1. Reflex Arc
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
NRIIMS
Email: dr.goothy@gmail.com

2. Definition
Involuntary response to a threshold stimulus obtained by
stimulating a sensory receptor

3. Definition
There must be a stimulus
There must be a motor effect – may be a contraction of
muscle
There must be integration – conversion of sensory
information to motor impulse

4. Spinal reflex
Decapitate a living frog

5. Spinal reflex
 Place the headless body of the frog on a plate
 With its belly facing up
 Touch a lower limb with a piece of cotton soaked in dilute acid
 Affected limb withdraws as if the limb is protecting itself from injury
 Centre of the reflex is spinal cord
 Spinal reflex

6. Spinal reflex
 In this experiment, it can not be argued that animal felt pain so the
withdrawn limb
 As there is no head (no brain)
 Acid stimulated the sensory nerves
 Stimulus reached the spinal cord
 Motor impulses from the spinal cord
 Contraction of muscles

7. The Reflex Arc
 For any reflex action to be brought about, the basic reflex arc should be intact
 Components of the reflex arc
1. Receptor
2. Afferent limb
3. Center
4. Efferent limb
5. Effector organ

8. The Reflex Arc
Center can be either brain or spinal cord
Damage to any part of the basic reflex arc results in loss
of reflex activity in that part of body

10. The Reflex Arc
Center can be either brain or spinal cord
Damage to any part of the basic reflex arc results in loss
of reflex activity in that part of body

13. Significance
Protective reflex that prevents tearing of the muscles to
excess stretching
Plays important role in regulating tension during normal
muscle activity

16. Plantar reflex
It is a superficial reflex
Tactile receptors participate in this reflex
Elicited by stroking the sole of the foot with a sharp edge of
the knee hammer or key along the lateral border
Starting from hell towards the base of the little toe and from
there towards the base of the big toe

17. Plantar reflex
Normal response is adduction and plantar flexion of all
toes including great toe
Centre is L5 and S1
This response is necessary to provide grip during
walking

18. Babinski sign
 Abnormal plantar reflex
 Dorsiflexion of big toe and fanning of other toes
 Normal in infants below two years and in deep sleep
 In infants myelination of the pyramidal tract is not completed below 2
years
 In deep sleep influence of higher centers on the reflex response is lost
 Babinski sign is present in UMN lesion

19. Classification of reflexes
Clinical classification
Number of synapses
Physiological classification

20. Clinical classification
Superficial reflexes
Deep reflexes
Visceral reflexes
Pathological reflexes

21. Superficial reflexes
These reflexes can be obtained by stimulating the
mucous membrane or skin that is from surface of the
body
Abdominal reflex, plantar reflex
They are polysynaptic reflexes

22. Deep reflexes
These reflexes can be obtained by stimulating a receptor
situated deep in the muscle (muscle spindle)
Knee jerk
These are also called tendon reflexes
These are monosynaptic reflexes

23. Visceral reflexes
Arise from visceral organs
Baroreceptor reflex
Chemoreceptor reflex
Micturition reflex
Oculocardiac reflex

24. Visceral reflexes
Arise from visceral organs
Baroreceptor reflex
Chemoreceptor reflex
Micturition reflex
Oculocardiac reflex

25. Pathological reflexes
Present only as an abnormality
Babinski reflex – UMN lesion
Mass reflex – Spinal cord lesions
Pendular reflexes – cerebellar lesion

26. Classification of reflexes
Clinical classification
Number of synapses
Physiological classification

27. Number of synapses
Asynaptic reflex
Do not have a single synapse
Axon reflex

28. Number of synapses
Mono synaptic reflex
One synapse
Knee jerk

29. Number of synapses
Poly synaptic reflex
many synapses
Crossed extensor reflex

30. Classification of reflexes
Clinical classification
Number of synapses
Physiological classification

31. Classification of reflexes
Flexor reflexes – produced when noci captive stimulus is
applied… withdrawal reflex
Extensor reflexes – stretch reflex

32. Classification of reflexes
Reflexes are also classified based on whether they are
conditioned or unconditioned
Unconditioned reflexes are in born ( presence since birth). Do
not depend on previous experience
Conditional reflexes develops upon previous experience

33. Properties of reflexes
Between the application of stimulus and starting of response,
there is a time interval – delay
Delay occurring in the synapse
Less delay in monosynaptic reflex
More delay in polysynaptic reflex

34. Summation
Spatial summation
Temporal summation

35. Occlusion

36. Subliminal fringe

37. Irradiation
 Low-intensity stimulus causes reflex contraction of only a few
muscles
 Strong stimulus causes reflex contraction of a large number of
muscles
 If there is a mild prick to the finger – withdraw of only that finger
 Strong prick – entire hand is withdrawn

38. Habituation and sensitization
Non injurious stimulus applied to snail
Snail shows withdrawal reflex
If this stimulus repeatedly continued
Intensity of response decline and finally ceases
Habituation

39. Habituation and sensitization
Injurious stimulus is applied to snail
Withdrawal response
Repetition of stimulus
Intensifies the response
Sensitization

40. THANK YOU