09 Control and Coordination - part 01 - Nervous System in Hydra

 

09 Control and Coordination - part 01 - Nervous System in Hydra


Introduction :
  • Unicellular organisms have a simple organisation 
  • Multicellular organisation of the body organs and organ systems required both, a control over their life processes as well as a coordination between the various systems in order to maintain homeostasis of the organism.
  • Plants and animals both show a control and coordination mechanism. 
  • In plants this done by sending chemical signals and brining about various types of movements (e.g. phototropic, chemotactic, thingmotactic, etc). 
  • Animals, specially the higher vertebrates show a gradual increase in the complexity of their control and coordination by giving both electrical and chemical singles. 
  • The porifera (sponges) are the most primitive of the animal phyla.
  • Even though there are different types of cells in sponges for carrying out different functions, a proper nervous system is lacking.
  • Multicellular animals show division of labour among the cells. 
  • This leads to specialization of cells for the various activities like digestion, respiration, excretion and others. 
  • Later in the higher animals, phyla the different cells form tissues, organs and systems which must work in coordination with each other for smooth internal functioning of the body. 
  • Also the organism must be able to respond and coordinate with respect to various stimuli or changes in the external environment.
  • In the lower animals like Hydra and Planaria the nervous system achieves this function, while in higher more complex animals, it is done by two coordinating systems - 
  1. the nervous (elctrical) system and 
  2. endocrine (chemical) control system. 
  • These two systems will be studied separately though they work in coordination with each other.
NERVOUS COORDINATION
Nervous System in Hydra :

  • Hydra, a cnidarian shows the diffused nervous system, which is the most primitive nervous system. 
  • First animal group showing true simple nervous system. 
  • Consists of - 
  1. Sensory cells and 
  2. Nerve cells or neurons along with their fibres.
1. Nerve cells :
  • Scattered or distributed throughout the body.
  • Inter connected to each other by synapses between their fibres to form the nerve net. 
  • There are two nerve nets both in the mesoglea, one connected towards the epidermis and second towards the gastrodermis.
  • Sensory cells scattered in the body wall and tentacles.
  • Sense organs are lacking. 
2. Neurons :
  • Have fibres but there are no sensory and motor nerves. 
  • The nerve impulse shows no polarity or direction. 
  • Activation of sensory cells can happen at any point, and from this point impulse can be carried through out the body in any direction, thus bringing movements of the body or tentacles
  • eg. catching of prey during feeding.
  • The diffused types of nervous system is the first important landmark in the nervous system.
  • It is seen in the ctenophora as well as in the enteric system or gut wall of higher animals including man.

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