Types of nerve fibres
May 26, 2018
- 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:
- 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.
Image source: jaypeedigital
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.
Image source: doctorlib
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.
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/