Postural reflexes Physiology

Regulation of posture ,postural reflexes,spinal animal,decerebrate rigidity,decorticate rigidity,MBBS lecture

1. POSTURE & EQUILIBRIUM Dr Raghuveer Choudhary

2. Objectives At the end of this lecture the student should : 1. be able to define human posture . 2. explain/define the concepts of “ center of gravity ’’ and “ support base, base of support “. 3. Explain what are postural reflexes and their overall function . 4. Know the centers of integration of postural reflexes . Describe decorticate rigidity and decerbrate rigidity and explain the mechanisms underlying them .

3. POSTURE & ITS REGULATION: POSTURE: A state of equilibrium in the space or position adopted by an individual in the environment.

4. POSTURE Posture : is defined as subconscious adjustment of tone in different muscles so as to maintain balance during displacement of the body caused by gravity or acceleration. Balance is the ability to maintain the body’s center of mass over its base of support. The erect posture is a prerequisite to most of the somatic motor activities of man and other higher animals. Posture control is required not only for holding the body in erect position but also for fixation of the body parts over adjoining body segments.

5. MECHANISMS OF POSTURE Two mechanisms involved in maintenance of posture are: 1. Muscle tone 2. Postural reflexes.

6. Role of muscle tone Largely , the posture is maintained through reflex adjustments of tone in the antigravity muscles. In human beings, flexors of upper extremity & extensors of lower extremity are the main antigravity muscles. The basic postural reflex involved in the control of muscle tone is stretch reflex.

7. The centre of gravity of head passes in front of the centre of gravity of atlanto-occipital joint . Thus head has got always a tendency to roll forwards. To hold the head in erect position cervico-occipital muscles are to maintained in a state of constant tension. In the upright position, gravity tends to displace the body downwards, stretching quadriceps muscles as the legs flex at the knee. Muscle stretch in turn evokes discharge from the muscles spindles of the muscle leading to its reflex contraction.

8. This ensures that the knee joints the main weight bearing joints do not give way under the effect of gravity . This maintains the leg as a pillar of support & thus counteracts the gravitational displacement of the body. Various postural reflexes influence the medial motor systems & the motor neurons of antigravity muscles. The inputs to this system through the postural reflexes significantly contribute to the maintainence of tone.

9. Thus , tone is the result of activity of various medial system pathways that descend to excite both alpha & gamma motor neurons that innervate antigravity muscles & their spindles. The two pathways of medial system that maintain muscle tone are Lateral vestibulospinal tract Pontine reticulospinal tract.

10. Maintenance of muscle tone Stretch reflex, plays main role in maintenance of muscle. It is a spinal reflex with supraspinal control. The centre for muscle tone lies in anterior motor neurons ( α - motor neurons) of spinal cord, which are stimulated through a constant γ - motor neuron discharge. Activity of both the neurons is modified by both pyramidal & extrapyramidal fibres which terminate on them.

11. Supraspinal control on muscle tone Bulboreticular facilitatory area – located in pons , has intrinsic activity of its own & discharges facilitatory impulses to gamma motor neurons . Bulboreticular inhibitory area - located in lower part of medulla & has no intrinsic activity of its own. Becomes active only if it recieves impulses from cerebellum or cerebral cortex & inhibits gamma motor discharge.

12. Areas in the cat brain where stimulation produces facilitation (+) or inhibition (-) of stretch reflexes . 1. motor cortex; 2. Basal ganglia; 3. Cerebellum; 4. Reticular inhibitory area; 5. Reticular facilitated area; 6. Vestibular nuclei.

13. Modulation of Muscle Tone Pontine & medullary reticular formation give rise to pontine reticular tract which is facilitatory (increases muscle tone) Descending medullary reticulospinal tract is inhibitory ( dereases muscle tone) These effects are exerted via Gamma efferents Vestibulospinal tract arising from Vestibular nuclei is excitatory. It directly stimulate Alpha motor neurons Increased tone produced by it is called Alpha rigidity

14. Motor cotex Basal ganglia cerebellum Inhibitory reticular formation Facilitatory reticular formation Vestibular nucleus

15. POSTURAL REFLEXES They help to maintain the body in upright & balanced position Provide adjustments necessary to maintain a stable posture during voluntary activity

16. REFLEX ARC OF POSTURAL REFLEXES Afferent pathways – eyes , vestibular apparatus, proprioceptors. Integrating center – neuronal networks in brainstem & Sp. Cord Efferent pathways – alpha motor neurons supplying various skeletal muscles.

