Exam 2: Ch5 Book Since Exam 1 Flashcards
every cell that is in a nonexcited or “resting” state has…
a potential difference (Vrest) across the membrane
usually lies between -20 and -100 mC depending on the kind of cell and ionic envionment
2 factors govern resting potential
presence of open selective ion channels
unequal extracellular and intracellular distribution of ions maintained by active transport (allows membrane potential)
ions influence the ________ across a membrane roughly in proportion to the _______ of the membrane to each ion species
potential, permeability
if a membrane is only permeable to one ion, will the distribution of that ion dictate the membrane potential?
yes, use Nernst equation
Vrest in frog skeletal muscle
-90 to -100
resting potential depends on which ions
Mostly K and some Na
resting potentials of muscle, nerve, and other cells are more sensitive to changes in [ ]… than the concentrations of other _____
[K] out, cations
consistent finding with high permeability of plasma membranes to K, compared to other cations
why do large changes in [Na] out have ______ effect on Vrest
little
b/c resting membrane is relatively impermeable to Na
how to calculate force acting on an ion
emf(x) = Vm - E(x)
depends on how different membrane potential is from equilibrium potential
based on [ ] gradient, there is a steady influx of which ion into the cell if not maintained by active transport?
Na, large electric driving force (extra: 120mM; intra: 10mM)
would accumulate inside cells if not removed, depolarizing the cell
if Na were allowed to leak in at rest, what would happen to K
it would leak out b/c internal negatively less able to hold K in
K high intracell, low extracell
which pump contributes to Vrest and how
Na/K pump (3 Na out / 2K in)
indirectly maintains high internal [K]
directly (very small effect) moves net pos charge out of cell
if you inhibit Na/K pump…
concentration gradients diminished
Na goes in, K eventually goes out and no Vrest b/c equilibrium will be reached
what is an action potential
a large, brief change in Vm that is propagated along an axon without decrement
all or none
3 key elements to producing an AP
active transport of ions to maintain [ ] gradients
[ ] gradient generates an electrochemical gradient that provides a reservoir of potential energy
electrochemical gradient drives ions across membrane when selective channels open (changes Vm)
what two voltage-gated ion channels are responsible for all the features of the AP
voltage gated Na and K
APs are generated by the membranes of ….
neurons
muscle cells
receptor cells, secretory cells, and some single-celled animals
local response
when a depolarization stimulus brings the membrane almost to threshold, there is an abortive, non-propagated excitation
beginning of an AP that died out before it reached threshold
threshold current
intensity of depolarizing stimulus that is just sufficient to bring membrane to threshold and elicit an AP
most neurons have threshold currents between…
-30 and -50 mV
what does regenerative mean
self-perpetuating AP
overshoot
brief period when interior of membrane is positive
after-hyperpolarization
transient period when Vm is more negative than Vrest
refractory
for a short time after an AP, it is difficult or impossible to trigger another AP
absolute refractory period
relative refractory period
absolute refractory period
if 2nd stimulus is delivered during an AP or immediately after, no AP is triggered
relative refractory period
slightly after the absolute refractory period, if stim is delivered an AP may be triggered but stim must be stronger than usual
amplitude of AP may be smaller (breaks all or none rule)
threshold potential decreases during the _____ refractory period until the membrane returns to its normal resting state
relative
what happens when a neuron is stimulated with a series of subthreshold depolarization
excitability progressively decreases
the stronger the series of subthreshold currents, the stronger they actually need to be to reach threshold
accommodation
the phenomena that the slower the rate of increase in the intensity of the stimulating current, the greater the increase in threshold potential
caused by time-dependent changes in the way membrane channels respond to depol
can some neurons accommodate rapidly?
