Membrane potential also known as transmemebrane potential is the electrical potential between the intracellular and extracellular space of a cell membrane. They are charge particles present within and outside the membrane which includes sodium, potassium, chloride and calcium. It is important to know that calcium play a minor role in membrane potential. Membrane potential performs two basic functions. It allows the cell function as a battery this means it enhances the generation of electrical power required to operate a variety of molecules devices embedded in it. In electrical excitable cells such as neurons or nerves, muscle cells, membrane potential function in transmitting signals between different regions or part of the cell. The signals are generated by opening and closing of the ion channels at a point in the membrane, producing a local change. Membrane potential can be determined using Nerst equations and Goldman Hodgkin Katz equation. Evaluation of sodium, potassium, chloride and the factors affecting their concentration in the body contribute to the use of membrane potential as a diagnostic tool. Membrane potential serves as a biomarker in the diagnosis of preeclampsia, sickle cell anaemia, diabetes mellitus, neurological diseases and renal disorders

Membrane potential also known as transmembrane potential or membrane voltage is the difference in electric potential between the interior and the exterior of a biological cell[1].With respect to the exterior of the cell, typical values of membrane potential, normally given in millivolts. Differences in the concentrations of ions on opposite sides of a cellular membrane lead to a voltage called the membrane potential. Typical values of membrane potential are in the range –40 mV to –80 mV [2]. Many ions have a concentration gradient across the membrane, including potassium (K+), which is at a high concentration inside and a low concentration outside the membrane. Sodium (Na+) and chloride (Cl−) ions are at high concentrations in the extracellular region, and low concentrations in the intracellular regions. These concentration gradients provide the potential energy to drive the formation of the membrane potential. This voltage is established when the membrane has permeability to one or more ions. In the simplest case, illustrated here, if the membrane is selectively permeable to potassium, these positively charged ions can diffuse down the concentration gradient to the outside of the cell, leaving behind uncompensated negative charges. This separation of charges is what causes the membrane potential.  Other ions including sodium, chloride, calcium, and others play a more minor role, even though they have strong concentration gradients, because they have more limited permeability than potassium [3]

The history of membrane potential stretches across multiple scientific disciplines; Membrane Potential plays a role in the studies of Chemistry, Physiology and Biology. The culmination of the study of membrane potential came in the 19th and early 20th centuries. Early in the 20th century, a man named professor Bernstein hypothesized that there were three contributing factors to membrane potential; the permeability of the membrane and the fact that [K+] was higher inside and lower on the outside of the cell [4].He was very close to being correct, but his proposal had some flaws. Walther H. Nernst, notable for the development of the Nernst equation and winner of 1920 Nobel Prize in chemistry, was a major contributor to the study of membrane potential. He developed the Nernst equation to solve for the equilibrium potential for a specific ion. Goldman, Hodgkin and Katz furthered the study of membrane potential by developing the Goldman-Hodgkin-Katz equation to account for any ion that might permeate the membrane and affect its potential. The study of membrane potential utilizes electrochemistry and physiology to formulate a conclusive idea of how charges are separated across a membrane [5]

All animal cells are surrounded by a membrane composed of a lipid bilayer with proteins embedded in it. The membrane serves as both an insulator and a diffusion barrier to the movement of ions [6]. Transmembrane proteins, also known as ion transporter or ion pump proteins, actively push ions across the membrane and establish concentration gradients across the membrane, and ion channels allow ions to move across the membrane down those concentration gradients. Ion pumps and ion channels are electrically equivalent to a set of batteries and resistors inserted in the membrane, and therefore create a voltage between the two sides of the membrane.

