APs and Synaptic Transmission Flashcards
what conditions are necessary for the initiation of an action potential?
both the activation gate and inactivation gate for the Na+ channels must be clear
what happens when the activation gate of Na+ channels open?
sodium flows into the cell until reaching its equil potential of +60mV
how does inactivation gate contribute to the action potential process?
inactivation gate closes Na+ channels by plugging the channel
what role do K+ channels play in the action potential process?
after Na+ channel action, K+ channels open, allowing K+ to leave the cell, leading to repolarization
what occurs during hyperpolarization?
K+ channels remain open, causing delayed closing and hyperpolarization of the cell
when does the closure of K+ channels occur in the action potential?
after repolarization and hyperpolarization, K+ channels eventually close
when happens to the cell after the closure of K+ channels in the action potential process?
returns to resting state
provide a step by step summary of the action potential
- Both activation and inactivation gates of Na+ channels must be clear for initiation. (depolar stimulus arrives at channel)
- Activation gate opens, allowing Na+ influx until reaching +60 mV.
- Inactivation gate closes Na+ channels.
- K+ channels open, leading to repolarization and hyperpolarization.
- K+ channels eventually close, and the cell returns to the resting state.
what is the equil potential of sodium during an aP?
+60mv, will flow into the cell until this si reached
why is the closure of the inactivation gate crucial in the action potential process?
the inactivation gate closes Na+ channels, preventing further influx, and is essential for proper termination of the action potential.
what biophysical properties does the phospholipid bilayer membrane of a cell exhibit?
displays properties akin to a resistor (ion channels) and capacitor (negative phosphate groups of the membrane acting as plates and fatty acids as insulators) in parallel.
how is resistance related to conductance?
is inversely proportional
what function do ion channels serve in terms of resistance in the cell membrane?
act as resistors by controlling the rate of ion flow
the more _____ there are, the ______
open channels, lower the resistance
explain how ion channels contribute to the conductance of the cell membrane
influence conductance, the more open channels the higher the conductance
what components of the cell membrane exhibit capacitor like behavior?
negative phosphate groups of the membrane function as plates, and fatty acids act as insulators, creating a capacitor-like structure.
describe how the resistor (ion channels) and capacitor (cell membrane components) are arranged biophysically.
resistor (ion channels) and capacitor (negative phosphate groups and fatty acids) are arranged in parallel in the phospholipid bilayer membrane.
explain the connection between the number of open channels and the resistance in the cell membrane
more open channels = lower resistance in the cell membrane
what is a capacitor and what components does it consist of?
capacitor is something that stores charge and consists of two metal plates separated by an insulator
equation for a capacitor
How is current related to the rate of change in voltage, and when do changes in capacitance/current occur?
Current is proportional to the rate of change in voltage. Changes in capacitance/current occur only when there are changes in voltage.
current will always take the ________; if possible, it will flow through a ____ rather than a _____
path of least resistance, capacitor, resistor
explain the hose analogy in the context of a neuron’s axon and membrane
The axon/axonal membrane is likened to a sponge surrounding a hose. Initially, water (current) is soaked up by the sponge (capacitor), and only once it’s “charged up” will it flow through the hose (resistor).
What does the graph illustrating the initiation of an action potential reveal?
An action potential doesn’t initiate instantly but slowly charges up the cell, presenting a challenge as neurons should respond instantly.
Define the time constant (T) and provide the equation representing it.
The time constant (T) is the time it takes for a neuron to charge up to (or discharge down to) 63% of its final voltage. The equation is τ = RC.
How can the time constant be modulated, and what are the effects of decreasing and increasing resistance (r)?
The time constant can be modulated by decreasing or increasing resistance (r). More channels result in lower resistance and a lower time constant, while fewer channels result in higher resistance and a greater time constant.
Define the length constant and describe what it represents in the context of a neuron.
The length constant is the distance at which 63% of Vmax has been reached during the rise or fall of voltage in a neuron. It depends on internal resistance (ri) and external resistance (rm).
What factors influence the length constant, and how do internal and external resistance play a role?
The length constant depends on internal resistance (ri) caused by cytoplasm and external resistance (rm), also known as membrane resistance. Increased ion channels lower membrane resistance, making it easier to leave the axon and reducing the length constant.
there is no ____ if there is no ____
current, driving force
In bioelectrical circuits, how can Ohm’s Law (V=IR) be alternatively expressed, and what does it represent?
