NERVE FIBER – CLASSIFICATION

NERVE FIBER – CLASSIFICATION

Two major prevalent nerve fiber classification:

• Erlanger & Gasser classification.

• Lloyd & Hunt 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:

• In decreasing order of susceptibility:

– For Pressure: A>B>C

– For Hypoxia: B>A>C

– For Local anesthesia: C>B>A.

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