Almost all plasma membranes have an electrical potential across them, with the inside usually negative with respect to the outside. The membrane potential has two basic functions. First, it allows a cell to function as a battery, providing power to operate a variety of "molecular devices" embedded in the membrane. Second, in electrically excitable cells such as neurons and muscle cells, it is used for transmitting signals between different parts of a cell. Signals are generated by opening or closing of ion channels at one point in the membrane, producing a local change in the membrane potential. This change in the electric field can be quickly affected by either adjacent or more distant ion channels in the membrane. Those ion channels can then open or close as a result of the potential change, reproducing the signal [7]

In non-excitable cells, and in excitable cells in their baseline states, the membrane potential is held at a relatively stable value, called the resting potential. For neurons, typical values of the resting potential range from –70 to –80 millivolts; that is, the interior of a cell has a negative baseline voltage of a bit less than one-tenth of a volt. The opening and closing of ion channels can induce a departure from the resting potential. This is called a depolarization if the interior voltage becomes less negative (say from –70 mV to –60 mV), or a hyperpolarization if the interior voltage becomes more negative (say from –70 mV to –80 mV). In excitable cells, a sufficiently large depolarization can evoke an action potential, in which the membrane potential changes rapidly and significantly for a short time (on the order of 1 to 100 milliseconds), often reversing its polarity. Action potentials are generated by the activation of certain voltage-gated ion channels [8]

In neurons, the factors that influence the membrane potential are diverse. They include numerous types of ion channels, some of which are chemically gated and some of which are voltage-gated. Because voltage-gated ion channels are controlled by the membrane potential, while the membrane potential itself is influenced by these same ion channels, feedback loops that allow for complex temporal dynamics arise, including oscillations and regenerative events such as action potentials.

The uncompensated positive charges outside the cell, and the uncompensated negative charges inside the cell, physically line up on the membrane surface and attract each other across the lipid bilayer. Thus, the membrane potential is physically located only in the immediate vicinity of the membrane. It is the separation of these charges across the membrane that is the basis of the membrane voltage [9]

Without membrane potentials human life would not be possible. All living cells maintain a potential difference across their membrane. Changes in membrane potential elicit action potentials and give cells the ability to send messages around the body. More specifically, the action potentials are electrical signals; these signals carry efferent messages to the central nervous system for processing and afferent messages away from the brain to elicit a specific reaction or movement. Numerous active transports embedded within the cellular membrane contribute to the creation of membrane potentials, as well as the universal cellular structure of the lipid bilayer. The chemistry involved in membrane potentials reaches to many scientific disciplines. Chemically it involves molarity, concentration, electrochemistry and the Nernst equation [10]

From a physiological standpoint, membrane potential is responsible for sending messages to and from the central nervous system. It is also very important in cellular biology and shows how cell biology is fundamentally connected with electrochemistry and physiology. The bottom line is that membrane potentials are at work in your body right now and always will be as long as you live.

Physical Basis of Membrane Potential

The membrane potential in a cell derives ultimately from two factors: electrical force and diffusion. Electrical force arises from the mutual attraction between particles with opposite electrical charges (positive and negative) and the mutual repulsion between particles with the same type of charge (both positive or both negative). Diffusion arises from the statistical tendency of particles to redistribute from regions where they are highly concentrated to regions where the concentration is [11]

Physiology of the Membrane Potential

The difference between the electrical and chemical gradient is important. Electrical Gradient opposes the chemical gradient and represents the difference in electrical charge across the membrane while Chemical Gradient opposes the electrical gradient and represents the difference in the concentration of a specific ion across the membrane. A good example is K+, the membrane is very permeable to K+ and the [K+] inside the cell is great, therefore a positive charge is flowing out of the cell along with K+. The [K+] inside the cell decreases causing the concentration gradient to flow towards the outside of the cell [12]. This also causes the inside of the cell to become more electronegative increasing its electrical gradient.

