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MEMBRANE POTENTIAL.pptx

- The membrane potential is caused by a difference in electrical potential across cell membranes, with the inside being negative relative to the outside. This is due to selective permeability of ions like Na+, K+, and Cl- across the membrane. - The Nernst equation and Goldman-Hodgkin-Katz equation can be used to calculate membrane potentials based on ion concentration gradients and permeabilities. Key ions involved are Na+, K+, and Cl-. - The sodium-potassium pump helps maintain the resting potential by actively transporting more Na+ out and K+ in, making the intracellular side more negative.

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Presented By (M.Sc. PG Student) Department Of Physiology Dr. Balasaheb Vikhe Patil Rural Medical College, Loni
Membrane potential is a difference in electrical potential across the cell membrane. There exists a potential difference across the membrane of all living cells with inside being negative in relation to the outside, i.e. the cations and anions arrange themselves along the outer and inner surfaces of the cell membrane.
The build up of charges occurs only very close to cell membrane. The ECF and ICF are else where contains equal number of +ve and –ve charges and is electrically neutral. Thus, the number of ions responsible for membrane potential is minute fraction of the total number present across the cell membrane in each respective compartment. The greater the difference across the membrane, the larger the membrane potential.
The membrane potential at rest is called resting membrane potential. It is written with minus sign, signifying that inside is negative relative to exterior. The membrane potential measured during the excited state of cell is called action potential. It is written with plus sign, signifying that outside is negative relative to interior.
 All living cells have resting membrane potentials. Whereas, action potentials are produced only in nerve and muscle cells. Cell Type Resting Potential Skeletal Muscle Cells -100 mV Neurons -70 mV Smooth Muscle Cells -60 mV Aorta Smooth Muscle tissue -45 mV Photoreceptor Cells -40 mV Chondrocytes -8 mV Erythrocytes -10 mV
In a particular cell also membrane potential varies according to its functional status. For example, a nerve cell has a membrane potential of -70 mV (inside negative) at rest , but when it gets excited, the membrane potentials becomes +30 mV (inside positive).
The essential instruments used in recording the activity of an excitable tissue are: 1. Microelectrode 2. Electronic amplifiers 3. Cathode ray oscilloscope (CRO i.e. voltage amplifier recorder system)
Measurement of the membrane potential of the nerve fiber using a microelectrode.
• When one electrode is placed on the surface of the cell and another electrode is inserted into interior of the cell, both are connected through a suitable amplifier to a cathode ray oscilloscope (CRO), a constant (or steady) potential difference is observed between the inside and outside of the cell at rest, called as the resting membrane potential.
• Factors involved in genesis of membrane potential: 1. Selective permeability of the membrane 2. Gibbs–Donnan effect 3. Nernst equation 4. Constant field Goldman equation 5. Sodium-potassium ATPase Pump
 The cell membrane is selectively permeable, i.e. to some ions it is freely permeable, to others impermeable to some others, it has variable permeability.
1. Ions like Na+, K+, Cl- and HCO3 - are diffusible ions. The cell membrane is freely permeable to K+ and Cl- and moderately permeable to Na+. The permeability of K+ is 50-100 times greater than that of Na+. 2. The cell membrane is practically impermeable to intracellular proteins and organic phosphate, which are negatively charged ions. 3. The presence of gated channels in the cell membrane is responsible for the variable permeability of certain ions in different circumstances.
According to Gibbs-Donnan membrane equilibrium, when two ionized solutions are separated by a semi-permeable membrane then at equilibrium: 1. Each solution shall be electrically neural, i.e.
2. The product of diffusible ions on each side of the membrane will be equal, i.e.
From the above the ratio of diffusible ions will be as below:  Thus, there will be symmetrical distribution of ions at equilibrium.
