ev‡qv‡gwW‡Kj BwÄwbqvwis wefvM h‡kvi weÁvb I cÖhyw³ wek¦we`¨vjq h‡kvi-7408, evsjv‡`k| ‡Uwj‡dvb:+042151081 d¨vKª :88-0421-61199 I‡qemvBU:www.Just.edu.bd Dept. of Biomedical Engineering Jashore University of Science & Technology Jashore -7408, Bangladesh, Phone: +042151081Ex-306 Fax : 88-0421-61199 Website: www.Just.edu.bd Lecture-01 July 9, 2019 Introduction to Bioelectricity and Excitable Cells: Bioelectric potentials and currents, ionic composition of excitable cells, Nernst-Planck equation, membrane structure, Nernst potential, parallel-conductance model. 3 BIOELECTRIC POTENTIALS In 1789 the Italian physicist Luigi Galvani touched an exposed sciatic nerve with a charged metal scalpel in a dead frog leg and observed the frog’s leg flex as if it were alive. Galvani believed that the muscular contractions were due to electrical energy emanating from the animal. However, Allesandro Volta was convinced that the electricity in Galvani’s experiments originated from the presence of the dissimilar metals. Both of these interpretations represent the two different aspects of electrical potential in biological system, the action potential and the steady source of electrical potential. Comparison of bioelectricity and man-made electrical systems: Man-made electrical systems Bioelectricity Charge carriers are electrons within a Charge carriers are ions within an electrolyte conductor Current flow within (insulated) conductors Current flow inside and outside of (partiallyinsulated) cell membranes CURRENTS IN SOLUTIONS In living tissues, ions within the electrolytes such as solutions of acids, bases, and salts, which conduct electricity are the charge carriers. These charge-carrying ions are present both inside and outside of cells, especially ions of sodium and potassium, allowing current to flow extensively throughout both intracellular and extracellular volumes. Charge movement within living tissue is similar to that in sea water, with its high dissolved salt content. But in wires, the charge carriers are electrons that move within the metallic structure in the wire, but not through the insulation into the surrounding space. Excitable cells Cells that can generate electrical potentials and currents are referred to as excitable cells. These potentials and currents can be observed in the cells’ interior volume, across their membranes, and in their surrounding conducting volume. Excitable cells include nerve cells (neurons), muscle fibers, and sensory receptor (transducer) cell. In case of any query or suggestion please contact with Md. Anas Ali, Lecturer, BME, JUST (Email:anas_bme@just.edu.bd) 1 ev‡qv‡gwW‡Kj BwÄwbqvwis wefvM h‡kvi weÁvb I cÖhyw³ wek¦we`¨vjq h‡kvi-7408, evsjv‡`k| ‡Uwj‡dvb:+042151081 d¨vKª :88-0421-61199 I‡qemvBU:www.Just.edu.bd Dept. of Biomedical Engineering Jashore University of Science & Technology Jashore -7408, Bangladesh, Phone: +042151081Ex-306 Fax : 88-0421-61199 Website: www.Just.edu.bd Lecture-01 July 9, 2019 IONIC COMPOSITION OF EXCITABLE CELLS The concentrations of ions in either the intracellular or extracellular volumes allow significant currents to flow in either place. The difference of ionic concentration between the intracellular and extracellular volumes are especially important to excitable cells. For all excitable cells the concentration of intracellular potassium greatly exceeds extracellular potassium. In contrast to potassium, the extracellular sodium and chloride concentrations greatly exceed intracellular sodium and chloride. The different ratios of intracellular to extracellular concentrations for sodium and potassium ions are of great importance to trans-membrane voltage, and to how it changes. The important thing is that these large differences exist despite the tendency of concentration to average out, due to diffusion. As an example of excitable frog muscle and squid nerve axon the ionic concentration difference of K+, Na+, and Cl− ions are given in