A reflex is a fast, unpremeditated, automatic response to the changes in the environment. It is a homeostatic mechanism. A reflex that involves the brain as the integrating centre is called a cranial reflex while that which involves the spinal cord is called the spinal reflex. The pathway followed by a nerve impulse resulting in a reflex action is called a reflex arc. It has the following components:
(i) Sensory receptor: which perceives the stimulus (a change in the environment).
(ii) Sensory neuron : which conveys the signal of this change to the CNS.
(iii) Integrating centre : a part of the CNS that receives and conveys information to other regions of the CNS and to the effector organ. When there is no interneuron between the sensory and motor neurons and they synapse directly such a reflex arc is known as a monosynaptic reflex arc. When there is a single or many interneurons between sensory and motor components it is known as a polysynaptic reflex arc.
(iv) Motor neuron : integrating centre sends impulses through the motor neuron to the effector organ.
(v) Effector: the ultimate organ responding to the initial stimulus (a change in the environment) is the effector organ. It can be a skeletal muscle where the reflex is known as a somatic reflex or it can be a gland, a smooth muscle or the cardiac muscle where it is known as an autonomic or visceral reflex.
Unconditioned reflex and conditioned reflex
• A simple or unconditioned reflex is inborn, unlearned response to a stimulus or any change in the environment.
• Breast feeding and swallowing in newly born babies and blinking of eyes are the examples of unconditioned reflexes.
• Conditioned reflexes are not inborn but are acquired on past experience, training or learning.
• It was first demonstrated by I.P. Pavlov. He presented a hungry dog with food, which elicited a reflex of salivation by the stimulation of the taste buds in the mouth. He then rang a bell each time the dog was given the food to associate the unconditioned response (salivation) with an additional stimulus (bell). After a few trials, he was able to demonstrate that the dog salivated at the sound of the bell without the sight or smell of food. This is thus known as conditioned reflex.
Characteristics of conditioned reflexes:
– They are acquired during lifetime.
– They depend on previous experience.
– They can be established or abolished.
– They are not transmitted by heredity.
– Cortical and subcortical centres are responsible for them. Importance of reflex action
• It controls a number of body activities.
• Response to harmful stimulus is quick as to reduce harm.
• The response is always accurate, useful/ purposeful.
• It avoids overtaxing of brain.