17. TYPES OF POSTURAL REFLEXES Static reflexes: These are elicited by gravitational pull . Maintain - Sustained contraction of muscles. Statokinetic reflexes (phasic reflexes): Elicited by acceleratory displacement of the body . Maintain - Stable postural background for voluntary activity.

18. TYPES OF POSTURAL REFLEXES Static reflexes : these are of 3 types Local static reflexes Segmental static reflexes General static reflexes

19. TYPES OF POSTURAL REFLEXES LOCAL STATIC REFLEXES - Exert their effect on the same limb from which the stimulus was initiated. Centre for these reflexes is located in the Sp. Cord. Reflex control of antigravity muscle tone. [Stretch Reflex] Positive supporting reaction. Negative supporting reaction.

20. REFLEX STIMULUS RESPONSE RECEPTORS INTEGRATING CENTRE IN CNS A.Static reflexes 1. Local static reflexes Stretch reflex     2. Positive Supporting reflex   3. Negative supporting reaction Stretch    Contact of skin of the sole of foot with ground Stretch of extensor muscles       Contraction of antigravity muscles   Contraction of flexors and extensors of the limb. Disappearance of positive supporting reaction   Muscles spindles     Touch and pressure receptors from skin of sole of foot. Proprioceptors in extensors Spinal cord and mid brain Spinal cord     Spinal cord   VARIOUS POSTURAL REFLEXES

21. TYPES OF POSTURAL REFLEXES 1. Reflex control of antigravity muscle tone: Stretch reflex operating in extensor muscles of lower legs control their tone & keep the body upright (by contracting antigravity muscles).

22. TYPES OF POSTURAL REFLEXES 2. Positive supporting reaction: Simultaneous contraction of both extensors & flexors of a limb converting it into a solid rigid pillar. Plays imp role in steadying the ankle joint in standing position. Afferent impulses are from touch- pressure receptors (from sole) & proprioceptors (muscles).

23. TYPES OF POSTURAL REFLEXES

24. TYPES OF POSTURAL REFLEXES 3. Negative supporting reaction: It refers to disappearance of positive supporting reaction. Initiated by stretch of extensor muscles . Helps the limbs to be used for activities other than supporting the body wt.

25. TYPES OF POSTURAL REFLEXES SEGMENTAL STATIC REFLXES - The center is located in the spinal cord. Characterized by bilateral reflex response when stimulus is applied to one limb. Example : crossed extensor reflex response component of withdrawal reflex. Role of crossed extensor reflex in control of posture: in the lower limb , it allows one limb to support the body while other is raised off the ground. Plays a imp role during walking.

26. REFLEX STIMULUS RESPONSE RECEPTORS INTEGRATING CENTRE IN CNS 2 .Segmental static reflexes Crossed extensor reflex       3.General static reflexes Attitudinal reflexes Tonic labyrinthine reflex Tonic neck reflex   Painful stimulus           Gravity (alternation of position of head relative to horizontal plan)    Stretch of neck muscles due to alternation of position of head relative to body. Contraction of flexors of the ipsilateral limb and extensors of contralateral limb to support the body. Extensor rigidity        Flexion of forelimbs and extension of hind limbs on ventroflexion of head .extension of fore limbs and flexion of hindlimbs.flexion of ipsilateral limbs and extension of contralateral limbs on turning the head side- ways. Nociceptors               Otolith organs             Pacinian corpuscles in the ligaments of cervical joint and muscles spindles of neck muscles. Spinal cord               Vestibular and reticular nuclei present in the medulla oblongata.       Medulla

27. TYPES OF POSTURAL REFLEXES GENERAL STATIC REFLXES - characterized by generalized reflex response in many muscle groups in the body in response to stimulus that arises at one side of body. Attitudinal or statotonic reflexes Long loop stretch reflexes Righting reflexes.

