Synapses Between Neurons

Question 1

What is the structure of the neuron-neuron synapse?

Answer 1

The presynaptic ending has an axon that expands into a varicosity or a terminal bouton with vesicles containing a neurotransmitter and the presence of an electron dense region.

There is a prominent synaptic cleft.

In the postsynaptic neuron, electron dense regions underlies that of presynaptic neuron, but the membrane does not have folds. THere are also receptors for the neurotransmitter

Question 2

How do the the sequence of events in transmission compare to a NMJ?

Answer 2

Essentially the same except the released transmitter may produce excitatory or inhibitory effects on the Post synaptic cell

Question 3

What occurs during excitatory transmitter action?

Answer 3

the conductance change drives the membrane potential to a level that is LESS negative (i.e. slight depolarization) than the threshold; thus increasing probability that the post synaptic cell will produce an action potential.

Question 4

What is the transient depolarization during excitatory transmitter action called?

Answer 4

Excitatory PostSynaptic Potential (EPSP)

Question 5

What occurs during inhibitory transmitter action?

Answer 5

the conductance change drives the membrane potential to a level that is MORE negative (i.e. slight hyperpolarization) than the threshold; thus decreasing probability that the post synaptic cell will produce an action potential.

Question 6

What is the transient hyperpolarization during the inhibitory transmitter action called?

Answer 6

Inhibitory PostSynaptic Potential (IPSP)

Question 7

What is the most important aspect about inhibitory transmitters?

Answer 7

They drive the membrane potential to a level MORE NEGATIVE than threshold.

Question 8

What is the structure of a neuron?

Answer 8

Dendrites –> Cell Body (Soma) –> Axon Hillock –> Axon (w/ trigger zone)

Question 9

Describe dendrites

Answer 9

highly branched process arising form cell body that conduct messgaes toward the cell body

Question 10

Describe cell body (soma)

Answer 10

Contains nucleus and syntehtic machinery. Possess the axon hillock - a place where the axon leaves the soma

Question 11

Describe an axon.

Answer 11

it’s a single process leaving the soma t the axon hillock region. The first part is the initial segment (called the trigger zone), where action potentials originate and carries information away from the soma.

Question 12

What are the three classifications of synapses on a neuron? Where are these synapses located?

Answer 12

1. Axodendritic : on dendrite
2. Axosomatic : on soma
3. Axoaxonic : on axon of neuron

Question 13

What is different about presynaptic ending releases in the NMJ and synapses between neurons?

Answer 13

In the NMJ, over 200 quanta produce an EPP of 40 mV by opening 1000s of channels, but in a CNS synapse, only one or a few quanta are released to produce a potential of <100 channels

Question 14

How is the action potential activity of a CNS neuron determined?

Answer 14

by the sum of all synaptic inputs impinging on the neuron at any givien time

Question 15

Are all inputs equal in their influence of action potential activity of a CNS neuron?

Answer 15

No! The synaptic potentials are decrementally inputted. Inputs closer to the site have a greater influence.

Question 16

What is temporal summation of CNS nerves? What does effectiveness depend on?

Answer 16

Temporal summation is the build of of synaptic potentials during repetitive stimulation of a SINGLE input because of an overlap IN TIME of the postsynaptic response.

Effectiveness depends on time constant. Higher time constant leads to more effective temporal summation.

Question 17

What is spatial summation of CNS nerves? What does effectiveness depend on?

Answer 17

when synaptic potential from two or more separate inputs occur at the same time and potentials add to one another or subtract from each other.

The effectiveness depends on the length constant of post synaptic neuron. Higher length constant leads to more effective spatial summation.

Question 18

What is causes presynaptic modulation of transmitter release in a CNS cell? What are the four types of Presynaptic modulation of transmitter release?

Answer 18

when the amount of transmitter released by a nerve terminal depends on the PREVIOUS activity of the terminal

Four types include:

1. facilitation
2. posttetanic potentiation (PTP)
3. Long term potentiation
4. Depression

Question 19

What is facilitation in terms of CNS presynaptic modulation? How long does it last? What is the cause? What is related to?

Answer 19

the increase in size of the post synaptic response during repetitive stimulation of the presynaptic neuron.

It lasts < 1 second

It is caused from an increased number of transmitter quanta being released with each succeeding stimulus.

It’s related to residual Ca2+: Ie. if another action potential occurs befor eall the Ca2+ from the preceding action potential is sequestered, the new Ca2+ adds to the residual Ca2+ causing a greater release.

Question 20

What is posttetanic potentiation (PTP) in terms of CNS presynaptic modulation? How long does it last? What is the cause? What is related to?

Answer 20

Enhancement of the post synaptic response AFTER subjecting the presynaptic neuron to high frequency stimulation for several seconds.

It lasts several minutes after cessation of the stimuli.

Again due to an increased number of transmitter quanta being released per stimulus after the tetanic stimulation

Since PTP outlasts the rise in [Ca2+]_i produced by tetanic stimulation, elevated [Ca2+]_i triggers some Ca-dependent signal transduction pathway, such as phosphoylation of some protein associated with vesicle release, that sustains the increased release

Question 21

What is long-term potentiation in terms of CNS presynaptic modulation? How long does it last? What parts of the synapse does it involve? What mediates it?

Answer 21

Enhanced transmitter release AFTER a strong tetanic stimulation

Lasts for days or longer

Involves both pre and post synaptic events and is mediated by NMDA receptors on the post synaptic cell.

Activation of NMDA receptor leads to a production of a retrograde messenger (like NO or CO), which diffuses from post synaptic cell to presynaptic ending causing an increase in transmitter output from the presnaptic ending by probably activating one or more second messenger systems.

Question 22

Does long-term potentiation require the cell to be depolarized?

Answer 22

Yes. NMDA activation needs the cell to be depolarized first.

Question 23

What is depression in terms of CNS presynaptic modulation? How long does it last? What parts of the synapse does it involve? What causes it?

Answer 23

A decrease in the amount of transmitter released after a train of stimuli?

Lasts only a few sec to a few minutes

Occurs at synapses with high quantal content.

Caused by depletion of the vesicles available for release in the presynaptic ending