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Membrane potential – Definition, Types, Equilibrium and Ions

Last Updated : 13 Jan, 2024
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Membrane potentials are defined by various ionic attention configurations outside and in the membrane of a cellular. These potentials are:

  • Resting membrane potential: the membrane ability at rest, steady-nation situations.
  • Action potential: a non-graded ability, similar to binary code (on/off).
  • Post-synaptic potentials: graded potentials, that may be summated/subtracted by using modulation from presynaptic neurons.

Resting Membrane 

Resting membrane potential is the difference between the electric potential inside the intracellular and extracellular matrices of the cellular whilst it isn’t exciting. Every cellular of the body has its very own membrane potential, however handiest excitable cells – nerves and muscle groups – are successful to exchange it and generate a motion capacity.

For this purpose, membrane ability for excitable cells while they’re no longer excited is called the resting membrane capability, at the same time as its modifications are related to a motion capacity.

Resting membrane potential (EM) originates from the exclusive concentrations of ions (expressed in mmol/l) on the inner and outer surface of the cellular membrane. There are four excitable tissues in our frame, and they all have exceptional EM values:

  • Skeletal muscle mobile = -90 millivolts (mV) 
  • Smooth muscle cellular = -55mV
  • Cardiac muscle mobile = -80mV
  • Neuron = -65mV 

The poor values imply that the cytoplasm is greater electronegative than the extracellular area. The values of EM rely on numerous elements:

  • The concentration of ions outside and inside the mobile. Ions that contribute the most are sodium, potassium, calcium, and chloride ions. 
  • The activity of the sodium-potassium pump.
  • Variable permeability of the cell membrane for ions.

Producing and maintaining Resting Membrane Potential (RMP)

RMP is produced and maintained by:

  • Donnan effect-described as large impermeable negatively charged intracellular molecules attracting positively charged ions (e. g.: Na+ and K+) and repelling negative ones (e. g.: Cl−)
  • Membrane selectivity is the difference in permeabilities between different ions
  • Active transport (Na+/K+ ATPase pump)-is the mediated process of moving particles across a biological membrane, against the concentration gradient.
    • Primary active transport – if it spends energy. This is how the Na+/K+ ATPase pump functions.
    • Secondary active transport – if it involves an electrochemical gradient. This is not involved in maintaining RMP.
Sodium- Potassium Pump

 

In the affection of resting membrane potential

RMP is created by the distribution of ions and their diffusion across the membrane. Potassium ions are important for RMP because of their active transport, which increases its concentration inside the cell. However, the potassium-selective ion channels are always open, producing an accumulation of negative charge inside the cell. Its outward movement is due to random molecular motion and continues until enough excess negative charge accumulates inside the cell to form a membrane potential.

Na+/K+ ATPase pump affection of the RMP

The Na+/K+ ATPase pump creates a concentration gradient by moving 3Na+ out of the cell and 2K+ into the cell. Na+ is being pumped out and K+ is pumped in against their concentration gradients. Because this pump is moving ions against their concentration gradients, it requires energy.

Ion channels affection of resting membrane potential

The cell membrane contains protein channels that allow ions to diffuse passively without the direct expenditure of metabolic energy. These channels allow Na + and K+ to move across the cell membrane from a higher concentration toward a lower. As these channels have selectivity for certain ions, there are potassium- and sodium- selective ion channels. All cell membranes are more permeable to K+ than to Na+ because they have more K+ channels than Na+.

The Nernst Equation- It is a mathematical equation applied in physiology, to calculate equilibrium potentials for certain ions.

Ei = (R.T/F.z).ℓn[X]1/[X]2

R = Gas Constant
T = Absolute temperature (K)
E = The potential difference across the membrane
F = Faradays Constant (96,500 coulombs/mole)
z = Valency of ion
 

The Goldman-Hodgkin-Katz Equation- Is a mathematical equation applied in Physiology, to determine the potential across a cell’s membrane, taking in account all the ions that are permeable through it.

