Nerve fibres, Biology


  1. Axon or dendrite of a nerve cell covered with one, two or three sheaths is called nerve fibre.
  2. Dendrites are surrounded only by one sheath. An axon may be surrounded by two or three sheaths.

(A) On the basis of structure, the nerve fibres are of two types: medullated or myelinated and non medullated or non-myelinated.

(i) Medullated (= Myelinated) Nerve fibres. They consist of the following parts.

(a)      Axis Cylinder. It is simply the axon or dendrite of a nerve cell. Its neuroplasm (= axoplasm) contains longitudinal neurofibrils and mitochondria. The plasma membrane surrounding the axis cylinder is called axolemma. Axolemma conducts the nerve impulses.

(b)      Medullary Sheath (= Myelin Sheath). The medullary sheath is composed of substance called myelin. Myelin contains lipids, proteins and water. The medullary sheath is continuous around the nerve fibres in the central nervous system but in the nerve fibres of the peripheral nerve fibres it is absent at certain points called the nodes of Ranvier. The part of a nerve fibre between two successive nodes of Ranvier is called inter node. Each segment of the myelin sheath is formed by one Schwann Cell.

(c)      Neurilemma (= Schwann Cell Sheath). Outside the myelin sheath there is a layer of Schwann cell cytoplasm which is called neurilemma. A single large and flat nucleus is present in the cytoplasm. The neurilemma is continuous over the nodes of Ranvier. As stated above Schwann's cells produce myelin sheath.

  1. A thin layer of connective (areolar) tissue, called the endoneurium, covers the neurilemma.
  2. The medullated nerve fibres of the brain and spinal cord lack neurilemma.
  3. Due to lack of neurilemma the nerve fibres of the brain and spinal cord do not regenerate after injury.
  4. These fibres have an incomplete covering of neuroglia cells which produce the myelin sheath.
  5. The medullated nerve fibres are found in the white matter of the brain and spinal cord and in the cranial and spinal nerves.
  6. When an impulse travels along a medullated nerve fibre, it does not proceed uniformly along the length of the axis cylinder, but jumps from one node of Ranvier to the next. This is called the saltatory conduction of impulses. Thus, the conduction of impulses is faster in medullated nerve fibres.

(ii)     Non-medullated (= Non-myelinated) Nerve Fibres. A non-medullated nerve fibre consists of an axis cylinder surrounded by neurilemma and connective tissue. The nuclei cause swelling along the fibre. The nonmedullated nerve fibres form grey matter. These fibres do not have the nodes of Ranvier and internodes. They also lack collateral fibres. The non-medullated nerve fibres are found in the autonomic nerves. They conduct nerve impulses much slower than the medullated nerve fibres.

On the basis of Function, the nerve fibres are again of two types

(i) Afferent (= Sensory) Nerve Fibres. They carry nerve impulses from the sense organs to the central nervous system (brain and spinal cord).

(ii) Efferent (= Motor) Nerve Fibres. They carry nerve impulses from the central nervous system to the effector organs (muscles and glands).

Posted Date: 10/1/2012 4:18:36 AM | Location : United States

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