Cell Membrane Transport III - Electrical properties of the cell membrane Flashcards
Learning outcomes
- Explain the ionic basis of membrane potentials
- Understand the principle of the Nernst equation and electrochemical
equilibrium - Understand the principle of the Goldman equation and how it relates to
the steady state membrane potential - Describe & explain the ionic basis of electrical signalling in excitable
cells - Understand that the electrical response of ‘excitable cells’ depends on
the type of membrane transport processes present in those cells
Further Reading (not essential)
Alberts et al. Molecular Biology of the Cell, chapter 11
Alberts et al. Essential Cell Biology, chapter 12
Electrical properties of the membrane
are important for cells
- Membrane potential (charge of the membrane) is a major force acting on ions and
molecules in all cells - Membrane potential of cells is generally around -70 mV (can vary depending on
cell type). - Ions are the most abundant dissolved solutes
- Electrical properties of membranes are important in:
- Muscle contraction, sensory signalling, CNS
- Fluid flows in specialized epithelia
- Intracellular enzyme cascades
- Gene expression, cell growth, cell death
- Gating of channels
- Venus fly traps????
Diffusion of ions is determined by:
Membrane permeability, concentration gradient and
voltage gradient
MCV
How do we generate a resting
membrane potential?
How do we generate a resting
membrane potential?
- Neutral membrane
impermeable to ions - But… membranes express
specific ion channels which
means selective
permeability of ions
How do we generate a resting
membrane potential?
How do we generate a resting
membrane potential?
How do we generate a resting
membrane potential?
A small number of charges
generate a large voltage
Ion channel involvement in
membrane potential
- Many ion channels are involved in
maintenance of membrane potential, K+
channel is a major one - Continued efflux of K+ builds up an excess
of positive charge outside of the cell and
excess of negative charge on inside of cell - Build-up of charge impedes further efflux of
K+
. Eventually a steady state is reached –
electrical and chemical driving forces
are equal and opposite - Electrochemical gradient: net driving force
tending to move an ion across a membrane
is the sum of the concentration and
electrical gradients
Ion channel involvement in
membrane potential
Nernst equation – for a single ion
Goldman equation – multiple ions
with varying permeabilities
- Cells have many different ion channels in their membranes
- Multiple ion gradients
- At a typical resting potential, the membrane is highly permeable to potassium
but less so to sodium and/or chloride - Allows determination of the membrane potential at steady state
- Membrane are permeable to more than one ion meaning a steady state rather
than equilibrium
Nernst equation – for a single ion
- Nernst equation gives the membrane
voltage which a single ion would be at
equilibrium - Considers the electrical gradient and
chemical gradient for a single ion - For K+
: Vm = -60 log10 (140/5) = -86.8 mV - But we have other ions with varying
membrane permeabilities!
Goldman equation – multiple ions
with varying permeabilities
Cells are in a steady state rather
than equilibrium
Cells are in a steady state rather
than equilibrium
Passive fluxes of Na+ into and K+ out of the cell are balanced by active
transport in the opposite direction by ATP-dependent Na+
-K+ pump
Utilization of electrical properties of
membranes for neuronal signaling
Utilization of electrical properties of
membranes for neuronal signaling
Resting membrane potential = ~ -60-70 mV. In nerve cells, membrane potential can be
quickly altered by changes in permeability to certain ions = action potential
Functional significance of action potentials:
* Fast signal transmission over long distances in the nervous system
* Control of hormone release from neuroendocrine and other cells
* Control of muscle contraction
* Coding of sensory stimulus features
Voltage-gated cation channels generate
action potentials in electrically excitable cells
- A single polypeptide chain with 4 homologous domains
- Green α helices form central ion conducting pore
- Dark green = selectivity filter
- Red S4 α helices form voltage sensor
- Green triangle forms inactivation gate that obstructs the
pore in the channel’s inactivated state
Voltage-gated cation channels generate
action potentials in electrically excitable cells
Voltage-gated cation channels generate
action potentials in electrically excitable cells
Voltage-gated Na+ channels can be in 1
of 3 states: Closed, open or inactivated
Action potentials caused by opening and
subsequent inactivation of voltage-gated
Na+ channels.
* Closed when membrane potential is at
the resting membrane potential
* Open when membrane potential
increases past a threshold
* Inactivated is a transient blocking of the
channel (separate from the closed state)
The membrane cannot fire a second
action potential until the Na+ channels
have returned from the inactivated to the
closed conformation
How do action potentials work at the
ion channel level?
Change in membrane potential
opens voltage gated Na+
channels, suddenly increasing
membrane permeability to Na+
,
Na+ influx caused depolarization
(increased membrane potential)
Na+ channels close as voltage
gated K+ channels begin to open
Membrane now much more
permeable to K+
, K+ efflux leads
to repolarization (decreased
membrane potential). Na+
-K+
pump also involved.
How do action potentials work at the
ion channel level?
Action potential are propagated over
long distances
Action potential are propagated over
long distances
Multiple dendrites and cell body
receive signals from axons of
other neurons
Single axon can conduct action
potentials over long distances
The axon terminals end on the
dendrites or cell body of other
neurons or on other cell types,
such as muscle or glandular cells
Propagation of an action potential
along an axon
AP is the same amplitude along the
length of the axon
AP continues in the same direction, ‘flow
back’ prevented by Na+ channel
inactivation
Propagation of an action potential
along an axon
Propagation of an action potential
along an axon
Myelination increases speed & efficiency
of action potential propagation in nerves
Schwan cells wrap around the axon to
form a myelin sheath
Insulates axonal membrane to reduce
current leak
Myelin sheath is interrupted by nodes of
Ranvier, highly concentrated Na+ and K+
channels
Action potential propagates along a
myelinated axon by jumping from node to
node = saltatory conduction
* Greatly increases conduction velocity
* Greatly reduces energy consumption
(less Na+
-K+ pump activity)
Myelination increases speed & efficiency
of action potential propagation in nerves
Action potentials cause release of
neurotransmitters at synaptic terminals
Action potentials reaches nerve
terminal and triggers release of
neurotransmitter into synaptic cleft
Neurotransmitter binds to and opens
the chemically-gated ion channels on
the postsynaptic cell
The resulting ion flows alter the
membrane potential of the
postsynaptic cell, thereby transmitting
the signal from the presynaptic to
postsynaptic cell
Action potentials cause release of
neurotransmitters at synaptic terminals
Summary
- Selectively permeable membranes can produce charge separation
- Very few ions move to generate resting potential
- Nernst equation gives the equilibrium potential if the membrane is only permeable to one
ion. - Resting membrane potential dominated by K+ permeability.
- Membrane are permeable to more than one ion meaning a steady state rather than
equilibrium - Goldman equation describes the membrane potential for multiple ions with differing
permeability. - Action potential is caused by the voltage activation of Na+ and K+ channels (and consequent
changes in membrane permeability). - Action potentials are propagated over long distances to cause release of neurotransmitters
and conduction velocity is increased by myelination and saltatory conduction
