Membrane and Action Potentials

1
Membrane and Action Potentials
membrane potential difference in charge on the
inside and outside of a cell usually
resting somewhere around -70 mV (/- 20) in
animals measured using an electrode whose tip is
'inside' of a cell inside means electrically
coupled directly to the cytoplasm
sharp electrode- high resistance
patch clamp- large surface hole
2
Membrane and Action Potentials
positive ions flowing in OR negative ions flowing
out depolarizes cell negative ions flowing in OR
positive ions flowing out hyperpolarizes cell
potassium and phosphates are usually high
inside cells and low outside cells sodium and
chloride are usually low inside cells and high
outside cells electrochemical potential
determines which direction the ions are going
to flow
3
Membrane and Action Potentials
depolarization moves the electrical potential
closer to 0 mV generally stimulates neurons!
hyperpolarization moves the electrical
potential more negative generally inhibits
neurons!
4
Membrane and Action Potentials
'stimulating' a neuron makes it more likely to
fire an action potential action potential
extreme depolarization of a neuron that
originates in a cell body and propagates down
an axon
action potentials are usually 'all or
nothing' can occur singly or in bursts of
activity can cause synapses out along the
axon to release neurotransmitter
5
Membrane and Action Potentials
neurons integrate all current sources when
deciding to fire an action potential hyperpo
larization lowers chances of firing an
action potential depolarization increases
chances of firing an action potential
6
Ion Channels and Membrane Potential
3 ions essentially determine membrane potential--
Na, K, Cl- ion channels control membrane
permeability different channels have different
ion selectivity-- all move according to their
electrochemical potential
7
Ion Channels and Membrane Potential
direction of ion movements determined by the
Nernst equation DG -RTln(iono/ioni)
zFVm under equilibrium conditions, DG0, then
solve for Vm Vm RT/zFln(iono/ioni)
reversal potential membrane voltage at which
an ion has no net movement across the
membrane ions will flow through an open ion
channel so that the cell's potential moves
toward the membrane potential
8
Ion Pumps and Membrane Potential
ion pumps have to regenerate the ion gradients
when channels close ion pumps are powered by
either the sodium ion gradient or ATP
sodium/potassium exchanger is the most common ion
pump chloride/bicarbonate ions form a second
ion pair across a membrane the brain uses a lot
of energy to maintain neuronal resting
potential lack of blood/oxygen to the brain
causes rapid unconsciousness/death more active
regions of the brain require more oxygen
anion exchanger AE1
9
one of the first things you notice in patch
clamps are action potentials 4 phases initial
slow rise, steep rise, steep fall, undershoot
10
rising phase
threshold
when threshold is reached, many sodium channels
in the cell body near the axon open, allowing
positive ions to enter and rapidly depolarize
the neuron sodium ions could continue entering
until they reach their Nernst potential, but
usually the channels shut somewhat before this
happens (55 mV)
11
peak
falling phase
near the peak (most depolarized part of the
action potential), sodium channels shut and
potassium channels open, allowing potassium to
leave the cell and causes the cell to become
repolarized when sodium channels shut, they
cannot immediately open again for another
action potential (refractory)
12
overshoot
potassium channels typically decrease Vp to about
-80 to -90 mV, the Nernst potential of
potassium for many neurons (overshoot) neurons
cannot fire well at this stage (relative
refractory period) because they are farther
from the threshold to generate another action
potential
13
equilibrium
The neuron returns to equilibrium and becomes
ready to fire again as ion pumps use ATP to
kick sodium out of the cell and potassium back
in time to fire the next action potential varies
depending upon what channels a neuron expresses
14
-70

outside inside
X-
X-
Cl-
Na
K
X-
Na
Na
Ca2
Cl-
Cl-
Na
X-
X-
Cl-
Cl-
Cl-
Ca2
Cl-
Cl-
Na
K
Na
Cl-
K
Ca2
K
K
Na
Cl-
Cl-
Cl-
K
K
Na
Cl-
Cl-
Na
X-
Ca2
Ca2
when the neuron is at rest, sodium is high
outside the cell and potassium is high inside
the cell
15
-70

outside inside
X-
X-
Cl-
Na
K
X-
Na
Na
Ca2
Cl-
Cl-
Na
X-
X-
Cl-
Cl-
Cl-
Ca2
Cl-
Cl-
Na
K
Na
Cl-
K
Ca2
K
K
Na
Cl-
Cl-
Cl-
K
K
Na
Cl-
Cl-
Na
X-
Ca2
Ca2
during the rising phase of the action potential,
sodium ions move in along their concentration
gradient and depolarize the neuron
16
-70

outside inside
X-
X-
Cl-
Na
K
X-
Na
Na
Ca2
Cl-
Cl-
Na
X-
X-
Cl-
Cl-
Cl-
Ca2
Cl-
Cl-
Na
K
Na
Cl-
K
Ca2
K
K
Na
Cl-
Cl-
Cl-
K
K
Na
Cl-
Cl-
Na
X-
Ca2
Ca2
At the peak of the action potential when sodium
is nearly at its Nernst potential, sodium
channels close and potassium channels
open sodium ions stop flowing into the cell and
potassium ions flow down their concentration
gradient and leave the cell
17
-70

outside inside
X-
X-
Cl-
Na
K
X-
Na
Na
Ca2
Cl-
Cl-
Na
X-
X-
Cl-
Cl-
Cl-
Ca2
Cl-
Cl-
Na
K
Na
Cl-
K
Ca2
K
K
Na
Cl-
Cl-
Cl-
K
K
Na
Cl-
Cl-
Na
X-
Ca2
Ca2
neurons hyperpolarize (go to an even more
negative Vp) as potassium leaves and reaches
it's Nernst potential before potassium channels
also close
18
-70

outside inside
X-
X-
Cl-
Na
K
X-
Na
Na
Ca2
Cl-
Cl-
Na
X-
X-
Cl-
Cl-
Cl-
Ca2
Cl-
Cl-
Na
K
Na
Cl-
K
Ca2
K
K
Na
Cl-
Cl-
Cl-
K
K
Na
Cl-
Cl-
Na
X-
Ca2
Ca2
ion pumps move sodium ions to the outside of the
cell and bring potassium back into the cell to
restore the ion concentration gradient and
bring the membrane potential back to -70 mV
19
Action Potentials and Myelination
myelin wrapped around an axon acts as an
insulator, reducing ion leakage through the
membrane and speeding up action
potentials between wrapped areas are nodes of
Ranvier where sodium and potassium channels
regenerate the action potential the refractory
period prevents backwards propagation of action
potential
20
Ion Channels Types and Properties
neurons show different patterns of intrinsic
activity depending upon what types of ion
channels are present
'trains' of action potentials 'bursting' trains
of firing short duration high frequency short
duration low frequency
21
Ion Channels Types and Properties
sodium channels may be either persistent or
transient transient currents function during
action potentials persistent currents are
usually smaller but last much longer and may
regulate how much a cell must depolarize to fire
an action potential
22
Ion Channels Types and Properties
potassium channels are very diverse some
activate and then quickly inactivate others fail
to inactivate may regulate the frequency of
action potential firing some potassium channels
are regulated by increasing intracellular
calcium or other second messengers 9 gene
families of potassium channels further
diversified by alternative splicing gene
duplication, and phosphorylation generally
hyperpolarizing due to their electrochemical
gradient
23
Ion Channels Types and Properties
calcium channels regulate other channels and
other cellular processes characterized as either
transient, long-lasting, and neither further
subdivided by 'high voltage' or 'low voltage' or
non-voltage gated even more diverse than
potassium channels!