Classification of nerve fibres

1. Classification of nerve
fibers
Dr. Sai Sailesh Kumar G

2. Peripheral nerve
It consists of bundle of nerve fibers
Three connective tissue sheets
Endoneurium –covers single nerve fibre
Perineurium – covers a bundle of nerve fibers
Epineurium – entire nerve trunk is surrounded

3. Peripheral nerve
May be purely sensory or motor or mixed nerve
Both myelinated and unmyelinated
Both small and large diameter fibers

4. Large diameter fibers
Conduct impulses faster than small diameter fibers
Less threshold for excitation
Refractory period is shorter
Concerned with proprioception, touch, pressure, and somatomotor function

5. Small diameter fibers
Conduct impulses slowly
Concerned with pain, temperature and autonomic functions

6. Properties of mixed nerve
Peripheral nerves usually is mixed nerves
Consists of nerve fibers of different diameters
Apart from the properties of nerve fibers, the peripheral nerve also has two
important properties
Compound action potential
Maximal stimulus

7. Compound action potential
Place two electrodes on the surface of the mixed nerve
AP recorded with multiple peaks
Mixed nerve has many nerve fibers with different conduction velocities
Hence each peak for a particular group of nerve fiber
First peak belongs to fastest conducting fiber
Last peak for slowest conducting fibers

8. Compound action potential
Helps to determine the conduction velocity of different types of nerve fibers
To evaluate effect of certain drugs such as local anaesthetics

10. Maximal stimulus
Mixed nerve has different types of nerve fibers
To excite all of them
Maximal stimulation is needed
threshold stimulus may activate only few fibers

11. Classification of nerve fibres
Numerical classification
Sensory nerve fibers are classified numerically
Ia- Muscle spindle, annulospiral endings
Ib- Golgi tendon organ
II – muscle spindle- flower spray ending, touch, pressure
III-pin, temperature, some touch receptors
IV- pain

12. Classification of nerve fibres
Erlanger- Grasser classification
Based on diameter and conduction velocity

14. Classification of nerve fibres
Physio clinical classification
Functional

16. On the basis of myelination
Myelinated nerve fibers – enveloped by myelin sheath
Unmyelinated nerve fibers – do not have myelin sheath

17. Depending on the function
Sensory fibers – carry sensory impulses to the CNS
Motor fibers – carry motor impulses to muscles and glands

18. Depending on NT released
Cholinergic fibers –Release Ach
Adrenergic fibers- Release nor epinephrine
Dopaminergic fibers – releases dopamine

19. Common causes of nerve injury
Compression of nerve – ischemia
Transection of nerve
Crush injuries
Toxic drugs
Hyperpyrexia -condition where the body temperature goes above 106.7 degrees
Fahrenheit (41.5 degrees Celsius) due to changes in the hypothalamus

20. Types of nerve injuries
According to Sunderland, nerve injuries are graded into five types
Seddons classification is given in brackets

21. First degree nerve injury (neurapraxia)
Commonly seen
Secondary to ischemia caused by pressure on the nerve for a definite perod
Nerve losses its function temporarily due to ischemia
Recovery is rapid and complete
No evidence of nerve degeneration
Saturday night paralysis

22. Second degree nerve injury (Axonotmesis I)
Compression of nerve for prolonged periods
Damage to nerve at pressure point
Axons undergo degeneration from the pressure point to the distal end
Regeneration is possible
Functional recovery is fairly rapid and complete
Eg: Carpal tunnel syndrome

23. Second degree nerve injury (Axonotmesis I)
Carpal tunnel syndrome is a common condition that causes numbness,
tingling, and pain in the hand and forearm.
The condition occurs when one of the major nerves to the hand — the
median nerve — is squeezed or compressed as it travels through the wrist.

24. Third degree nerve injury (Axonotmesis II)
Along with axon
Endoneurial tube also disrupted
Recovery is slow, poor and incomplete

25. Fourth degree nerve injury (Axonotmesis III)
Fascicle is disrupted
Damage to perineurium
Regeneration is poor and incomplete

26. fifth degree nerve injury (Neurotmesis)
Along with axon all the three sheaths are damaged
Complete section of nerve trunk
Regeneration is not possible unless the cut ends are brought closer by surgery