The Role of Membrane Potential in Human Biological System

A. Contractility

Despite many being non-excitable, smooth muscle cells are heavily dependent on their RMP for control of contraction. Some of the best-studied cells are vascular smooth muscle cells (VSMs) as they have considerable plasticity in their phenotype [13]. They have the ability to change phenotype depending on their microenvironment from contractile cells to non-contractile cells and vice versa. VSMs function to drive the contraction of the vascular wall and regulate the luminal diameter and vascular tone. Unlike skeletal and cardiac muscle, whose contraction depends on “all or nothing” action potentials, VSMs contraction typically depends on the RMP changes through the KC channel feedback loop mechanism to contract. The activation of KC ion channels for example, the K2P, causes the efflux of KC ions and hyperpolarization which brings the membrane potential to a value more negative than the threshold for activation of Ca2C channels. This results in decreased Ca2C influx, leading to the relaxation of the VSM and hence vasodilation. On the other hand, the inhibition of K2P channels leads to the depolarisation of the membrane potential and the contraction of smooth muscle cells. Other non-excitable cell types that contract due to changes in the RMP include myofibroblasts and ventricular fibroblasts [14]. The role of the membrane potential in contraction is crucial. One such example mentioned above is its modulation of the contraction of VSMs, since failure of the VSMs to contract can lead to detrimental effects in the blood flow to the heart. Perhaps, this explains why there are many ion channels that maintain the

RMP as the heart’s change in metabolic demand needs to be met promptly and accurately.

B. Cell Migration and Wound Healing

Tissue wounding is an interesting phenomenon because the electrical potential generated by the ion movement in healthy tissue is disrupted and a significant EF is generated that is necessary for wound healing [15]. Indeed, the EF over-rides other well-accepted physiological cues and initiates directional cell migration into the wounded area. Wound generated EFs are produced by the directional flow of charged ion species. Some of this ion flux will be due to leakage from damaged cells which themselves have membrane potential dependent repair mechanisms [16] and tissue immediately after injury. However, large currents are generated for days after wounding that are not accounted for by immediate injury.

Epithelial wounding has been extensively studied, however, little is reported on the role of the membrane potential in response to wounding and healing. The maintenance of the structural integrity of epithelia is crucial to the function of this tissue type, and healing after injury has been described by two major mechanisms, cell migration and cytoskeleton reorganization [17]. It was demonstrated that the epithelial sodium channel ion enhanced membrane depolarization occurs at the leading edge of wounds, gradually extending toward neighboring cells, and that this depolarization supports the development of the characteristic actin reorganization found in healing cells. Actin reorganization is evident by the formation of actin cables that form at the leading edge of cells, analogous to the tightening of a purse string as the cells close the wound.

C. Pigmentation

Finally, pigmentation in mammals is generally a membrane potential dependent process. In mammals, pigment cells such as skin and uveal melanocytes, and retinal pigment epithelial (RPE) cells are non-excitable cells that contain melanosomes which are lysosome-related organelles that synthesize melanin, the main pigment that colors eyes, skin and hair [18]. Melanin is essential for the protection of the skin and eyes against solar ultraviolet (UV) radiation. The changes in intracellular Ca2+ regulates the concentration of melanin in pigment cells. These intracellular free Ca2+ concentrations are regulated by the membrane potential of the pigment cells. Membrane depolarisation mediated by different ion channels causes the delay of transient potential receptor potential (TRPA1) ion channel inactivation, this leaves the channels open for longer and causes the sustained Ca2+ response required for melanogenesis [7].It has been reported that the activation of TPC1 ion channels, which are highly permeable to Ca2+, mediate changes in the membrane potential of the melanosomes, could facilitate the fusion between the melanosomes and other organelles, the plasma membrane, or protein transport vesicles [10]. This could initiate the melanin transfer in the skin and hence enable protection of the genetic material of keratinocytes against UV radiation damage.

D. Secretion

One of the most extravagantly studied secretory mechanisms is the pancreatic β-cell insulin secretion system. This system has membrane potential level control of secretion and is perhaps a model of secretion that is more widely known. β-cells express ATP-sensitive K+ (KATP) channels; that is, Kir that close in response to elevation of intracellular ATP. These KATP channels play a key role in the glucose-stimulated insulin release in pancreatic β-cells. When the blood glucose levels rise, there is an increase in the ATP concentration and a decrease in the ADP concentration which causes the KATP channels to close and the cell’s membrane to become depolarised leading to Ca2+ influx and therefore insulin release. channels are seen in subgroups of diabetic patients . The KATP channels can also regulate the insulin release through their interaction with phosphoinositides in particular with PIP2 which stimulates KATP channels by decreasing their sensitivity to ATP, causing the cells to become more hyperpolarized and not secrete insulin properly when glucose levels are high [1] This highlights the importance of the RMP as mutations in the ion channels that contribute to the RMP can have detrimental effects that lead to disease.

