2. Electrophysiological Principles Flashcards
information transfer along neurons occurs via the movement of:
ions
neurons can produce a change in membrane potential known as the:
action potential
stimulus intensity can be encoded by the _____ of action potentials a neuron produces
frequency
information is passed from neuron to neuron at the:
synapse
the plasma membrane is a _____ that has many types of proteins associated with it
lipid bilayer
water and ions move into and out of the cell through:
ion channels and pores in the membrane
the two main types of lipids found in nerve cell membranes are:
glycerophospholipids and sphingolipids
what types of cells have a membrane potential?
all cells (including plants and single celled organisms)
excess ions collect along a thin shell on the inner and outer surfaces of the:
plasma membrane
true or false: the bulk of the intracellular and extracellular fluid is electrically neutral
true
what is the resting membrane potential?
-60mV
why is the resting membrane potential negative?
the inside of the cell is approximately 60mV negative with respect to the outside
tells you about the relationship between current, resistance, and potential
Ohm’s law
what is the formula for Ohm’s law?
V = IR
what is voltage according to Ohm’s law?
the voltage between two points is the cost in energy (work done) required to move a unit of charge from the more negative point (lower potential) to the more positive point (higher potential)
voltage is also called the:
potential difference
a Joule of work (J) is needed to move a Coulomb of charge (C) through a potential difference of:
one Volt (V)
what is current (I)?
the rate of flow of electric charge past a point
by convention, current in a circuit always flows from a more _____ point ot a more _____ point even though the actual electron flow is _____
positive, negative, in the opposite direction
a current of one ampere (A) equals a flow of:
one coulomb of charge per second (C/s)
what is the formula for current?
I = Q/t
Q = charge
t = time
the current through a conductor is proportional to the voltage across it. this constant of proportionality is known as:
conductance (g)
what is the formula for conductance?
g = I/V or g = 1/R
what is a capacitor?
something that stores charge. it is an insulator that separates two conductors
how is the cell membrane like a capacitor?
the insulator is the membrane itself, while the conductors are the intracellular and extracellular solutions
what is the unit of capacitance?
the farad (F)
the farad depends on the:
- distance apart of the two conductors
- the surface area of the cell membrane
- the materials of the insulator and conductors
the thickness of the membrane is relatively constant, and the nature of the conductors is relatively constant, therefore:
the total capacitance depends on the surface area of the cell membrane
how do we measure charge separation across the cell membrane?
with electrodes
charge separation of the cell membrane is brought about by 3 main factors:
1) the cell membrane is semi-permeable to K+
2) the cell membrane is not permeable to many ions
3) ATP-dependent transporters pump ions (Na+ and K+) across the membrane in order to redistribute and maintain charge separation
true or false: the cell membrane is permeable to K+ but impermeable to Cl-
true
in a model cell, the system is in equilibrium and there is no net flux of K+ when the:
electrical and the concentration gradients are equal
what are the three main requirements for the model cell?
1) the intracellular and extracellular solution must each be electrically neutral
2) the cell must be in osmotic balance
3) there must be no net movement of any ion into or out of the cell
what is the Nernst equation?
Eion = (RT/ZF)ln([ion]o/[ion]i)
when Z = 1, (RT/ZF) =
25 and Eion is in mV
the Nernst equation for an ion gives you the potential (Eion) at which there will be:
no net flux of that ion across the cell membrane
what are the two additional forms of the Nernst equation when (RT/ZF) = 25mV
E(K+) = 25ln([K]o/[K]i)
or
E(K+) = 58log([K]o/[K]i)
how does an increase in extracellular K+ affect the membrane potential?
- reduces the concentration gradient for K+, allowing it to move into the cell
- membrane polarizes due to accumulation of +ve charges on the inner surface
- Cl- ions also move into the cell
- K+ and Cl- flow into the cell until a new equilibrium is established and both ions are at a new concentration consistent with the new membrane potential
how does a decrease in extracellular Cl- affect the membrane potential?
