- Animals show two types of actions voluntary and involuntary.
- The involuntary actions are known as the reflex actions.
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.
MECHANISM OF REFLEX ACTION:
- A reflex action is brought about in the following way.
- When acid is applied to a toe of a decapitated frog.
- The stimulus is received by a receptor in the skin, receptor is a general term for any type of sense organ.
- On receiving a stimulus, the receptor sets up a sensory impulse.
- The latter is carried to the spinal cord through the dorsal sensory root of a spinal nerve, i.e. sciatic nerve, in the above example.
- The spinal cord transforms the sensory impulse into a motor impulse.
- The latter is transmitted to the leg muscles.
- The muscles then contract and the leg is withdrawn to avoid the stimulus.
- The muscles are referred to as the effectors, where the impulse ends and response is given.
- The path travelled by an impulse in a reflex action is called the reflex arc.
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.