Nerve Fiber – Classification

Nerve Fiber – Classification


NERVE FIBER – CLASSIFICATION

Two major prevalent nerve fiber classification:

  • Erlanger & Gasser classification.
  • Lloyd & Hunt classification.

1. ERLANGER & GASSER CLASSIFICATION:

  • It covers all nerve fibers.
  • Divides them into A, B & C.
  • A and B fibers are myelinated.
  • Only C fibers are nonmyelinated.
  • Group A subgroups:
  • Divided into four: Aα, Aβ, Aγ & Aδ.

2. LLOYD & HUNT CLASSIFICATION:

  • Classifies only sensory fibers.
  • Divide them into I, II, III & IV groups.

FEATURES OF FIBERS:

Classification

Erlanger & Gasser

Lloyd & Hunt Characteristics of fibers Function High susceptibility to
A alpha I

Diameter – 13-20

Myelination – Heavily myelinated & thick

Conduction – 70-120 (maximum velocity)

  • Proprioception –
  • Due to fiber thickness
  • Motor supply to skeletal muscle (extrafusal to muscle spindle)

 

Pressure

 

A beta II

Diameter – 4-13

Myelination – Present

Conduction – 25-70 

  • Touch
  • Kinesthesia
  • Pressure
Pressure
A gamma – No comparable entity –

Diameter – 3-6

Myelination – Slightly myelinated

Conduction – 15-30

  • Motor supply to intrafusal muscle fibers (Muscle spindles)
Pressure
A delta  III

Diameter – 1-5

Myelination – Some myelination

Conduction – 5-30

  • Pain –
  • “Fast/Epicritic/First” pain.
  • Since fibers are relatively fast
  • Temperature
  • Pressure
  • Touch
Pressure
B fiber – No comparable entity –

Diameter – 1-3

Myelination – Some myelination

Conduction – 3-14

 

  • Preganglionic autonomic fibers
  • (both sympathetic & parasympathetic)

 

Hypoxia
C fiber IV

Diameter – 0.2-1.0

Myelination – Unmyelinated

Conduction – 0.2-2 (minimum)

  • Pain –
  • Slow pain (“Protopathic /Second pain)
  • Temperature
  • Pressure
  • Postganglionic autonomic fibers.

Local anesthetics 

Divisions based on differences in susceptibility to sensations:

  • Local anesthesia, hypoxia & pressure can block the conduction of nerve impulses.

1. Susceptibility to pressure:

  • Most susceptible – “A” fibers (especially Aα type)
  • Least susceptible – “C” fibers.

2. Susceptibility to Hypoxia:

  • Most susceptible  – “B” fibers.
  • Least susceptible – “C” fibers.
3. Susceptibility to local anesthesia:
  • Most susceptible – “C” fibers.
  • Least susceptible – “A” fibers.
SUMMARY:
  • In decreasing order of susceptibility:
  • For Pressure: A>B>C
  • For Hypoxia: B>A>C
  • For Local anesthesia: C>B>A.
Exam Question
 

NERVE FIBER – CLASSIFICATION

ERLANGER & GASSER CLASSIFICATION:

  • A & B type fibers are myelinated.
  • Only C fibers are nonmyelinated.
FEATURES OF FIBERS:
 

Classification

Erlanger & Gasser

Characteristics of fibers Function High susceptibility to
A alpha

Myelination – Heavily myelinated & thick

Conduction – 70-120 (maximum velocity)

  • Proprioception.
  • Motor supply to skeletal muscle (extrafusal to muscle spindle)

 

Pressure

 

A beta

Diameter – 4-13

Kinesthesia

Pressure
A delta 

Diameter – 1-5

 

“Fast”/”Epicritic”/”First” pain

Pressure
B fiber

Diameter – 1-3

Preganglionic autonomic fibers

 

Hypoxia
C fiber

Myelination – Unmyelinated

Conduction – 0.2-2 (minimum)

  • Pain –
  • Slow pain (“Protopathic/Second pain)
  • Temperature
  • Pressure
  • Postganglionic autonomic fibers.

Local anesthetics 

 

  •  Local anesthesia, hypoxia & pressure can block the conduction of nerve impulses.
  • Most susceptible fibers to pressure – “A” fibers.
  • Especially, Aα type

Hypoxia:

  • Most susceptible – “B” fibers.
  • Least susceptible – “C” fibers.

Local anesthesia:

  • Most susceptible – “C” fibers.
  • Least susceptible – “A” fibers.
SUMMARY:
  • For Pressure: A>B>C
  • For Hypoxia: B>A>C
  • For Local anesthesia: C>B>A
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