Cl- and K+ will leave the cell, but in very minimal amounts, so there is pretty much no change in the membrane potential
only the _____ concentrations change in response to adjusting ion concentration outside the cell
intracellular
if we increase [K+]o then the membrane potential becomes:
more positive
if we decrease [Cl-]o then the membrane potential:
does not change (essentially)
what is the equilibrium potential of K+ in mammalian skeletal muscle cells?
-92mV
what is the equilibrium potential of Na+ in mammalian skeletal muscle cells?
+63mV
what is the equilibrium potential of Cl- in mammalian skeletal muscle cells?
-85mV
what is the equilibrium potential of Ca++ in mammalian skeletal muscle cells?
+121mV
what is the Goldman-Hodgkin-Katz equation (constant field equation)?
Vm = 58log((P[K]o + P[Na]o + P[Cl]i) / (P[K]i + P[Na]i + P[Cl]O))
the constant field equation determines the exact membrane potential by taking into consideration:
the activity of all permeant ions
active transport of ions across the cell membrane ensures:
that the ions do not run down their concentration gradient
Na+ tends to flow _____ the cell, and K+ tends to _____ the cell?
into, out of
the Na+/K+ exchange pump ensures that Vm does not slowly run down by:
using energy (ATP) to pump Na+ out of the cell and K+ into the cell
the Na+/K+ ATPase pumps 3Na+ ions out for every 2K+ ions pumped into the cell. Therefore, the pump is said to be:
electrogenic
ATPases can be inhibited by a toxic compound called:
ouabain
go review slide 65
you got this
when Hodgkin and Huxley perfused the axon with a salt solution, they were still able to:
evoke action potentials in the axon
the peak of the action potentials occurs close to:
the equilibrium potential of Na+
for Na+, depolarization is positive feedback. Why?
the more positive charge, the more channels open
how did Hodgkin and Huxley investigate the activity of Na+ and K+ voltage-gated ion channels and the conductances of Na+ of K+?
they developed the voltage clamp
this method permits us to set the membrane of the cell almost instantaneously to any level and hold it there while at the same time recording the current flowing across the membrane
voltage clamp
what are the three main components of a voltage clamp?
- electrode that measures membrane potential
- current-passing electrode
- voltage clamp amplifier
compares membrane potential to the desired (command) potential
voltage clamp amplifier
by convention, inward current is:
negative current
by convention, outward current is:
positive current
current flow through a channel is determined by the difference between:
the cell membrane potential (Vm) and the ion equilibrium potential (Eion)
what is Eion?
the equilibrium potential for an ionic species
(Vm - Eion) is called the ______ for an ion
driving force
the first current to occur since it represents a redistribution of charge across the membrane
capacitative current
capacitative current is followed by a small, steady outward current which flows through the resting membrane potential. this current is called the:
leak current
the leak current is usually small and represents:
ion flow through various different channels
the early current is carried by _____ ions flowing through voltage-gated channels _____ the cell
Na+, into
the late current is carried by _____ ions flowing through voltage-gated channels _____ the cell
K+, out
how did Hodgkins and Huxley find out that the currents are carried by Na+ and K+?
they replaced Na+ in the bath with equimolar choline. this abolished the early inward current and left only the outward current. they then subtracted the outward current from the original current trace and were left with the inward current.
the early current is seen when the late current is:
subtracted from the complete current trace
the late current can be seen when Na+ is replaced with:
equimolar choline
a poison which physically blocks voltage-gated Na+ channels
tetrodotoxin (TTX)
a poison which physically blocks voltage-gated K+ channels
tetra ethyl ammonium (TEA)
how is current affected when TTX blocks Na+ channels?
there is no early negative current
how is current affected when TEA blocks K+ channels?
