Synaptic Neurotransmission Flashcards by Miguel Merida

The postsynaptic potential (PSP) is a transient change in membrane potential (ΔVm) caused by?

An ionic current across the membrane.

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The ionic current is determined by the membrane conductance, which is governed by?

Neurotransmitter stimulation of the postsynaptic receptors.

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The magnitude of the PSP (ΔVm) is determined by?

The number of ion channels that open.

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The duration of the PSP is determined by?

How long the ion channels remain open

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Which one applies to the “magnitude of PSP”?

A) determines the peak strength of the ionic current, and is dependent upon how much neurotransmitter is present in the synaptic cleft

B) depends upon how long the neurotransmitter “remains resident” in the synaptic cleft

A) determines the peak strength of the ionic current, and is dependent upon how much neurotransmitter is present in the synaptic cleft

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Which one applies to the “duration of PSP”?

A) determines the peak strength of the ionic current, and is dependent upon how much neurotransmitter is present in the synaptic cleft

B) depends upon how long the neurotransmitter “remains resident” in the synaptic cleft

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The PSP is therefore dependent upon the ____ of neurotransmitter to stimulate the postsynaptic receptors

availability

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The amount of neurotransmitter present in the synaptic cleft is continuously dependent upon the relative balance between two rates:

the rate of ____ from the nerve terminal
the rate of ____ from the synaptic cleft

release; removal

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As long as the neurotransmitter remains ____ within the synaptic cleft, it will continue to re-stimulate the receptors

resident

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The neurotransmitter molecule does not permanently bind to a receptor, due to?

A) after a variable length of time (ms) it is released

B) the kinetics of binding and release are stochastic the duration the molecule remains bound is random, within limits

C)“re-stimulate” it is probable that a transmitter molecule just released will once again bind to an open receptor

D) the ion channels controlled by each receptor will therefore remain open for a variable length of time

All these statements are true

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What are the three general mechanisms for the removal of neurotransmitter from the synaptic cleft?

1) Reuptake by the presynaptic terminal
2) Enzymatic degradation within the synaptic cleft
3) Diffusion, i.e. the neurotransmitter diffuses out of the cleft into the surrounding interstitium

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Of the three general mechanisms that removes the neurotransmitter form the synaptic cleft does the following belong to?

this mechanism is most common, and occurs via high affinity transporter mechanisms along the presynaptic membrane
may rapidly pump released transmitter out from the cleft
typically requires energy (ATP-dependent) and/or an ion exchange mechanism

Reuptake

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Of the three general mechanisms that removes the neurotransmitter form the synaptic cleft does the following belong to?

typically via an esterase that is locally available within the cleft
rapidly reduces the available neurotransmitter within the cleft

Enzymatic

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Of the three general mechanisms that removes the neurotransmitter form the synaptic cleft does the following belong to?

accounts for at most 10-20% of transmitter removal
not rapid enough to support optimal synaptic coding

Diffusion

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Normal synaptic structure will present a certain number of?

Postsynaptic receptors

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We normally assume that there will be a fixed population of postsynaptic receptors, but we also recognize that physiological regulatory mechanisms can either __-____ or __-____ the population size (i.e. the number of receptors).

up-regulate; down-regulate

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Recall that the strength of the ionic current across the postsynaptic membrane is determined by?

strength of the electrochemical forces acting upon the ions (Vm-E)
the conductance of the postsynaptic membrane

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Given that neurotransmitter is continuously drained from the synaptic cleft at a constant rate …

The amount of neurotransmitter present within the synaptic cleft at any moment in time is determined by?

the rate of release from the nerve ending

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in response to the arrival of each action potential at the nerve terminal, the amount of neurotransmitter that is released is determined by?

how much Ca++ enters the nerve ending

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How much Ca++ enters the nerve ending is determined by?

how long the Ca++ channels remain open the duration of the action potential
the strength of the electrochemical gradient on Ca++ –> dependent upon the resting Vm of the nerve ending

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Recall: that within the nerve ending, the cytosolic [Ca++] is dependent upon the temporal interaction between the rates of ____ ____ and ____ ____ from the cytosol

Ca++ entry; Ca++ removal (buffering)

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Is the rate of influx variable or stable?

variable

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The rate of influx is dependent upon the _____ ____ of the nerve ending.

stimulation frequency

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The kinetics of Ca++ buffering are part of the normal structure of the nerve ending, and therefore remain?

a) variable
b) stable

Stable

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increases in the frequency of arriving action potentials --> ____ rate of Ca++ entry

increased

decreases in the frequency of arriving action potentials --> ____ rate of Ca++ entry

decreased

Ca++ accumulation within the nerve terminal causes increased neurotransmitter release, which, in turn, causes an ____ in the magnitude of the postsynaptic response

increase

What is potentiation?

The increase in the postsynaptic response, due to an increased amount of transmitter released into the synaptic cleft

What does tetanic stimulation mean? a high rate = 400+ per second

Above this frequency there will be no further increase in transmitter release

What is post-tetanic potentiation?

transient persistence of the enhanced postsynaptic response following a brief period of rapid stimulation

With an increase in the frequency arriving action potentials, the magnitude of the PSP will?

Increase

With a decrease in the frequency arriving action potentials, the magnitude of the PSP will?

