Membrane Potential (MP) in Excitable Tissues PDF

Dr. Ahmed Abdelmonaem Assistant professor of medical physiology Dr.Ahmed Alsabih Assistant Professor and Consultant of Physiology MSc (Glasgow, UK), PhD (Leicester, UK) Dr Taha Sadig Ahmed , Clinical Neurophysiologist  Objectives Define the excitable ti...

Dr. Ahmed Abdelmonaem Assistant professor of medical physiology Dr.Ahmed Alsabih Assistant Professor and Consultant of Physiology MSc (Glasgow, UK), PhD (Leicester, UK) Dr Taha Sadig Ahmed , Clinical Neurophysiologist  Objectives Define the excitable tissues and why they are called “ excitable ”. Describe the types of ionic membrane channels. Explain how the membrane potential ( MP) and resting membrane potential ( RMP) can be measured Analyze the origin of RMP. Mention the effects of changing extracellular sodium and potassium concentrations on the RMP. 0 The ability of the living tissue to respond to changes in its environment (stimulus). Types of stimuli: 1. Electrical 2. Chemical 3. Mechanical 4. Thermal Q : Why are they called excitable ? Because their cell-membrane is polarized ( inside negative & outside Positive ) and carries electric charge stored in it : thus it behaves as a capacitor 0  Definition: It is the potential difference between inside & outside the membrane at rest. Value : it is – 70 mv. Causes of RMP: Unequal Distribution of ions Selective permeability Na-K pump Ionic equilibrium Equilibrium potential is the membrane potential that puts an ion in electrochemical equilibrium. It can be calculated using the Nernst equation , which computes the equilibrium potential for any ion based upon the Concentration gradient.  Nernst equation E X + : equilibrium potential [X + ]o : concentration outside (extracellular) [X + ]i : concentration inside (intracellular) Z : value of the charge Constituents of Extracellular and Intracellular Fluids  Resting memb. Is 50-100 times more permeable to K+ than to Na+….. Why?? -Inside: Potassium, protien, phosphate, Sulphate (non diffusible ions), magnesium. (-ve) -Outside: chloride,bicarbonate, Sodium(+ve)  POTASSIUM ION DIFFUSION : The cell membrane is about 100 times more permeable for potassium ions than for sodium ions. Potassium ions are concentrated inside 30-40 times more than outside. Potassium ions tend to move from inside the cell to outside. Potassium ion diffusion continue untill the positive charge outside the membrane reach a level that repells the outflow of more potassium ions ( equilibrium of potassium ion diffusion ).  SODIUM ION DIFFUSION : Sodium ion concentration outside the membrane is 10-15 times the concentration inside. Sodium tends to diffuse to inside the cell. Sodium ion diffusion is limited by the low permeability of the resting membrane to sodium. Leak ( Passive ) channels ( what are other types ? Remember.  Chloride ions tend to diffuse from outside to inside the cell (concentrated 25 times outside more than inside ). This diffusion is prevented by repulsion force caused by the negativity inside & also by the attraction force between chloride & sodium ions outside the membrane.  The cell membrane offers absolute barrier to the passage of organic anions (proteins), because the protein molecules are of large size. They cannot pass through the cell membrane. movement of ions at rest occur through the leak protein channels which are always opened. ** The most important ion at rest is the potassium ion because of highest concentration gradient & highest membrane permeability  2.REPOLARIZATION K+ IC F ECF K+ Na- ATPas K 1. e Na+ DEPOLARIZATION (ECF by convention is 0 mV) Net Deficit: +ve outside, - ve inside.  0 2 Depolar  Measurement : it is measured by special apparatus with microelectrodes , amplifier ,and sensitive voltmeter. One microelectrode is put in the inner side of the membrane & the other microelectrode is put on the outside surface. The difference in voltage between the inside and outside of a resting nerve cell will be = -70 mV : this is called Resting Membrane Potential ( RMP ). Cathode ray oscilloscope  Intracellular Recording of MP CRO ( Oscilloscope ). Potential difference = O mV When both electrodes ( recording & reference electrodes) are in ECF , outside the cell Potential difference = -70 mV Refrence Record When the recording electrode enters electrode electr the cell while the other ( reference ) electrode remains in in the ECF Student John Reeves , winner of student’s prize of intracellular recording competition , University of Cardiff RMP Stimulation Action Potentials Two questions should be asked : Q1: What are the 3 factors that make the inside of the cell negative ? Q2: and give the RMP the value of -70 to -90 mV ? Answer to both questions : the 3 factors are (1) At rest , K+ leak channels are more effective than Na+ leak channels more K+ diffuses to outside than Na+ to inside i.e , the membrane is 50 -100 times more permeable to K+ than to Na+ more potassium lost than sodiumgained net loss of +ve ions from inside the cell more negative inside 0  (2) Large intracellular anions ( proteins , sulphates & phosphates) have much higher concentration inside the cell than outside it (3) Active Transport : The sodium-potassium pump ( 3 Na+ pumped out in exchange for 2 K+ pumped in ) net loss of +ve charge from cell interior makes inner side of membrane negative 0  Important points of K+ ↑ extracellular K+ ions will reduce the efflux of K+ ions or even create an influx of K+ ions , the net result of which will be DEPOLARIZATION. ↓ extracellular K+ ions will accelerate the efflux of K+ ions, the net result of which will be HYPERPOLARIZATION Thus a cell’s Resting Membrane Potential is very sensitive to change in Extracellular K+ ions !!  Important points of Na+ In most cells, including the excitable cells under resting conditions, there is not a significant number of Na+ channels (CONDUCTANCE is close to zero), Sodium flux is minimal despite large net force. An increase in membrane conductance to Na+ ions will produce an influx of Na+ ions and DEPOLARIZATION. Extracellular Na+ will not affect the Resting Membrane Potential. Thus, a cell’s Resting Membrane potential is NOT sensitive to changes in Extracellular Na+ ions. Thank You !  HOME WORK :) Why dose K more permeable than Na(other than causes mentioned)?