Physiology Lecture (13) PDF

Physiology Membrane Potentials & RMP LECTURE (13) DR. El-Sawy 0  Physiology Membrane Potenti...

Physiology Membrane Potentials & RMP LECTURE (13) DR. El-Sawy 0  Physiology Membrane Potentials & RMP Def  Structural and functional unit of nervous system. Number  One trillion neurons Sensory neurons Motor neurons Interneurons (afferent) (efferent) Classification  Conduct  Conduct  Small integrative impulses from impulses from neurons located receptors to CNS to effector in CNS CNS. organs. DR. El-Sawy 1  Physiology Membrane Potentials & RMP Nerve trunk  Composed of a larger number of nerve fibers.  Each nerve fiber is an axon covered by: Structure a) Myelin sheath b) Schwann sheath. Nerve fibers  Conduction of nerve impulses : a) Sensory nerve fibers: Function Conduct sensory information from sensory receptors to CNS b) Motor nerve fibers: Conduct motor information from CNS to effector organs 1. Excitability: The ability of nerve fibers to respond to stimuli and convert them into nerve impulse or action potential Properties 2. Conductivity: The ability of nerve fibers to conduct nerve impulse or action potential from one place to another DR. El-Sawy 2  Physiology Membrane Potentials & RMP Def :  Potential difference between inside & outside of nerve during rest.  RMP is present not only in nerves but also in muscle fibers and all cells of the body with the inside negative relative to outside. Value : Variable acc. to excitable tissues : Nerves Skeletal ms. Cardiac ms. Smooth ms. -70mV -90mV -90mV -60mV Measurement:  Recorded by use of two microelectrodes with fine tips ( > 1 µm) Connected with a special voltmeter.  One electrode inserted into nerve.  Other electrode placed on the surface. DR. El-Sawy 3  Physiology Membrane Potentials & RMP Causes of RMP : Unequal distribution of ions inside & outside nerve : Na+ K+ Cl- Protein- Outside 140 4 100 2 Inside 14 140 4 16 Unit m.Eq / Litre gm / dl ECF : ICF ratio 10 : 1 1 : 35 25 : 1 1:8 Selective permeability of cell membrane :  The cell membrane contains ion channels allow only specific ions to pass while blocking others because of their size, charge, or state of hydration  Diffusion of ions thorough leak channels is the cause of RMP  Types of ion channels : Leak or passive Responsible for resting membrane potential (RMP). channels Voltage gated channels Responsible for Action Potential (AP). Ligand gated channels Act as receptors. DR. El-Sawy 4  Physiology Membrane Potentials & RMP K+ Na+ Cl-  From inside.  From outside  From outside Diffusion  To outside  To inside  To inside  Concentration  Concentration  Concentration gradient : gradient : gradient : (Inside > outside (Outside > inside (outside > inside Helped by 30-40 times). 10 times) 25 times).  High permeability  Electric gradient to K (100 times (inside is negative) more than Na+).  This diffusion  Na ions pass through  K+ ion is the main prevent by repulsion NB passive Na channels cause of RMP. force by negative with difficulty charges inside cell. Calculation of Resting Membrane Potential  When a selectively permeable membrane is permeable to one ion : This ion diffuses across this membrane according its concentration gradient from high concentration to low concentration. This should continue till concentration is equal on both sides.  However, since this ion is charged: The ionic charges will accumulate on side of membrane where ion diffuses to (the side of low concentration).  This accumulation results in repulsion with the diffusing ions which does not allow the concentrations to be equal on both sides. DR. El-Sawy 5  Physiology Membrane Potentials & RMP We have two forces in opposite directions: Diffusion force Repulsion force According to concentration gradient in According to electrical gradient in direction of low conc. direction of high conc. When the forces become equal in magnitude, net diffusion stops it is said that "equilibrium is reached".  Equilibrium potential: Def: Electrical potential caused by charge separation due to diffusion of ions through a selectively permeable membrane that balances the concentration gradient in the opposite direction.  NB: In a cell where only one type of ion can cross the membrane, RMP will equal the equilibrium potential for that particular ion. DR. El-Sawy 6  Physiology Membrane Potentials & RMP Calculation Nernst equation (Nernst Potential): 61 𝐂𝐢𝐧 ± log 𝑧 𝐂𝐨𝐮𝐭 Where:  E ion = equilibrium potential in mv.  Cin and Cout = ion concentration inside and outside membrane.  Z = valence: For Na & k → it is +1, so it is omitted. For Ca → it is +2, so 61 must be divided by +2. For Cl → it is -1, so 61 must be divided by -1. Sodium and potassium pump (Na - K pump) :  Electrogenic Pump. Function  Pumps 3 Na+ outside the cells and 2 K+ inside the cell.  Maintain high Na conc. outside & high K conc. inside.  By conditions which ↓ metabolic activity ( cooling ) : a. Na+ ions will accumulate inside the cell & neutralize negative charges of protein Inhibition b. K+ ions which were held on the outer surface escape away  Resting membrane potential becomes progressively lost. DR. El-Sawy 7  Physiology Membrane Potentials & RMP Note  90-95 % of RMP : Passive process due to diffusion of ions through leak ion channels  5-10% of RMP : Active process due to the electrogenic Na-K pump i.e. about 4-5 mV DR. El-Sawy 8  Physiology Membrane Potentials & RMP Resting membrane potential. Action potential :  Electrical changes occur in RMP due to stimulation by effective stimulus.  These changes propagate along nerve fibers → response or actions. Graded potential :  Local change.  Result from stimulation of nerve by ineffective (Sub-miniml )stimulus.  Duration & magnitude are variable according to stimulus. Example Site Receptor potential In the beginning of the sensory nerves. Synaptic potential In synapses inside CNS. Local excitatory state In cell bodies of neuron. DR. El-Sawy 9