to suprathreshold activity and thus LTP.
- Associativity: stimulation of strong and weak input together can lead to LTP in both synapses.
- Synaptic Specificity: only synapses that are stimulated will undergo LTP.
What can be used to bias a synapse towards LTP or LTD?
Firing rate and/or spike timing
High-frequency firing → LTP
Low-frequency firing → LTD
Calcium (Ca2+) concentration outside the cell is ____ than inside the cell
much higher
Is the following statement true or false?
Signal transmission in an electrical synapse has no threshold
True
Glutamate is an excitatory or inhibitory neurotransmitter?
excitatory
Reminder: Glutamate neurotransmitter activates both AMPA and NMDA receptors
Is GABA (gamma-Aminobutyric acid) is an excitatory or inhibitory neurotransmitter?
inhibitory
Three main types of neurotransmitters are:
(a) Amino acids
(b) Amines
(c) Peptides
What is an active zone of a presynaptic terminal?
A site of neurotransmitter release
What is the main mechanism of synaptic transmission?
The action potential arrives at the presynaptic terminal ->
Calcium channels open ->
Calcium enters the presynaptic terminal ->
vesicles fuse with the membrane ->
the neurotransmitters are released into the synaptic cleft ->
neurotransmitters bind with the postsynaptic receptors -> channels open ->
current comes into the postsynaptic cell
What is long-term potentiation (LTP)?
LTP is a process by which synaptic connections between neurons become stronger with frequent activation of the synapse.
It is often thought to be the biological substrate of Hebbian learning.
What is short-term potentiation (STP)?
STP is the transient enhancement of synaptic transmission as a result of an increase in synaptic vesicle release probability.
What is the role of the NMDA receptor in long-term potentiation?
The NMDA receptor acts as a coincidence-detector.
The induction of LTP is due to the accumulation of postsynaptic calcium as a result of calcium influx from NMDA receptors.
The influx of calcium leads to the expression of more AMPA receptors, enhancing the post-synaptic response to glutamate released from the presynaptic cell.
What is the role of the AMPA receptors in long-term potentiation?
The expression of LTP depends on dynamic changes in AMPA receptors.
AMPA receptors are (dynamically) inserted into the postsynaptic membrane by synaptotagmin.
The subsequent rise in AMPA receptors increases postsynaptic response to glutamate, strengthening the synaptic transmission.
Explain spike-timing-dependent plasticity (STDP)
The temporal relationship between activity in the pre- and post-synaptic cells determines the amount of long-term plasticity.
Is the following sentence true or false?
A presynaptic action potential causes a postsynaptic excitatory potential because it synchronizes the release of many neurotransmitter quanta.
True.
How is postsynaptic transmission calcium-dependent?
The amount of extracellular calcium determines the amplitude of the postsynaptic potential.
This relationship is non-linear: a small decrease in the calcium concentration decreases the size of the postsynaptic potential to a significantly higher degree.
What are the different ways to model a synapse?
We can use:
(1) current-based models - a little current coming in from the presynaptic cell to a postsynaptic cell
(2) voltage-based models - activity from a presynaptic cell arrives at a postsynaptic cell in the form of voltage deflection
(3) conductance-based models - we can model the proportion of channels open as a function of the driving force for a particular synapse.
(4) gap junctions
What is the spiking version of Hebbian learning?
STDP (Spike time dependent plasticity)
What are three computational properties of STDP?
STDP has several fascinating computational properties:
Spike correlations: spikes between pre- and post-synaptic neurons become correlated through reinforcement.
network latency: STDP can reduce network latency by reinforcing presynaptic neurons which consistently fire prior to the postsynaptic neuron.
regulation: STDP regulates network firing rates, achieving a homeostatic effect , despite being quite stable at the local level.
