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Neural Control and Coordination
All living organisms including plants and animals respond and react to environmental factors or stimuli. There is a necessity to develop some system for control and coordination of various body organs.
To carry out a simple function such as picking up an object from the ground, there has to be coordination of the eyes, hands, legs and the vertebral column. The eyes have to focus on the object, the hands have to pick it up and grasp it, the legs have to bend and so does the back bone (vertebral column). All these actions have to be coordinated in such a manner that they follow a particular sequence and the action is completed. A similar mechanism is also needed for internal functions of the body.
The individuals also have to adjust to the changing conditions around them and vary their responses. At the same time, the internal conditions of the body should be maintained constant.
There are two modes of control and coordination, chemical and nervous. Plants do not have a nervous system. They possess only chemical control and coordination. Animals have both chemical and nervous control and coordination. The two constitute neuroendocrine system.
Stimulus: An agent or a change in external or internal environment that induces reaction in the body.
Response: A change in activity in the body of an organism due to stimulus.
Coordination in Animals
Animals have the ability of locomotion. This ability is probably developed as they have to search for food. Since they move from one place to another, the animals have to continuously encounter changes in their environment. In order to maintain a steady state within the body, all animals should be able to perceive these changes and adapt to them.
With increasing complexity in their structure, the number and types of cells in the animal body increased. Thus, it became necessary to have some coordination mechanism. Two systems have been developed for better control and coordination of the various activities of the organisms. These systems are the Nervous System and the Endocrine System.
1.0Nervous System
The nervous system in vertebrates is highly evolved. It is the control system for all our actions, thinking and behaviour.
It is concerned with receiving stimuli from the external or internal environment of the body, interpreting these stimuli and producing the appropriate response to these stimuli. To achieve this, highly specialized cells are required which can receive the message and conduct them to the chief centres of the nervous system where they are to be interpreted and then returned to the relevant part of the body. Neurons or nerve cells are the structural and functional unit of the nervous system. Each neuron has following two parts:
(i) Cyton or cell body - Contains a central nucleus and cytoplasm with characteristic deeply stained particles called Nissl's granules [i.e. clumps of ribosomes].
(ii) Cell processes - These are of two types
(a) Dendrites: These may be one to many, generally short and branched cytoplasmic processes. Dendrites are afferent processes because they receive impulse from receptor or other neuron and bring it to cyton.
(b) Axon: It is a single generally long efferent process which conducts impulse away from cyton to other neuron.
The longest cell in the body is neuron because axons can be more than one metre long. Axon has uniform thickness, but it has terminal thin branches called telodendria. Terminal end buttons or synaptic knobs occur at the end of telodendria.
Unlike body cells which reproduce by cell division, neurons are incapable of cell division because they lack centrioles.
Electrochemical Mechanism of Transfer of Nerve Impulse
Any two neurons in the nervous system do not join to one another completely, there is always a very small gap between the two neurons. This gap is called synapse. The nerve impulse is carried over this small gap between a pair of neurons by means of a chemical substance called neurotransmitter.
Neurotransmitter is a chemical secreted by axon terminal for transmission of impulse to the next neuron, gland or organ.
When a stimulus acts on the receptor, a chemical reaction is set off which produces an electrical impulse in it. This impulse travels from the dendrite of sensory neurons to its cell body and then along its axon.
Impulse is a self-propagated electrical current that travels from one end to another of a neuron for the passage of a message.
At the end of the axon of sensory neuron, the electrical impulse releases tiny amount of a chemical substance in the synapse. This chemical substance crosses the gap and starts a similar electrical impulse on the dendrite of the next neuron. This process continues till the electrical impulse reaches the relay neurons in the brain and spinal cord. These relay neurons connect in a similar way from the brain and spinal cord to the effector muscles and glands via motor neuron.
2.0Neuromuscular Junction
A neuromuscular junction is a place in the body where the axons of motor nerves meet the muscle, thus transmitting message from the brain or spinal cord which causes the muscle to contract or relax.