ANIMALS – NERVOUS SYSTEM
- The multicellular animals (except sponges) have specialized cells called nerve cells (or neurons) to respond to stimuli and coordinate their activities.
- A system made up of nerve cells is called the nervous system.
The nervous system of an animal is composed of
1) Specialized cells called neurons or nerve cells which can detect, receive and transmit different kinds of stimuli.
2) The nerve fibers which are certain bundles of extended processes of nerve cells.
- Nerve cells or neurons are the structural and functional units of the nervous system.
- Billions of nerve cells make up our brain.
- A nerve cell is a microscopic structure consisting of three major parts namely cell body, dendrites and axon.
- It is the cell structure irregular in shape or polyhedral structure, it is also called cyton.
- Cell body contains cytoplasm with typical cell organelles and certain granular bodies are called Nissl granules.
- Dendrites or Dendrons are shorter fibres which branch repeatedly and project out of the cell body.
- Dendrites transmit electrical impulses towards the cyton.
- One of the fibres arising from the cell body is very long with a branched distal end and it is called Axon.
- The distal branches terminate as bulb-like structures called synaptic knobs filled with chemicals called neurotransmitters.
- Axon contains axoplasm inside and is covered by a membrane called neurilemma.
- Neurilemma encloses the axon except at the branched distal ends.
- In some neurons called myelinated neurons an additional white fatty fibre called myelin sheath covers the neurilemma.
- Myelin sheath is not continuous over the neurilemma.
- The gaps left by the myelin sheath on the axon are called Nodes of Ranvier.
- Over the myelin sheath are found certain cells called Schwann cells.