Physiology of Nervous System (Assiut 2024) - PDF

Physiology of Nervous system Dr. Eman Negm Neuron structure: Unit of Nervous system. Cell body Dendrites Axon (nerve N. (soma) fiber(N.F.) terminal Plasma mem. Myelin sheath Neurilemma...

Physiology of Nervous system Dr. Eman Negm Neuron structure: Unit of Nervous system. Cell body Dendrites Axon (nerve N. (soma) fiber(N.F.) terminal Plasma mem. Myelin sheath Neurilemma Dr. Eman Negm Types of N.F.: 1-Myelinated (Insulation, most central nervous system (CNS), faster, less E, ion exchanges only at nodes, less ions, saltatory movement). 2-Unmyelinated. Dr. Eman Negm  Axon hillock (called trigger zone( as A.P. usually begins on it as it contains most voltage gated ions channels. Dr. Eman Negm  AP directed only from neuron soma to axon. Dr. Eman Negm Membrane potential: It is the difference in ions concentrations between the intracellular and extracellular fluid. Extracellular Intracellular fluid fluid Most: K+, organic anions (phosphate, Most: Na+ & sulfate and –ve Cl- proteins) Few: K+ Few: Na+ & Cl- Dr. Eman Negm Resting mem. Potential (polarized mem.): Mem. pot. of Large N.F. is -90 mv more negative than outside. *Causes: 1- Leakage of K+ outflow is 100 times permeable than Na+ permeability (as Na+ channel guarded by Ca+ from outside that repel Na+, also Na+ is thicker in diameter than K+). 2- Na+-K+ pump (3 Na+ outside & 2 K+ inside). 3- Organic anions (-ve) inside that cannot diffuse through Dr. Eman Negm mem. *Action potential (AP): It is rapid transient Electrical changes in mem. Pot. of the excited tissue when stimulated. Stages Dr. Eman Negm Depolarization Re- (ascending Repolarization establishment limb) (descending of Na+ K+ limb) conc. grad. *Voltage gated Na+ channel activation (rapidly). *Voltage gated K+ *Acc. to conc. grad. channel activated outward diffusion of *Max. activ. “threshold”, Na+ & inward of K+, irreversible AP : at -50: -60 mv. (slowly) at +35 mv. until equal reached *”Isopotential”: zero potential difference(Na begin to closed). then. *”Overshoot”: +35 mv *Na-K+ pump potential (reversed), activated: 3 Na+ ion (completely closed Na). outward, 2 K+ inward. Dr. Eman Negm  Stimulus Threshold: is the minimum stimulus power required to open the voltage gated Na channel. At threshold -50: -60: Na+ channel opened maximumly. Once first Na channel opened, more Na channel to opened subsequently. So, the initial effect of A.P. is due to Na channel. AP spread in one direction only from soma to axon because of that voltage gated channel is more in axon hillock. -90 mV →large nerve fiber and skeletal muscle fiber -70 mV → medium-sized neuron Dr. Eman Negm Na+ Channel has TWO gates: Outer gate and Inner gates. K+ Channel has only ONE gate, Inner gate only. Dr. Eman Negm Video 2 Spike potential: It is the sharp rise of depolarization followed by rapid fall of repolarization. Repolarization stage also includes: *Negative after potential: Inner surface become more negative than resting (-94 versus – 90) because of slowly K+ channel closure, causing continuous outward diffusion of K+. (slowly closure). Is hyperpolarized & need stronger signal than normal stimulus to be excited. Dr. Eman Negm Dr. Eman Negm Video 3 Dr. Eman Negm *Stimulus strength: sub minimal, minimal (threshold), Maximum. *Stimulus types: Mechanical, thermal, chemical, electrical (like nature, not harmful, easily controlled). Dr. Eman Negm *The strength- duration curve: Is the relationship between stimulus strength and time needed by it to produce response. Rheobase: Min. strength needed to stimulate M., below that no excitation occurred whatever duration prolonged. (ex: 0.4 v) Utilization time: Min. time needed for any voltage to stimulate fiber. (ex: 1.5 ms) Chronaxie: Least duration (ex: 0.5 ms) needed by stimulus twice in strength (ex: 0.8 v) than rheobase to stimulate M. Dr. Eman Negm *Curve significance (Sig. of chronaxie): Give an idea about M. excitability, compare exc. of different tissues. Excitability of nerve is high, as Chronaxie is short. Normal Complete Partial denervation: innervation: denervation: *Kink. *All fibers *all fibers *combination of 2 intact. degenerated. curves, left *smooth and * Curve rise steeply innervated one & normal curve. and right than right denervated (A) normal (B). one (C). Dr. Eman Negm Dr. Eman Negm