Membrane potential is classified as: 1. Resting Membrane Potential 2. Action Potential 3. Graded Potential.
All cells in animal body tissue are electrically polarized, in other words they maintain a voltage difference across the plasma membrane known as membrane potential. The cell membrane acts as a barrier that prevents intracellular fluid from mixing with the extracellular fluid. Therefore, the electrical potential difference results from a complex interplay between protein structures embedded in the membrane called ion pumps and ion channels.
Type # 1. Genesis of Resting Membrane Potential:
The membrane potential across the cell membrane when the cell is at rest is called resting membrane potential (RMP). We always express RMP by comparing ICF potential to ECF potential keeping ECF potential to be zero. For example: RMP for large nerve fibre is –90 mV. That is the potential inside the fiber is 90 millivolts more negative than the ECF potential. The nerve cell at this state is said to be in polarized state.
RMP in a cell is generated due to two reasons:
i. Simple diffusion, and
ii. Sodium potassium pump.
i. Contribution of Simple Diffusion to the Genesis of RMP:
Simple diffusion through protein channels like sodium and potassium channels, which allows movement down the concentration gradient, is influenced by factors such as size, charge on surface of protein, hydration of the ion, etc.
Biophysical basis for membrane potential is caused by simple diffusion alone.
a. Gibbs-Donnan Effect:
The Gibbs-Donnan effect (also known as the Donnan effect, Donnan law, Donnan equilibrium, or Gibbs-Donnan equilibrium) is a name for the behavior of charged particles near a semipermeable membrane which sometimes fail to distribute evenly across the two sides of the membrane. The usual cause is the presence of a different charged substance that is unable to pass through the membrane and thus creates an uneven electrical charge.
In the body, it is the Gibbs-Donnan effect of intracellular negatively charged protein forms the basis of the negative resting membrane potential. If it was not for the electrogenic activity of Na/K ATPases the resting membrane potential would be even more negative. This forms the basis for equilibrium potential of ions.
b. Equilibrium Potential and Nernst Equation:
Particular ion will flow across a membrane from the higher concentration to the lower concentration (down a concentration gradient), causing a current. However, this creates a voltage difference across the membrane that opposes the movement of ions. When this voltage reaches the equilibrium value, the two balances (concentration gradient and the voltage) and the flow of ion stops.
The voltage at which the flow of the ion stops is called the equilibrium potential of that ion. The equilibrium potential for any ion can be calculated by an equation called Nernst equation. Equilibrium potential for potassium is –94 mV; equilibrium potential for sodium is +61 mV.
Example:
Equation for calculating the equilibrium potential for potassium is as follows:
Eeq K+ = RT In [K+]o/zF [K+]i
Where;
i. Eeq K+ is the equilibrium potential for potassium in volts
ii. R is the gas constant
iii. T is the absolute temperature
iv. Z is the number of elementary charge of the ion
v. F is Faraday constant
vi. [K+] o is ECF concentration of potassium
vii. [K+] i is ICF concentration of potassium
viii. RT is a constant and the value is calculated as 61 and formula can be written as-
ix. ZF
c. Goldman-Hodgkin-Katz Equation:
If membrane is permeable to only one ion, the membrane potential is the equilibrium potential of that ion. But in reality, animal cell is permeable to many ions. So the membrane potential has to be calculated taking equilibrium potential of all ions into consideration.
Hence, membrane potential depends on:
i. Polarity of electric charge of each ion
ii. Permeability of membrane
iii. Concentration of ion in ICF and ECF.
The major ions involved in generating membrane potential are sodium, potassium and chloride. Membrane potential can be calculated from Goldman-Hodgkin-Katz equation ―
Where P is permeability of the cell membrane to the ion. Due to the negative charge of chloride ion, the concentration of chloride in ECF is written in the numerator.
Note:
At rest the cell membrane is 100 times more permeable to potassium diffusion than sodium because the hydrated form of potassium is smaller in size compared to that of hydrated form of sodium ion. This is reason why RMP will be close to equilibrium potential of potassium.
ii. Contribution of Sodium Potassium Pump for the Genesis of RMP:
It is an electrogenic pump which pumps three Na+ to ECF and two K+ to ICF against concentration gradient leaving a net deficient of one positive ion on the inside, which causes a negative voltage inside the cell membrane.
Let us calculate the RMP of the large nerve fiber:
i. The resting membrane potential of large nerve fiber, if potassium alone is considered as permeable is –94 mV which is calculated from Nernst equation.
ii. The RMP if sodium alone is considered will be +61 mV.
iii. But actually RMP is due to both sodium and potassium which can be calculated from Goldman equation to be –86 mV, which is nearest to equilibrium potential of potassium.
iv. But still we have –4 mV left to get –90 mV as RMP in large nerve fiber. What it is due to? It is due to the sodium potassium pump which leaves negativity inside the cell contributing to –4 mV by pumping an extra positive charge outside the cell.
v. So totally –86 mV and –4 mV negativity inside the cell is due to simple diffusion and active transport respectively which contributes to –90 mV RMP in large nerve fiber.
