Synapses and Synaptic Models

Question 1

What is a synapse?

Answer 1

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

Question 2

What are the two main types of synapses?

Answer 2

Electrical and chemical synapses

Question 3

What are the two main (ionotropic) receptors for glutamate? And what are their dynamics?

Answer 3

AMPA and NMDA

AMPA current activates/deactivates faster.

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

Question 4

Is transmission in an electrical synapse is unidirectional or bidirectional?

Answer 4

Bidrectional

The current crosses from one cell to another as a function of the difference of potential between cell 1 and cell 2

The driving force for cell 1 is proportional to (V2-V1) and for cell 2 is the driving force for cell 2 is proportional to (V1-V2)

Question 5

Which is faster, an electrical or a chemical synapse?

Answer 5

An electrical synapse is instantaneous, a chemical synapse takes longer.

(because neurotransmitter released has to trigger receptors, and ionic current is exchanged immediately and continuously)

However: the electrical synapse is ‘low pass filtered’ and has a much smaller amplitude than the original signal, whereas the chemical synapse can amplify signals.

Question 6

What is the cause of short-term synaptic depression?

Answer 6

Depletion of the synaptic vesicles in the presynaptic terminal.

Question 7

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

Answer 7

The hypothesis holds that post-synaptic currents are generated with discrete quanta, i.e., individual neurotransmitter-containing vesicles, are released at the presynaptic terminal (probabilistically).

Question 8

How can a facilitating synapse reduce noise in information transfer?

Answer 8

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 9

In an electrical synapse, ____ flow between cells

Answer 9

ions, or current.

Question 10

What is classical Hebbian plasticity?

Answer 10

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

Question 11

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

Answer 11

Recruitment of AMPA receptors

Facilitation of synaptic release

Synthesis of new synapses

Question 12

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

Answer 12

Depression and facilitation.

Question 13

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

Answer 13

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 14

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

Answer 14

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 15

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

Answer 15

Firing rate and/or spike timing

High-frequency firing → LTP

Low-frequency firing → LTD

Question 16

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

Answer 16

much higher

Question 17

Is signal transmission in an electrical synapse graded or all or none?

Answer 17

Graded

Question 18

Is glutamate an excitatory or inhibitory neurotransmitter?

Answer 18

excitatory

Reminder: Glutamate neurotransmitter activates both AMPA and NMDA receptors

Question 19

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

Answer 19

inhibitory

Question 20

What is an active zone of a presynaptic terminal?

Answer 20

A site of neurotransmitter release

Question 21

What is the main mechanism of chemical synaptic transmission?

Answer 21

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 22

What is long-term potentiation (LTP)?

Answer 22

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 23

What is short-term potentiation (STP)?

Answer 23

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

Question 24

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

Answer 24

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 25

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

Answer 25

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 26

Explain spike-timing-dependent plasticity (STDP)

Answer 26

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

Question 27

Is the following sentence true or false?

A presynaptic action potential synchronizes the release of many neurotransmitter quanta.

Answer 27

True.

Question 28

How is postsynaptic transmission calcium-dependent?

Answer 28

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 29

What are the different ways to model a synapse?

Answer 29

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 30

What is the spiking version of Hebbian learning?

Answer 30

STDP (Spike time dependent plasticity)

Question 31

What are three computational properties of STDP?

Answer 31

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.

Question 32

What is STDP?

Answer 32

Spike Time Dependent Plasticity: the idea that the connection between two neurons changes as a function of the time interval between pre and post-synaptic spikes.