Nerve/synapse III

Question 1

What are the 3 broad categories of synapses?

Answer 1

Axodendritic, axosomatic, and axoaxonic

Question 2

What is an axosomatic synapse? Is it excitatory or inhibitory or both?

Answer 2

It is a synapse that is on the cell body. It is usually inhibitory.

Question 3

What is an axoaxonic synapse?

Answer 3

It is a synapse whose presynaptic terminal makes a synapse with the presynaptic terminal of another neuron.

Question 4

What are the two types of axodendritic neurons? Explain the difference between them.

Answer 4

Spine synapses: on a little structure on the dendrite. These are excitatory synapses.
Shaft synapses: directly on the dendrite. These are inhibitory synapses.

Question 5

How does an action potential behave when it reaches a branch point? What consequence does this have on neurotransmission?

Answer 5

It goes down both branches at the same intensity as it arrived, as you can’t have half an action potential. This means that the message of the neuron can be sent to several different parts of the nervous system/brain at once.

Question 6

Describe the major components of the structure of the synapse.

Answer 6

There is the presynaptic terminal, which is connected to the axon of the previous neuron, and the postsynaptic spine, which is attached to the dendrite of the next neuron. The presynaptic terminal contains presynaptic vesicles, and the postsynaptic spine contains postsynaptic densities.

Question 7

What is the purpose of presynaptic vesicles?

Answer 7

They contain chemicals called neurotransmitters.

Question 8

Describe how presynaptic vesicles are organized in the presynaptic terminal.

Answer 8

They can be either docked, which is when they are lined up along the membrane facing the synaptic cleft. These areas are called active zones, and these vesicles are involved in synapse function There are also other vesicles scattered in the presynaptic terminal.

Question 9

Describe the organization of postsynaptic densities and their purpose.

Answer 9

They are concentrations of specialized proteins located in the postsynaptic spine, facing the synaptic cleft.

Question 10

What type of channels are found in the presynaptic terminal? What is their purpose?

Answer 10

Voltage-gated calcium channels. Their purpose is to release neurotransmitters into the synaptic cleft.

Question 11

What type of channels are found in the postsynaptic spine? What is their purpose?

Answer 11

Ligand-gated ion channels (aka neurotransmitter receptors). These are activated when the neurotransmitter binds to them.

Question 12

Describe the 3 major steps of chemical synaptic transmission.

Answer 12

1. the action potential invades the presynaptic terminal. This opens the voltage-gated calcium channels, allowing Ca2+ to flow in.
2. Synaptic vesicles fuse with the presynaptic membrane, releasing the neurotransmitter into the synaptic cleft.
3. The transmitter diffuses across the cleft and activates receptors in the post-synaptic membrane. The chemical signal gets converted into an electrical signal, causing a change in the electrical properties of the postsynaptic spine.

Question 13

What is the fate of presynaptic vesicles once they have emptied their contents into the synaptic cleft?

Answer 13

They get recycled by the cell and dock again.

Question 14

Explain the process by which presynaptic vesicles fuse with the membrane.

Answer 14

When the calcium channel opens, the Ca2+ flows in and binds to proteins that are part of this complex. This causes the proteins to charge their conformation, which drives the fusion of the vesicle with the presynaptic membrane.

Question 15

What are the two types of postsynaptic responses to a neurotransmitter?

Answer 15

Excitatory postsynaptic potential (EPSP) and Inhibitory postsynaptic potential (IPSP)

Question 16

What is the difference between an EPSP and an IPSP?

Answer 16

An EPSP causes the depolarization of the membrane, making it more likely to fire an action potential. An IPSP hyperpolarized the membrane and prevents it from firing an action potential.

Question 17

[…] synapses are found on spines, while […] synapses are mainly found on the shaft

Answer 17

Excitatory, inhibitory

Question 18

What is the main excitatory neutransmitter?

Answer 18

Glutamate

Question 19

Where is glutamate found (before an action potential is fired)?

Answer 19

It is packaged in the presynaptic vesicles.

Question 20

What are the two types of glutamate receptors found in the postsynaptic spine?

Answer 20

AMPA and NMDA receptors.

Question 21

AMPA receptors and NMDA receptors are […] receptors. What does this mean?

Answer 21

Ionotropic. This means that they are ion channels that open in response to the binding of neurotransmitters to their external surface receptors = ligand gated

Question 22

Explain how AMPA receptors work.

Answer 22

When the presynaptic vescile dumps the glutamate into the synaptic cleft, the glutamate binds to the AMPA receptor. This causes the AMPA receptor to open a channel to sodium, which is allowed to flow into the postsynaptic terminal and depolarize it, pushing it towards the action potential threshold.

Question 23

How big is an EPSP?

Answer 23

A single EPSP is very small and brief, only depolarizing the postsynaptic terminal by a few millivolts. Many are needed to reach an action potential in the postsynaptic cell.

Question 24

How long does a typical EPSP last?

Answer 24

Only around 20 msec

Question 25

How many EPSPs are required to initiate an action potential? Do they all come from the same place?

Answer 25

At least 50-100. They don’t necessarily all come from the same place; they can come from multiple synapses or from one synapse.

Question 26

Describe the key properties of NMDA receptors.

Answer 26

They are ion channels. At resting membrane potentials, their pore is blocked by Mg2+. When depolarization occurs, the Mg2+ is expelled, which allows Ca2+ through and into the postsynaptic terminal

Question 27

Compare the activity of ionotropic receptors during resting potential vs when depolarization is taking place.

Answer 27

At resting potential, only AMPA receptors are active. NMDA receptors are still blocked by Mg2+.
At a potential of around -50 mV, the NMDA receptor becomes active as well, allowing Ca2+ to flow into the cell.

Question 28

What is the purpose of the Ca2+ that flows into the postsynaptic terminal via NMDA receptors? What is this process called?

Answer 28

It allows the EPSPs to become larger, allowing synapses to control their strength. This is called synaptic plasticity.

Question 29

Which type of ionotropic receptor causes the “fast” depolarization of the postsynaptic terminal?

Answer 29

The AMPA receptor