Midterm 2 Flashcards
Week 2 (Action potentials and propagation)
Neurons are electrically excitable, the membrane potential deviates from the resting membrane potential of _____________.
-70 mV
Decreasing membrane potential is associated with depolarization, true or false?
False, Increasing
Increasing membrane potential is associated with depolarization, true or false?
True
Increasing membrane potential is associated with hyperpolarization, true or false?
False
Neurons that are only capable of generating graded potentials are involved in what type of communication?
Local or short distance communication, as signal transmission distance is limited.
Neurons that generate action potentials are capable of ______________ signalling.
Long-range
The generation of action potentials depends on the generation of graded potentials, true or false?
True
The generating of graded potentials depends on the generation of action potentials, true or false?
False, vice versa (action potentials depend on graded potentials)
All neurons that can generate action potentials can also generate graded potentials, true or false?
True
All neurons that can generate graded potentials can generate action potentials, true or false?
False
How is the resting membrane potential generated?
An ion pump actively (requires energy, ATP) moves potassium ions (K+) from the outside (extracellular space) to the inside of the neurons. K+ concentration inside is elevated.
The pump also moves sodium (Na+) ions from the inside, keeping the concentration of sodium low relative to the outside.
At rest, there are ion channels that allow some of the potassium to move from the inside to the outside, down the ________________________.
Concentration gradient
If Na+ moves from the outside of the cell to the inside, down the concentration gradient, would the inside of the cell become more positive or more negative?
More positive
The influence of K+ is much greater because the K+ channels are more ______________ than the Na+ channels.
Peremeable
Action potentials depend on graded depolarization, what causes this depolarization in a simple sense?
The permeability of the Na+ channel increases, Na+ moves down the concentration gradient (from the outside of the cell to the inside), the membrane voltage than becomes more positive (depolarizes).
Action potentials are produced by ion channels that are unlike those that produce graded potentials, what makes them different?
Action potentials are produced by ion channels that are voltage-gated. Graded potentials are produced by channels that are activated by a change in membrane potential.
Voltage-gated Na+ channels are subject to intrinsic inactivation, meaning…
Soon after activation, voltage-gated Na+ channels inactivate (contributing to the falling phase after the peak).
The falling of the action potential is due not only to the inactivation of voltage-gated Na+ channels but also due to…
A delayed activation of the voltage-gated K+ channels
What is the absolute refractory period?
Voltage-gated Na+ channel inactivation imposes a refractory period on action potential generation. The absolute refractory period is the period during which, no matter how strong the stimulus, another action potential cannot be generated. It is due to the inactivation of the voltage-gated Na+ channels.
What and when is the relative refractory period?
The relative refractory period is the time following Na+ channel reactivation, but when voltage-gated K+ channels are still sufficiently active to oppose the depolarization (increased hyperpolarization, moves in negative direction) to the threshold. A sufficiently strong stimulus could overcome such opposition.
Voltage-gated K+ channels inactivate, true or false?
False, voltage-gated K+ channels do not inactivate, it takes a long time for voltage-gated K+ channels to return to resting levels, as membrane potential hyperpolarizes
Explain what factors impose limits on the frequency that action potentials can be generated
Refractory periods
After hyperpolarization due to voltage-gated K+ channels (not inactivating)
Name the two-mechanisms that allow for action potentials to travel further distances (active regeneration of the signal)
- Voltage-gated channels expressed along the fiber so the action potential is constantly being boosted, this is the case in unmyelinated axons
- Insulate the axon with myelin, to decrease the decay of the signal
Does myelin cover the axon completely?
No, there are small gaps (Nodes of Ranvier)
On a myelinated axon, voltage-gated Na+ and K+ channels are found only at which part?
The nodes. Myelin reduces the decay of the electrical signal so that the voltage is above threshold when it reaches the node, at the node the signal is regenerated.
Conduction velocity is affected by axon diameter, what does this mean?
Larger axons have less longitudinal resistance and thicker myelin
What are the two basic types of synapses?
1) Electrical synapses (gap junctions)
2) Chemical synapses (release neurotransmitter)
What is an electrical synapse (gap junction)?
Transmembrane channels that connect the interior of one cell with the interior of another cell
Which type of synapse is bidirectional and essentially has no synaptic delay?
Electrical synapse (gap junction)
What is a chemical synapse?
A neurotransmitter is released at axon terminals, crosses the synaptic cleft, and then acts on receptors on a postsynaptic membrane.
What type of synapse is unidirectional?
Chemical synapse
Chemical synapses are bidirectional, true or false?
False, chemical synapses are unidirectional (presynaptic to postsynaptic)
Neurotransmitters are contained within __________________________, which fuse with the presynaptic membrane and release contents into the synaptic cleft
Synaptic vesicles
There is partial fusing of vesicles, true or false?
False, there is NO partial fusing of vesicles, if a vesicle fused, all of the neurotransmitter in the vesicle is released.
The amount of neurotransmitter released by a single vesicle is called a __________________.
quantum (quanta if more than one vesicle fuses)
The total amount of neurotransmitter released by a single action potential corresponds to the number of quanta, the number of vesicles released depends on?
