Final Lecture 5 Action Potential Functions of Bodysem1 PDF

Foundations of Body Functions and Biophysics Initiation and Propagation of Action Potential in Neurons Prof Dr. Abdelaziz M. Hussein Prof and Chairman of Medical Physiology Department Lecture Objectives At the end of this lecture yo...

Foundations of Body Functions and Biophysics Initiation and Propagation of Action Potential in Neurons Prof Dr. Abdelaziz M. Hussein Prof and Chairman of Medical Physiology Department Lecture Objectives At the end of this lecture you will be able to: 1. Define action potential and describe its phases 2. Explain the ionic basis of action potential in neurons 3. Identify the bases of biphasic recording of AP in neurons 4. Identify the methods and record of monophasic of AP in neurons 5. Explain how to AP propagates along myelinated and unmyelinated nerve fibers Contents 01 definition, phases and ionic basis of AP in neurons 02 Recording of AP in neurons (biphasic and monophasic recording) 03 Propagation of AP in myelinated and unmyelinated nerve fiber A) Def. It is the electrical changes, which occur in the RMP as a result of stimulation of N.F. by an effective stimulus ++++++++++++++++++++ B) Phases of action potential --------------------------------- RMP or i) Depolarization Polarization Na+ ii) Repolarization --------------------------------- ++++++++++++++++++++ Depolarization Action Potential ++++++++++++++++++++ -------------------------------- Repolarization K+ 5 Depolarization Repolarization 6 Membrane potential (mv) - 55 mv Threshold - 63 mv - 70 mv Time (ms) Na+ Na+ ++++++++++++++++----+++++ Active depolarization -------------------------++++-------- 7 Def., Depolarization means loss of normal polarized state of the membrane i.e. ↓ed difference ( ) outer and inner surface of the membrane) Mechanism: Stimulus ↑es the permeability of the cell membrane (several hundred folds) to Na+ ions → Na diffusion changes the membrane potential from the RMP (- 70m.v) to +35 mV i.e. reversal of polarity 8 Mechanism: Steps: a) Electrotonic potentials and firing levels: i) The stimulus ( As cathode) adds negative charges to the outer surface of the membrane →↓es the potential difference between the inner and outer surface of the membrane by about 7 m.v. (from – 70 m.v. to – 63 m.v.) i.e. passive depolarization 9 ii) At – 63 mV, some voltage gated Na channels open allowing Na influx →↓ es the membrane potential more → opening of more voltage-activated Na+ channels → more Na+ influx → more decreased membrane potential → opening of more and more Na+ channels i.e. +ve feedback mechanism →↑ es membrane potential to – 55 mV (firing level) i.e. active depolarization Mechanism: Steps: b) At – 55 mV All voltage gated Na channels open allowing Na influx → rapid Na influx → the membrane potential quickly reaches zero potential and then overshoots to about +35 m.v, so there is a momentary reversal in polarity 11 12  Repolarization is restoration of membrane potential Mechanism 1- Stoppage of Na+ influx due to: a) Closure of the voltage-activated Na+ channels by closure of h gate b) Reversal of the electrical gradient for Na 2- Opening of voltage-activated K+ channels: At the threshold potential (-55 m.v), the voltage-activated K+ channels start to open but after a slight delay time Diffusion of K ions outside returns the inside of the membrane to its original -ve potential (-70m.v) i.e. restore the RMP i.e. repolarization 13  Also, K ions continue to diffuse to outside due to delayed closure of its channels leading to hyperpolarization Nerve and muscle, Abdelaziz Hussein 14  After depolarization and repolarization, the ionic composition inside and outside the cell membrane is slightly disturbed. Redistribution of Na and K ions to the normal resting condition is established by the Na-K pump which actively transports Na out and K into the cell. 15 Apparatus: Cathode ray oscilloscope (CRO) 17 a) Method: - By inserting microelectrode into the nerve fibers (recording) and placing the 2nd electrode (reference) outside away from the stimulated nerve fibers 18 b) Record: - Consists of 3 parts;  Latent period,  Spike potential  After potentials 1) Latent period: Def Is the time between the stimulation of the nerve (marked by stimulus artifact) and the start of the action potential. Cause: Represents the time taken by the impulse to travel from the site of stimulation to the site of recording electrodes b) Record: 1) Latent period: Duration: Is affected by: 1. The distance between the stimulating and recording electrodes. 