PHYS20008 Lecture Notes - Lecture 4: Threshold Voltage, Membrane Potential, Action Potential
21 Jun 2018
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Lecture 4: Action potential
What is an action potential?
An action potential is the rapid change in electric potential that parts of a nerve cell undergo
when a nerve impulse is generated- the wave movement from one end of the axon to the
other.
Instead of using the terms ‘more/less positive/negative’ to describe membrane potential
changes, we can use ‘polarity’.
Polarity refers to the distance away from the membrane potential of 0mV:
Hyperpolarisation (Greater concentration gradient and charge): This is when the cell
becomes more polarised and, therefore, has a membrane potential moving further
and further away from 0mV (In either the negative or positive direction)
Depolarisation (Lower concentration gradient and charge): This is when the cell
becomes less polarised and, therefore, has a membrane potential moving closer to
0mV.
So far, we know of the Na+ and K+ leak pumps with the ability to open and increase
permeability of the particular ion into or out of the cell.
Although opening, for example, an Na+ leak pump we would expect Na+ to come into the
cell and make the cell more positive, this charge diffuses from the initial site of potential
charge very quickly and as you move across the membrane, this charge is less and less felt.
This is why action potentials occur- so the change is maintained across the
membrane instead of diffusing out.
Voltage gates:
An action potential is triggered when the membrane potential reaches the threshold voltage
of -50mV. The action potential released will graph the same sudden upward and then
downward jolt in every case.
There are essentially three steps in a neuron firing an action potential- involving the
individual ion pumps.
Voltage-gated sodium channel:
Here is where the action potential begins (when -50mV is hit). This channel has 2 gates: The
activation and inactivation gates with name-obvious functions:
*Note: Inactivation gate actually opens first to steadily leak Na+ into the cell so threshold voltage is
reached
The activation gate is fast- opens immediately when membrane potential reaches
-50mV. Na+ is allowed to enter into the cell, ultimately, resulting in more Na+
coming in than K+ going out. From the threshold voltage, Na+ will continue to come
into the cell until it reaches about +30mV
The inactivation gate is also opened fully at threshold voltage and proceeds to close
from this point. This process is extremely slow which is why enough Na+ would have
made it into the cell to make the membrane potential positive before it closes the
channel. Once closed, Na+ and K+ will resume crossing the membrane at RMP rate
until RMP is reached.
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Document Summary
An action potential is the rapid change in electric potential that parts of a nerve cell undergo when a nerve impulse is generated- the wave movement from one end of the axon to the other. Instead of using the terms more/less positive/negative" to describe membrane potential changes, we can use polarity". Polarity refers to the distance away from the membrane potential of 0mv: Hyperpolarisation (greater concentration gradient and charge): this is when the cell becomes more polarised and, therefore, has a membrane potential moving further and further away from 0mv (in either the negative or positive direction) Depolarisation (lower concentration gradient and charge): this is when the cell becomes less polarised and, therefore, has a membrane potential moving closer to. So far, we know of the na+ and k+ leak pumps with the ability to open and increase permeability of the particular ion into or out of the cell.
