Classification of Fibres in Peripheral Nerves according to Diameter and Velocity of Conduction
In a transverse section across a peripheral nerve it is seen that the nerve fibres vary considerably in diameter. Fibres of larger diameter are myelinated while those of smallest diameters are unmyelinated. It is well established that by and large fibres of larger diameter conduct impulses more rapidly than those of smaller diameter. Various schemes for classification of nerve fibres on the basis of their diameter and their conduction velocity have been proposed. The best known classification is as follows.
The fastest conducting fibres are called Type A fibres. Their conduction velocity is 30 to 120 m/sec; and their diameter varies from 1 to 2 μm. They are myelinated.
Type A fibres are further divided (in descending order of diameter and conduction velocity) into three subtypes: alpha (Aα), delta (Aδ) and gamma (Aγ). Type A fibres perform both motor and sensory functions as follows:
Motor Type A fibres
a. Aα fibres supply extrafusal fibres in skeletal muscle.
b. Aγ fibres supply intrafusal fibres in muscle spindles.
c. Aδ fibres are collaterals of Aα fibres (to extrafusal fibres) that innervate some intrafusal fibres.
1. Aα sensory fibres carry impulses from encapsulated receptors in skin, joints and muscle. They include primary sensory afferents from muscle spindles (also called Group I fibres); and secondary afferents from spindles (also called Group II fibres). Some of them carry impulses from the gut.
2. Aδ sensory fibres are afferents from thermo-receptors and nociceptors (pain receptors).
Type B fibres have a conduction velocity of 4 to 30 m/sec; and their diameter is less than 3 μm. They are myelinated. They are either preganglionic autonomic efferent fibres (motor), or afferent fibres from skin and viscera, and from free nerve endings in connective tissue of muscle (also called Group III fibres).
In contrast to type A and type B fibres, type C fibres are unmyelinated. They have a conduction velocity of 0.5 to 4 m/sec; and their diameter is 0.5 to 4 μm. These are postganglionic autonomic fibres, and some sensory fibres conveying pain. (These include nociceptive fibres from connective tissue of muscle: Group IV fibres. Note that the terms Group I to IV all refer to afferents from muscle tissue). Some fibres from thermoreceptors and from viscera also fall in this category.
Unmyelinated axons are numerous in dorsal nerve roots and in cutaneous nerves. Many unmyelinated axons are also present in nerves to muscles and in ventral nerve roots. Most autonomic nerve fibres are unmyelinated although myelinated fibres are also present in preganglionic nerves.