Neural System A neuron is a microscopic structure composed of three major parts namely cell body (soma), dendrites and axon. The cell body is the spherical part of the neuron that contains all the cellular organelles as a typical cell (except centriole) . The plasma membrane covering the neuron is called neurilemma and the axon is axolemma. The repeatedly branched XI Std Biology-Zoology Chapter- short fibres coming out of the cell body are called dendrites, which transmit impulses towards the cell body.
The cell body and the dendrites contain cytoplasm and granulated endoplasmic reticulum called Nissl’s granules. An axon is a long fibre that arises from a cone shaped area of the cell body called the Axon hillock and ends at the branched distal end. Axon hillock is the place where the nerve impulse is generated in the motor neurons. The axon of one-neuron branches and forms connections with many other neurons.
An axon contains the same organelles found in the dendrites and cell body but lacks Nissl’s granules and Golgi apparatus. Figure . Neuron Nucleus with nucleolus Soma Trigger zone Axon hillock Initial segment Direction of signal transmission Synaptic knobs Node of Ranvier Schwann cell Myelin sheath Nucleus Nissl bodies Dendrites The axon, particularly of peripheral nerves is surrounded by Schwann cells (a type of glial cell) to form myelin sheath, which act as an insulator. Myelin sheath is associated only with the axon; dendrites are always non-myelinated.
Schwann cells are not continuous along the axon; so there are gaps in the myelin sheath between adjacent Schwann cells. These gaps are called Nodes of Ranvier . Large myelinated nerve fibres conduct impulses rapidly, whereas non- myelinated fibres conduct impulses quite slowly (Figure . ).
The longest cells in the human body are the neurons . The longest axons in the human body , for example, are those of the sciatic nerve, which run from the base of the spine to the big toe of each foot. These single-cell fibers may extend a meter or even longer. The axons of the inter neurons in the CNS