week 11 Flashcards
what is ohms law
Movement of a dissolved, charged particle - i.e. an ion - across a
lipid membrane depends on:
▪ The charge of the particle
▪ The difference in distribution of charges across the
membrane – this separation in charges is represented by
voltage
* Voltage is a type of potential energy → how much work it
takes to move a charged particle through an electric field
▪ The permeability of the membrane to the charged particle
Ohm’s law is most
useful when thinking about
unequal distributions of
charges very close on either
side of a membrane
The Nernst potential is the membrane potential at
which the ……
A balance is reached between?
inward and outward movement of an ion
through a channel is balanced and equal
- The diffusional force (movement of an ion down
its concentration gradient)
▪ The electrical force (attraction or repulsion based
on the charge of the ion and the charge across the
membrane)
Diffusional forces and electrical fields are very small at
large distances
what does nernst potential not include?
- flow of ions (current) or the
resistance of the membrane to flow…
▪ It describes the energy gradient
the electric field declines very rapidly as charges are separated by
distance
(ohms law)
what is needed for nernst potential
60 mvl / the charge and valence of P (anions are negative)
log 10
= ratio of intracellular:extracellular concentrations of X
Describes the voltage across a membrane that is
permeable to X given the ratio of [X] inside:outside
the ions …. to the membrane have the most effect on nernst potential
closest
At rest, neurons typically have a membrane potential that is close to the Nernst potential for
K+
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
If the membrane potential is close to the Nernst
potential of a particular ion, it usually means that
the membrane is more permeable to that ion
The membrane potential is about …. in many neurons
-75 mV
Why is the membrane potential of a neuron close to, but not the same, as the equilibrium (Nernst) potential for K+?
because there are other ions
what is the concept of the Goldman Field equation
that the concentration of one electrolyte has effects on the others
The potential across the membrane depends on
concentration gradients and the permeability (or its
inverse, the resistance) of the membrane to each ion
Channels are often
dynamic
-They can open or close in response to a variety of stimuli…
▪ which means membrane permeability and the membrane
potential can change, often very quickly
what are the main four types of channels
- Voltage – voltage-gated channels
▪ Stretch or mechanical deformation – mechanoreceptors or
osmoreceptors
▪ Intracellular messengers
▪ Extracellular messengers – ionotropic receptors
- A ligand binds to a receptor which is also a channel –
binding opens the channel, and allows an ion across 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 potentials occur?
The axon hillock, the axon (or in myelinated axons the nodes of Ranvier) and the synaptic terminals possess a large population of sodium voltage-gated channels (Na+ VGC) in the membrane
K+ VGC are also present in these areas – they help to
quickly terminate the action potential
…. starts an action potential …. ends an action potenial
sodium
potasssium
… Na+ out … K+ in
3
2
K+ concentrations are …inside the axon, and ….outside
high
low
K+ is high inside the axon, therefore ..
it diffuses out
what is the resting membrane potential
-70mV
what helps to keep the resting membrane potential
Na/ K+ATPase pump
what is depolarization
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)
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 msec
what happens during repolarization
- Na+ VGC are closed, no further Na+ entering the axon
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 membrane potential is -70 mV (repolarization)
▪ after they’re “unlocked” (1 – 2 msec after closing)
what are the two gates of action potenials
The activation gate – this gate opens as soon as threshold is
reached (i.e. the membrane depolarizes to -55 mV)
The inactivation gate – this gate closes very soon after the activation gate opens, after Na+ has rushed into the cell
* The inactivation gate will not open again unless:
▪ 1-2 msec has passed since it has closed (it’s “locked”)
▪ The cell membrane becomes inside-negative again
(repolarized)
The potassium voltage-gated channel does not have an inactivation gate – it opens when the cell ……., and closes once the cell is ………….
depolarizes
inside-negative again
It is slower to open than the sodium voltage-gated channel
what is the absolute refractory period
- Inactivation gate of
the Na+ VGC is
closed - Another action
potential is
impossible until this
gate opens
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
* A larger stimulus is
necessary to reach
threshold
what are the actions of a action potential
All-or-none events
▪ Begin when a threshold voltage (usually 15 mV positive to resting
potential) is reached
▪ There are no “small” or “large” APs – each one involves maximal
depolarization → all Na+ channels open once threshold is reached
- Initiated by depolarization
- Have constant amplitude
▪ Action potentials don’t summate – information is coded by
frequency, not amplitude
▪ the size of the depolarization stays the same size no matter how far
it travels along axon - Have constant conduction velocity along a fiber
▪ Fibers with a large diameter conduct faster than small fibers. - Myelinated fiber velocity in m/s = diameter (um) x 4.5
- Unmyelinated fiber velocity in m/s = square root of diameter
(um)
why does myelin increase conduction speed
what is continuous conduction
no jumping, every channel has to open, no mylien
no gaps, repolarization already happening
slowest process
what is saltatory conduction
jumping conduction - nodes of ranvier
the myelin insulation allows the electrical field to from depolarization to jump to the next ranvier
very fast
The portions covered by myelin do not
experience action
potentials – they can’t, there’s no ion channels and myelin keeps ions from crossing the cell membrane