E. Cell Cycle

As discussed earlier, the membrane potential can regulate proliferation levels within cells through regulating the cell cycle progression. A hyperpolarized membrane potential inhibits mitosis as it blocks quiescent cells in the G1 phase of the cell cycle from entering the S phase and hence blocks the DNA synthesis. It is hypothesized that voltage-gated Na+ and/or Ca2+ channels open to depolarise cells and promote transition from G0/G1 to the S-phase. Whereas opening of K+ voltage-gated channels (such as KV11.1, the human ortholog also known as hERG or KCNH2) and closure of Na+/Ca2+ channels during the S-phase tend to re-polarize the cell and lead back to G0/G1 [5].It is postulated that there may be a threshold RMP level that cells need to overcome in order to drive DNA synthesis in cells. For example the expression of certain ion channels in proliferating astrocytes can be upregulated or downregulated depending on the RMP levels of the cells and the cell cycle stage that they are in. A cell cycle arrest in the G1/G0 phase of proliferating astrocytes induces a premature upregulation of an inwardly rectifying K+ channel (Kir) that results in the hyperpolarization of the membrane potential.

Determination of the Electrical State of a Cell Membrane

The electrical state of the cell membrane can have several variations and can be determined. These are all variations in the membrane potential. A potential is a distribution of charge across the cell membrane, measured in millivolts (mV). The standard of determination is to compare the inside of the cell relative to the outside, so the membrane potential is a value representing the charge on the intracellular side of the membrane based on the outside being zero, relatively speaking [7]. The concentration of ions in extracellular and intracellular fluids is largely balanced, with a net neutral charge. However, a slight difference in charge occurs right at the membrane surface, both internally and externally. It is the difference in this very limited region that has all the power in neurons (and muscle cells) to generate electrical signals, including action potentials.

1. Calculation of Membrane Potential by Nernst Equation

The calculation for the charge of an ion across a membrane, membrane potential can be calculated using Nernst equation, this equation gives a cell resting membrane potential, the Nernst equation helps us to reveal the numerical values, for the concentration of electrical gradient it mediate a baseline to measure the function or behavior in a given state, which is at rest or not at rest.  The Nernst Potential, is relatively easy to calculate. The equation is as follows: 

(RT/zF) log ([X]out/[X]in). RT/F is approximately 61, therefore the equation can be written as (61/z) ln([X]out/[X]in)

• R……… is the universal gas constant (8.314 J.K-1.mol-1).

• T……… is the temperature in Kelvin (°K = °C + 273.15).

• z............ is the ionic charge for an ion. For example, z is +1 for K+, +2 for Mg2+, -1 for F-, -1 for Cl-, etc. Remember, z does not have a unit.

• F……. is the Faraday's constant (96485 C.mol-1).

• [X]out….. is the concentration of the ion outside of the species. For example the molarity outside of a neuron.

• [X]in…… is the concentration of the ion inside of the species. For example, the molarity inside of a neuron.

Also equilibrium potentials of a particular ion is calculated using Nernst equation

E.g:  Eeq K+ =RT ln [K+]0

                                   ZF    [K+]i

Where Eeq is the equilibrium potential of potassium.

1. Calculation of Membrane Potential by Goldman-Hodgkin Katz Equation

This equation is used to determine the resting membrane potential in red cells in which potassium (K+), sodium (Na+) and chloride (Cl-) are the major contributors of membrane potentials. It is important to account for any ion that might permeate membrane and affects its potential and formulate a conclusive idea on how charges are separated across membrane.

Vm= RT  ln (Pk[K+]0 + PNa[Na+]0 + Pcl[Cl-]0

        F        Pk[K+]i + PNa[Na+]i + Pcl[Cl-]i

Moreover this equation can predict the reversal potential of current voltage relationship obtained from a cell in which the predominant ion channel and plasma membrane are the Na+ and Cl- channels.     