28. TYPES OF POSTURAL REFLEXES Attitudinal Reflexes - Initiated when attitude of the body is changed i.e. while standing on an inclined plane. Tonic labyrinthine reflex Tonic neck reflex.

29. TYPES OF POSTURAL REFLEXES Tonic Labyrinthine Reflex - Produced in response to alteration in position of head relative to horizontal plane e.g. while standing on an inclined plane . Stimulus : Gravity Receptors : otolith organs present in labyrinthine apparatus. Afferents : impulses generated in the receptors travel along vestibular nerves. Center : vestibular & reticular nuclei present in medulla oblongata. Efferents : vestibulospinal & reticulospinal tracts.

30. TYPES OF POSTURAL REFLEXES Reflex response : particularly effective in extensors muscles. Depending upon the position of head in relation to horizontal plane the reflex response produced is: Dorsiflexion of Head → Flexion of fore limbs & Extension of hind limbs Ventroflexion of Head → Extension of fore limbs & Flexion of hind limbs

31. Tonic Labyrinthine Reflex

32. TYPES OF POSTURAL REFLEXES Tonic Neck Reflex - Are produced in response to alteration in position of head relative to body . Depending upon the position of head in relation to horizontal plane the reflex response produced is: Stimulus: stretch of neck muscles. Receptors: pacinian corpuscles in the ligaments of cervical joints particularly atlanto-occipital joint & muscle spindle of neck muscles. Center: medulla oblongata. Efferent: corticospinal tracts.

33. TYPES OF POSTURAL REFLEXESRESPONSE Reflex response: depends on position of head in relation to body. Dorsiflexion of head → extension of forelimbs & flexion of hind limbs. Ventroflexion of head → flexion of forelimbs & extension of hind limbs. Turning of head sideways i.e. towards right or left produces flexion of ipsilateral limbs & extension of contra lateral limbs.

34. Tonic Neck Reflex

35. Tonic Neck Reflex

36. Tonic Neck Reflex

37. Medulla Reflexes Tonic neck reflex (primitive reflex found in newborn) head up causes hind limbs to flex fore limbs to extend head down fore limbs flex hind limbs extend head turn to one side limbs on the side where head is turned to extend/straighten opposite side is bent/flexed

38. REFLEX RESPONSE Combined response of Tonic Neck / Labyrinthine reflexes – Dorsiflexion of head → Actual result is extension of forelimbs & little change in hind limbs. Ventroflexion of head → Actual result is flexion of forelimbs & little change in hind limbs

39. IMPORTANCE Both the reflexes bring about a redistribution of muscle tone in all the limbs & ensure that the body is not thrown off balance even when standing on an inclined plane. In man , tonic labyrinthine reflex is active during erect posture, because in erect posture the vestibular apparatus is thrown 30º backwards. This results in slight flexion of UL & extension of LL . When the head is tilted 30º forwards , tonic labyrinthine reflex ceases & flexion of neck triggers the tonic neck reflex .

40. TYPES OF POSTURAL REFLEXES LONG LOOP STRETCH REFLEXES [Righting Reflexes] Polysynaptic reflexes . Centre – cerebral cortex. They are continuously active during erect posture & continuously correct the sways that occur from moment to moment during standing.

41. RIGHTING REFLEXES Righting reflexes help to correct the position of the body when it goes off balance & falls down. They help to maintain the head & body in erect position under all circumstances.

42. RIGHTING REFLEX Head righting reflex Body righting reflex or body on head righting reflex. Neck righting reflex or neck on body righting reflex. Body on body righting reflex. Optical righting reflex.

