The document discusses the stretch reflex and muscle tone. It defines the stretch reflex as an involuntary response to stretching a muscle, which stimulates sensory receptors and causes contraction of the muscle. The key components of the reflex arc are: receptor, afferent nerve, spinal cord or brain center, efferent nerve, and effector organ (muscle). Muscle tone refers to the partial contraction of muscles at rest, which is regulated by spinal and supraspinal mechanisms. The stretch reflex plays an important role in maintaining proper muscle tone and protecting muscles from overstretching.

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

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

4. 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

5. Spinal reflex
Decapitate a living frog

6. 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

7. 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withdrawn limb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

8. 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

9. 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

11. 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

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

16. Muscle tone
 Sustained partial contracted state of muscles is called muscle tone
(in living muscles)
 Tone may be normal
 Decreased tone (hypotonia)
 Increased tone (hypertonia)
 Estimation of tone is must in a person suffering with neurological
disorder

17. Muscle tone
Tone of the muscle is maintained by two mechanisms
Spinal mechanisms
Supraspinal mechanisms (control by brain via
descending tracts- pyramidal and extra pyramidal tracts)

18. Stretch reflex arc
 Stimulus – Stretch
 Receptor – Muscle spindle
 Afferent nerve – Ia fibers
 Center – Spinalcord
 Efferent nerve – Alpha motor neuron
 Effector – Skelatal muscles
 Response - contraction

19. Mode of action of stretch reflex
When the central region of spindle called nuclear bag
region is stretched
Afferent nerve Ia fibers are stimulated
Alpha motor neurons are stimulated
Muscle contracts

20. Muscle spindle
Every skeletal muscle contains variable number of muscle
spindles
Spindles are found within the muscle belly
The muscle fibers of main muscle are called extra fusal fibers
The muscles present in muscle spindle is called intra fusal
fibers

21. Muscle spindle
Intrafusal fibers are attached with the fibrous capsule of
spindle
The capsule inturn attached with extrafusal fibers
Spindle is thinner at its ends – polar regions
Broad central region – equatorial zone

23. Muscle spindle
Intrafusal fibers are two types
Nuclear bag fibers
Nuclear chain fibers

24. Nuclear bag fibers
 Extend from end to end of the spindle
 At the equatorial zone, large number of nuclei are present
 Zone of nuclear bag
 At this zone no contractile materials ( no actin and no myosin)
 When intrafusal fibers contracts, the nuclear bag will be stretched
 At nuclear bag zone, annulospiral ring – primary afferent (Ia) fibers

26. Nuclear bag fibers
Intrafusal fiber contracts
Stretching of the zone of nuclear bag
Stimulation of Ia fibers
Reflex stimulation of alpha motor neuron
Contraction of extrafusal fibers ( stretch reflex)
Shortening of the main muscle

27. Nuclear bag fibers
Extrafusal fibers are passively stretched
The stretch is transmitted to intrafusal fibers
Stimulation of Ia fibers
Reflex stimulation of alpha motor neuron
Contraction of extrafusal fibers ( stretch reflex)
Shortening of the main muscle

28. Nuclear bag fibers
In the anterior horn cell of spinal cord there are two types of
nerve cells
Alpha motor neuron
Gamma motor neuron
Alpha motor neurons terminate on extrafusal fibers
Gamma motor neurons terminate on intrafusal fibers

29. Nuclear bag fibers
Stimulation of alpha motor neuron results in???

30. Nuclear bag fibers
Stimulation of gamma motor neuron results in???

31. Nuclear bag fibers
Stimulation of gamma motor neurons
Contraction of intrafusal fibers
Stretch of nuclear bag zone
Ia fibers stimulation
Reflex activation of alpha motor neuron
Contraction of extrafusal fibers

32. Nuclear chain fibers
 Attached with nuclear bag fibers
 Do not contain nuclear bag
 Has chain of nuclei at their polar areas
 Nuclei appear as chains the name nuclear chain fibers
 In addition to primary ending, secondary ending ( flower spray
ending) also supplies here
 Gamma 2 fibers motor supply

