Definition of Reflex Action: A reflex action may be defined as a spontaneous, automatic and mechanical response to a stimulus acting on a specific receptor without the will of an animal.
Examples of Reflex Action: examples of reflex actions in man are knee-jerk reflex, movement of diaphragm during respiration, blinking of eyes, coughing, yawning, sneezing etc.
In knee-jerk reflex, a gentle strike below the knee cap, while sitting with freely hanging legs, kicks the leg forward.
It consists of five parts:
(i) A specific receptor, the neurons of which receive a stimulus and set up a sensory impulse.
(ii) An afferent nerve, which brings the sensory impulse from the receptor to the central nervous system.
(iii) A portion of the central nervous system, brain or spinal cord, the neurons of which analyse and interpret the sensory impulse and set up an appropriate motor impulse. Brain and spinal cord are called modulators.
(iv) An efferent nerve, which carries the motor impulse from the central nervous system to the specific effectors
(v) An effector, where impulse terminates and response is given as per instructions received from the modulators.