43. 1. HEAD RIGHTING REFLEX Labyrinthine righting reflex . Initiated when animal’s head is in lateral position . Impulses arising from the saccules , reflexly stimulate the appropriate muscles to bring the head back to upright position. How to test the existence of this reflex - Thalamic Animal

44. midbrain level labyrinthine righting reflex Stimulus: gravitational pull Response: attempt to maintain head level receptors: vestibular organs neck righting reflex Stimulus: stretch of neck muscles Response: righting of thorax and shoulders receptors: muscle spindles eg . A newborn's reflex to turn his trunk and shoulders to the same side his head is turned

45. 1. HEAD RIGHTING REFLEX

46. 2. BODY RIGHTING [BODY-ON-HEAD] REFLEX Body-on-head righting reflex When an animal lies on ground, the side in contact with the ground is constantly stimulated while the other side is not . This differential stimulation of deep structures in body wall reflexly rights the head. How to test the existence of this reflex - Thalamic Animal with bilateral labyrinthectomy

47. 2. BODY RIGHTING REFLEX

48. 3. NECK RIGHTING [NECK-ON-BODY] REFLEX Also called Neck–on-Body righting reflex. As a result of labyrinthine righting reflex & body on head righting reflex , the head is righted but the body remains in lateral position . This leads to twisting of neck , which leads to neck righting reflex. Brings thorax & lumbar region successively into upright position . How to test the existence of this reflex - Thalamic Animal with bilateral labyrinthectomy.

49. 4. BODY-ON-BODY RIGHTING REFLEX When body is lying on the ground & Righting of head is prevented , differential stimulation of the body surface provides necessary cues for righting the body directly . Body directly rights [not secondary to twisting of the neck]. How to test the existence of this reflex - Thalamic Animal with bilateral labyrinthectomy & put the neck in plaster to avoid neck reflex.

50. 5. OPTICAL RIGHTING REFLEX Optical impulses cause righting of head . In animals with visual cortex intact, righting of head can take place with eyes open even after denervation of the labyrinths & neck muscles . In humans optical righting reflexes are far more imp than the labyrinthine righting reflex. How to test the existence of this reflex - Normal animal with bilateral labyrinthectomy

51. CENTERS OF RIGHTING REFLEX Center for all righting reflexes except optical righting reflex lies in Red Nucleus in Mid Brain. It controls them by following tracts: Rubro spinal tract Rubro reticular tract Center for optical righting reflex- visual cortex

52. REFLEX STIMULUS RESPONSE RECEPTORS INTEGRATING CENTRE IN CNS 2.Long loop stretch reflex       3.Righting reflexes Labyrinthine righting reflex Body righting reflex Neck righting reflex Body on body righting reflex Limbs righting reflex optical righting reflex Stretch of the muscle due to swaying of body         Gravity     Pressure on side of body   Stretch of neck muscles   Pressure on side of the body     Stretch of limb muscles   Visual cues Continuous moment to moment corrections of sways which occurs during standing.    Brings the head in upright level   Righting of head.     Righting of thorax and shoulders and then pelvis Righting of body even when righting of head is prevented.  Appropriate posture of limbs   Righting of head Muscle spindles(monosynaptic reflex) visual receptor(long loon reflex)   Otolith organs in saccules of labyrinth.exteroceptors Exteroceptors    Muscle spindles     Exteroceptors       Muscle spindles     Eyes Spinal cord   Cerebral cortex         Mid brain       Mid brain     Mid brain     Mid brain       Mid brain     Cerebral cortex  

53. Reflexes Integrated at the level of Cerebral Cortex ( Cortical Reflexes ) Examples : (1) Optical righting Reflex, (2) Placing Reaction , (3) Hopping Reaction

54. STATO-KINETIC REFLEXES Elicited by angular (rotatory), linear acceleratory (progressive) stimuli to labyrinthine receptors of vestibular apparatus. These are programmed reflexes that depend on motor cortex . They maintain equilibrium of body during movements. Mediated by lateral vestibulospinal tracts. Vestibular placing reaction Visual placing reaction Hopping reaction

55. REFLEX STIMULUS RESPONSE RECEPTORS INTEGRATING CENTRE IN CNS B.Statokinetic reflexes Vestibular placing reaction   Visual placing reaction   Hopping reactions Linear acceleration      Visual cues       Lateral displacement while standing Foot placed on supporting surface in position to support body. Foot places on supporting surface.    Hops, maintains the limb in position to support the body Receptors in utricle and saccule .   Eyes       Muscle spindle Cerebral cortex       Cerebral cortex       Cerebral cortex

56. 1. VESTIBULAR PLACING REACTION Evoked by linear acceleration . Receptors in utricle & saccule. Reflex response that prepares the animal for appropriate support by the limbs on surface contact . As soon as the foot comes in contact with firm surface , the leg muscles are adjusted so as to support the body.

