Lab 2 Action Potential PDF - 2024-2025 - Helwan National University

Faculty of Medicine Academic Year: 2024-2025 Year: 1 Semester: 1 Module: Human Body Function (HBF) 102 Action Potential By: Eman Mohamed Ali Department: Physiology 12/7/2024 22  Objectives By the end of this lab, the stu...

Faculty of Medicine Academic Year: 2024-2025 Year: 1 Semester: 1 Module: Human Body Function (HBF) 102 Action Potential By: Eman Mohamed Ali Department: Physiology 12/7/2024 22  Objectives By the end of this lab, the students should be able to ✓ Discuss Action Potential and Its Ionic Basis ✓ Explain excitability changes during action potential ✓ Deduce effect of the changes in plasma ionic concentrations on the action potential 12/7/2024 HBF - 102 33  Introduction 12/7/2024 HBF - 102 44 Resting Membrane Potential (RMP) Definition: It is the difference in potential between the outer surface and the inner surface of the membrane of excitable tissues (nerve & muscle) under resting condition. *The inside of the membrane is negatively charged with respect to the outside. 12/7/2024 Helwan Special Medical Program 5 Resting Membrane Potential (RMP) Electrode CRO inside Electrode outside How to measure (RMP) ? 12/7/2024 Helwan Special Medical Program 6 Causes of the resting membrane potential Passive Forces (93%) Active force (7 %) Selective permeability of the cell Na+- K+ Pump. membrane. The membrane is not permeable to the negatively charged proteins, organic phosphate and sulfate ions present inside the cell 12/7/2024 HBF - 102 7 Passive causes of the resting membrane potential (A) Permeability of the membrane to potassium (K+) ions 50-70 times its permeability to sodium (Na+) ions at rest. At rest, K+ ion concentration is greater inside the membrane than outside the concentration of Na+ ions is the opposite. The membrane is leaky (through K+ and Na+ leak channels). 12/7/2024 Helwan Special Medical Program 8 Passive causes of the resting membrane potential (B) The membrane is not permeable to the negatively charged proteins, organic phosphate and sulfate ions present inside the cell 12/7/2024 Helwan Special Medical Program 9 Active causes of the resting membrane potential Na+ - K+ pump It is electrogenic pump (with coupling ratio 3/2), transfers more positive charges to the outside, helping in the maintenance of RMP. 12/7/2024 Helwan Special Medical Program 10  Interactive Question 2 3 What is the 1 name and function of 1, 2 and 3 ? 12/7/2024 HBF - 102 11 11  Action Potential Depolarized state (depolarization) Polarized state (RMP) Hyperpolarized state: (hyperpolarization) 12/7/2024 Helwan Special Medical Program 12  Action Potential Definition: It is a transient reversal in the membrane polarity of an excitable cell (nerve or muscle) in response to threshold stimulus. 12/7/2024 Helwan Special Medical Program 13  Phases of Action Potential and Its Ionic Basis: 1.Latent period 2.Depolarization phase 3.Repolarization phase 12/7/2024 Helwan Special Medical Program 14  Action Potential 1.Latent period ▪ Application of the stimulus is marked by stimulus artifact. ▪ is the isopotential interval that follows the stimulus artifact and ends with the start of the depolarization phase of the action potential. ▪ Latent period corresponds to the time needed by the stimulus to travel along the axon to reach the recording electrode. 12/7/2024 Helwan Special Medical Program 15  Action Potential 2.Depolarization phase At the beginning of action potential, the depolarization is slow - 55 12/7/2024 Helwan Special Medical Program 16  Action Potential 2.Depolarization phase when the membrane potential reaches (-55 mv) the firing level is reached, the rate of depolarization increases markedly and the membrane potential overshoots beyond the zero level and become (+35 mv) Reversal of polarity occurs = inner surface of the membrane becomes positive in relation to outside. The magnitude of action potential=105 mv. 12/7/2024 Helwan Special Medical Program 17  Ionic basis of depolarization At the beginning of action potential, when depolarization exceeds 7 mv, the voltage gated Na+ channels start to open at an increasing rate (opening of the m gate). 12/7/2024 Helwan Special Medical Program 18  Ionic basis of depolarization When depolarization exceeds 15 mv = (the membrane potential reaches (-55 mv)) the firing level is reached and more activation of voltage-gated Na+ channels take place. The influx of Na+ increases more due to its inwardly directed concentration and electrical gradients. 