2. The velocity of conduction of the nerve fibers. Importance: Used to calculate the velocity of conduction of a nerve fiber b) Record: 2) Spike potential: Is a large wave (105 mV from – 70 to +35 mV) of a short duration (0.5 -1 msec in thick myelinated nerve fibers) Consists of: a) Ascending limb: Represents the process of depolarization Due to Na+ influx which consists of 2 phases: i) Slow depolarization Due to gradual Na influx form the resting potential (-70 mV) to the threshold potential (-55 mV) ii) Rapid depolarization (upstroke) At the firing level, the voltage sensitive Na channels open suddenly → rapid Na influx, to (+35 mV) → reversal of polarity or overshoot b) Record: 2) Spike potential: b) Descending limb: Represents the major part of the process of repolarization (about 70%). Is due to sudden opening of the voltage sensitive K channels → rapid K efflux→ rapid repolarization. b) Record: 3) After potentials: Are small waves with relatively longer durations and are of two types i) Negative After-Potential ii) Positive After-Potential Short duration (4 msec) Long duration (40 m sec) Membrane is partially Membrane is hyperpolarized depolarized Due to slow K+ efflux Due to continuous excess K+ efflux due to slow closure of K + channels  Action potential is initiated at the initial segment (axon hillock) and propagates along the axon down to the terminal ending The AP must be propagated in order to transfer information from one place in the nervous system to the other There are 2 types of propagation (continuous and salutatory) 24  Continuous Conduction Salutatory Conduction Site Unmyelinated nerve fibers Myelinated fibers Speed 0.5 -2.0 m/sec Up to 120 m/sec Energy More energy consumption Less energy consumption (1% of continuous conduction) Mechanism Occur Step-by-step or sweeping i.e. Occurs by jumping of impulses from continuous conduction node to node i.e. salutatory (jumping) conduction 25 Mechanism: Stimulation of the nerve fiber makes the nearest node of Ranvier depolarized (+35 mV) The potential difference between the depolarized (active) node (+ 35 mv) and adjacent polarized (resting) node (- 70 m.v) makes a local current flows between the 2 nodes helping generation of an action potential at the resting node, which by turn becomes the stimulus for the adjacent node & so on 26  The potential difference between the depolarized (active) area (+ 35 mv) and adjacent polarized (resting) area (- 70 m.v) makes a local current flows This generates an action potential at the resting area, which by turn becomes the stimulus for the adjacent region & so on 27 N.B. The speed of conduction of action potential depends upon the degree of myelination: ↑ing the myelin sheath thickness → increases the membrane resistance to current, so the charge will jump from one node to another and increase the conduction velocity In multiple sclerosis, loss of myelin sheaths causes decrease in conduction velocity 28  Phases of action potentials include depolarization and repolarization  Voltage gated Na and K channels are responsible action potentials Summary & Wrap up  Biphasic recording of AP occurs when electrodes are placed on the nerve fiber surface  Monophasic record of AP consists of latent period, spike potential and after potential  Conduction of AP in myelinated nerve fiber is saltatory and in unmyelinated is continuous conduction  Orthodromic conduction occurs in the same  Questions 1. During depolarization phase of action potential in nerve fibers: 2. During repolarization phase of action potential a) Voltage activated Na+ channels open. in nerve fibers: b) The membrane becomes impermeable to a) Voltage activated Na+ channels open. Na+. b) The membrane becomes impermeable to c) The membrane potential increases from – Na+. 70 mv to +10 mv c) The membrane potential increases from – 70 d) K+ ions diffuse to outside the nerve fibers mv to +10 mv e) Sodium ions diffuse to outside the nerve d) K+ ions diffuse to outside the nerve fibers fibers e) Voltage gated K channels are closed 3. Continuous conduction in nerve fibers: a) Occurs in myelinated nerve fibers. b) Occurs by jumping of charges from one node of Ranvier to another. c) Is relatively slow 0.5-2.0 meter / second. d) Is fast conduction e) Is responsible for transmission of organelles inside the axons  References 1. Costanzo, Linda S. "BRS Physiology (Board Review Series)." (2018).‫‏‬ 2. Ganong, William F. "Review of medical physiology." (2020).‫‏‬ Discussion and Feedback THANK YOU