Membrane and Action Potential Flashcards
Physiology
How is the resting membrane potential generated?
When at ‘rest’ or electrically inactive, cells are negatively charged on the inside compared to the outside.
- At some point in the development of the cell, the numbers of positive (+ve) potassium (K+) ions equal the number of large negative (-ve) anions (A-) inside the cell. An
example of a large anion would be a protein - There is a concentration (or chemical) gradient for K+ ions, (there are more inside the cell than there are outside the cell) and since the cell membrane is permeable to K+ its leaves the cell. There is also a concentration gradient for the A-, however the anions are too large to leave the cell. It is important to note that the concentrations of K+ either side of the membrane do not equalise. This is because as K+ leaves, an electrical gradient (i.e. increasing –ve charge), keeps it within the cell. Eventually the effects of the
chemical and electrical gradients balance each other out, but the point at which this occurs is before the [K+] equalises on either side of the membrane. - What is left is a situation where there are less K+ ions inside the cell and still the same number of A-. This means that there is a surplus of negative charges inside the cell and so its overall charge is –ve.
Nerve cells have a REm of -70mV, smooth muscle cell -40mV and cardiac and skeletal muscle -90mV.
Equilibrium potential for K+ (explain)
When K+ crosses the membrane down its concentration gradient it leaves a negative charge behind (in the form of Cl-).
However because opposite charges attract K+ movement will eventually stop when the attraction of the –ve charges inside the cell precisely counter the outward driving force of the concentration gradient (electrochemical equilibrium).
The potential at which equilibrium is established is known as the equilibrium potential for K+ (Ek).
Equilibrium potentials can be calculated for any membrane permeant ion using the Nernst Equation.
Nernst Equation
- A way of mathematically determining the contribution of a particular ion to the REm.
- It does this by calculating what is referred to as the equilibrium potential (E) for a particular ion (x).
- This value indicates the membrane potential at which the opposing forces of the chemical and electrical gradients for an ion balance each other out.
- By comparing the equilibrium potential of an ion (Ex) to the REm you can get an idea of how much that ion contributes to it.
Nernst Equation :
E(x) = RT/zF loge [X]o/[X]i
example:
E(K) = 25/1 loge [4]o/[140]i = -88.8mV
R= ideal gas constant
T= absolute temp
z= valency of the ion
F= Faradays constant.
[X]o= concentration of an ion outside the cell
[X]i= concentration of an ion inside the cell
When the equation is calculated it will be noticed that the figure calculated for EK is not the same as the REm for a neurone, therefore other ions must also contribute the REm.
The diagram below identifies Na+, Ca2+ and Cl- as being other contributing ions, however since EK is the closest to the REm you can see that it makes the largest contribution. The reason for this is that the cell membrane is more permeable to K+ than the other ions.
Potential Changes (explain)
The cell is –ve on the inside and it is +ve outside; this is referred to as the cell being polarized.
If the charge within the cell becomes +ve then the cell is now referred to as depolarized.
When the charges returns to their normal state i.e. –ve inside and +ve outside, this is referred to as repolarization.
This is essentially what
electrically excitable cells use to communicate, they are polarised (at rest) then become depolarized (when excited) then undergo repolarization (return to resting state) waiting for next stimulus.
There are two types of potential changes, both are depolarization followed by repolarization, but one is a small potential change which doesn’t get very far along the cell, and the other is a larger change which is carried along the entire length of the cell and beyond. It is this larger change, referred to as the action potential which acts as a signal for cell to cell communication i.e. you need an action potential for neurotransmitter to be released!
In a neurone a local potential is produced when a neurone is activated by a particular stimulus, a chemical for example.
-> When this happens Na+ channels in the membrane open and Na+ diffuses into the cell, this influx of +ve charge is
what causes the cell to depolarize.
-> Local potentials vary in
size depending on how many ion channels open, the more
Na+ channels open the greater the size of the depolarization.
-> They also do not depolarize the entire length of the cell, since they decline as they spread out from the point of stimulation. Local potentials begin at the dendrites.
If they are strong enough when they reach the soma/cell body of the neurones they can trigger an action potential.
We call the membrane potential at which this occurs the threshold potential.
The significance of the soma is that it is the location of
an area of membrane called the trigger zone. This is an area with a particularly high density of Na+ channels. Since many
more Na+ channels can open here a much greater potential change is possible.
Action Potential (explain)
An action potential can be defined as a ‘rapid and uniform electrical signal conducted down (along) a cell membrane’.
As a chemical signal reaches the Na+ ion channels open causing an influx of Na+, the cell becomes more +ve causing depolarization.
Repolarization begins as Na+ channels close and K+
channels remain open. This prevents any more Na+ entering the cell but allows K+ to leave permitting the cell to become more –ve / less +vely charged.
Once the desired –ve charge returns the cell is repolarized and the K+ channels
close.
At the end of the repolarization phase the charge inside the cell fall below the REm (-70mV), known as hyperpolarization.
The reason for this is the K+ channels take longer to
close than the Na+ channels, letting slightly more K+ out of the cell than the Na+ that entered. However, inward diffusion of Na+ through leakage channels soon brings the Em back to resting levels.
It is also important to note that action potentials are different from local potential in that they are not a graded response. If the threshold potential is reached then an action potential will occur.
The action potential never
deviates in size and so is referred to as an ‘all-or-none / nothing’ response, it will also travel the entire length of the axon (think of it travelling as a wave down the length of the neuron).
Refractory Period (explain)
During the action potential and for a short period after it is either impossible or difficult to stimulate the membrane to depolarize again.
This is referred
to as the refractory period.
There are two types of refractory periods. During the absolute refractory period it is impossible to stimulate an action potential in the section
of membrane that has just been active, no matter how strong the stimulus.
This is because Na+ channels are only open for a short
period of time before they shut and become inactivated i.e. impossible to open for a while.
This period extends from the start of depolarization until the REm is reached again. The relative refractory period lasts until hyperpolarization is over.
During this period Na+
channels may be opened but K+ channels are also open so it requires a greater stimulus than normal to open enough Na+ channels to overcome the opposing effects of K+ efflux.
Propagation of the Action Potential
When action potentials spread across the cell membrane.
The way in which they are propagated depends on whether the axon is coated with a fatty substance called myelin.
In an unmyelinated axon (axon not covered by myelin) a section of cell membrane depolarizes Na+, enters the cell and spreads underneath the cell membrane.
-> This activates the voltage
gated ion channels in the next section of membrane along
and initiates another action potential.
-> This process repeats
itself until the end of the neuron is reached (like a wave
travelling down the length of the axon).
-> The significance of the refractory period becomes clear if you consider that
without it an action potential could be initiated in the section
of membrane behind the active one. In other word the refractory period ensures that the action potential (signal)
travels in only one direction!
Propagation of an action potential along a myelinated axon (axon with a myelin covering, a myelin sheath) is faster.
-> The myelin is not a continuous covering along the entire length of the axon, there are gaps called the nodes of Ranvier.
-> These nodes allow the action potential to jump along the axon, which takes less time than propagation of an action potential along an unmyelinated axon as every part of the membrane does not have to be depolarized.
-> This type of propagation is called saltatory conduction after the French word saltare (meaning to jump).
The speed at which an action potential travels along an axon is also dependent on the axon diameter (the greater the
diameter of a nerve fibre the faster the conduction velocity.