12/7/2024 Helwan Special Medical Program 19  Ionic basis of depolarization The increase in Na+ influx will cause further depolarization with more and more opening of voltage-gated Na+ channels with more and more Na+ influx. This occurs in a positive feedback mechanism. Na+ influx increases dramatically and very rapidly up to several hundred folds (reversal of polarity) 12/7/2024 Helwan Special Medical Program 20  Action Potential 3.Repolarization phase At the end of depolarization, the membrane potential falls rapidly to return back to the resting level. The membrane potential may become more negative (-ve) than original resting membrane potential (after hyperpolarization) and Finally, the resting membrane potential (RMP) is restored. 12/7/2024 Helwan Special Medical Program 21  Ionic basis of repolarization the voltage-gated Na+ channels h gate become inactivated (closure of the h gate). voltage-gated Na+ channels K+ efflux that is due to opening of the voltage-gated K+ channels n gate (opening of n gate). voltage-gated K+ channels 12/7/2024 Helwan Special Medical Program 22  Ionic basis of repolarization The opening of n gate is slower and more prolonged. The net diffusion of (+ve) charges out of the cell, due to K+ efflux, completes repolarization. Some of voltage-gated K+ channels are still open with slow return to the closed 12/7/2024 Helwan Special Medical Program 23  Ionic basis of repolarization Finally, both Na+ and K+ voltage gated channels regain their responsiveness and become ready for a new action potential Repolarization Resting voltage-gated K+ channels 12/7/2024 Helwan Special Medical Program 24  Interactive Question match a 1-Inactive Na+ channels 2-Active Na+ b channels 3-resting voltage-gated Na+ channels c 12/7/2024 HBF - 102 25 25  Action Potential 12/7/2024 Helwan Special Medical Program 26  Excitability Changes During Action Potential During different phases of action potential, the ability of the nerve to respond to a new stimulus and to produce another action potential is variable. 1.Absolute refractory period(ARP) 2.Relative refractory period (RRP) 3.Supranormal period 4.Subnormal period 12/7/2024 Helwan Special Medical Program 27  Absolute refractory period(ARP) ❑ During this period, the nerve excitability is completely lost=zero= no stimulus can excite the nerve whatever its strength. It corresponds to the depolarization phase and early part of repolarization = ascending limb of depolarization and the upper 1/3 of repolarization. 12/7/2024 Helwan Special Medical Program 28  Absolute refractory period(ARP) fractory period(ARP) 12/7/2024 Helwan Special Medical Program 29  Relative refractory period (RRP): ❑ During this period, nerve excitability is only partially recovered, thus stronger stimulus than normal is required to excite nerve. It corresponds to the remaining part 2/3 of the descending limb of repolarization. 12/7/2024 Helwan Special Medical Program 30  Relative refractory period (RRP): 12/7/2024 Helwan Special Medical Program 31  Supranormal period ❑ During this period nerve excitability is increased above normal, thus a weaker stimulus (below threshold) can excite the nerve.. 12/7/2024 Helwan Special Medical Program 32  Supranormal period 12/7/2024 Helwan Special Medical Program 33  Subnormal period ❑ During this period, nerve excitability is decreased below normal, thus stronger stimulus than normal is required to excite nerve (as in RRP). It corresponds to the after hyperpolarization. 12/7/2024 Helwan Special Medical Program 34  Subnormal period 12/7/2024 Helwan Special Medical Program 35 Effect of the changes in plasma ionic concentrations on the action potential no effect on RMP Hypernatriemia Facilitates the process of depolarization. no effect on RMP Hypornatriemia Delays the process of depolarization & decreases amplitude of A.P 12/7/2024 Helwan Special Medical Program 36 Blockage of voltage gated Na+ channels : blocking Na+ channels = no action potential 1- Tetradotoxin (TTX) ▪by closure of m-gate (activation gate) (outside). 2- Local anesthetics ▪by closure of h-gate (inactivation gate) (inside). ▪Anesthetics can cross the membrane because they are lipid soluble. 12/7/2024 Helwan Special Medical Program 37 Effect of the changes in plasma ionic concentrations on the action potential decreases RMP Hyperkaliemia Initial increase of excitability of the nerve causing depolarization, but without firing due to closure of Na⁺ channels (h gates). Slower repolarization. Hypokaliemia Decreases excitability of the nerve 12/7/2024 Helwan Special Medical Program 38  Blockage of voltage gated K+ channels can be done by: Tetraethylammonium ▪Note that blocking K+ channels prolongs repolarization. 12/7/2024 Helwan Special Medical Program 39 Effect of the changes in plasma calcium concentrations on the action potential: ❑ Decreased Ca++ concentration (hypocalciemia): increases excitability by increasing Na+ influx into excitable cells ❑ Increased Ca++ concentration the(hypercalciemia): decreases excitability of the nerve & stabilizes the membrane 12/7/2024 Helwan Special Medical Program 40