NeuroPhys 2 Flashcards
the movement of a dissolved, charged particle across a lipid membrane depends on:
- the charge of the particle
- the difference in distribution of charges across the membrane
- the permeability of the membrane to the charged particle
the rate of flow of charges across a membrane is known as ?
current (I) and is defined by Ohm’s law I = V/R
what is the nerst potential?
the membrane potential at which the inward and outward movement of an ion through a channel is balanced and equal
at rest, neurons typically have a membrane potential that is close to?
Nerst potential for K+
at rest, the only ion channels that are open are what?
K+ channels
the membrane potential of any cell depends on?
- the relative permeability of the membrane to each ion
- the concentration of the ion on either side of the membrane
an action potential:
○ Requires the presence of sodium voltage-gated channels (or sometimes calcium voltage-gated channels)
○ Relies on positive feedback
○ Always results in a membrane voltage change that is the same size
○ Occurs very quickly - the membrane becomes more positive (depolarized) in a matter of milliseconds
where do action potential occur?
the axon hillock, the axon, and the synaptic terminals
what is the first step of action potential?
- The Na+/K+ ATPase uses ATP to pump Na+ out of the axon, and K+ in
○ K+ concentrations are high inside the axon, and low outside (vice-versa for Na+) - K+ is high inside the axon, so it diffuses out
○ Diffusional, or chemical, force acting on K+ - Membrane becomes negative inside the axon
○ Negatively-charged proteins, ions cannot leave the cell with K+ - The attractive force of the negatively-charged membrane balances out the diffusional force driving K+ out
○ This balance establishes the resting membrane potential at about -70 mV (inside membrane negative)
what is step 2 (depolarization) of action potential?
- The inside of the axonal membrane becomes more positive, and a Na+ VGC opens
○ Channels are opened by more positive charges inside membrane
○ Threshold = membrane potential at which all Na+ VGC will end up opening (~ -55 mV) - Na+ VGC opening leads to other Na+ VGC opening, eventually all open
○ Positive feedback, Na+ diffuses into the cell, making membrane more positive, allowing more Na+ in - Inside of the axon becomes completely depolarized
○ Diffusion gradient (high Na+ outside, low inside) as well as electrical force (inside negative) drives Na+ into the cell - K+ VGC open, Na+ VGC close after ~ 1 millisecond
what is step 3 (repolarization) of the action potential?
- Na+ VGC are closed, no further Na+ entering the axon
○ Close after about 1 millisecond
○ Are unable to open for 1-2 millisecond - they are “locked”
○ After 1-2 millisecond, Na+ VGC will “unlock” - but only if the membrane is repolarized (becomes inside-negative again) - K+ rapidly leaves the axon
○ High K+ inside axon and positive charge inside the membrane strongly drive K+ out
○ K+ VGC and regular K+ channels are both open, allowing rapid K+ exit - Na+ VGC are ready to re-open:
○ When the membrane potential is -70 mV (repolarization)
○ After they’re “unlocked” (1-2 millisecond after closing)
the sodium voltage gated channel has 2 gates which are?
the activation gate and inactivation gate
when does the activation gate of the NaVGC open?
as soon as threshold is reached (the membrane depolarizes to -55 mV)
what does the inactivation gate do for the NaVGC?
this gate closes very soon after the activate gate opens, after Na+ has rushed into the cell
the potassium voltage gated channel does not have what?
an inactivation gate, it opens when the cell depolarizes, and closes once the cell is inside-negative again, much slower to open than the sodium voltage-gated channel
what is the absolute refractory period?
inactivation gate of the Na+ VGC is closed (at the peak of depolarization)
what happens after the absolute refractory period?
relative refractory period
what is the relative refractory period?
inactivation gate is open, activation gate is closed for the Na+ VGC
- the cell is hyperpolarized - the membrane potential is lower than resting membrane potential
what are the properties of action potentials?
- all or none event
- initiated by depolarization
- have constant amplitude
- have constant conduction velocity along a fiber
explain continuous conduction
when one part of the membrane depolarizes, it reaches threshold and an action potential occurs, the neighboring part of the axon needs to depolarize -> reach threshold -> before the action potential progress further down the axon
explain saltatory conduction
in a myelinated axon, the nodes of ranvier are the only parts of the axon expressing voltage-gated channels, the myelin insulation allows the electrical field from the depolarization to jump to the next node ranvier hence being faster than continuous conduction
which fibers are the largest?
A fibers
which fibers are the medium sized ones?
B fibers
