Posture & Its Regulation

7/31/2019 Posture & Its Regulation

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POSTURE: A state of equilibrium in

the space or position adopted by anindividual in the environment.


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It is active muscular resistance against displacement of body by the gravity &

acceleratory forces.By posture regulation, not only an uprightbalanced posture is maintained , but also

a background for motor activity isprovided.


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In posture regulation, sensory systems of body help, these include:

visual system , vestibular apparatus (semicircular canals, utricle & saccule),stretch receptors in muscles & ligaments

of neck, pressure receptors in sole &palm & also on the surface of body.


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Posture is regulated by a number of postural reflexes which may be static or

dynamic.Static reflexes involve sustained musclecontraction.

Dynamic reflexes involve transientmuscle contractions.


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These postural reflexes are integrated atvarious levels in CNS (from cerebral

cortex to spinal cord).Integration of these postural reflexeshave been studied by giving sections at

various levels of CNS & then studying theeffects produced.


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It is produced by giving a mid-collicular section (sectionin midbrain between superior & inferior colliculi).

This procedure is called decerebration.

Sherington introduced this section in cats & dogs &observed the effects:


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In this case following features are seen: All 4 limbs are extended.

Back or spine is hyper-extended.Head is dorsi-flexed.Tail is lifted.Jaws are tightly closed.Similar features in humans as well.


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Legs are extended. Arms are extended.

Flexion at wrist.Head is dorsiflexed.Back is hyper-extended.Due to increased tone in extensors.Increased gamma efferent discharge.Stretch reflex or myotatic reflex becomeshyperactive.


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When gamma efferents are cut decerebrate rigiditydisappears.When reticulospinal & vestibulospinal tracts are cut rigidity also disappears.In decerebrate rigidity the influence of cerebral cortex &basal ganglia is cut .*normally cerebral cortex & basal ganglia inhibit reticular formation ( bulbo-reticular facilitatory area ).

When cut no inhibition increased gamma efferentdischarge & stretch reflex.This mid-collicular decerebrate rigidity = classicaldecerebrate rigidity .


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There is another type of this rigidity:It is produced by ligating I.C.A between pons &medulla.

Same effects are produced but mechanism is different.In ischemic decerebrate rigidity, ischemic parts are :

cerebral cortex, basal ganglia & cerebellum .There is increase in alpha efferent discharge instead of

gamma efferent discharge.When we cut gamma efferents in this type of rigidity no effect.


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Incase of classical decerebrate rigidity, if cerebellum is cooled down, classical type

ischemic type.


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Classical / mid-collicular de-cerebrate rigidity:Increased gammaefferent discharge.

Ischemic de-cerebrate rigidity:

Increased alphaefferent discharge.


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Wide spread lesion of cerebral cortex .Patient is unconscious .

Legs are extended . Arms are moderately flexed (A)When head is turned to right side, on that sidearm becomes extended (B)

We can also give transection in spinal cord atlower cervical level & study various reflexes(here spontaneous respiration is possible).


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The animal that receives this transectionof spinal cord at lower cervical level to

study various reflexes.


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In spinal section, reflexes with centers inthe spinal cord are intact (e.g, withdrawal

reflex).


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Posture is maintained through variation inmuscle tone &

Also by active muscular contraction.STIMULUS: Stretch of muscle.RECEPTORS: Muscle spindles.

RESPONSE: Muscle contraction.CENTRE / SEGMENTS: Spinal cord & medullaoblongata.


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When pressure is applied on sole or palm

limbs become extended like a pillar tosupport the body (Magnet Reaction).When pressure is applied on toe pad limbs of animal are extended.RECEPTORS: Proprioceptors in flexors& pressure receptors in sole / palm.


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A relaxation which allows the limb to move to anew position.Whenever there is passive flexion there isrelaxation of extensors which allow the limb tomove to new position.

RECEPTORS: Proprioceptors in extensors.CENTRES for + & - supporting reactions are inspinal cord.


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To study this reflex, tonic neck reflex must beabolished.We can abolish it by applying a plaster jacket aroundthe neck in humans or in animals by cutting upper 3cervical nerves (C1, C2, C3).STIMULUS: Gravity.RECEPTORS: Autolith organs, especially in utricle &saccule (in absence of neck movement).

When head is dorsiflexed extension of all 4 limbs inanimals.When head is ventroflexed decreased extension of all 4 limbs.CENTRE: Vestibular nuclei of medulla.


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To study this reflex tonic labyrinthine reflex is abolished, by cuttingvestibular nerves or damaging the labyrinth.Here is no role of labyrinth, only neck moves.When neck is extended forelimbs extention & hind limbs flexed.(posture of animal looking above a shelf).When neck is flexed forelimbs become flexed & hindlimbs becomeextended. (posture of animal looking below a shelf).When head is turned to 1 side, on that side limb extend, other side isflexed (typical posture of a boxer), through tonic neck reflex.

RECEPTORS: Muscle spindles, paccinian corpuscles in ligaments of neck & vertebral joints.CENTRE: Spinal cord & medulla.These reflexes are used in posture maintenance in humans.