yes
some generate 1-2 APs (phasic response)
some fire repeatedly but with decreasing frequency (tonic response)
adaptaion
the decrease in frequency of APs seen in a neuron that responds tonically during sustained stimulus
visualized as increasing distance between APs
what causes the rapid depolarization of an AP
inward Na current caused by a sharp increase in Na conductance (rising phase)
opening of voltage-gated Na channels
when voltage-gated Na channels close and permeability of Na drops (peak of AP), what happens to K
memb. much more permeable to K b/c voltage-gated K channels are still open
falling phase
why do animals have giant axons
rapid conduction of AP
escape response
what did Hodgkin and Huxley discover about resistance and capacitance
membrane resistance during an AP decreases, but capacitance remains constant
therefore, change in conductance is entirely responsible for change in ionic current
what did Hodgkin and Huxley discover about voltage during AP
goes past 0 toward ENa of nernst equation (58 mV in squid giant axon)
when Na removed (artificial seawater), magnitude of AP decreased as more Na removed
why use a squid giant axon (1mm in diameter)
thin electrode wires could be threaded longitudinally
equation for current carried by ionic species
I(x) = g(x)*(Vm-E(x))
I(x) = current carried by ionic species x
g(x) = membrane conductance for X (proportional to # of open channels selective for x)
V(m) = membrane potential
E(x) = equilibrium potential of x
what does equation for current carried by ionic species demonstrate
there is only an ionic current across the memb. if there is a force driving the movement of ion x, and a conductance for x
if g(x) or emf acting on X is -0, there is no I(x)
what does it mean if g(x) is 0
no channels are open
changes in ionic ________ play a major role in controlling the electric currents carried across biological membranes
conductance
4 pieces of evidence that Na is the major ionic species responsible for AP
[Na] out exceeds [Na] by ~10, ENa ~55-60 mV so emf acting on Na is large and drives Na into cell
entry of pos charged Na produced pos shift in Vm
overshoot of AP approaches ENa
magnitude of overshoot changes as a function of [Na] out (artificial seawater)
voltage clamping
method used on giant squid axon allows you to change the Vm abruptly to any value, and hold it there
can therefore measure ionic current flowing across memb.
Ohms law to calculate conductance
when Hodgkin and Huxley abruptly depolarized an axonal memb. from its resting level they were able to measure…
initial transient inward current of Na to drive rising phase followed by a sustained outward current of K for falling phase
voltage clamping method
electronic feedback system holds voltage difference across plasma membrane at a constant
electrode inserted into neuron and control amplifier compares potential across membrane
use solutions of different ion [ ] or agents that block particular ion channels
tetrodotoxin (TTX) pufferfish fugu
blocks voltage-gated Na channels (know from radiolabel)
are there many voltage gated sodium channels in cell memb
no, makes up small surface area
however, 10^7 Na ions or more per second can pass through
unitary current
current through a single channel
which technique elucidated how voltage gated Na channels operate
patch-clamping
gating current
I(g)
small measurable current associated in time with opening and closing of Na channels
mechanism of voltage gated Na channels
gating takes place with movement of charges
activation and inactivation are coupled processes
selectivity of channels
determine relative permeability to each ion
channel filters partly by size, net charge, and ease of dehydration
selectivity filter: part of channel that determines selectivity
positive feedback loop during AP (Hodgkin’s cycle)
Na channels respond to depol but opening, allowing Na to enter cell, which depol it more
this extra depol causes more channels to open, allowing even more Na to enter causing an explosive regenerative event
once AP started it needs no more stimulus to continue
limits of Hodgkin’s cycle
as membrane potential approaches ENa, driving force on Na is reduce so less is driven into the cell
open Na channels become inactivated after a short time even if depol is maintained
how are Na channels opened
activation gate closed and inactivating particle located away from channel
when voltage clamped to depol, activation gate opens and inward current is maxed when most Na channels are open
as depol continues, inactivating particle blocks channels
after membrane repolarization, inactivation particle unblocks channels and activation gate closes (memb. must be inside-negative)
what is patch-clamping
variation of voltage clamping that allows currents through a single membrane channel to be recorded
do voltage gated K channels respond to changes more quickly or slowly than Na channels
slowly
when does membrane conductance of K change in an AP
only near its peak (very slow rise before then)
remains elevated during falling phase
during after-hyperpolarization, membrane potential moves even closer to ___
E(K) equilibrium potential of K
are voltage gated K channels needed to generate APs?
no
benefit of rapid membrane repolarization produced by current through K channels
shortens AP, but allows neurons to generate them with higher frequency
how do glial cells help maintain Vrest
remove K from extracellular fluid surrounding axons
slowly release it allowing it to be reclaimed by neurons
lidocaine blocks
Na channels
dentists use for pain… no signals conducting
K channel structure
each gene codes for a subunit (4) monomeric alpha arranged around a central pore
4 beta subunits associated with each channel modulate function
Na channel structure
1 large alpha protein with 4 same transmembrane domains
central pore
Ca channels
help depol w/ inward current a little
open more slowly and conduct less current
Ca can act as a
intracellular messenger