Em = 58log(PNa.[Na]out+PK.[K]out)/(PNa..[Na]in+PK.[K]in)

E = The potential difference across the membrane
P = Permeability of the membrane to sodium or potassium
[ ] = Concentration of sodium or potassium inside or outside

Measuring resting potentials

In some cells, the RPM is always changing. For such, there is never any resting potential, which is only a theoretical concept. Other cells with membrane transport functions that change potential with time, have a resting potential. This can be measured by inserting an electrode into the cell. Transmembrane potentials can also be measured optically with dyes that change their optical properties according to the membrane potential.

Resting membrane potential varies according to the types of cells
For example:
Skeletal muscle cells: −95 mV
Smooth muscle cells: −50 mV
Astrocytes: −80/−90 mV
Neurons: −70 mV
Erythrocytes: −12 mV

Action Potential

If we introduce one electrode inside the axon and one to the cytoplasmic surface of the axon, hyperpolarization (in the case of negative internal electrodes) or depolarization (in the case of negative external) occurs. If we increase the membrane potential to the threshold potential (in membrane with resting membrane potential, from -70mV to about -55 mV), nerve fiber responds with the emergence of an action potential (sudden opening voltage-gated sodium ion channels, thus allowing ions of sodium to enter through the membrane, causing the inside of the cells to become positive – there is transpolarization). If the increment in the membrane potential doesn’t reach “threshold potential”, the sodium voltage-gated channel will not open. In this case, no action potential is generated. In the next phase, the membrane again becomes permeable for potassium ions, and the potential returns to resting value despite a slight hyperpolarization.

Ions 

There are many ions within the cell and extracellular area, but now not all of them can skip through the cellular membrane. Those who can, are called diffusible ions (sodium, potassium, calcium, and chloride), and people who can’t are non-diffusible ions (proteins). Nonetheless, each group of ions contributes to membrane capability. Why? Ions are chemical elements that convey strength, a few tremendous (+) and some poor (-). Usually, there are greater bad ions inside the mobile than outside, that’s why the EM has the bad values. This negativity is ordinarily because of non-diffusible proteins (-).

Diffusible ions are responsible for the change of the membrane potential. During movement capability, a redistribution of the ions happens, in which large quantities of sodium (+) input the cell, making the membrane capability less negative and closer to the threshold for the movement capacity.

Distribution of ions  
Intracellular space Sodium = 14 mmol/l
Potassium = one hundred forty mmol/l
Calcium = 0.0001 mmol/l
Chloride = five mmol/l
Extracellular space Sodium = 142 mmol/l
Potassium = 4-five mmol/l
Calcium = 2.Five mmol/l
Chloride = 103 mmol/l

Sodium-potassium pump (Na-K pump)

Another thing that controls membrane capacity is the Na(+)-K(+) pump. This pump uses power to expel 3 molecules of sodium in exchange for 2 molecules of potassium. This is vital because this pump creates attention gradients for sodium and potassium, allowing extra sodium within the extracellular space, and more potassium inside the intracellular space.

The awareness gradient will later make contributions to producing an action potential, due to one of the legal guidelines of physics. By awareness gradient definition, every detail modifies its attention gradient to seek equilibrium. For example, ions will diffuse from a place of better awareness to a place of decreased awareness till the awareness of the element is equal in each aspect. In this approach that the sodium will diffuse from more- to the intracellular area, and the potassium will do the alternative.

Cell membrane permeability

The 0.33 component that impacts the membrane capability is the permeability of the membrane for sodium and potassium, which relies upon the ion channels. Ion channels are specialized proteins of the cell membrane that allow the migration of the ions. There are two forms of ion channels:

  • Passive channels – which are the pores within the mobile membrane, via which the molecules skip relying on their attention gradient. 
  • Active channels – which open and permit the ion delivery either depending on the trade of the membrane potential (capacity-gated channels), after binding of some different protein (ligand-gated channels), or after mechanical stimulation.
     

Pores contribute to setting up resting membrane ability, and they may be located alongside the whole excitable cellular membrane. When the mobile isn’t exciting, diffusion of ions occurs handiest through the pores. Note that during rest, lots of extra potassium pores are open then for the sodium. For this, the potassium efflux is larger than the sodium inflow, which contributes to preserving the negativity of the intracellular area and EM.