57. VESTIBULAR PLACING REACTION

58. VISUAL PLACING REACTION Initiated by visual cues Many postural reflexes mediated by vestibular system can be stimulated by visual stimuli. Receptors : eyes Centre : cerebral cortex

59. HOPPING REACTION Occur in form of hopping movements that keep the limbs in position to support the body when a standing animal is pushed laterally. Stimulus : lateral displacement while standing. Receptors : muscle spindles Centre : cerebral cortex

60. Optical righting reflex : Visual stimuli that enable an animal to maintain the correct position of the head in space, by bringing about movements of the muscles of the neck and limbs . visual clues –> righting of the head

61. Placing reflex   : when the infant is held erect and the dorsum of the foot is drawn along the under edge of a table top  flexion followed by extension of the leg Appears by 4 days in the newborn Stepping Reflex When feet touch the ground , the infant appears to take some steps Stepping reflex disappears before walking :

62. Hopping reaction lateral displacement while standing cause animal to hop to maintain balance  

63. Postural Control Centers : Cerebral cortex Optical Righting reflex Placing Reaction Hopping Reaction Midbrain : the Midbrain Righting Reflexes  (a) Labyrinthine (b) Neck (c) Body on Head (d) Body on Body Medulla (1) Tonic Labyrinthine Reflex (2) Tonic Neck Reflex Spinal Cord Positive and Negative Supporting Reaction Stretch Reflex

64. Experimental evidence : Role of spinal cord: spinal animal Role of brain stem: decerebrate animal Role of mid brain: mesencephalic animal or high decerebrate animal Role of cerebellum Role of basal ganglia: decorticate animal

65. SPINAL ANIMAL: The animal that receives this transection of spinal cord at lower cervical level to study various reflexes. In spinal section, reflexes with centers in the spinal cord are intact ( e.g , withdrawal reflex).

66. transection of brain stem at superior border of pons decerebrate rigidity midbrain pons medulla

67. decerebrate rigidity no spinal shock spasticity results in the form of decerebrate rigidity tonic reflexes are seen tonic labyrinthine reflex tonic neck reflexes spasticity is mainly found in limb extensors these reflexes support the body against gravity

68. Decerebrate Rigidity Site of lesion  between the superior and inferior colliculi of the midbrain , lesion below Red Nucleus , resulting in extensive extensor posture of all extremeties  Rigidity of all 4 limbs All limbs extended arms extended by the sides  & rotated internally (outward) ( hallmark  elbows extended) Head may be arched to the back It is due to  (1) increased general excitability of the motor neuron pool  especiallly Gamma efferent discharge ( due to facilitatory effects of the un inhibited Vestibulospinal Tract ) .

69. In a decerebrate animal : head arched back arms extended by the sides  & rotated internally (outward) extended elbows* legs extended & rotated internally rigid with clenched teeth damage to (level below red nucleus) brain stem midbrain cerebellum Reflexes that are lost/absent Righting Reflexes ( optical) Reflexes that are retained /still present ( i.e., reflexes the do not depend primarily cerebrum on cerebrum : (1) Tonic Labyrinthine reflexes (2) Tonic Neck Reflexes

70. Mechanism of decerebrate rigidity: 1.Classical decerebrate rigidity 2.Ischaemic decerebrate rigidity Classical decerebrate rigidity: First produced by Sherrington in cats in 1898. Here basal nuclei & cerebral cortex removed.

71. Mechanism of classical decerebrate rigidity: Exaggerated stretch reflex due to increased activity of gamma motor neurons. If dorsal roots are cut then rigidity disappears.

72. Ischemic decerebrate rigidity Pollock & Davis (1930) produced decerebrate rigidity by tying carotid artery & basilar artery at junction of pons medulla. Considerable part of pons as well as more rostral part brain stem & cerebral hemisphere & about half of cerebellum becomes ischemic.

73. Mechanism of ischemic decerebrate rigidity : Ischemic decerebrate rigidity is due to disinhibition of alpha-motor neuron Rigidity is not lost by deafferentation.

