Properties and Classification of Nerve Fibres Notes

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:

1. Depending upon structure
2. Depending upon distribution 
3. Depending upon origin
4. Depending upon function
5. Depending upon secretion of neurotransmitter
6. Depending upon diameter and conduction of the impulse (Erlanger-Gasser classification).

Read And Learn More: Medical Physiology Notes

1. Depending Upon Structure: Based on the structure, the nerve fibers are classified into two types:
   1. Myelinated Nerve Fibers: Myelinated nerve fibers are the nerve fibers that are covered by myelin sheath.
   2. Nonmyelinated Nerve Fibers: Nonmyelinated nerve fibers are nerve fibers that are not covered by myelin sheath.
2. Depending Upon Distribution: Nerve fibers are classified into two types on the basis of the distribution:
   1. Somatic Nerve Fibers: Somatic nerve fibers supply the skeletal muscles of the body.
   2. Visceral or Autonomic Nerve Fibers: Autonomic nerve fibers supply the various internal organs of the body.
3. Depending Upon Origin: On the basis of origin, the nerve fibers are divided into two types.
   1. Cranial Nerves: Nerve fibers arising from the brain are called cranial nerves.
   2. Spinal Nerves: Nerve fibers arising from the spinal cord are called spinal nerves.
4. Depending Upon Function: Functionally, the nerve fibers are of two types
   1. 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.
   2. 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.
5. Depending Upon Secretion Of Neurotransmitter: Depending upon the neurotransmitter substance secreted, the nerve fibers are divided into two types
   1. Adrenergic Nerve Fibers: Adrenergic nerve fibers secrete noradrenaline.
   2. Cholinergic Nerve Fibers: Cholinergic nerve fibers secrete acetylcholine.
6. 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:
   1. Type A nerve fibers
   2. Type B nerve fibers
   3. 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:

1. Type A alpha nerve fibers
2. Type A beta nerve fibers
3. Type A gamma nerve fibers
4. Type A delta nerve fibers.
5. 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.