But if one or more non-diffusible ions ‘X’ are present on one side (A side) of the membrane, then according to to Gibbs-Donnan membrane equilibrium the distribution of diffusible ions will be as follows: 1. Each solution shall be electrically neural, i.e. (Na+)A = (Cl-)A + (X-)A (Na+)B = (Cl-)B
2. The product of diffusible ions on two sides will be equal, i.e. From the relationship of (1) and (2), it is found that: • (Na+)A > (Na+)B , and • (Cl−)A < (Cl−)B
The Gibbs–Donnan effect is named after the American physicist Josiah Willard Gibbs who proposed it in 1878 and the British British chemist Frederick George Donnan who studied it experimentally in 1911.
Frederick George Donnan
Josiah Willard Gibbs
Membrane Potentials Caused by Diffusion  A diffusion potential is the potential difference generated across a membrane when a cation or anion diffuses down its concentration gradient. A diffusion potential can be generated only if the membrane is permeable to the ion.
The size of the diffusion potential depends on the size of concentration gradient The sign of the diffusion potential depends on whether the diffusing ion is positively or negatively charged. Diffusion potentials are created by diffusion of very few ions and, therefore, do not result in changes in concentration of diffusing ions.
2. The Nernst Potential— Relation of the Diffusion Potential to the Concentration Difference. The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the Nernst potential for that ion. The magnitude of this Nernst potential is determined by the ratio of the concentrations of that specific ion on the two sides of the membrane.
The greater this ratio, the greater the tendency for the ion to diffuse in one direction, and therefore the greater the Nernst potential required to prevent additional net diffusion. The following equation, called the Nernst equation, can be used to calculate the Nernst potential for any univalent ion at normal body temperature of 98.6°F (37°C):
Walther Hermann Nernst German physical chemist who formulated the Nernst equation in 1887
The Nernst equation helps in calculating the equilibrium potential for each ion individually. However, the magnitude of the membrane potential at any given time depends on the distribution of Na+, K+ and Cl- and the permeability of each of these ions.
The integrated role of different ions in generation of membrane potential can be describe accurately by the Goldman’s constant field equation or the so called Goldman-Hodgkin-Katz Eqaution:
• Interference of Goldman constant field equation The following important inferences can be drawn from gold-man constant field equation: 1. Most important ions for development of membrane potentials in nerve and muscle fibres are Na+, K+ and Cl-. The voltage of membrane potential is determined by concentration gradient of each of these ions.
2. • Degree of importance of each of the ions in determining the voltage depends upon the membrane permeability of the individual ion. • Each permeable ion attempts to drive the membrane potential towards its equilibrium potential. • Ions with highest permeability will make greatest contribution to the membrane potential and those with lowest permeability will make very little or no contribution. • For example, if the membrane is impermeable to K+ and Cl-, then the membrane potential will be determined by the Na+ gradient alone and the resulting potential will be equal to the Nernst potential for sodium.
3. Higher concentration of cations in the intracellular fluid as compared to extracellular fluid is responsible for electronegativity inside the membrane. This is because of the fact that due to concentration gradient, the cations diffuse out outside leaving the non-diffusible anion inside the cell.
4. Signal transmission in the nerves is primarily due to change in the sodium and potassium permeability because their channel undergo rapid change during conduction of the nerve impulse and not much change is seen in chloride channels.
The discoverers of Goldman Equation are David E. Goldman of Columbia University, and the Medicine Nobel laureates Alan Lloyd Hodgkin and Bernard Katz.
David E. Goldman
Sir Alan Lloyd Hodgkin English physiologist and biophysicist
Sir Bernard Katz German-born Nobel Prize winning physiologist
• Na+-K+ pump provides an additional contribution to the resting potential. • There is continuous pumping of three sodium ions to the outside for each two potassium ions pumped to the inside of the membrane. • The fact that more sodium ions are being pumped to the outside than potassium to the inside causes continual loss of positive charges from inside the membrane; this creates an additional degree of negativity (about −4 millivolts additional) on the inside beyond that which can be accounted for by diffusion alone.
MEMBRANE POTENTIAL.pptx