Reflex action

                        Reflex action

➢ It is spontaneous, automatic and involuntary response of the body to a
stimulus acting on a receptor.
➢ Reflex actions were discovered by Marshall Hall.
➢ In reflex actions, the conscious will of the animal is not involved.

Reflex arc

➢ It is the path followed by the stimulus generated at receptors to reach the 

effector organ. It has following 5 components.

1. Sensory Receptor

2. Sensory or Afferent Neuron

3. Part of CNS involved

4. Motor or Efferent Neuron

5. Effector organ(muscle/gland)

Types of reflexes

1.unconsitional reflexes

➢ These are present right from birth.

➢ These are same in all the individuals.

➢ These have survival value.

Examples:

➢ Withdrawal of hand or foot on being picked or when touched by hot object.

➢ Wide opening of mouth on hearing a loud sound.

➢ Knee jerk reflex (stretch reflex) to maintain erect posture.

➢ Blinking of eyes when approached by some object.

➢ Constriction and dilation of pupil in response to light.

➢ Salivation in response to taste of food.

2.Acquired reaction

➢ Not present right from birth.

➢ These are acquired by an individual through regular learning, experience or 

association with a stimulus during lifetime. 

➢ These reduce burden on brain.

➢ Such reflex actions are likely to disappear or even reappear later in life.

Examples:

➢ Salivation in response to smell, thought, sight, feel of food or even sound of 

food being cooked.

➢ Knitting without looking at needles.

➢ Cycling, driving a vehicle.

➢ Playing musical instrument, typing.

➢ Reading, writing.

➢ Depending upon the path of the CNS involved, reflex actions can be :

1.cerebral reflexes

➢ Superior colliculi of midbrain regulate reflexes from iris or eyelids.

➢ Inferior colliculi of midbrain regulate reflexes from tympanic cavity of middle 

ear.

➢ On the basis of number of synapses involved, the reflex actions can be :

1.monosynaptic reflexes

Stretch reflex is monosynaptic. In 

these, one synapse and one motor 

neuron is involved.

2.polysynaptic reflexes

Majority of reflexes are 

polysynaptic reflexes. In them, 

more than one sensory, motor and 

association neurons are involved.