Decrease

The rate of neurotransmitter release from a nerve ending is governed by the ____ of action potentials arriving at the nerve ending

frequency

The amount of neurotransmitter that remains resident within the synaptic cleft results from the temporal interaction between the rates of ____ and ____

release; removal

The neurotransmitter at a particular synapse is acetylcholine. Suppose that the availability of acetylcholinesterase at that synapse is only one-half its normal value. For a given frequency of arriving action potentials at the nerve ending, the magnitude of the PSP will: A) be greater than expected B) be less than expected C) not be affected

A) be greater than expected

A train of action potentials arrive at the nerve ending at a stead frequency; therefore, the cytosolic [Ca++] remains?

Stable

A train of action potentials arrive at the nerve ending at a steady frequency, but with one “missing” action potential the cytosolic [Ca++] will?

decrease in one “step” from a higher to a lower level the rate of neurotransmitter release also decreases in one “step”

A train of action potentials arrive at the nerve ending at a steady frequency, but with one “extra” inserted action potential the cytosolic [Ca++] will?

increase in one “step” from a lower to a higher level the rate of neurotransmitter release also increases in one “step”

neurotransmitter availability in the synaptic cleft will therefore vary ____ ____ with changes in the frequency and pattern of the arriving action potentials

in step

Given a steady rate of removal of neurotransmitter from the synaptic cleft, the PSP (whether an EPSP or an IPSP) is determined by the ____ ____ ____ ____ ____ ____ ____ ____, which is continuously governed by the frequency and pattern of arriving action potentials.

rate of neurotransmitter release from the nerve ending

Sustained ____ via trains of arriving action potentials gives rise to a ___ ____ in the postsynaptic Vm.

Stimulation; sustained change

What is modulation?

Strengthen or lessen the normal response

What is a normal response?

a fixed amount of neurotransmitter is released from the nerve ending in response to the arrival of each action potential

How can this normal response be modulated?

the amount of Ca++ entering the nerve ending will vary relative to changes in the resting membrane potential (Vm) at the nerve terminal

Altering the ____ membrane potential of the presynaptic nerve ending provides the means to alter (“____”) the amount of neurotransmitter released in response to arrival of ____ ____ action potential

resting; modulate; any one

Is this an example of hyperpolarization or depolarization? Ca++ influx will be increased, leading to increased transmitter release, and a larger postsynaptic potential

Depolarization

Is this an example of hyperpolarization or depolarization? Ca++ influx will be decreased, leading to decreased neurotransmitter release, and a smaller postsynaptic potential

Hyperpolarization

The resting membrane potential of the presynaptic nerve ending can be altered via other nerves providing a synaptic input with the presynaptic nerve. Give an example:

axo-axonic synapse

Presynaptic inhibition: Input from an “____ neuron” (c1) causes the nerve terminal of the presynaptic neuron (a) to ____, causing ____ neurotransmitter release from the presynaptic neuron to its postsynaptic neuron (b)

inhibitory; hyperpolarize; reduced

Presynaptic facilitation: Input from an “____ neuron” (c2) causes the presynaptic terminal to ____, causing ____ neurotransmitter release from the presynaptic to the postsynaptic neuron (a to b)

excitatory; depolarize; increased

Which has the capability to vary the amount of neurotransmitter that is released in response to each action potential arriving at the nerve ending? A) Presynaptic inhibition B) Presynaptic modulation C) Presynaptic facilitation

B) Presynaptic modulation

Name the two forms of modulation:

Inhibition | facilitation

Which form of modulation provides "more neurotransmitter release"?

facilitation

Which form of modulation provides less neurotransmitter release?

inhibition

Which of the two "regulated" presynaptic modulations would strengthen the postsynaptic response of neuron “b” to its input from neuron “a”?

facilitation

Which of the two "regulated" presynaptic modulations would lessens the postsynaptic response of neuron “b” to its input from neuron “a” ?

inhibition

Presynaptic modulation provides the means for control signals to be fed into a synapse to either ____ or ____ the transmission of information encoded by the ____ and ____ of the arriving action potentials from the presynaptic neuron over to the postsynaptic neuron

diminish or augment; frequency and pattern

Synaptic inputs are mainly to what region of a neuron?

Dendritic

As the ionic current flows through the synaptic channels, it then flows along the inner membrane via ____ ____ to spread away from the immediate synapse

electrotonic conduction

The PSP will therefore spread along the membrane due to this electrotonic flow of current, but due to “____ ____” and active pumps along the membrane, the magnitude of the PSP will ____ as it spreads along the neuronal membrane, ____ from the synapse

current leakage; diminish; away

Assume that an excitatory synaptic input to the dendritic region, what occurs?

The PSP (ΔVm) will then spread along the neuronal membrane towards the cell body, but with decreasing amplitude

Where are action potentials generated at?

Axon hillock (trigger zone)

T/F: The region at the axon hillock has a relatively low threshold potential, due to the presence of many voltage gated Na+ channels

True

For excitatory inputs to cause the neuron to “fire” action potentials, there must be sufficiently strong “____ current flow” at the synapses along the ____ membrane to drive enough current flow along the ____ membrane to cause the axon hillock to become ____ above its threshold potential

depolarizing; dendritic; neuronal; depolarized

A given neuron will typically receive 100’s to 1000’s of ____ synaptic inputs

individual

a depolarizing ionic current across a ____ synapse will not be strong enough to drive a spread of current along the inner membrane of the neuron to depolarize its axon hillock above its threshold for triggering an action potential

single

to cause a neuron to start “firing” action potentials, multiple ____ inputs must ____ ____ to cause a sufficient level of ____ at the axon hillock

excitatory; sum together; depolarization

What is synaptic integration?

the process through which multiple synaptic responses are “summed together” to determine whether or not the postsynaptic neuron responds by firing action potentials