Types of nerve fibres

  • Nerves are the communication lines for transmitting impulses from one part of the body to another.
  • They are elongated, slender, thread-like structures arising from the brain and spinal cord.
  • They vary in length i.e. from 0.1 mm to several metres.
  • In certain cases they may be almost equal to the body length.
  • They are of various types. They are as follows:
  1. On the basis of presence and absence of myelin sheath

i) Myelinate or medullated fibre

  • The axons are provided with myelin sheath.
  • It constitutes of the white matter.
  • These are present in the central and peripheral nervous system.

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ii) Amyelinate or non-medullated fibre

  • The axons are without myelin sheath.
  • It constitutes the grey matter.
  • These are present in the autonomic nervous system.

2) On the basis of number of cytoplasmic extensions

i)Unipolar

  • These are found only in embryo.
  • They are marked by the presence of only one axon and no dendrite.

ii)Bipolar

  • These are found in retina of eyes.
  • They are marked by having one axon and one dendron.

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iii)Multipolar

  • These are found in brain and spinal cord.
  • They are characterized by having one axon and many dendrites.

 

3)On the basis of nature of working

i) Sensory or afferent fibres

  • These carry stimulus from different parts of the body to the brain or spinal cord.

 

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ii)Motor or efferent fibres

  • These carry impulses from brain or spinal cord to different parts of the body.

iii)Inter neurons

  • They convert sensory stimuli to motor stimuli.
  • They are located in the brain and spinal cord.

4) On the basis of size and speed of conduction

i) Type A fibres

  • These fibres are thickest and fastest conducting.
  • They are myelinated.
  • They have diameter of 1.5- 20 micron.
  • Their speed of conduction is 4-120 m/s, which shows that they have a really fast conduction of impulse.
  • e.g., skeletomotor fibres, fusimotor fibres and afferent fibres to skin.

ii) Type B fibres

  • These fibres are medium sized i.e. they are larger than type C and smaller than type A.
  • They are myelinated.
  • They have a diameter of 1.5-3.5 microns.
  • Their speed of conduction is 3-15 m/s, which shows that they are slower than type A fibres.
  • e.g., preganglionic autonomic efferents.

iii) Type C fibres

  • These are the smallest and thinnest.
  • They are non-myelinated.
  • They have a diameter of 0.1-2 microns.
  • Their speed of conduction is 0.5-4 m/s, which shows that they have the slowest conduction.
  • e.g., postganglionic autonomic efferents and afferent fibres to skin.

References: 

i) https://www.sciencedirect.com/science/article/pii/B9780124103900000081

ii) https://www.pharmacy180.com/article/classification-of-nerve-fibers-3543/

Types of nerve fibres