L7 Membrane Potential - Physiology PDF

‫بسم الله الرحمن‬ ‫الرحيم‬ PHYSIOLOGY OF FOUNDATION BLOCK L7: MEMBRANE POTENTIAL  Objectives By the end of this lecture the students should be able to: 1. Define the resting membrane potential 2. Describe the measurement of resting membrane poten...

‫بسم الله الرحمن‬ ‫الرحيم‬ PHYSIOLOGY OF FOUNDATION BLOCK L7: MEMBRANE POTENTIAL  Objectives By the end of this lecture the students should be able to: 1. Define the resting membrane potential 2. Describe the measurement of resting membrane potential 3. Outline the factors responsible for genesis of membrane potential Membrane Potential Cell membrane of all living cells has a MEMBRANE POTENTIAL (polarized electrically) Why? Separation of opposite charges across plasma membrane Due to differences in concentration and permeability of key ions  Membrane Potential Membrane Potential : The difference in ionic distribution between inside and outside of cells which results in electrical potential difference across the cell membrane. Units of this potential are volts (millivoltes) o _ + Voltmeter _ +0 CELL  Charged Substances in Body Fluids -Many molecules in our body fluids have electrical charges due to the presence of negative groups (e.g. phosphate) or positive groups such as RNH3 +. -Also most mineral elements such as sodium, potassium, chloride, and calcium (Na+, Cl-, K+, Ca++) present in our body fluids as ions. Positive Charges Negative Charges _ Na + Cl - K+ HCO3 Ca++ - PO42 & Organic anions Mg++ Proteins RNH3+ Fixed Anions Distribution of Main Charged Substances in Body Fluids _ _ _ Cl _ _ _ FIXED _ Cl Cl _ Na+ K+ K+ _ _ _ Cl _ + + K+ K+ _ Cl Na K _ FIXED _ Cl _ Na+ Na+ _ _ + Cl _ K+ Na _ Na+ _ FIXED _ _ _ _ _ K+ Na+ Na+ Cl Cl _ K+ _ _ K+ _ FIXED _ _ _ FIXED _ _ + Na Cl Cl Na+ K+ _ + K K+ ECF CELL  Properties of Charged Chemicals Distribution of various charges outside and inside the cell play a significant role in cell functions. Excitable cells such as muscle and nerve use this phenomenon to generate and propagate electrical signals. Electrical Like charges repel each other. + Force + This creates force at opposite direction of each charge. _ Electrical _ Force Opposite charges attract each other. Electrical _ The force of attraction between these + Force charges has the potential of creating force and hence performing work. This is called electrical potential.  Distribution of Charges -The membrane potential is due to small excess of negative ions inside of the cell and small excess of positive ions outside of the cell. -The negative charges are attracted to the positive charges and they form a thin layer of negative inside and positive outside of the cell. The rest of intracellular and extracellular fluid remain neutral.  Membrane Potential Can be Measured The potential difference across the cell membrane can be measured using microelectrodes and a sensitive voltmeter.  Resting Membrane potential Definition : in excitable cells the potential difference(ionic concentration ) between inside and outside of the cell during rest is called Excitable cells: -Nerve cells (-70mv) - Muscle cells (-90mv) What is importance of resting membrane potential occurs in these excitable cells to perform a function. Usually this change indicate a signal. For example nerve cells produce  Genesis of Resting Membrane Potential Restring Membrane potential is produced by: 1- Ion Channels which cause leakage of ions across the cell membrane. 2- The action of Na/K pump at the cell membrane. 3.- Proteins and other organic molecules in the cell are negatively charged (anions), and can not cross the cell membrane therefore this makes inside of the cell negative.  Genesis of Resting Membrane Potential 1)Selective permeability of the membrane : The resting membrane of 25 to 30 times more permeable to K + than to Na+ K + moves easily but Na + with difficulty why ? 1- The membrane Na+ channels are closed under resting condition, while the K+ channels are open (leak channels) 2-The Na+ channels are guarded by ca2+ from outside that form gates which repel Na+ and prevent its entry under resting condition  Genesis of Resting Membrane Potential 2)The Na+ -K+ pump The cell membrane contains a Na+ - K+ pump. It transmits 3 Na+ ions to the outside for each 2 K+ ions transmitted to the inside of the membrane. This pump is electro-genic causing a potential difference (creating negativity inside & positivity outside).  Genesis of Resting Membrane Potential 3)Presence of non diffusible anions These anions include: Proteins and other organic molecules in the cell are negatively charged (anions), and can not cross the cell membrane therefore this makes inside of the cell negative These fixed anions does not pass through the cell membrane due to: 1.Their large molecular weight 2.Repelling by proteins in the cell membrane  Let us Revise Non-gated K+ Channel K+ Sodium potassium leak channels K+ Gated K+ 2 K+ Channel _ _ _ _ FIXED _ ATP Na+ Na+ 3 Na+ Gated Na+ Channel  References Human physiology by Lauralee Sherwood, 9th edition Text Book Of Physiology by Guyton & Hall, 12th edition Review of Medical Physiology by Ganong, 24th edition