The only difference in the Goldman-Hodgkin-Katz equation is that is adds together the concentrations of all permeable ions as follows

(RT/zF) log([K+]o+[Na+]o+[Cl-]o /[K+]i+[Na+]i+[Cl-]i) [4]

Evaluation of Sodium, Potassium and Chloride ion Concentration

A. Sodium

Sodium is the major positive ion found in body fluid outside our cells. The chemical notation for sodium is Na+. Excess sodium such as that obtained from dietary sources is excreted in the urine [15]

Sodium regulates the total amount of individual cells and play a role in critical body functions. Many processes of the body especially the brain, nervous system and muscle require electrical signals for communication. The movement of sodium is critical in the generation of these electrical signals. Therefore too much or too little sodium can cause cells to malfunction and be fatal.

Increased sodium (hypernatraemia) in the blood occurs whenever there is excess sodium in relation to water. There are numerous caused of hypernatremia, these may include kidney disease, too little water intake and loss of water due to diarrhoea or vomiting [16]

Decrease concentration of sodium (hyponatremia) occurs whenever there is a relative increase in the amount of body water relative to sodium. This happen with some diseases of the liver and kidney in the patients with congestive heart failure, in burn victims, and in other numerous conditions.

A normal blood sodium level is 135-145millieqivalent/litre or in international units, 135-145millimoles/litre.

B. Potassium

Potassium is major positive ion (cation) found inside of cells. The chemical notation of potassium ion is K+. The proper level of potassium is essential for normal cell function. Among the many functions of the potassium in the body are regulation of the heart beat and the function of the muscles. A seriously abnormal increase in potassium ion (hyperkalaemia) or decrease in potassium (hypokalemia) can profoundly affect the nervous system and increases the chance of irregular heartbeats (arrhythmias), which, when extreme can be fatal [17]

Potassium is normally excreted by the kidneys, so disorders that decrease the function of the kidneys can result in hyperkalemia. Certain medications may also predispose an individual to hyperkalemia. Hypokalemia or decreased potassium can arise due to kidney diseases, excessive losses due to heavy sweating, vomiting, diarrhoea, eating disorders, certain medications or other causes.The normal blood potassium level is 3.5-5.0milli Equivalents/litre or in international units, 3.5-5.0millimoles/litre.

C. Chloride

Chloride is a major anion (negatively charged ion) found in the fluid outside the body to maintain a normal balance in fluids. The balance of chloride ion (Cl-) is closely regulated by the body significant increases or decreases in chloride can be deleterious or even fatal consequences

Increased chloride (hyperchloremia) can be seen in diarrhoea, certain kidney diseases and sometimes in overactivity of the parathyroid glands [18]

Decreased chloride (hypochloremia) is due to heavy sweating, vomiting, and adrenal gland and kidney diseases.

The normal serum range for chloride is 98-108mmol/L

All cells maintain a potential difference across their membrane. Changes in membrane potential indicates action potentials and aid cells in their ability to send electrical signals across the cells which involve many active transports embedded in cellular membrane. Membrane potential plays an important role in the body such as in the healing of wounds, contractility, biological sensing, cell proliferation etc. The use of membrane potential in diagnosis is relevant in diabetes diagnosis, as a biomarker of pre-eclampsia, Sickle cell disease, cardiovascular disease, neurological disease and renal disorders.

Due to the differences in electrical potential charged particles are able to move with the aid of active transport through pumps, ion channels, ion pumps, across a

concentration gradient to enhance the movement of charged particles in and out the cell membrane.

Increase or decrease on the concentration of a specific charged particle due to ion loss through leakage channels, blockage of the channels by tumours, delay of ion channel to open or close, retention of specific ion in the cell membrane, inadequate intake of meals rich in macro nutrients may lead to a detrimental effect in the human body system.

Monitoring of membrane potential activities can serve as a valuable tool in the management and diagnosis of disease in clinical laboratories. Life is not possible without membrane potential