How much Calcium enters the terminal, which is related to … the extent of voltage-gated calcium channel activation, which is related to … the number and frequency of action potentials that reach the terminal
How is synaptic transmission halted?
In most cases, transporters on the pre-synaptic membrane can remove neurotransmitter from the synaptic cleft and recycle it.
In some cases, the neurotransmitter is broken down by an enzyme and used to synthesize more of the neurotransmitter.
The neurotransmitter released from synaptic vesicles produces graded potentials that can either transiently depolarize (producing a(n) ___________) or transiently hyperpolarize (producing a(n) ______________) the postsynaptic membrane.
Depolarize = excitatory post-synaptic potential
Hyperpolarize = inhibitory post-synaptic potential
Glutamate is the most common excitatory neurotransmitter in the central nervous system. Glutamate is synthesized from the amino acid glutamine, and vesicles are loaded with glutamate by a _________________________.
Vesicular glutamate transporter (vGlut)
To remove glutamate from the synaptic cleft, it is transported by _____________________ into ______________________.
Excitatory amino acid transporters into astrocytes
Astrocytes covert glutamate into __________________.
Glutamine
After astrocytes convert glutamate into glutamine, the glutamine is then transported (by other transporters) back to the _____________________ where it is converted into glutamate (by the enzyme glutaminase) and loaded into vesicles.
presynaptic neuron
For excitatory neurotransmission, glutamate acts primarily at post-synaptic __________________. When glutamate binds to ______________________ a channel allows _____ to enter the postsynaptic neuron, leading to depolarization.
AMPA receptors
Na+
A type of glutamate receptor that is permeable to Na+ and Ca2+. The influx of these cations (especially Na+) can contribute to excitatory post-synaptic potential.
NDMA receptors
Explain the mechanism of the NDMA receptor.
Under resting conditions, the NDMA receptor channel is blocked by Mg2+. To remove the Mg2+ block requires that the receptor is depolarized (e.g., by AMPA receptors).
Activation of NDMA receptors also requires the presence of co-transmitter glycine.
The AMPA and NDMA receptors are both involved in excitatory post-synaptic potentials, what is the main (broad) difference between the two?
The AMPA receptor is less “complicated” than the NDMA. NMDA receptor requires voltage-dependent removal of the Mg2+ block, and require partnership from AMPA and co-transmitter glycine.
Excitatory postsynaptic potentials produced by AMPA alone can be brief, those produced by NDMA (+AMPA) receptors can be of longer duration.
How are inhibitory post-synaptic potentials generated?
Inhibitory post-synaptic potentials are generated by inhibitory neurotransmitters, the most common inhibitory transmitter is GABA.
GABA is synthesized from _______________ and loaded into synaptic vesicles by the vesicular GABA transporter.
Glutamate
Once released GABA is cleared from the synapse by __________________ either on the presynaptic membrane (where it can be loaded again into vesicles) or on ________________ where it is converted to glutamine and can be returned for synthesis of GABA.
GABA transporters
Astrocytes
GABA can act exert an inhibitory influence by acting either at GABA(A) receptors, increasing the influx of ________, or at GABA(B) receptors that increases the efflux of ________ through channels.
Cl-
K+
GABA(A) receptors (as well as AMPA and NDMA receptors) are ________________ receptors
Ionotropic (directly control permeability of the membrane)
GABA(B) receptors are _______________ receptors.
Metabotropic receptors (indirectly control permeability of the membrane, binds to receptor through G-protein second messenger)
What are the two types of neurotransmitter receptors discussed in class?
Ionotropic receptors
Metabotropic receptors
What are the characteristics of ionotropic receptors?
Ions move rapidly across the membrane through ligand-gated ion channels (channels are selective to certain ions).
Effect requires transmitter to be bound to the receptor (many desensitize even in the continued presence of neurotransmitter, so ion flow is brief)
What are the characteristics of metabotropic receptors?
Typically involves G-protein activation that either directly, or indirectly (second messenger system) influences ion channel permeability or other intracellular processes.
Slower than ionotropic
Second messengers may persist after removal of transmitter.
Ionotropic receptors work faster than metabotropic receptors, true or false?
True
Ionotropic receptors work slower than metabotropic receptors, true or false?
False, ionotropic receptors work faster than metabotropic receptors.
The amplitude of an excitatory post-synaptic potential depends on the strength of the signal arriving at the synaptic terminal which affects…
The amount of neurotransmitter (glutamate) release.
The neuron that receives the signal is called the presynaptic neuron, true or false?
False, the neuron that receives the signal is called the postsynaptic neuron.
An excitatory postsynaptic potential may not be sufficient to depolarize the postsynaptic neuron to the threshold required for action potential generation, true or false?
True
Excitatory postsynaptic potentials can summate if sequenced quickly enough (__________________ summation) or through multiple inputs (_________________ summation).
Temporal summation = sequenced quickly
Spatial summation = multiple inputs
The summation of excitatory postsynaptic potentials allows for…
The postsynaptic membrane potential to reach threshold
Inhibitory postsynaptic potentials can prevent excitatory postsynaptic potentials from reaching threshold, true or false?
True, postsynaptic inhibition
