Properties of nerve fibre

1. PROPERTIES OF NERVE FIBRES

2. IMPORTANT PROPERTIES Excitability Conductivity Unfatigability Refractive period All or none response Summation Accommodation

3. Excitability: nerve fibres are highly excitable tissue respond to various stimuli Capable of generating electrical impulse Conductivity: action potential is generated in the nerve fibre, which is propagated along its entire length to the axon terminal.

4. Refractive period: during action potential the excitability of a nerve become reduced i.e a new impulse cannot be generated during a AP Types: a. Absolute refractory period (ARP) b. Relative refractory period ( RRP) Note :- once initiated moving impulse wont depolarize the area behind it

5. Action Potential

6. Unfatiguability : Nerve fibres can not be fatigued even when they are stimulated continuously. All or none response: Either all of the action potential is seen or none at all If a stimulus of threshold strength is applied AP will be generated Further increase in strength of stimulus or duration has no effect on amplitude of AP But can affect frequency

7. Summation: Application of a sub threshold stimulus does not evoke an action potential. However if sub threshold stimuli are applied in rapid succession they are added and they produce an action potential. Accommodation: Application of continuous stimuli may decrease the excitability of nerve fibre.

8. Nerve fiber types in mammalian nerve Fiber Type Function Fiber Diameter ( μm ) Conduction Velocity (m/s) Aα Proprioception ; somatic motor 12-20 70-120 Aβ Touch, pressure 5-12 30-70 Aγ Motor to muscle spindles 3-6 15-30 Aδ Pain, cold, touch 2-5 12-30  

9. Nerve fiber types in mammalian nerve Fiber Type Function Fiber Diameter ( μm ) Conduction Velocity (m/s) B Preganglionic autonomic <3 3-15 Dorsal root (C) Pain, temperature, some Mechano-reception , reflex responses 0.4-1.2 0.5-2 Sympathetic Postganglionic sympathetic 0.3-1.3 0.7-2.3

10. Nerve Susceptibility Pressure – A> B > C Hypoxia – B > A > C Local Anesthesia – C > B > A

11. Myelination It’s a protective insulator covering of the axon Formed by schwann cells Double layer membrane of a single schwann cell wraps itself several times around axon 1 mm in length , 8 -10 micrometer in thickness Layers stick to each other due to protein p0 Schwann cell nucleus lies in the outermost layer

12. Saltatory Conduction

13. Conduction Of Nerve Impulse Orthodromic Antidromic Axoplasmic Transport Fast antegrade – 400mm/day ( kinesin ) Fast retrograde – 200mm/day ( dynein ) Slow antegrade

14. Wallarian Degeneration Distal fragment degenerates fully Proximal frag . Until previous node of Ranvier Swollen myelin sheath – (appear as beads ) Schwan cells does not die Debris taken by macrophages Soma swells and become round Extrusion of nuclei Disintegration of Nissle granules – Chromolysis Disappearance of golgi apparatus Completed by 3 – 4 weeks

15. Wallarian Degeneration

16. Regeneration Soma tries to repair by synthesizing new protein (axonal reaction) Chromatolysis reversible Axonal sprouts may form Axonal cone (growth 1-4 mm/day) Schwan cells myelinate new axon Nucleus occupy center , RER , GA reappears Denervation hypersensitivity seen in target organ