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