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Electrophysiology of Cell

  1. Donnan effect / Gibbs – Donnan equilibrium
  2. Nernst equation
  3. Goldman – Hodgkin – Katz (G-H-K) equation or, Goldman constant field equation or chord conductance equation

1. Donnan effect

Presence of an impermeant ion (e.g A- in side 2) on one side of the membrane repels similarly charged permeant ions to the other side and holds opposite charged permeant ions to the same side.    
                     


2. Gibbs – Donnan equilibrium

  1. This can be considered as the ‘mathematics’ of the Donnan effect.
  2. The effects the presence of the impermeant ion on the distribution of permeant ions are
  3. It causes an asymmetric distribution of the permeant ions
  4. More osmotically active particle on the side containing the impermeant ion
  5. The product of the concentration of the permeant ions on one side equal, the product of the concentration of the permeant ions on the other side
  6. However, the total number of positive charges on one side equals the total number of negative charges.

Illustrate example

 

 

 

A is the impermeant anion

 

3. Nernst equation

Gives the value of equilibrium potential or isoelectric potential.  (E) Equilibrium potential is the membrane potential at which equilibrium is reached (i.e. there is no net flux of that ion).    
 

Examples
E Na+              =      +60mv(tendency of Na+ is to diffuse in till potential reaches +60 mv)
E K+                   =      - 90mv(tendency of K+ is to diffuse out till potential reaches -90 mv)
ECl-        =      - 70mv

 

Note: The ECl is the closest to the RMP.

 

4. Goldman constant Field equation

  1. Gives the magnitude of the membrane potential. It depends on
    1. The distribution of Na+, Cl-, K+
    2. Permeability of the membrane to each of these ions
  2. Concept: Resting Membrane potential (R.M.P)
     
    Definition: Every cell shows a potential difference, with the inside being negative. Its value varies from cell to cell
  3. Genesis of R.M.P.
  1. Diffusion of K+ : This is the most important cause as membrane is more permeable to K+ than Na+ so it diffuses out and membrane potential becomes negative.
  2. Na+ - K+ ATPase: 5-10% of RMP
  3. By itself, it contributes a small percentage; its contribution towards RMP is more in those cells with low RMP
  4. Note: Pacemaker tissues have a low ‘R’MP
  5. More importantly, it maintains the diffusion gradient for K+
  1. Donnan effect: This also maintains the diffusion gradient for K+

Value (mV) : Neuron – 70, skeletal muscle – 80, SA node –30 to –40, ventricle –90, smooth muscle –30 to –40, thyroid –50, RBC –10

Effect of change in Na+/K+ on RMP 

 

1. Changes in Na+: No change in RMP

Reason : Low permeability of membrane to sodium

 

Note: However, a decrease in external Na+ concentration decrease the height of the action potential

 

2. Change in K+

Increase in K+ concentration in ECF decreases RMP

 

Example: Eg. From – 70 mV, it may become – 65 mV

 

Note: While commenting on the change in the membrane potential (eg. From – 70 mV to – 65 mV) the sign (positive or negative) has to be ignored. Thus, -70mV to – 65mV should be considered as a decrease in potential or depolarization. (-70mV to – 90mV is hyperpolarization

 

The patch clamp technique is a laboratory technique in electrophysiology that allows the study of single or multiple ion channels in cells. The technique can be applied to a wide variety of cells, but is especially useful in the study of excitable cells such as neurons, cardiomyocytes, muscle fibers and pancreatic beta cells. It can also be applied to the study of bacterial ion channels in specially prepared giant spheroplasts. The patch clamp technique is a refinement of the voltage clamp. Neher and Sakmann received the Nobel Prize in Physiology or Medicine in 1991 for this work.
 

Techniques:     

  1. Cell-attached or on-cell patch                       
  2. Inside-out patch
  3. Whole-cell patch                           
  4. Outside-out patch
  5. Perforated patch

Some Important Data

 

 

 ECF ( in mEq/litre)

 

 Cations

 

 

 Anions

 Na

 145

 Cl

 100

 K

 5

 HCO3

 27

 Ca

 2

 PO4---

 2

 Mg

 2

 SO4--

 1

 

 

 

 

 

 

 Organic acids

 5

 

 

 Proteins

 19

 Total

 154

  Total

 154

 

 

 ICF ( in mEq/litre)

 

 Cations

 

 

 Anions

 Na

 10

 Cl

 10

 K

 150

 HCO3

 10

 Ca

 3

 PO4---

 90

 Mg

 15

 SO4--

 15

 

 

 Organic acids

 -

 

 

 Proteins

 52

 Total

 177

 Total

 177

 

 

 Total

Exchangeable

 Na

 3900 mEq (90 gm)

80%

 K

 3400 mEq (90 gm)

95%

 *(only one-third of Na in bone is exchangeable

 

 

 Na

 3900 mEq in 70 kg or 56 meq/Kg

 K

 3400 mEq in 70 kg or 50 meq/Kg

 

ELECTROLYTES

 

 

Losses

 

 

 

(in mEq/day in a 70 kg man in temperature climate)

 

Na

K

Cl

Urine

40-90

20-60

40-120

Sweat

50-100

5

50-100

Faeces

1.5

4

0.5

Total

140 (3.2 gm)

60 (2.4 gm)

200 (7 gm)

 





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