properties of nerve fibre
TRANSCRIPT
Excitability
Conductivity
Unfatigability
Refractive period
All or none response
Summation
Accommodation
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.
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
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
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.
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
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
Pressure – A> B > C
Hypoxia – B > A > C
Local Anesthesia – C > B > A
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
Orthodromic
Antidromic
Axoplasmic Transport
› Fast antegrade – 400mm/day (kinesin)
› Fast retrograde – 200mm/day (dynein)
› Slow antegrade
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
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