(1) A receptor—it is a sensory structure which receives the stimuli.
(2) A afferent or sensory neuron—it passes into the spinal cord by way of a dorsal root of the spinal cord. Its primary function is to convey the impulses received from the receptor to the central nervous system (spinal cord).
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(3) Interneuron—it is present in the central nervous system (spinal cord). It is generally one but sometimes two interneurons may be present.
It simply serves to transmit the impulses from the afferent or sensory neuron to the motor neuron.
(4) A motor neuron—it is also known as efferent neuron a is situated in the ventral root of spinal cord.
It transmits the impuli to the effector organs which may include muscles or glands.
(5) An effector organ—it may be a muscle or gland which responds to impulse received.
The reflex arc constitutes the functional unit of the central nervous system.
Reflexes are of common occurrence in animals. In higher animals a large number of reflex actions are performed in daily life.
They are performed unconsciously; therefore, they are of simple nature and termed simple reflexes. Now we shall discuss few examples of reflex action.
Suppose that you have placed your hand in a bowl of cold water. Receptors in the skin are stimulated by the coldness of the water, nerve impulses pass via the sensory nerve fibres through the dorsal root of the spinal nerve to the spinal cord which is a part of central nervous system where the water temperature is appreciated.
Impulses from the brain are carried down to the arm muscles through motor neurons. The arm muscles act accordingly. They contract and withdraw the hand from the water.
Similarly, if you place your hand in boiling water without knowing that it is hot, immediately withdraw it rapidly and then realize how hot it was.
To protect your hand from damage a mechanism has acted in an involuntary manner by making use of a “short circuit”.
The path of the nerve impulses is as follows:
Sensory fibres convey impulses from the receptors into the spinal cord as before, but they pass not only to the brain but also by appropriate connector or interneurons to motor neurons which carry them to arm muscles which contract to remove the hand from the water.