there is no late negative current
the early inward current goes back to:
zero after 3-4ms
true or false: both the early and late current go back to zero
false, the late current does not go back to zero
the early inward current is said to:
inactivate
Hodgkins and Huxley studied inactivation of the early current by:
giving prepulses to the cell before voltage-clamping the membrane at a predetermined voltage
true or false: inactivation of the early current and activation of the late current happen at about the same time
true
the more positive the membrane potential, the more:
inactive the Na+ channels
what percent of voltage-gated Na+ channels are inactivated at the resting membrane potential
~40%
Na+ and K+ conductances have essentially the same ____, thus the channels are activated in a ____
voltage dependence, similar manner
the voltage at which inward and outward currents are equal and exactly balance each other
threshold
the period of time during and immediately following an action potential in which another action potential is either difficult or impossible to initiate
the refractory period
if an action potential does occur during the refractory period, its amplitude will be:
smaller than usual
what are the two types of refractory periods?
- the absolute refractory period (AFR)
- the relative refractory period (RRP)
during the absolute refractory period, action potentials cannot occur because:
- Na+ channels have been inactivated
- there is a large K+ conductance forcing Vm to negative potentials
during the relative refractory period, an action potential can occur because:
- Na+ channels are moving back into a state where they can be activated
- the K+ conductance is smaller than in the ARP
are the m-gate and h-gate in Na+ channels dependent on one another?
no, they are independent
in Na+ channels, the m-gate is closed at _____, and open during _____
rest, depolarization
in Na+ channels, the h-gate is open during _____, but closes during _____
depolarization and repolarization, refractory period
the inactivation gate closes the Na+ channel through:
diffusion and binding
what is the sequence of ion channel gating events during an action potential?
1) Na+ channels open (activate)
2) Na+ channels inactivate
3) K+ channels open (activate)
4) K+ channels close
5) Na+ channels move back to rest
a technique where a small patch of membrane is sealed to the tip of a micropipette, enabling currents through single membrane channels to be recorded
patch-clamp technique
small currents across the patch membrane can be recorded by connecting the pipette to:
an appropriate amplifier
using patch-clamp techniques, one can record:
single channel activity and whole cell currents
what are the four main modes of patch-clamp recording?
1) cell attached recording
2) inside-out recording
3) whole cell recording
4) outside-out recording
a type of patch-clamp where you can record single channel activity of a few channels
cell- attached recording
a type of patch-clamp where the inside of the patch is exposed to the external environment and you can record single channel activity of a few channels
inside-out recording
a type of patch-clamp where the cell membrane within the patch is ruptured and there is complete access to the inside of the cell and you can record action potentials, macro-currents, and miniature currents and potentials
whole cell recording
a type of patch-clamp where the outside of the patch is the outside surface of the cell and is exposed to the external environment and you can record single channel activity of a few channels
outside-out recording
when using a patch-clamp technique, current flow through ion channels depends on:
the potential of the patch of membrane
describe the structure of Na+ channels
a continuous protein with 4 domains and six transmembrane helices (S1-S6) per domain
describe the structure of K+ channels
composed around 4 alpha subunits and 4 beta subunits (multiple proteins coming together to form a channel)
the region between S5 and S6 forms:
part of the pore of the channel
the S4 region is lined with:
positive charges (voltage-sensing region of the channel)
true or false: Na+ channels are universal across many animal species and behave very similarly
true
the Na+ channels spends more time in the open state at:
depolarized potentials
a poison frog toxin that blocks Na+ channels
batrachotoxin
batrachotoxin shifts the voltage-dependence of Na+ channel gating, so now the channels open at:
more negative potentials (makes it so that channels are consistently open at resting potential)
how would the Na+ ion channel detect voltage changes across the membrane?
