NEUROPHYSIOLOGY SMV LECTURES
Ion Channels
Passive and active properties of neurons
- Neurons all have a negative resting membrane potential.
- Hyperpolarization (cell gets more negative) and Depolarization (cell gets more positive).
Ion channels
- Neurons use proteins for their functions.
- Allow ions to diffuse down concentration gradient.
- Cause selective permeability to certain ions.
Ligand-gated vs. voltage-gated ion channels
- Ligand-gated ion channels use chemical ligands to be opened when a ligand binds to the
outside of the channel. Upon binding of a ligand, an electrical signal will be induced leading
to an action potential.
- Voltage-gated ion channels open due to a change in potential difference.
o Na+, Ca2+, K+, and Cl- voltage-gated ion channels.
o It is based on passive diffusion.
o The selectivity of the channels is based on the amino acids in the channel structure.
Measuring ionic currents through ion channels
- Erwin Neher and Bert Sakmann.
- Cell-attached recording recording pipette tight contact between pipette and
membrane mild suction.
- With this technique, it was possible to measure the very small currents in the proteins.
- It was possible to measure single ion channel function.
Activation and inactivation: Sodium currents
- Microscopic Ik with open and closed channels (slides 10 and 11).
- When the current is down, the channel is closed. When the current is up, the channel is
opened (slide 10).
Sodium currents rapidly activate and inactivate (slide 12)
- With more depolarization, the chance of opening of the ion channels increases (it is
stochastic). So the more positive, the higher the probability of opening the ion channels.
Therefore, the probability will never reach 1.0 because there is a possibility that an ion
channel will not open upon rising membrane potential.
- At 15 ms, there is a macroscopic decrease in current, this is called ‘the capacitive current’.
This is the redistribution of charges across the axonal membrane.
,Activation and inactivation gates
- During depolarization of Na+ channels:
o The closed channel opens and Na+ flows in depolarization The channel comes
in an inactivating state During the inactivated state Na+ cannot flow in anymore
The Na+ channel is closed.
- During depolarization of K+ channels:
o During depolarization, there is a closed state when Na+ channels are already opened
During inactivation of Na+ channels, the K+ channels open, and K+ is released
outside the cell This open state occurs even when Na+ channels are already
inactivated for a while K+ channels also close.
- So, potassium channels open slower and do not inactivate.
Sodium channels have all these subunits (see slide 14) all covalently linked together. The sodium and
potassium channel are quite similar, but potassium channels needs 4 of these 6-subunit structures to
form a channel that is produced by 4 different genes. Therefore, potassium channel structures are
more diverse due to the fact that K+ channels are formed from all type of different subunits (see
slide 15). This combination of different subunits is highly regulated by gene transcription.
Voltage dependence
- Voltage sensor measures the currents outside and inside the cell.
- Depolarized: pore open
- Hyperpolarized: pore closed
- Diffusion of ions takes place when the pore opens. Diffusion from high to low concentration.
Gating: movement of S4 domain
- The third protein is always on the outside of the helix. Thereby, this protein determines how
the structure communicates with the aqueous surroundings and the ions that bind to the
channel.
- The S4 domain in voltage-gated ion channels responds to changes in membrane potential
and controls channel opening.
,Ion selectivity
- They have an aqueous pore, which becomes accessible to ions after a conformational change
in the protein structure that causes the ion channel to open. Ion channels are selective
meaning that they only allow certain ions to pass through them, and they play critical roles in
controlling neuronal excitability.
- The pores only allows specific ions. Potassium is always coupled to an H2O molecule when
passing through the channel. The H2O surrounding sodium should first be removed before it
can pass the channel.
- S4 domain: voltage-sensitivity domain.
- S5 domain/S6 domain: make up the pore and determine selectivity-filter.
- Sodium channels also have inside a domain that can block ion movement during inactivation.
Ion channels and toxins in venoms: Animal venoms contain various toxins that act on ion channels,
responsible for either sodium, potassium, calcium, or chloride permeation.
Ion channel pharmacology: the venom of puffer fish acts on sodium channels in nerve and skeletal
muscle. TTX binds to sodium channels and thereby blocks the propagation of action potentials
resulting in death. As shown in the figure on slide 20, the sodium channels are more in a closed
condition after TTX exposure. It is an open channel blocker, so it sits inside the channel and physically
blocks the flow of sodium ions.
Conotoxin (from snails) is used as venom or venomous pharmacology to block potassium channels.
Ion channels and disease
- Cystic fibrosis – Chloride channel
- Epilepsy – Potassium channel
- Migraine – Calcium channel, K/Na pump
- Obesity – TRP (calcium) channel
Migraine, ion channels, and pumps (see pictures on slide 23)
- Migraine is often caused by cortical spreading depression which is basically a wave of activity
that in humans often starts in the visual part of the brain (backside). It is a wave of
depolarization to other cortical regions. It is caused by too much glutamate and calcium
channels.
- Familial migraine is caused by a point mutation in the gene encoding voltage-gated calcium
channels. With this point mutation, there is less voltage change needed to induce
depolarization. Therefore, glutamate is released by lower membrane potentials and that will
cause cortical spreading depression.
, Resting membrane potential
All neurons have negative resting membrane potential, about -60 to -80 mV. With a negative
potential they attract sodium ions through the channels. The resting membrane potential is created
by ion pumps, they create ion gradients across the membrane. This way ions can diffuse through the
ion channels. Ions face a chemical driving force (concentration gradient) and an electrical driving
force for their diffusion. Electrical driving force can overcome the chemical driving force. This means
the membrane potential determines ion flux.
Nernst equation: based on concentration of ions outside and inside of the cell. Different ions have
different concentration gradient, which means there are different equilibrium potential for different
ions. Cells make a lot of effort to put these concentration differences in place. Calcium concentration
as low as possible in cells because calcium is an important second messenger.
When you know the equilibrium potentials of ions, you can calculate the membrane potential with
the Goldman Hodgkin Katz equation. This includes concentration differences and permeability of
different ions. The membrane potential is continuously changing because permeability for ions is
continuously changing because of voltage gated ion channels. When permeability gets bigger for
certain ions, the membrane potential goes to the equilibrium potential of that certain ion.
During an action potential, permeability of Na+ increases, then
decreases while permeability of K+ increases. The Na+/K+ pump puts
this resting membrane potential in place. They use energy to pump K+
inside the cell and Na+ out of the cell. Some K+ channels open during
resting conditions, Na+ channels are closed during resting conditions.
K+ concentration is important for the resting membrane potential,
however the Na+ concentration does not affect the resting membrane
potential. In resting conditions permeability is very high for K+, so it
has a direct effect on membrane potential.
Membrane potentials are regulated during different stages of development. Before birth the chloride
pump is active, which creates a high concentration of chloride inside the cell. When GABA binds to its
receptor, chloride goes out of the cell. Before birth the equilibrium potential of chloride is -40 mV,
after GABA binds it quickly depolarizes to 40 mV. During birth oxytocin blocks the chloride pump, no
chloride into the cell, so when GABA binds no chloride is going out so almost no depolarization. At
postnatal development chloride pump is replaced by another to create the normal equilibrium
potential of chloride.
