Action Potentials + Synaptic Transmission Flashcards
what was proposed by Johannes Peter Muller?
- in 1835 johannes peter muller proposed the law of specific nerve energies
what is the law of specific nerve energies?
nature of perception is defined by the pathway over which the sensory information is carried
->each type of nerve has different properties that determine how we sense a stimulus
how is the brain like an encoder?
similar to morse code via action potentials
-> must be able to infer from beeps what the message is, like how APs go to the brain
what is an action potential?
ions moving through the membrane causing changes in membrane potential
what is a depolarized state?
more positive than resting potential
what is a hyperpolarized state?
more negative than resting potential
Na+/K+ ATPase
sets ionic gradients.
-> uses ATP to bring 3Na out, 2K in
K equilibrium potential
must have permeability to create potential
- when channels are introduced, K goes from high to low
1. concentration gradients push K out
2. electrostatic force pushes it back in
steady-state potentials
net inward current = net outward currnt
- net flow of all charges
equation for work required to move an ion in an electric field
We = (zF)*V
We = work required
z = valence of the ion
F = Faraday’s constant
V = voltage
work required to change concentration equation
Wc = RT*ln([X]2/[X]1)
-sum is 0 at equilibrium
- can predict what membrane potential will be for a certain ion
- nernst equation
what is the nernst equation @37c?
(61.5/z)(log([X]out/[X]in))
what is the nernst equation referring to?
a mathematical relationship used to calculate an ionic equilibrium potential
work required to change concentration = work required to move ion through electrical field
potassium flows in the direction that would drive Vm towards ……
Ek, wants to be at equilibrium
if you move resting potential more positive….
thats depolarization
if you move resting potential more negative…
thats hyperpolarization
when you move away from equilibrium, you create…
a driving force
driving force equals?
membrane potential - eq potential
Na flows in the direction that would tend to…
push Vm towards Eq potential
Cl flows in the direction that would tend to…
push Vm towards Eq potential of Cl
currents can go through the membrane via two distinct pathways…
channels (R= resistance) and the capacity of the neuron offered by the lipid bilayer membrane (C=capactiance)
resistance is proportional to….
channel properties such as pore size
resistance is the same as…?
conductance, they are inversely proportional. stronger resistance = harder to push current through
what is the difference between resistance and conductance?
resistance is how much you resist the flow of charge, conductance is how much you conduct
current depends on….
voltage, which is the driving force
larger voltage =
larger current
larger resistance =
less current
what is ohm’s law?
V= IR
current = conductance * voltage
net driving force tells you which direction the current will flow in
I = G*DF
what is a capacitor?
two metal plates separated by an insulator and connected by a battery. the capacitor has the ability to store charge.
current flow
flows through a resistor and capacitor set in parallel
what does capacity depend on?
size of the plates (bigger = more charge you can store) and the gap between plates and insulator
capacitance is…
charge per volt (C = q/V)
how much charge you can store to get a volt across the capacitor
what is the time constant?
the amount of time required for the voltage to reach 63% of its final value
if the time constant of a neuron is 5ms, it will take 5ms for it to reach 63% of its voltage
how can you find capacitance with time constant and resistance?
Tau = R*C
delayed electrical signals
convey sense energies to the brain
- we live in the past
- time constant to charge the membrane
which charges faster, a neuron with fewer channels or a neuron with more channels?
the one with more channels, because there is smaller resistance (less membrane)
more channels =
less resistance because there are more ways for the electricity to flow through
capacity current equation
C = q/V
q= CV
dq/dt= I = CdV/dt
voltage along an axon
current will go some distance and leak out, length constant determines how far the signal spreads in time
length constant equation
lambda = Sqr (Rm/Ri)
- the easier it is to leave the axon, the lower the length constant
what is the length constant?
length constant the distance at which there is 63% of leakage; axial resistance.
aka if there is a length constant of 1mm, then 1mm away from the cell body down the axon, only 37% of the voltage magnitude remains
what dictates leakage?
the ratio of resistance
if resistance is high, the current flows through the sides through the membrane resistance. the ratio of the two determines the length constant.
action potentials
solves length problem:
- long range communication is possible
- AP continue because of active propogation
- all or none events generated by voltage gated ion channels
Na+ channels:
for an action potential to be initiated, both activation gate and inactivation gate must be clear
activation gate
once open, sodium flows into the cell until its equil potential (+60mv)
inactivation gate
closes Na+ hannels by plugging the channel
voltage gated Na channel role in AP
has an inactivation gate that turns off even even in depolarization which allows you to take AP back down
K channel role in Ap
ensures unidirectional propagation:
- K channels activate with a delay and pull membrane potential back which overshoots resting
- activate and immediately inactivate so you cannot go backwards
saltatory conduction
myelin decreases capacity current, tightens resistance and concentrates ion channels to nodes which allows current to spread faster
why doesnt the action potential go backwards?
sodium inactivation gate + potassium channels being open (they have a high conductance for potassium, and there isnt a huge driving force for sodium downstream anymore)
AP role in sensory systeems
- some (mechano) use AP directly
- some dont (photoreceptors would lose sensitivity)
what is the difference between a chemical vs electrical synapse?
- electrical goes directly from presynaptic cell to post via connexin
- chemical uses transmitter to get from pre to post
morphological correlates for the quantum hypothesis
support for chemical synapses came from being able to see the vesicles at the synaptic cleft
what is the quantal hypothesis?
proposes that NTs are released in discrete packets or ‘quanta’ at synapses. it is an all or none event and the activation of postsynaptic receptors by said quanta leads to consistent, discrete responses in the postsynaptic cell
what will decreasing calcium do to the probabilistic and quantal nature of synaptic release?
reducing Ca outside the EC matric will give no response or minimal response.
what are the steps in synaptic transmission?
- bring the presynaptic neuron to threshold at the axon hillock
- conduction down presynaptic axon
- opening of voltage-gated calcium channels at the nerve terminals
- diffusion and action of Ca2+ at release machinery
- exocytosis and diffusion of transmitter in cleft
- activation of postsynaptic receptors
SNARE hypothesis
proteins in vesicle membrane and presynaptic terminal are SNARE proteins that form complex (core) that primes vesicle for release
- requires energy
molecules in SNARE
- synaptobrevin - synaptic vesicle
- syntaxin, snap 25 - presynaptic membrane
- synaptotagmin - Ca sensor
- mediate docking and priming release and retrieval
what are the three steps involved in SNARE
- docking and priming
- release (exo)
- retrieval (endo)
how is SNARE formed?
syntaxin in the membrane of the cell acts as a zipper using SNAP-25, when this complex comes together you get the SNARE complex
what happens when Ca2+ comes in and binds to synaptotagmin?
it inserts itself into the transmembrane
- this pulls the v- and t- membrane together, eventually leading to the fusion of the two membranes and exo of transmitter molecules
what happens when synaptotgamin is knocked out?
no fast synchronous release but instead slow asynchronous.
- Ca binding leads to insertion into the membrane which pulls v and t together and leads to fusion