the S4 regions is the voltage sensor (sensitive to changes in membrane potential)
the S4 region of the Na+ channel responds to depolarization by:
translating/rotating within the membrane, effecting a transfer of charge between the inner and outer aqueous compartments
every third amino acid in the S4 region is:
positively charged
at rest, the activation gate is ____ and the inactivation gate is ____
closed, open
refers to the opening of voltage-gated channels caused by a depolarization, or a change in voltage
activation
the closing of voltage-gated channels through movement of an inactivation gate that is a separate and distinct gate from the activation gate
inactivation
the closing of voltage-gated channels through movement of the opening gate back into a closed position
deactivation
true or false: inactivation is strictly voltage dependent
false
at rest, when the m-gate is closed, the channel is in such a conformation that the h-gate:
cannot bind to the pore and block the channels
when the m-gate opens, the channel changes conformation and opens a binding site for the:
h-gate (can now bind to the pore and block the channel)
do K+ channels inactivate?
yes
addition of _____ to the interior of cells removes inactivation of K+ channels
proteinase (trypsin –> eats proteins)
deletion mutations shows that _____ are important for inactivation of K+ channels
the first 20 a.a. of the N-terminus
how is inactivation of K+ channels restored after deletion mutations?
intracellulary adding a peptide that resembles the 20 a.a. of the N-terminus
how many inactivation gates does a K+ channel have?
4 (one per domain)
how do K+ channels inactivate?
the inactivation gates diffuse into place
describe the structure of Ca++ channels
a continuous protein with 4 domains and six transmembrane helices (S1-S6) per domain
ensures that only one type of ion can move through the channel
the selectivity filter
why can’t K+ flow through an Na+ channel?
the hydration radius for Na+ is greater than the hydration radius for K+ (the unhydrated K+ is larger than Na+ and cannot fit through the Na+ channel)
the K+ pore has enough energy to strip H2O from ____ but not enough energy to strip H2O from ____
K+, Na+
the Na+ pore is capable of stripping H2O from both ____ and ____, but ____ is too large to fit through the pore
Na+, K+, K+
when current is injected into a cell we get a voltage change across the membrane. the maximum voltage change occurs at:
the site of current injection
the voltage change increases exponentially to a plateau but then:
decays after the current has ended (decay phase is exponential)
the inverse of the input conductance
rinput
the cell membrane can be modelled as:
a capacitor and resistor in parallel
the time it takes for the voltage to rise to 63% of its final value
time constant (tao)
once the current pulse has ended, the voltage falls back to baseline with a time course of:
the time constant (tao)
true or false: the time constant is independent of cell or axon size
true
the capacitance of one square centimeter of membrane
Cm
true or false: Cm is approximately the same in all cells
true
the resistance of one square centimeter of membrane
Rm
list the formulas to calculate the time constant
- (tao) = (Rm)(Cm)
- (tao) = (rm)(cm)
- (tao) = (rinput)(cinput)
the time constant (tao) can be determined _____, it usually varies between _____ and _____
experimentally, 1ms, 20ms
positive current injected into the cytoplasm of an axon (cable) flows:
longitudinally along the fibre and outward across the cell membrane
when measuring current in a cable (axon), as the recording electrode is moved further from the current source, the potentials become:
smaller and slower
when measuring current in a cable (axon), the voltage decrease with respect to distance is:
exponential
go review the formulas on slides 133-135
I hate formulas
a measure of the resistance through the axoplasm and then across the cell membrane
rinput
the distance over which potential falls to 37% of the original value
the length constant (lambda)
what is the effect on the length constant as the membrane resistance increases?
the length constant increases because the more membrane resistance the longer it takes for the current to fall off
what is the effect on the length constant as the axoplasmic resistance increases?
length constant decreases
what are the three main parameters that specify the behaviour of an axon?
1) the input resistance (rinput)
2) the length constant (lambda)
3) the time constant (tao)
the action potential moves along the axon as:
a wave of depolarization
do large fibres or small fibres conduct signals more rapidly?
large fibres
how does the myelin sheath affect the cell membrane resistance and cell capacitance?
increases the cell membrane resistance and decreases the cell capacitance
myelin sheaths restrict membrane current flow to the:
nodes of Ranvier
in an axon, Na+ channels are concentrated at:
the nodes of Ranvier
in an axon, K+ channels are concentrated at:
the parts of the axon covered in myelin
