Nervous Tissue

The nervous tissue is composed of densely packed nerve cells commonly known as neurons. The neurons are capable of conducting nerve impulses. The nervous tissue arises from ectoderm of a growing embryo.

Neurons

  • There are more than 100 billion neurons in a nervous system.
  • They vary in shape and size.
  • All neurons have 3 main parts – a cell body, few dentrites, and an axon.

  • The cell body contains a large nucleus and several other structures such as mitochondria, Lysosomes, Nissl bodies and neurofilaments, which are involved in the metabolism, growth and repair of the neuron.
  • Dentrites are thread-like branched cytoplasmic extensions arising from the cell body. They are involved in the conduction of nerve impulses towards the cell body.
  • Axon is a long unbranched cytoplasmic extension which conducts an impulse away from the cell body.

Based on the number of dentrites, the neurons are classified into 3 groups

  • Unipolar
  • Bipolar
  • Multipolar

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Neuroglial cells

  • The rest of the nervous system is composed of non- conducting cells called neuroglial cells.
  • These cells make up more than one-half of the volume of the total nervous system.
  • Their main function is to protect, nurture and support the neurons.
  • There are 4 types of neuroglial cells – Astrocytes, Oligodendrocytes, Microglial cells and Ependymal cells.
  • Astrocytes: It is the largest and most abundant of all glial cells. These cells have long star-like processes. Astrocytes form the blood-brain barrier.
  • Oligodendrocytes: They are small glial cells with several processes. They are found in the grey and white matter of CNS.
  • Microglial cells: These are the smallest of all glial cells. They are either cuboidal or columnar cells. Functionally, macroglial cells are macrophages. They engulf damaged nerve cells.
  • Ependymal cells: They are elongated cells found as a single layer in the lining of spinal cord and ventricle of the brain.