Table 1. These relative ratios are similar to those generally found in other excitable muscle and nerve. Diffusion is the movement of a substance from an area of high concentration to an area of low concentration. It is not an electrical phenomenon i.e., no electric field is required for diffusion to occur. Rather, diffusion arises as a consequence of the pronounced random motion of molecules that occurs at ordinary temperatures. NERNST–PLANCK EQUATION The Nernst–Planck equation relates the flow of ions to spatial differences in concentration or in the electric potential. The individual effect of a concentration gradient is described by Fick’s law of diffusion as Here, In case of any query or suggestion please contact with Md. Anas Ali, Lecturer, BME, JUST (Email:anas_bme@just.edu.bd) 2 ev‡qv‡gwW‡Kj BwÄwbqvwis wefvM h‡kvi weÁvb I cÖhyw³ wek¦we`¨vjq h‡kvi-7408, evsjv‡`k| ‡Uwj‡dvb:+042151081 d¨vKª :88-0421-61199 I‡qemvBU:www.Just.edu.bd Dept. of Biomedical Engineering Jashore University of Science & Technology Jashore -7408, Bangladesh, Phone: +042151081Ex-306 Fax : 88-0421-61199 Website: www.Just.edu.bd Lecture-01 July 9, 2019 𝑗̅𝑑 = Ionic flux due to diffusion is the number of particles (ions) moving per unit time through a unit crosssectional area. C = Concentration of some substance as a function of position. D = Proportionality constant. Sometimes it is called as “Fick’s coefficient” since its value is not quite independent of concentration but increases slightly with increases in C. And The individual effect of an electric potential gradient is described by Ohm’s law of drift as- Here, 𝑗̅𝑒 =Ionic flux due to an electric field. −𝛻Φ is the electric field, up is the mobility of pth ion. Zp/|Zp| is the sign of valance of pth ion [positive for positively charged ions (cations) and negative for negatively charged ions (anions).] Cp is the concentration of pth ion. Flux is expressed as moles per unit area per second. Diffusion and drift are impeded by the same molecular processes, i.e., collisions with solvent molecules, and consequently a physical connection exists between the parameters up and D. This relation was mathematical described by Einstein as Here, p signifies the pth ion species with valence |Zp|, and up is its mobility, T is the absolute temperature, F is Faraday’s constant, and R is the gas constant. Their numerical value is given below In case of any query or suggestion please contact with Md. Anas Ali, Lecturer, BME, JUST (Email:anas_bme@just.edu.bd) 3 ev‡qv‡gwW‡Kj BwÄwbqvwis wefvM h‡kvi weÁvb I cÖhyw³ wek¦we`¨vjq h‡kvi-7408, evsjv‡`k| ‡Uwj‡dvb:+042151081 d¨vKª :88-0421-61199 I‡qemvBU:www.Just.edu.bd Dept. of Biomedical Engineering Jashore University of Science & Technology Jashore -7408, Bangladesh, Phone: +042151081Ex-306 Fax : 88-0421-61199 Website: www.Just.edu.bd Lecture-01 July 9, 2019 Ion movement across the membrane is subject to both diffusion and electric field forces. Therefore, the total flux when both diffusional and electric field forces are present is The above equation is known as the Nernst–Planck equation. It describes the flux of the pth ion under the influence of diffusion and an electric field. This flux can be converted into an electric current density when multiplied by FZp, the number of charges (Coulombs) carried by each mole. The resulting electric current density is Substituting the value of Dp Alternatively, using Einstein’s equation to substitute for Dp, In the diffusion term the concentration gradient controls the direction of the flow, but the current will be in the same or opposite direction, depending on whether Zp is negative or positive. Conversely, in the electric field term the electric field itself determines the direction of positive current flow, so knowledge of Zp is necessary only to determine current magnitude. In case of any query or suggestion please contact with Md. Anas Ali, Lecturer, BME, JUST (Email:anas_bme@just.edu.bd) 4