Maharashtra Board Class 12 Biology Chapter 9 Control and Coordination PDF Download

Control And Coordination

Unicellular organisms have a simple organisation of their life processes. However a 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. In this chapter now you will study about development of nervous system in different animal groups and details of the system in humans.

The porifera (sponges) are the most primitive of the animal phyla.

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 - the nervous (elctrical) system and endocrine (chemical) control system. These two systems will be studied separately though they work in coordination with each other.

Nervous Coordination

9.1 Nervous System In Hydra

Hydra, a cnidarian shows the diffused nervous system, which is the most primitive nervous system. The cnidarians are thus the first animal group showing true simple nervous system. It consists of the sensory cells and the nerve cells or neurons along with their fibres. The nerve cells are scattered or distributed throughout the body and 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. There are sensory cells scattered in the body wall and tentacles, but sense organs are lacking. Neurons have fibres but there are no sensory and motor nerves. The nerve impulse shows no polarity or direction. Thus in Hydra, 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.

Teacher's Note

The nerve net in Hydra is very simple but works very well. In India, we can see hydra in small ponds. It catches small water animals to eat. This shows how a simple nervous system helps animals survive.

Exam Trick

Remember: Hydra has a "nerve net" - just like a fishing net. Messages travel in all directions, not in one way. This is different from our brain which has clear pathways.

Points To Remember

Hydra has the most primitive nervous system called diffused nervous system.
The nerve cells are scattered throughout the body.
Nerve impulses have no fixed direction.
There are no sense organs in Hydra.
The nerve net helps Hydra catch prey and move.

9.2 Nervous System In Planaria (Flatworm)

Planaria is a flatworm belonging to the phylum platyhelminthes. It is the most primitive animal with a central nervous system (CNS) located on the ventral side of body. It consists of a mass of cerebral or cephalic ganglion appearing like an inverted U shaped brain. These lie in the anterior or head region and from each ganglion arise nine branches towards the outer side. Ventrally from below the ganglia arise a pair of Ventral Nerve Cords (VNC) or long nerve cords. These are inter connected to eachother by transfer nerve or commissure in a ladder like manner. The peripheral nerve plexus arising laterally from VNC. The PNS include sensory cells arranged in lateral cords in the body. A pair of photosensory structure, the eyes are located on dorsal side of the brain. Also there are single sensory cells scattered in the body.

In the above examples of Hydra, Planaria and the earlier studied examples of cockroach and humans, we have seen the gradual evolution or changes of the neural system. There is a high level of specialization in the formation of neurons as electrically signalling cells and also in the entire system, gradually from a diffuse neural system to a centralized nervous system. The expansion into a properly organized system involving the brain, its gradual expansion in size and functions. This has lead to centralization of various sense organs assisting in coordinating the internal environment with that of the external environment. Also there evolution of a complex networking system which efficiently transmits signals between one part or organ of the body and another.

Teacher's Note

Planaria is a flatworm we find in freshwater. It has a small brain and two nerve cords. It can move towards food and away from light. This simple brain helps it survive better than Hydra.

Exam Trick

Remember: Planaria has a "ladder-like nervous system" - like a ladder with two long side rails (nerve cords) and many cross bars. This allows messages to travel faster than in Hydra.

Points To Remember

Planaria has the first true central nervous system (CNS).
The brain is located at the head end.
Two ventral nerve cords run along the body.
It has a ladder-like structure with cross connections.
Planaria has simple eyes and can sense light.

9.3 Neural Tissue

The neural tissue consists primarily of two types of cells viz the neurons and the neuroglia or glial cells. While a nerve is bundle of axons, the word 'nerve' is used for a bundle of axons outside the CNS while inside the CNS for the same, word 'tract' is used. The nerves may be sensory or motor or mix type i.e. having both the types - sensory and motor fibres.

All these along with nervous organs make up the nervous system of the higher animal and bring about coordination and control of various activities of the body. This is done through the receptors which bring in sensory inputs towards the central nervous system. Processing is carried out in the CNS and then through the motor commands, the response is sent out. The nerves arising from the cytons of the CNS, travel throughout the body transmitting the nerves impulses to or from the CNS.

Teacher's Note

Neural tissue is made of neurons and supporting cells. Think of neurons as telephone wires and supporting cells as insulators that protect them. Just like a phone needs wires to work, our body needs neural tissue to feel and move.

Exam Trick

Remember: A "nerve" is outside the brain and spinal cord. A "tract" is inside the brain and spinal cord. This simple rule helps you answer many questions correctly.

Points To Remember

Neural tissue has two main types of cells - neurons and glial cells.
Neurons carry messages as electrical signals.
Glial cells support and protect neurons.
A nerve is a bundle of axons outside the CNS.
A tract is a bundle of axons inside the CNS.

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