The axon along with its enveloping sheath is called as a nerve fibre. The nerve fibres located outside the brain and spinal cord are covered by a thin membrane called neurilemma or sheath of Schwann. The neurilemma intern is made up of sheath cells or lemnoblasts or Schwann’s cells. These sheath cells are present in a series.
Each sheath cell with its cytoplasm and flattened nucleus surrounds the section of the axon. The presence of these cells is essential for the proper functioning of the axons of peripherial nerves. The nerve fibres are classified into two types. Medullated nerve fibres and Non-medullated nerve fibres.
Medullated nerve fibres:
These are having a central core (neuraxis) surrounded by two sheaths – inner thick medulary sheath and outer thin nurilemma. The axis cylinder is nothing but the axon nerve cell. It contains longitudinal neurofribrils in its neuroplasm. The medulary sheath is composed of a shining white fatty substance called myelin.
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This sheath serves as an indicating layer preventing the loss of energy of the nerve impulses during the passage. The medulary sheath is not continuous, at some places it is absent and these regions are known as the nodes of Ranvier. The part of the nerve fibre between two nodes of Ranvier is called the internode.
Non-medullated nerve fibres:
These nerve fibres lack the myelin sheath and the neurilamma directly surrounds the axon. The nuclei of the neurilamma bring about swellings around the fibre. Electron microscopic studies however have shown a trace of myelin surrounding the axon in the non-medulated fibres also. The fibres appear grey in colour in contrast to the glistening white appearance of the medullated fibres. The medullated fibres occur in the brain, spinal cord, Cranial and spinal nerves whereas the nonmedullated nerve fibres occur in the autonomous nervous system.