5.14 - Synapses and Synaptic Models

Question 1

Which is faster, an electrical or a chemical synapse?

Answer 1

An electrical synapse is instantaneous, a chemical synapse takes longer

Question 2

What is the major cause of short-term synaptic depression?

Answer 2

Depletion of the synaptic vesicle in the presynaptic terminal.

Question 3

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

Answer 3

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

Question 4

How can a facilitating synapse (a synapse that shows facilitation) reduce noise in information transfer?

Answer 4

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

Question 5

What is Hebbian plasticity?

Answer 5

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

Question 6

What are the typical mechanisms of long-term potentiation?

Answer 6

Recruitment of AMPA receptors

Facilitation of synaptic release

Synthesis of new synapses

Question 7

What is “cooperativity” in the context of ligand-protein binding? Give an example of positive cooperativity.

Answer 7

Cooperativity in ligand-protein binding refers to the phenomenon that for a protein with multiple binding sites, the binding of the first ligand can change the binding/unbinding rate of the second, third, etc. ligand.

For example, hemoglobin has positive cooperativity, in which the binding of the first oxygen molecule makes the binding of the next molecule easier.

Question 8

How can you model a protein with multiple binding sites using ODE?

Answer 8

By specifying all the bound states (unbound, single bound, double bound, etc) and the rate of transition between the different states.

The rate of change of each state can be described as a differential equation.

Question 9

What are the two main types of synapses?

Answer 9

Electrical and chemical synapses

Question 10

What is the major molecule that forms an electrical synapse?

Answer 10

Gap junctions in an electrical synapse are formed by connexins.

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

Question 11

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

Answer 11

Depression and facilitation.

Question 12

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

Answer 12

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 13

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

Answer 13

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 14

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

Answer 14

Firing rate and/or spike timing

High-frequency firing → LTP

Low-frequency firing → LTD

Question 15

In an electrical synapse, ____ flow between cells

Answer 15

ions

Question 16

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

Answer 16

much higher

Question 17

Transmission in an electrical synapse is bidirectional or unidirectional?

Answer 17

bidirectional

Question 18

Is the following statement true or false?

Signal transmission in an electrical synapse has no threshold

Answer 18

True

Question 19

Glutamate is an excitatory or inhibitory neurotransmitter?

Answer 19

excitatory

Reminder: Glutamate neurotransmitter activates both AMPA and NMDA receptors

Question 20

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

Answer 20

inhibitory

Question 21

Three main types of neurotransmitters are:

Answer 21

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

Question 22

What is an active zone of a presynaptic terminal?

Answer 22

A site of neurotransmitter release

Question 23

What is synaptotagmin?

Answer 23

Calcium (Ca²⁺) sensor, involved in vesicle fusion

Question 24

What is the main mechanism of synaptic transmission?

Answer 24

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 25

What is a synapse?

Answer 25

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

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.

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](https://knowingneurons.com/2013/05/30/what-a-coincidence/).** 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 size 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

GABA (γ-aminobutyric acid) activates two inhibitory conductances in the brain. What are they? What can you say about their activation?

Answer 34

These are GABA_A and GABA_B. GABA_A receptors produce a comparatively fast ionotropic conductance of chloride. GABA_B receptors, which are metabotropic, produce a slower, long-lasting potassium conductance.

Question 35

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

Answer 35

AMPA and NMDA AMPA current activates/deactivates faster. NMDA current is slower to activate and it takes even longer to deactivate.