Synaptic Neurotransmission Flashcards
The postsynaptic potential (PSP) is a transient change in membrane potential (ΔVm) caused by?
An ionic current across the membrane.
The ionic current is determined by the membrane conductance, which is governed by?
Neurotransmitter stimulation of the postsynaptic receptors.
The magnitude of the PSP (ΔVm) is determined by?
The number of ion channels that open.
The duration of the PSP is determined by?
How long the ion channels remain open
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
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
B) depends upon how long the neurotransmitter “remains resident” in the synaptic cleft
The PSP is therefore dependent upon the ____ of neurotransmitter to stimulate the postsynaptic receptors
availability
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
As long as the neurotransmitter remains ____ within the synaptic cleft, it will continue to re-stimulate the receptors
resident
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
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
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
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
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
Normal synaptic structure will present a certain number of?
Postsynaptic receptors
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
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
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
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
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
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)
Is the rate of influx variable or stable?
variable
The rate of influx is dependent upon the _____ ____ of the nerve ending.
stimulation frequency
The kinetics of Ca++ buffering are part of the normal structure of the nerve ending, and therefore remain?
a) variable
b) stable
Stable
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