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When an animal who is asleep, wakes up (animal maybe lying on its side or on the back) the animalassumes an upright posture, through a series of

reflexes called righting reflexes:1) LABYRINTHINE RIGHTING REFLEX acting on theneck.2) BODY RIGHTING REFLEX acting on neck.3) NECK RIGHTING REFLEX acting on body.4) BODY ON BODY RIGHTING REFLEX.5) OPTICAL RIGHTING REFLEX.


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1) LABYRINTHINE RIGHTING REFLEXacting on the neck:

When the animals head is in the lateral

position, due to stimulus of gravity autolith organs are stimulated

contraction of neck muscles to correcthead position.


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2) BODY RIGHTING REFLEX acting onneck:

When body of animal is on its side, oneside of body is in contact with surface of the ground, on that side pressure

receptors or exteroceptors are stimulatedreflex contraction of neck muscles

correction of head position.


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3) NECK RIGHTING REFLEX acting onbody:

When head has become upright, whilebody is in lateral position there istwisting of neck stimulation of muscle

spindles in neck muscles impulsesfrom here, reflexly right (correct) theposition of body.


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4) BODY ON BODY RIGHTINGREFLEX:

Position of body can be righted without rightingthe position of head by this reflex.When body is on its lateral side pressurereceptors / exteroceptors on that side of thebody are stimulated these impulses reflexly

right position of body e.g., a wrestler is caught& neck is locked by the other neck cant bemoved, but he tries to right the body by thisreflex.


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Brain, above the level of mid-collicular region.

These reflexes will be absent inDECEREBRATE RIGIDITY.


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V) OPTICAL RIGHTING REFLEX:(Important in humans)

Position of head can be righted with thehelp of visual impulses

CENTRE (AREA 17)

AFFERENTS

EFFERENTS (RIGHTING IMPULSES)


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Cerebellum is important in execution of righting reflexes.

In cerebellar lesion righting reflexesare not adequate.


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When a standing animal is pushed fromits lateral side animal makes hoping

movements to maintain its uprightposture.RECEPTORS: Muscle spindles

CENTRE: In cerebral cortex


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When an animal falls from a height animal places its feet on the surface and then assumes an upright posture.RECEPTORS: Visual, proprioceptors &exteroceptors.CENTRE: In cerebral cortex.This reflex is seen even in humans, whenthey jump.


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In addition to visual system (sensorysystems), postural reflexes, cerebral

cortex, basal ganglia & cerebellum arealso involved in postural regulation.


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1ST order neurons in vestibular ganglion&

1 st order nerve fibers in vestibular nerve.Most of these nerve fibers synapse investibular nuclei, which are located at

junction of medulla & pons.


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From vestibular nuclei 2nd order nerve fibersfestigeal nucleus, uvula, flocculo-nodular

lobe & cortex of cerebellum.Fibers also go to spinal cord as vestibulo-spinal tract.Fibers go into medial longitudinal bundle & alsoto nuclei of reticular formation in brain stem.Some of the vestibular nerve fibers bypassvestibular nuclei & go to cerebellum &reticular formation.


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These fibers go to festigeal nucleus, uvula &flocculo-nodular lobe, not to the cortex of cerebellum. Fibers from vestibular nuclei, goingto medial longitudinal bundle, synapse onto themotor nuclei of 3 rd , 4 th & 6 th cranial nerves.These fibers control compensatory eye

movements during head rotation (nystagmus)


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In vestibular pathway, there are 18,500 nervefibers & most of these fibers are afferent,sensory fibers.Only 1-2% fibers are efferent & these fibersarise from lateral vestibular nucleus of sameside & reticular formation on both sides.

These efferent fibers are inhibitory in nature.


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From vestibular pathway, fibers also goto cerebral cortex through thalamus &these fibers help in perception of motion& orientation in space.Various features of vestibular stimulationlike vertigo, nausea, vomiting, heart ratechanges, B.P changes, sweating & skinpallor are produced through connectionsof vestibular nuclei with brain stem.


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There are 4 vestibular nuclei within brainstem.

There are 4 vestibular nuclei:1) Superior 2) Medial3) Lateral4) Inferior


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Superior & medial vestibular nuclei receivefibers mainly from semicircular canals.

From these 2 nuclei, fibers go into mediallongitudinal bundle to control compensatoryeye movements.Fibers also go into medial vestibulo spinal tract

to control movements of head & neck.


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Lateral nucleus is also called Deiters nucleus. It receives fibers mainly from utricle & saccule

& from this nucleus, fibers go to lateral spino-thalamic tract.Inferior / descending nucleus receives fibersfrom semi-circular canals & also from utricle &

saccule & from here fibers go to cerebellum &reticular nuclei of brain stem.


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They are characterized by signs &symptoms:

Vertigo or dizzinessNausea & vomiting Actual falling

Diplopia (double vision)Changes in heart rate & B.PSweating & skin pallor


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It is feeling of rotation in the presence of actual rotation.

Patient may feel that the external world ismoving or his body is being rotated.(hum bhi ghoomtay hain or ghoomta hay

sara jahan !!)Or their may be unsteadiness of legs.


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In this disease attacks of vertigo &progressive impairment of hearing.

Cause = ???

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Posture & Its Regulation

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