The nervous system receives a stimulus, through a receptor organ, integrates or coordinates it, and effects a response through the effector organ. Thus, a coordinated behaviour has six main components stimulus, impulse, receptor, coordinator, effector, and response.
In such a coordinated behaviour, any stimulus of sound, sight, smell, etc. is perceived by receptor organs like eyes, ears, skin, etc.
A stimulus is an agent or sudden (external or internal) change that results in a change in activities of an organism.
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An impulse is a wave of chemical disturbance that travels through the nerve cell.
Receptors are sensory organs which receive stimulus and send wave in form of impulse towards CNS (coordinator).
Effectors are the muscles or organs which show response due to motor nerves. Response is a change that occurs in an organism due to stimulus.
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The brain and spinal cord are the coordinators which receive information in the form of messages called nerve impulses, from receptor organs via neurons. The information flows to the effector organs, i.e. muscles, which contract or relax or secrete substances to show response.
Transmission of the Nerve Impulses:
Nerve impulses pass along a neuron in one direction only. At one end, the neuron is connected to a sensory receptor that receives the stimulus and converts it into electrochemical waves which are carried by the neuron. The fibre at this stage is said to be excited.
The events that take place during the conduction of an impulse along a nerve are given below.
At Resting State Polarised State:
At normal (resting) state, the outer side of nerve fibres carry more positive (+) charge due to more Na+ ions outside the axon membrane. This is called polarised state.
At Stimulated (Excited) State Depolarization:
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On receipt of a stimulus, the axon membrane at the place of stimulus become more permeable to Na+ ions and as a result, the Na+ move inside causing loss of polarity, i.e. depolarisation. This region, thus, becomes excited region. This region of depolarisation moves forward to next area which in turn becomes depolarised.
Returning to Normal State Repolarisation:
The previous area (Which has received stimulus) becomes repolarized due to active transport of Na+ ions outside. This transport is achieved by sodium pump’ for which energy in the form of ATP is required. Thus, conduction of nerve impulse is a wave of depolarisation followed by repolarisation.