Ligand-gated channels are placed near the synapses and are accountable for neighborhood hypo- or hyperpolarization of the cellular after the neurotransmitter binds to them. Potential-gated channels are chargeable for the era and propagation of an action capacity, which ultimately reasons the release of a neurotransmitter. They are discovered in the membranes of axons and axon terminals.

Equilibrium Capacity

From the component of the attention gradient, we would anticipate that each diffusible ion skips via the mobile membrane till their concentrations are equal from each facet. But still, that doesn’t take place. Why? There is every other physical factor in this complete process that opposes the awareness gradient, known as the electrical gradient, that works just like a magnet.

Let’s take potassium for instance. The intracellular concentration of potassium is one hundred forty mmol/l, whilst the extracellular is 4-5 mmol/l. We might assume that the potassium diffuses outdoor of the mobile till there are around 70 mmol/l of potassium from each side of the membrane. But, seeing that potassium is a fine ion (+), its efflux increases the positivity of the extracellular space and increases the negativity of the intracellular space. This ends in the factor wherein the extracellular space is high quality sufficient to repel the potassium, and the intracellular space becomes negative sufficient to draw the nice potassium. This factor is known as the electrochemical equilibrium. Physiologists calculated the fee of the EM whilst the potassium can not diffuse out of the cellular anymore, and it’s far -ninety-four mV.

Now, permit’s take a look at sodium, which is also a tremendous ion. Because of the attention gradient, sodium has a tendency to influx into the cellular. At some point, the mobile turns into electropositive enough to repel the brand new sodium ions, and therefore opposes the sodium attention gradient, reaching the electrochemical equilibrium. The price of electro positivity that stops the sodium inflow is +sixty-one mV.

As we stated earlier, potassium diffusion typically impacts the resting membrane ability. On the alternative hand, the sodium diffusion is large in the course of movement potential. This implicates  matters:

  • Membrane potential can’t be extra negative than -94 mV
  • Membrane capability can’t be more positive than +61 mV

Conceptual Question

Question 1: What does Membrane potential mean?

Answer:

Membrane potential is the capability gradient that forces ions to passively circulate in a single path: Positive ions are attracted by the negative ions and negative ions are attracted by the positive ions.

Question 2: What are the different types of Membrane potential?

Answer:

  • Resting membrane capacity: the membrane ability at rest, steady-nation situations.
  • Action capacity: a non-graded ability, similar to binary code (on/off).
  • Post-synaptic potentials: graded potentials, that may be summated/subtracted by using modulation from presynaptic neurons.

Question 3: What are the two different ion channels of cell membrane permeability?

Answer:

  • Passive channels – which are the pores within the mobile membrane, via which the molecules skip relying on their attention gradient. 
  • Active channels – which open and permit the ion delivery either depending on the trade of the membrane potential (capacity-gated channels), or after binding of some different protein (ligand-gated channels), or after mechanical stimulation.

Question 4: What is the role of pores in cell membrane permeability?

Answer:

Pores contribute to setting up resting membrane ability, and they may be located alongside the whole excitable cellular membrane. When the mobile isn’t exciting, diffusion of ions occurs handiest through the pores

Question 5: What is the Donnan effect?

Answer:

Donnan effect is described as large impermeable negatively charged intracellular molecules attracting positively charged ions (e. g.: Na+ and K+) and repelling negative ones (e. g.: Cl−)

Question 6: Explain the Nernst equation of Membrane potential?

Answer:

 Ei = (R.T/F.z).ℓn[X]1/[X]2

  • R = Gas Constant
  • T = Absolute temperature (K)
  • E = The potential difference across the membrane
  • F = Faradays Constant (96,500 coulombs/mole)
  • z = Valency of ion

Question 7: What are 3 elements that contribute to the resting membrane ability being? 

Answer:

Membrane potentials in cells are determined primarily by three factors: 

  • The concentration of ions on the inside and outside of the cell; 
  • The permeability of the cell membrane to those ions (i.e., ion conductance) through specific ion channels; 
  • By the activity of electrogenic pumps 


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