Action Potentials Flashcards
leak channels
always open
unregulated ion movement in one direction of electrochemical gradient
gated channels
slide 5
typically ion specific
depolarization
flow of (+) ions into cell (membrane potential becomes less negative)
repolarization
flow of (+) ions out of cell (potassium) brings back to resting membrane potential
-90 mV
resting membrane potential
0 mV
overshoot phase
what increases membrane excitability
when membrane becomes more POSITIVE (closer to threshold)
closer to threshold means closer to excitability
an increase in EC potassium (K+) concentration would lead to the membrane becoming….
more excitable!
potassium is high in the cell and retains its positive charge inside the cell
K+ ions moving out of the cell?
will be the repolarization stage of the action potential
Ungated K+ channels
always open
K+ efflux (until Ek+ is reached)
3 states of voltage gated sodium channels?
Closed but capable of opening (-70)
Open (activated) (-50 mV to +30 mV)
Closed and not capable of opening (inactivated) (+30 mV to -70 mV (ball and chain)
Two states of potassium gated channels?
closed but capable of opening
open from peak potential through after hyperpolarization
when do K voltage gated channels open?
slowly!
open around peak of the action potential
potassium will be leaving the cell for repolarization phase
how is the action potential generated?
by the rapid opening and subsequent voltage inactivation of voltage-dependent Na+ channels and the delayed opening and closing of voltage-dependent K+ channels.
what happens when Na+ ions move into the cell?
the membrane becomes depolarized into the positive direction
-occurs at more negative membrane voltages
versus the potassium which occurs at more positive membrane voltages
if the duration of the stimulus is short….
the intensity must be high1
if the duration of the stimulus is long….
a lower intensity stimulus can trigger an action potential
phases of depolarization
initial depolarization (reaches threshold)
Na+ channels open
Na+ influx (depolarizes)
Peak Na+ conductance
repolarization phases
early depolarization (voltage gated Na channels close and lock AND voltage gated K channels are still opening)
K+ efflux (repolarizes membrane in direction of Ek+)
Peak K + conductance
slide 20
what would happen if voltage gated K+ channels didn’t open?
it would eventually get back down to rest (b/c of leak channels) BUT it would take much longer
Refractory period
Time period after AP when a subsequent AP either cannot, or likely will not, be generated
Key for ensuring unidirectional propagation of APs
absolute refractory
no matter how large the next stimulus you are still NOT going to get another AP b/c of the inactivation gates of voltage gated Na channels
relative refractory
another AP can be produced only if the stimulus is large enough (stronger than normal)
directly related to delay or slower gating kinetics of voltage gated K channels (delayed closing of K+ voltage gated channels)
Why do we have refractory periods? x2
ensures unidirectional propagation
set upper limit to AP firing frequency
AP potential characteristics
undiminshed propagation
All-or-none law
if stimulus is greater than required to reach threshold…
AP is still same size
cannot sum AP’s
nervous system does not differentiate intensity of stimulus by size of AP but rather….
by AP frequency and/or number of AP’s
Graded potential
less than threshold (SUB THRESHOLd)
usually produced by Na+ influx
change in potential can vary (duration and strength is directly proportional to duration and strength of stimulus)
spread by passive current flow
DECREMENTAL- not propagated and can die out
resistance hinders flow
CAN BE SUMMATED
do graded potentials have refractory period?
NO b/c they are sub threshold
Decremental current flow? what is that
not propagated
die out over short distances
resistance hinders flow of electrical current (V=IR)
differences b/w graded and action potentials
slide 31
dendrites
signal toward cell body
axon
signal away from soma
axon hillock
initial segment
lowest threshold for AP initiation
which characteristic of an axon is most dependent on diameter?
conduction velocity of action potentials
Conduction methods (x2)
Contiguous
saltatory
contiguous conduction
Local event of reaching threshold causes enough Na+ influx to open adjacent channels → more Na+ flows into adjacent channels……..down entire length of membrane
Positive-feedback entire membrane has been depolarized & repolarized
an AP is conducted then it triggers neighboring AP and so and so on…. so one AP doesn’t travel all the way down, there are multiple AP’s
saltatory conduction
Property of MYELINATED nerve fibers which increases conduction velocity
Nodes of Ranvier (not myelinated): Concentrated Na+ channel expression
Impulse “jumps” between nodes down the length of the axon
increases conduction velocity up to 50x faster
Myelin function
insulating resistance
high trans-membrane resistance to internode region
forces current to travel node to node where there is lower resistance
Axon diameter effects what?
increase in axon diameter actually increases axon potential propagation
larger diameter –> decreased internal resistance to current flow
what increases conduction velocity
myelination
increased axon diameter
2 types of synaptic communication
electrical synapses
chemical synapses
electrical synapses
allow for direct communication of electrical signal from cell to cell
gap junctions (which have connexon channels)
low-resistance to current flow
chemical synapses
unidirectional signal transmission
electrical signal –> chemical signal
ionotropic
slide 44
metabotropic
slide 44
voltage gated calcium channels
are expressed at synaptic terminal of pre-synaptic neuron
can tell when AP is coming in
capable of responding to depolarzing stimulus (by opening up–> calcium influx)
10,000:1 Ca outside to inside of cell
convergance
multiple synapses on one postsynaptic cell body
sensory neurons such as feedback coming back to spinal cord
divergance
branching axon terminals can affect thousands of postsynaptic cells
(Ex: motor neuron → multiple fibers of motor unit)
prior to axon hillock what type of potentials?
graded
what does the axon hillock possess that allows it to transmit/make AP?
high concentration of voltage-gated Na+ channels
how does the neuron decide what to transmit?
tally the votes of EPSP’s and IPSP’s
excitatory synapse
depolarizes/hypopolarizes
always excitatory
EPSP- subthreshold event bringing membrane closer to threshold
inhibitory synapse
IPSP
hyperpolarizes membrane (says no don’t do it)
this is always inhibitory
how to get an EPSP?
Na influx or K efflux
how to get an IPSP?
Potassium efflux
Cl influx
spatial summation
EPSP’s initiated simultaneously allowing threshold to be reached and AP to be generated
temporal summation
if an excitatory presynaptic input is stimulated a second time before the first EPSP has died off, the second EPSP will add onto, or sum with the first EPSP, resulting in temporal summation
EPSP-IPSP cancellation
activation of ex1 and inn 1 presynaptic input does not change the postsynaptic potential b/c these two results (EPSP and IPSP) cancel each other out
look at supplemental summary posted after lecture
do it
hypocalcemia
hyperexcitability/spontaneous muscle twitch
decreased threshold voltage of Na channel gating
reach threshold at a lower potential than normal
hypercalcemia
when EC of calcium is elevated it can lead to hypo excitability/muscle weakness
decreased membrane excitability
K + has what effect usually?
inhibitory role in excitatory cells due to very negative equilibrium potential