Note:
For calculating RMP in a muscle, calcium ion should also be considered for calculating Goldman equation (Fig. 2.18).
Type # 2. Action Potential (AP):
An action potential is a short lasting event in which the electrical membrane potential of cell rapidly rises and fall after sufficient strength of a stimulus is applied (during action). This occurs in excitable cell like neurons and muscle cell. In neurons, they play a central role in cell to cell communication (Fig. 2.19). In muscle cell, AP is the first step in chain of events leading to contraction.
The stages of action potential:
I. Resting (Polarized) Stage:
This is the membrane potential before the stimulus, i.e. RMP and the membrane is said to be polarized.
II. Depolarization Stage:
After the sufficient stimulus to an excitable cell, there occurs a change in voltage which opens the voltage gated sodium channel, (Sodium permeability is more than potassium during action in contrast to what it is at rest) allowing tremendous flow of positively charged sodium ions through voltage gated sodium channels by simple diffusion to the interior of cell.
The membrane potential which was negative compared with outside, now rapidly shifts to positive side due to flow of positive charge sodium ion. This shift from negative to positive potential is called as depolarization (i.e. polarized to depolarized state).
III. Repolarization Stage:
The gated channel is open only 1/10000th of a second. Within a few 10000th of a second the sodium channels begin to close. But the voltage for opening potassium channel is attained, and therefore, voltage gated potassium channel opens. Since K+ is more inside, the potassium flows tremendously to the outside of the cell (Fig. 2.20). This rapid diffusion of K+ which is also a positive charge to outside of the cell, re-establishes the negative RMP. This stage is called repolarization stage. After a fraction of second the gated potassium channel closes.
The ionic basis with an example of action potential in a large nerve fiber is explained as:
RMP nerve fibre is –90 mV (resting stage) → Sufficient strength of a stimulus is applied → Opening of voltage-gated sodium channel → Sodium flows slowly from outside to inside through these channels thereby slowly decreasing the negativity inside the cell to –50 mV to –70 mV (threshold potential) → Once firing potential or threshold potential is reached, more voltage gated sodium channels is recruited which causes rapid inflow of sodium (sodium permeability increases by 500 to 5000 times) → The voltage inside the cell shoots up to +35 mV. This is depolarization stage → Inactivation of sodium gate at +35 mV occurs, but this is voltage required for opening the potassium gates → Potassium pours from inside to outside through these channel because ICF potassium is more than ECF concentration of potassium. There is rapid fall of potential. This event brings back the potential towards negativity. This is repolarization stage (sharp rise and fall of potential is called spike potential) → At the end there is slow fall of potential towards RMP called after depolarization → Potassium channel is slow to close, so there is extra outflow of the potassium ion to the ECF → More negativity than RMP is created called after hyperpolarization → Here comes Na+ K+ pump which is re-establish RMP
Note:
There will be some extra flow of K+ ions outside the cell because they are very slow to close. This causes after hyperpolarization. The gates will not reopen until the membrane potential returns to the original RMP. This is possible only with the help of Na+ K+ pump which helps to re-establish the RMP.
Propagation of Action Potential:
An action potential elicited at any one point on an excitable membrane usually excites the adjacent portion of the membrane, resulting in propagation of action potential over the membrane. The local change in potential is carried inwards to several millimeter of adjacent membrane in both the directions, which slowly opens more Na+ channel. This newly depolarized area in the same manner propagates the action potential. This transmission of depolarization process is called an impulse.
Saltatory Conduction:
The need for fast transmission of electrical signals in nervous system results in myelination of neuronal axons. Myelin sheath around axons is separated by intervals known as nodes of Ranvier. In saltatory conduction, an action potential at one node of Ranvier caused inward current that depolarize the membrane at the next node, provoking a new action potential, the AP hops from node to node because myelin offers resistance in internodal intervals.
The significance of this is that:
i. The conduction of AP is faster.
ii. The energy is conserved, which otherwise requires lot of energy if it travels through the entire neuron.
Properties of Action Potential:
I. All-or-None Law:
It states that a threshold or sub-threshold stimulus is capable of producing action potential will produce the maximum possible amplitude of action potential or will not produce an action potential if the stimulus is sub-threshold. In other words, large strength do not create large action potential, therefore, action potential are said to be all-or-none.
II. Refractory Period:
It is the period during which excitability of excitable tissue to second stimulus is decreased.
It is divided into:
i. Absolute Refractory Period (ARP):
It is the period during which even a second strongest stimulus cannot produce an action potential. The period extends from firing level to one-third of repolarization stage of first action potential. This is responsible for unidirectional conduction of action potential in axon.
