Basis Of Classification Of Nerve Fibers
The nerve fibers are classified by different methods. The basis of classification differs in each method. Nerve fibers are classified by six methods:
- Depending upon structure
- Depending upon distribution
- Depending upon origin
- Depending upon function
- Depending upon secretion of neurotransmitter
- Depending upon diameter and conduction of the impulse (Erlanger-Gasser classification).
Read And Learn More: Medical Physiology Notes
- Depending Upon Structure: Based on the structure, the nerve fibers are classified into two types:
- Myelinated Nerve Fibers: Myelinated nerve fibers are the nerve fibers that are covered by myelin sheath.
- Nonmyelinated Nerve Fibers: Nonmyelinated nerve fibers are nerve fibers that are not covered by myelin sheath.
- Depending Upon Distribution: Nerve fibers are classified into two types on the basis of the distribution:
- Somatic Nerve Fibers: Somatic nerve fibers supply the skeletal muscles of the body.
- Visceral or Autonomic Nerve Fibers: Autonomic nerve fibers supply the various internal organs of the body.
- Depending Upon Origin: On the basis of origin, the nerve fibers are divided into two types.
- Cranial Nerves: Nerve fibers arising from the brain are called cranial nerves.
- Spinal Nerves: Nerve fibers arising from the spinal cord are called spinal nerves.
- Depending Upon Function: Functionally, the nerve fibers are of two types
- Sensory Nerve Fibers: The sensory nerve fibers carry sensory impulses from different parts of the body to the central nervous system. These nerve fibers are also known as afferent nerve fibers.
- Motor Nerve Fibers: The motor nerve fibers carry motor impulses from central nervous system to different parts of the body. These nerve fibers are also called the efferent nerve fibers.
- Depending Upon Secretion Of Neurotransmitter: Depending upon the neurotransmitter substance secreted, the nerve fibers are divided into two types
- Adrenergic Nerve Fibers: Adrenergic nerve fibers secrete noradrenaline.
- Cholinergic Nerve Fibers: Cholinergic nerve fibers secrete acetylcholine.
- Depending Upon the Diameter And Conduction Of Impulse (Erlanger-Gasser Classification): Erlanger and Gasser classified the nerve fibers into three major types on the basis of diameter (thickness) of the fibers and the rate of conduction of impulses:
- Type A nerve fibers
- Type B nerve fibers
- Type C nerve fibers.
Among these fibers, type A nerve fibers are the thickest fibers and type C nerve fibers are the thinnest fibers. The velocity of impulse through a nerve fiber is directly proportional to the thickness of the fibers. The different types of nerve fibers along with diameter and velocity of conduction are given in the Table.
Type A nerve fibers are divided into four types:
- Type A alpha nerve fibers
- Type A beta nerve fibers
- Type A gamma nerve fibers
- Type A delta nerve fibers.
- Type A alpha fibers are also called Type 1a fibers.
- Type A beta nerve fibers are called Type 2 fibers and Type A-delta fibers are known as Type 3 fibers.
- Type C fibers are also known as Type 4 fibers.
- Except for ‘C’ type of fibers, all the nerve fibers are myelinated.
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