5.14 - Synapses and Synaptic Models

Question 1

What is synaptotagmin?

Answer 1

Calcium (Ca²⁺) sensor, involved in vesicle fusion

Question 2

What is a synapse?

Answer 2

A junction between two cells that permits for the transmission of electrical or chemical signals

Question 3

What are the two main types of synapses?

Answer 3

Electrical and chemical synapses

Question 4

What are the two principle ionotropic receptors for glutamate? And what are their dynamics?

Answer 4

AMPA and NMDA

AMPA current activates/deactivates faster.

NMDA current is slower to activate and it takes even longer to deactivate.

Question 5

True or False: Transmission in an electrical synapse is unidirectional?

Answer 5

False

Question 6

Which is faster, an electrical or a chemical synapse?

Answer 6

An electrical synapse is instantaneous, a chemical synapse takes longer

However: the electrical synapse is often ‘low pass filtered’

Question 7

What is the cause of short-term synaptic depression?

Answer 7

Depletion of the synaptic vesicle in the presynaptic terminal.

Question 8

What is the quantal hypothesis/quantal theory of synaptic transmission?

Answer 8

The hypothesis holds that discrete quanta, individual neurotransmitter-containing vesicles, are released at the presynaptic terminal probabilistically.

Question 9

How can a facilitating synapse reduce noise in information transfer?

Answer 9

Single, isolated action potential (noise) in the presynaptic cell will not be well transmitted to the postsynaptic cell, while bursts of activities (signal) will.

Question 10

In an electrical synapse, ____ flow between cells

Answer 10

ions

Question 11

What is classical Hebbian plasticity?

Answer 11

If neuron A repeatedly or persistently takes part in firing neuron B, the efficiency with which neuron A excites neuron B will increase.

Question 12

What are three typical mechanisms for LTP (long-term potentiation)?

Answer 12

Recruitment of AMPA receptors

Facilitation of synaptic release

Synthesis of new synapses

Question 13

How can you model a protein with multiple binding sites using ODE (ordinary differential equations)?

Answer 13

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.

Question 14

What is the major molecule that forms an electrical synapse?

Answer 14

Gap junctions in an electrical synapse are formed by connexins.

One connexon (a hemichannel) is made up of six connexins.

Question 15

How are the two types of short-term synaptic plasticity called?

Answer 15

Depression and facilitation.

Question 16

What properties of the NMDA receptor help mediate long-term potentiation?

Answer 16

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.

Question 17

Name and explain the three properties of LTP long-term potentiation.

Answer 17

1. Cooperativity: the simultaneous activation of multiple weak inputs can lead to suprathreshold activity and thus LTP.
2. Associativity: stimulation of strong and weak input together can lead to LTP in both synapses.
3. Synaptic Specificity: only synapses that are stimulated will undergo LTP.

Question 18

What can be used to bias a synapse towards LTP or LTD?

Answer 18

Firing rate and/or spike timing

High-frequency firing → LTP

Low-frequency firing → LTD

Question 19

Calcium (Ca²⁺) concentration outside the cell is ____ than inside the cell

Answer 19

much higher

Question 20

Is the following statement true or false?

Signal transmission in an electrical synapse has no threshold

Answer 20

True

Question 21

Glutamate is an excitatory or inhibitory neurotransmitter?

Answer 21

excitatory

Reminder: Glutamate neurotransmitter activates both AMPA and NMDA receptors

Question 22

Is GABA (gamma-Aminobutyric acid) is an excitatory or inhibitory neurotransmitter?

Answer 22

inhibitory

Question 23

Three main types of neurotransmitters are:

Answer 23

(a) Amino acids
(b) Amines
(c) Peptides

Question 24

What is an active zone of a presynaptic terminal?

Answer 24

A site of neurotransmitter release

Question 25

What is the main mechanism of synaptic transmission?

Answer 25

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

Question 26

What is long-term potentiation (LTP)?

Answer 26

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.

Question 27

What is short-term potentiation (STP)?

Answer 27

STP is the transient enhancement of synaptic transmission as a result of an increase in synaptic vesicle release probability.

Question 28

What is the role of the NMDA receptor in long-term potentiation?

Answer 28

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.

Question 29

What is the role of the AMPA receptors in long-term potentiation?

Answer 29

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.

Question 30

Explain spike-timing-dependent plasticity (STDP)

Answer 30

The temporal relationship between activity in the pre- and post-synaptic cells determines the amount of long-term plasticity.

Question 31

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.

Answer 31

True.

Question 32

How is postsynaptic transmission calcium-dependent?

Answer 32

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.

Question 33

What are the different ways to model a synapse?

Answer 33

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

Question 34

What is the spiking version of Hebbian learning?

Answer 34

STDP (Spike time dependent plasticity)

Question 35

What are three computational properties of STDP?

Answer 35

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.