Reason ― Inactivation of sodium gates.
ii. Relative Refractory Period (RRP):
It is the period during which a stronger than normal stimulus can produce action potential. It extends from one-third of repolarisation stage to after depolarization.
Reason ― The excitability is increased but threshold is decreased, so you need stronger stimulus.
III. Strength-Duration Curve:
This is a curve plotted to show the relationship of action potential with strength of a stimulus and duration of stimulus. A stimulus must be of adequate intensity and duration to evoke a response. If it is too short, even a strong pulse will not be effective. A long pulse below certain strength will evoke only a local non- propagated response.
Rheobase:
It is the minimum threshold strength which can produce an action potential.
Chronaxie:
It is duration for which twice the rheobase strength has to be applied to produce an action potential. It is the measure of excitability of the tissue. Chronaxie is inversely proportional to excitability.
Variations in Action Potential in Other Tissues:
I. Plateau in Action Potential:
For example, in cardiac muscle. This type action potential happens in cardiac muscle.
The stages are:
a. Resting Phase
b. Depolarization Phase:
Due to rapid sodium influx through voltage-gated sodium channel which is same as nerve action potential, but it is followed by plateau phase.
c. Plateau Phase:
The membrane is held at high voltage for a few milliseconds prior to being repolarized.
This is due to two reasons:
(i) Voltage-gated calcium sodium channel, which is slow to open causing slow inflow of calcium and sodium,
(ii) Voltage-gated potassium channel are slow to open which causes slow outflow of potassium, and therefore, potential remains in positivity for some time. This delays the return of membrane potential to resting level.
d. Repolarization Phase:
Due to rapid outflow of potassium through K+ channels and closing of slow calcium sodium channel which returns the action potential to resting level.
II. Pacemaker Potential:
For example ― cardiac pacemaker and smooth muscle (Fig. 2.25).
The cardiac pacemaker cells of the sinoatrial node in the heart provide a good example. They have self-induced rhythmical action potential without any stimulus. This is due to spontaneous excitability.
This is attributed to two reasons:
a. The resting membrane potential of pacemaker cells is between -55 mV to -60 mV. It is very close to the threshold potential which makes the cells to depolarize easily.
b. The resting sinoatrial nodal cells have sodium leaking channel called funny channels that are already open to sodium. Without any stimulus, the sodium leak to the inside of the pacemaker cell causes a slow rising RMP between heart beats, this slow rising membrane potential when reaches -40 mV attains threshold for firing due to rapid entry of sodium and calcium ions at -40 mV. After repolarization the cycle continues. Thus the inherent leaking of sodium ion causes self-excitation.
Type # 3. Graded Potential:
It is the localized change (depolarization or hyper-polarization) in the potential difference across a cell surface membrane. Strength of the graded potential varies with the intensity of the stimulus and causes local flows of current which decrease with distance from the stimulus point (Fig. 2.26).
Graded potentials are given different names according to their function (Fig. 2.27). The membrane potential at any point in a cell’s membrane is determined by the ion concentration differences between the intracellular and extracellular areas and by the permeability of the membrane to each type of ion.
The ion concentrations do not normally change very quickly (with the exception of calcium, where the baseline intracellular concentration is so low that even a small inflow may increase it by orders of magnitude), but the permeability can change in a fraction of a millisecond, as a result of activation of ligand-gated or voltage-gated ion channels.
The change in membrane potential can be large or small, depending on how many ion channels are activated and what type they are. Changes of this type are referred to as graded potentials, in contrast to action potentials, which have a fixed amplitude and time course. Graded membrane potentials are particularly important in neurons, where they are produced by synapses ― a temporary rise or fall in membrane potential produced by activation of a synapse is called a postsynaptic potential.
Comparison between Graded Potentials and Action Potentials:
Graded Potentials:
1. Origin ― Arise mainly in dendrites and cell bodies
2. Types of channels ― Chemical, Mechanical or light
3. Conduction ― Not propagated, localized, thus permit communication over few mm
4. Amplitude ― Depends on strength of stimulus; varies from less than 1 mV to more than 50 mV
5. Duration ― Longer, ranging from msec to several minutes
6. Polarity ― May be hyperpolarizing, depolarizing
7. Refractory period ― No.
Action Potentials:
1. Origin ― Arise at trigger zones and propagate along axon
2. Types of channels ― Voltage-gated ion channels
3. Conduction ― Propagated, thus permit communication over long distance
4. Amplitude ― All-or-none, typically 100mV
5. Duration ― Shorter, ranging from 0.5-2 msec
6. Polarity ― Always consist of depolarizing phase followed by repolarizing phase and then return to resting membrane potential
7. Refractory period ― Yes.