Ohm’s Law can be expressed as I = GV, where resistors are considered linear elements (y = mx + b).
What relationship exists between conductance and the slope of the current vs. voltage graph in a channel?
If there is high conductance through a channel, it will have a steep current vs. voltage slope.
What happens to the slope of the current vs. voltage graph in a channel with low conductance?
If there is low conductance through a channel, it will have a shallow current vs. voltage slope.
How do action potentials (APs) regenerate as they travel down the membrane?
APs decay as they travel, but activation of channels along the membrane regenerates them. Local depolarization triggers neighboring Na+ channels to open, generating further action potentials.
What role does local depolarization play in the regeneration of action potentials?
Local depolarization causes neighboring Na+ channels to open, initiating further action potentials and contributing to regeneration.
with APs, ______ is possible due to their regeneration; however they are still ___
long range communication, slow
____ speeds up AP propagation and allows for long range communication via _____
myelin, saltatory conduction
myelin prevents ____ as it is wrapped in ____
leakage, oligodendrocytes
without _____ signals would be _____
saltatory conduction, delayed
soup:
chemical synpases are the primary mode of synaptic transmission
spark:
eletrical synpases are the primary mode of synaptic transmision
which is true, soup or spark?
both, but soup wins; there is mostly chemical trans in the cell but small amount of electrical
what do both chemical and electrical transmission help do?
synchronize activity in neuronal populations
what was the morphological correlation in the quantum hypothesis?
was seen that vesicles fuse to the membrane during chemical transmission
Fatt and Katz (1952): what did they do?
monitored postsynaptic electrical activity in the frog neuromuscular junction
Fatt and Katz (1952): what was observed?
Observed miniature evoked potentials (MEPs) that lessened the further away the electrode was from the synapse.
Fatt and Katz (1952): what was hypothesized?
that there was neurotransmitter released from the axon.
Del Castillo and Katz (1954): what did they do?
Reduced the amount of calcium in the extracellular medium and found that there was less NT released after stimulation
Del Castillo and Katz (1954): what did they find?
found that the evoked amplitudes occurred as multiples of discrete units → called this quanta.
steps in synaptic transmission
- Bring the presynaptic neuron to threshold at the axon hillock.
- Conduct the AP down the presynaptic axon.
- Open voltage-gated Ca2+ channels at the nerve terminal
- Diffusion and action of Ca2+ at the release machinery.
- Exocytosis and diffusion of neurotransmitters in the synaptic cleft.
- Activation of postsynaptic receptors.
what are the steps in synaptic vesicle cycle
- docking and priming
- release of NT (exocytosis)
- retrieval of NT (endocytosis)
what proteins are used in docking and priming step?
SNARE proteins - involved in vesicle docking
what are the 3 major snare proteins?
Synaptobrevin, Syntaxin & SNAP-25
what do the snare proteins do?
Synaptobrevin: vesicle snaring
Syntaxin & SNAP-25: membrane snares
These 3 proteins come together to dock the vesicle on the membrane.
what is one problem associated w/ synaptic vesicle cycle?
negatively-charged phosphates in the vesicle/cell membrane should repel each other
how is this problem solved?
- SNARE circuitry works against energy barriers to push barriers together.
- Alpha helices are looped into each other like a zipper mechanism to bring the vesicle towards the membrane and dock.
what is caused by the AP and voltage gated channels?
release of NT (exocytosis)
what snare protein is involved in release of NT?
synaptotagmin-1
what is synaptotagmin-1?
the calcium sensor
a vesicle is released when there is an ____ and _____
action potential, calcium channels cause a cloud of Ca2+ near the vesicular machinery
Ca2+ binding to ____ leads to _____
synaptotagmin, its insertion into the membrane
what happens after insertion into the membrane?
pulls the vesicle membrane and cell membrane together, eventually leading to the fusion of the two membranes and exocytosis of NTs.
when synaptotagmin is KO’d ____
there is less and irregular NT release
what is shown in this graph?
syt1 KO vs wildtype
-> Syt1 knockout completely ablates fast synchronous release but not slow asynchronous responses
what happens in retrieval?
Ca interacts w/ SNARE and drives fusion
