Reflex action is a rapid, automatic action carried out without the intervention of the will of the animal. It is independent of the will of the animal.
Marshall Hall first observed such action in the year 1833.
During reflex action the impulse travels through a path known as reflex arc. A simple reflex arc (monosynaptic) involves a sensory or afferent neuron, an interneuron present within the spinal cord and a motor or efferent neuron. The afferent is connected to the receptors (such as skin) and the efferent is connected to the effectors (muscles or glands).
The stimulus detected by the receptors passes into the sensory or afferent neuron. These impulses enter the spinal cord through the dorsal root and initiate impulse in interneuron or association neuron. From the spinal cord the impulse is carried through the ventral root and travel along the afferent or motor nerve fibres to reach the effect or organ.
Some Important Reflex Actions:
(i) Narrowing of the pupil of eye on seeing bright light
(ii) Withdrawal of limbs when it touches hot object
(iii) Quick closing of eye lids when a flying object suddenly approaches the eye
(iv) Coughing, sneezing, yawning
(v) Shivering with cold
(vi) Opening of the mouth on hearing a sudden loud noise.
Monosynaptic Reflex Arc:
It is a simple reflex arc involving one sensory and one motor nerve fibre. It is generally not found in vertebrates.
Polysynaptic Reflex Arc:
Reflex arc involving more than one sensory and one motor neuron.
Types of Reflexes:
If the controlling nerve centre of reflex are is located in spinal cord, the action is called spinal reflex. If it is brain, the action is described as cerebral reflex. There are inborn reflexes which are due to unfamiliar or unconscious stimulus such as unknowingly touching hot object, sudden appearance of a flying object in front of the eye are called unconditioned reflexes.
There are other reflexes which are acquired by the animal by learning process or experience such as secretion of saliva on looking at delicious food, getting the smell of it. Such reflexes are called conditioned reflexes. Reflex actions have many advantages. It relieves the brain of such unnecessary works by taking care of minor daily activities of the body. They are automatic and protective in nature.