LECTURE 2 Flashcards
What forces move ions across membranes?
Chemical and electrical
How do chemical forces move ions across membranes
Differences in concentration: diffusion from a region of high concentration to a region of low concentration
How do electrical forces move ions across membranes
Interior of cell is negatively charged so positively charged cations are retained and negative ions will be expelled
What is the electrochemical driving force
The electrochemical driving force is a combination of the chemical and electrical forces acting on any particular ion
2 broad categories of ion channels
- Channels that are gated and require a stimulus to open
ligands, mechanical force or voltage
specific to particular ion(s) - Channels that are always open and allow free movement of ions
resting concentration Na+ and K+
Under resting conditions, the concentration of Na+ ions is ~ 10x higher outside the neuron compared to the concentration of Na+ ions inside
At the same time, levels of K+ ions are ~ 15x higher inside the neuron compared to the extracellular environment
Potassium movement
There is a constant flow of K+ ions down their concentration gradient, from the inside of the neuron to the outside
This movement occurs via open (or leaky) K+ channels that are situated in the membrane of the neuron
Na+/K+ ATPase pump
The ion gradient is maintained by the continuous operation of the Na+/K+ ATPase pump
It moves 3 Na+ ions from the inside of the neuron to the outside of the cell
At the same time, 2 K+ ions are moved from outside the neuron to the inside of the cell
At each cycle of the Na+/K+ ATPase pump, the cell loses one positively charged ion from the intracellular environment
polarisation meaning
The difference in charge across the membrane of the neuron is referred to as polarisation
Where is there more positive charge at rest
At rest, there is more positive charge outside the neuron compared to the inside of the neuron
What is the resting membrane potential (meaning and number)
The difference in voltage across the plasma membrane when the neuron is at rest is called the resting membrane potential
For most neurons, the resting membrane potential is ~ -70mV
Electrochemical gradients of sodium
When Na+ channels open:
chemical gradient drives ion movement into the cell
electrical force pulls + ions into the cell
both act in the same direction = Na+ will enter the cell
Equilibrium of sodium movement
As Na+ moves into the neuron, the charge inside the cell starts to become positive and the electrical gradient decreases, along with the chemical gradient
Eventually, the chemical and electrical forces will be exactly in balance and there will be no net flow through any open channels
What is the equilibrium potential
The equilibrium potential (E) is the membrane potential required to exactly counteract the chemical forces acting to move one particular ion across the membrane.
Electrochemical gradient of potassium
When K+ channels open:
chemical gradient drives ion movement out of the cell
but electrical force pulls + ions into the cell
two forces act in opposite directions
chemical force > electrical force, so K+ moves out of the neuron
Equilibrium of potassium movement
As K+ moves out of the neuron, the charge inside the cell starts to become even more negative, so the electrical gradient becomes stronger
Eventually, the chemical force that drives K+ out of the cell = the electrical force driving K+ back into the cell
At this point, there will be no net flow of K+ ions
What equation is used to calculate the equilibrium potential
Nernst equation
E = 61 log Co
___ ___
z Ci
E= equilibrium potential in millivolts (mV)
z = charge of ion
Co = concentration of the ion outside the neuron
Ci = concentration of the ion inside the neuron
How are action potentials triggered or made less likey
If the membrane potential is depolarised beyond a certain critical level (threshold potential = -55mV) then an action potential is triggered in the neuron
Other incoming signals can do the reverse and hyperpolarise the membrane (i.e. cause the membrane potential to decrease), so making an action potential less likely
What are Voltage-gated ion channels
Embedded in the plasma membrane of the neuron are ion channels that are sensitive to the voltage of the cell
These channels open only when the voltage in the cell reaches a certain value
Voltage-gated Na+ & K+ channels
Voltage-gated Na+ channels have both an activation gate and an inactivation gate. At rest, the activation gate is closed and the inactivation gate is open
Voltage-gated K+ channels have one activation gate, which opens to allow the flow of K+ ions through the channel and closes to stop the flow of K+ ions
- Initial stimulation of neuron
When the neuron receives an excitatory signal or stimulus, ligand-gated ion Na+ channels open
Small amounts of Na+ will move down their concentration gradient into the neuron and the resting potential will start to become more positive
- Depolarisation of neuron
Once the membrane potential reaches a critical threshold of -55 mV, voltage-gated activation gates in the Na+ channel open quickly, allowing Na+ to flood into the neuron
As a result of the large influx of positively charged Na+, the neuron quickly loses its negative charge and undergoes depolarisation
Is the neuron positive or negative after it undergoes depolarisation
positive
- Inactivation of Na+ channels
When the inside of the neuron become highly positive, the pore of the voltage-gated Na+ channels is plugged by the inactivation gate and the flow of Na+ into the neuron stops
- Repolarisation
Eventually the intracellular environment of the neuron becomes sufficiently positive that voltage-gated K+ channels begin to open slowly
Opening of these channels allows K+ to flow down its concentration gradient out of the cell
This movement of K+ causes the inside of the neuron to quickly regain its negative charge in a process called repolarisation
- Hyperpolarisation
In response to the increasingly negative charge inside the neuron, the voltage-gated K+ channels close.
Because this process is slow, some K+ ions continue to move outside the cell while the channel is closing
This extra efflux of K+ causes the membrane potential to become more negative than the resting potential of -70 mV. This process is called hyperpolarisation
- Refractory period
During the period of hyperpolarization, the neuron will not be able to fire another action potential. This is termed the refractory period
Eventually, the action of the Na+/K+ ATPase pump will restore the resting membrane potential to -70mV and the neuron will be ready to fire another action potential