74. Classical decerebrate Ischemic rigidity decerebrate rigidity 1)transection of brain 1)ligation of common stem bet superior & carotid & basilar Inferior colliculi. artery. 2) Gamma rigidity 2) alpha rigidity 3)Cutting post root 3) does not abolish abolishes rigidity rigidity. 4)Local injection of 4) does not reduce procaine into nerve rigidity Trunk reduces spasticity

75. Classical decerebrate Ischemic rigidity decerebrate rigidity 5) Systemic 5) has no effect on administration of rigidity Chlorpromazine reduces spasticity 6)Removal of ant lobe 6) Has no effect of cerebellum increases rigidity.

76. Decerebrate rigidity in man :

77. Decerebrate rigidity is not commonly found in man. The pattern in true rigidity is extensor in all 4 limbs. The actual decerebrate rigidity generally shows extensor rigidity in legs & moderate flexion in arms due to lesions of cerebral cortex with most of brain stem intact.

78. DECORTICATE ANIMAL Decorticate animal is one in whom the whole cerebral cortex is removed but the basal ganglia and brain stem are left intact Characteristic features: ( i ) Moderate rigidity is present Cortex inhibit medullary reticulospinal tract Therefore, removal of cortex results in facilitation of γ motor neuron discharge Seen only when the animal is at rest

79. (ii) Typical posture in decorticate man consists of - full extension of legs - arms lying across the chest - semiflexion at elbow - slight pronation of forearm - flexion of wrist and fingers (iii) Righting reflexes are present

80. removal of cortex decortication

81. In a decorticate animal : Reflexes that are lost/absent (1) Placing Reaction , Hopping Reaction (2) Visual righting reflex Reflexes that are retained /still present ( i.e., reflexes the do not depend primarily on cerebral cortex : (1) Tonic Labyrinthine reflexes (2) Tonic Neck Reflexes (3) Other Righting Reflexes

82. A- Decerebrate and B,C,D- Decorticate rigidity in man

83. Decorticate posture Level of lesion lies above the Red Nucleus. So the Rubrospinal Tract remain intact. It supplies flexor group of motor neurons in upper limb So in decorticate rigidity where the rubrospinal tract is intact the upper limbs are flexed Decerebrate posture In midcollicular section, the rubrospinal tract is sectioned as thelevel of midcollicular section passes below the Red Nucleus . So flexion of upper limb is absent and all 4 limbs are extended

84. In humans , where true decerebrate rigidty is rare , since the damage to the brain centers involved in it are lethal. However decorticate rigidity can be caused by bleeding in the internal capsule which causes UMNL (damage to upper motor neurons) . Symptoms & Signs : Flexion in the upper limbs and extension in the lower limbs. Arms flexed & bent inwards towards chest Feet turned inward Seen in corticospinal animal : damage above Red Nucleus ( in cerebral hemispheres , internal capsule or thalamus ).

85. Motor cotex Basal ganglia cerebellum Inhibitory reticular formation Facilitatory reticular formation Vestibular nucleus

86. removal of cortex decortication

87. Decorticate Animal Decerebrate Spinal Animal Levels of Lesions

88. decortication righting reflexes are present temperature regulation & other visceral homeostatic mechanisms integrated at the hypothalamus are present rigidity is seen (decorticate rigidity)

89. postural adjustments vestibular nuclei cerebellum pressure & other receptors neck receptors Retina Occulomotor system vestibular system complex pathways

90. Types of Rigidity Animal Experiments show increased tone or rigidity. Different Types 1 Decerebrate Rigidity -Sherrington and ischemic. 2 Decorticate Rigidity . 3 Lead Pipe Rigidity. 4 Cogwheel Rigidity .- 3 &4 are found in Parkinson’s Disease

91. Types of Rigidity 5 Decerebellate Rigidity-found in animals.In humans cerebellectomy is associated with hypotonia . True rigidity is hypertonia in both extensor and flexor group of muscles and is found in lead pipe and cogwheel rigidity of Parkinsonism. All other types of rigidity mentioned above are really spaticity where only one group of mucle show hypertonia

92. Thank You