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Where the medullated multi polar neurons are found. Give their structure with help of labelled diagram.

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Types of neurons :

On the basis of number of axon & dendron, the neurons are of three types-

(a) Unipolar neurons – Presence of only one axon. Dendrone is absent. They are found in retina.

(b) Bipolar neurons – Presence of one axon and one dendrone. The}- are found in retina, olfactory epithelium, internal ear etc.

(c) Multipolar neurons – Presence of one axon & two or more dendrones. They are common in brain & spinal cord. The cytoplasm in the c}ton is called as neuroplasm. The neuroplasm consists of special Nissl’s bodies in addition to nucleus & other orgenelles but lacks centrosome.

The axon is covered by whitish & non-cellular myelin sheath which is made up of sphingolipids. There is neurolemma outside the myelin sheath.

The myelin sheath may be absent in some neurons. On the basis of presence/absence of myelin sheath, the neurons are of two typcs-

  • Medullated – Presence of myelin sheath around the axon.
  • Non – Medullated- Absence of myelin sheath.
    The myelin sheath is discontinuous & the gaps are called as nodes of Ranvier. The structure between two nodes is called as internode. The neurolemma of one internode is made up of one Schwann cell. The axon at few places gives out collateral fibres.

The neurons with the help of synapses form nerves. These nerves are of three types-

  • Sensory nerves -Transmit nerve impulses from sense organs to the brain.
  • Motor nerves- Transmit nerve impulses from brain to the muscles/ glands
  • Mixed nerves – Both way transmission of nerve impulses

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