Physiology of The Nerve. Week 4 Dr. Walid Daoud A. Professor. Neuron. Neuron or nerve cell is specialized for transfer and integration of information . Spinal Motor Neuron: - Cell body (soma). - Dendrites for perception of signals from other neurons.
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Physiology of The Nerve Week 4 Dr. Walid Daoud A. Professor
Neuron Neuron or nerve cell is specialized for transfer and integration of information. Spinal Motor Neuron: - Cell body (soma). - Dendrites for perception of signals from other neurons. - Axon (nerve fiber) carries impulse away from the cell body.
Types of Nerve Fibers 1-Myelinated Nerve Fibers: . Surrounded by myelin sheath made by Schwann cells. . Myelin sheath is insulator to ion flow. . Nodes of Ranvier are non-insulated areas. e.g Preganglionic autonomic nerve fibers. 2-Unmyelinated Nerve Fibers: . Axons not surrounded by myelin sheath. e.g Postganglionic autonomic nerve fibers.
Electrical properties of Neuron Excitability: It is the ability of generating electrochemical impulse (action potentials) at the cell membrane in response to any stimulus. Conductivity: It is the ability to propagate action potential from the point of generation to the rest of the membrane.
Nerve Excitability Selective permeability of cell membrane causes difference in ionic composition of intracellular and extracellular fluids creating a membrane potential which is the basis of excitability. Mainly intracellular ions: Potassium, proteins, magnesium, phosphate Mainly extracellular ions: Sodium, chloride, bicarbonate
Factors affecting effectiveness of a stimulus 1- Strength: Threshold stimulus or Rheobase is the minimal stimulus needed to excite the nerve and produce action potential. 2- Duration: Utilization time is the minimal time needed by Rheobase to give a response. Chronaxie is the time needed by a stimulus of 2 Rheobase to excite the nerve. It is a measure of nerve excitability.
Resting Membrane Potential (RMP) It is the difference in electric potential (voltage) between the inside and outside the membrane under resting conditions with the inside negative and the outside positive. RMP is recorded by 2 microelectrodes connected to a voltmeter. RMP in large nerve fibers -90 mv RMP in medium nerve fibers -70 mv RMP in red cells -20 mv
Causes of RMP 1-Selective permeability of cell membraneBeing 100 times more permeable to K+ than to Na+ and impermeable to protein anions. 2-Na+ – K+ pump Against concentration and electric gradient and this needs energy derived from cell membrane ATPase activity. 3 Na+ ions pumped out while 2 K+ ions pumped in leading to positive charge outside.
Calculation of RMP Nernst Equation: equilibrium potential (E) is membrane potential at which movement of a certain ion (Na+ or K+) stops. This occurs when the 2 opposing forces (electrical and chemical gradients) become equal. E = +/- 61 log conc.inside / conc.outside E for K+ = - 94 mv E for Na+ = +61 mv This means that if K+ is the only factor causing RMP, RMP would equal -94 mv.
Calculation of RMP Goldman Equation: more accurate as it consider: - Na+, K+ and Cl- conc.inside and outside the nerve fiber. -K+ permeability is 100 times as that of Na+. RMP by diffusion of ions = -86 mv (near to K+ equilibrium potential).
Action Potential - It is the rapid change in the resting membrane potential following stimulation of a nerve by threshold stimulus. - It is a process of depolarization (Na+ influx) followed by repolarization (K+ efflux). - Can be recorded by 2 microelectrodes connected to cathode ray oscilloscope.
Steps and Phases of Action Potential 1- Stimulus artifact. 2- Latent period. 3- Ascending limb (Depolarization). 4- Descending limb (Repolarization)
Ionic Basis of Action Potential Depolarization is caused by Na inflow (influx) Repolarization is caused by K outflow (efflux) through 2 types of voltage-gated ion channels: 1-Voltage-gated Na channel: 2 gates. 2-Voltage-gated K channel: one gate. Once nerve is stimulated: - Outer Na gates open & inner Na gates close - K gates open.
Successive movements of gates is essential for production of action potential 1-During Depolarization: - Slow stage from -90 mv to -65 mv which is the firing level. - Rapid stage from -65 mv to +35 mv. 2-During Repolarization: - Inactivation of Na channels. - Activation of K channels. 3-Hyperpolarization: Slow closure of K channels.
Action Potential in Nerve Trunk Nerve trunk or peripheral nerves are made of many nerve fibers. Action potential has many peaks (compound) because each nerve fiber in the nerve responds with All or None Rule. Nerve fibers vary in: - Threshold of stimulation. - Distance from stimulating electrode., - Speed of conduction.
Local response (local excitatory state) It is a slight depolarization caused by subthreshold stimulus which opens few Na channels not enough to produce action potential and repolarization follows rapidly. - Does not obey All or None Rule. - Can be graded. - Can be summated. - Not propagated.
Excitability Changes During Action Potential 1- Absolute Refractory Period (ARP): During which a second action potential can not be elicited whatever is the strength of the stimulus (excitability is zero). 2- Relative Refractory Period (RRP): During which the nerve can not be stimulated unless the stimulus is stronger than usual (excitability is below normal).
Factors Affecting Excitability of Nerve 1-Increase excitability: -Increase Na permeability (Depolarize): Low extracellular Ca, Veratrine - Increase extracellular K concentration. 2-Decrease excitability (membrane stabilizers) - Decreased Na permeability: High extracellular Ca, local anesthesia:Cocain - Decrease extracellular K concentration.
Nerve Conductivity Action potential once generated, it travels along the length of the nerve fiber in both directions. Normally impulse travels in one direction i.e. Orthodromic conduction: from cell body along its axon to its termination. Antiorthodromic conduction: is opposite. Velocity of conduction is increased by increased diameter of nerve fiber & myelin
Conduction in myelinated Nerve Fibers (Salutatory Conduction) The action potentials are only generated at the nodes of Ranvier i.e action potential in one node acts as stimulus for generation of new action potentials at adjacent nodes. The positive charges jump from the resting node to the adjacent active one. Importance of Salutatory Conduction: -It increases velocity of conduction 50 folds. -It conserve energy for the axon.
Neuromuscular Transmission Neuromuscular Junction: It is the area between nerve endings of alpha motor neurons and skeletal muscle fibers. Axon terminals of motor neuron (end feet or Knobs) contain acetyl choline vesicles. Motor end plate membrane contain acetyl choline receptors and extracellular space contains choline esterase enzyme.
Steps of Neuromuscular Transmission 1- Arrival of action potential. Nerve impulse increases permeability of the membrane to Ca triggering rupture of acetyl choline vesicles. 2- Postsynaptic response. 3- End plate potential. 4- Acetyl choline degradation.
Properties of Neuromuscular Junction 1- Unidirectional. 2- Delay. 3- Fatigue. 4- Effects of ions. 5- Effects of drugs: - Drugs stimulate NMT. - Drugs block NMT.