33. Gamma motor neuron
Gamma1 neuron – dynamic fibers- supply nuclear bag
fibers
Gamma 2 neuron – static fibers – supply nuclear chain
fibers

34. Gamma motor neuron
Motor pathways stimulates both alpha and gamma motor
neurons
Co-activation
Skeletal muscle contraction is effective

35. Gamma motor neuron
Gamma motor neuron receives impulses from the
descending fibers from the brain which controls the
stretch reflex
Damage of these descending fibers leads to alteration of
tone – spasticity, rigidity…

36. Supraspinal mechanism
Descending motor fibers – pyramidal tract and
extrapyramidal tract terminate on anterior horn cell
Either directly or via interneurons
Influence alpha and gamma motor neuron activity
Modify muscle tone

37. Extra pyramidal system
EPS consists of
Basal ganglia
Motor nuclei of reticular formation of brain stem
Vestibular nucleus
Other descending fibers that convey to spinalcord

38. Extra pyramidal tracts
Reticulospinal tract
Vestibulospinal tract
Rubrospinal tract
Tectospinal tract

39. Reticulospinal tract
Arises from pontine nuclei and medullary nuclei
Tract remain uncrossed
Terminates on both alpha and gamma motor neurons of
anterior horn
Has both inhibitory and facilitatory fibers

40. Vestibulospinal tract
 Arises from lateral vestibular nucleus
 Descends through spinal cord
 Remains uncrossed
 Terminates on alpha motor neurons on alpha motor neurons on
anterior horn cells
 Facilitatory tract
 Increases muscle tone

41. Rubrospinal tract
Arises from red nucleus of mid brain
Decussates almost immediately
Terminates on anterior horn cell
Red nucleus receives information from cerebellum and
motor cortex

42. Role of pyramidal tract
pyramidal tract lesion
Claspknife spasticity
hypertonia

43. Basal ganglia
Especially substansia nigra
Gives inhibitory fibers
Supress the muscle tone
Part of extra pyramidal system

44. Cerebellum
Inhibits stretch reflex on the same side

45. Cerebral cortex
Has both facilitatory and inhibitory effects on stretch
reflex
Inhibitory effect dominates over facilitatory effect

47. Decerebrate rigidity
Sherrington did this experiment
1890
Transection is made between the superior and inferior
colliculus
After that the animal developed decerebrate rigidity

48. Decerebrate rigidity
Animal stands hyper extended
All four limbs become rigid like pillars
Both extensors and flexors of the limbs show hypertonia
Tail and neck hyper extended

50. Decerebrate rigidity
The result of transection
Only facilitatory impulses from the motor pathway survive
Excessive stimulation of gamma motor neurons
Excessive contraction of muscles

51. Decorticate rigidity
Decortication – removal of cortex
Produces decorticate rigidity
Flexion of upper extremities at elbow
Extensor hyper activity in lower extremeties
Rubrospinal excitation of flexor muscles in upper extremities

52. Decorticate rigidity
Seen on hemiplegic side in humans after hemorrhages or
thromboses in the internal capsule
Small arteries in the internal capsule are especially prone
to rupture or thrombotic obstruction
60% of intracerebral hemorrhages occurs in internal
capsule

54. Hypotonia
Accompanied by weakness
Atrophy
Decreased reflex response
The resistance to passive movement is less than normal

55. Hypotonia
Destruction of efferent fibers- poliomyelitis
Destruction of dorsal column – tabes dorsalis
During sleep
Thalamic and cerebellar lesions

56. Hypertonia
Stiffness and increased resistance to the passive
movement of limbs
Due to stimulation of facilitatory areas of the brain
Increased activity of gamma motor neuron
Increased activity of alpha motor neuron

57. Hypertonia
Hypertonia is two types
Spasticity
Rigidity

58. Spasticity
1) Pyramidal tract lesion
2) Involves only one group of
muscles, either agonist or
antagonists
3) Clasp knife rigidity
4) Hypertonia
Rigidity
1) Extra pyramidal tract lesion
2) Involves both agonists and
antagonists. Hypertonia is uniform
throughout the body
3) Lead pipe rigidity or cogwill rigidity
4) Hypertonia

59. THANK YOU