Posture & Equilibrium by dr anita teli

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POSTURE AND EQUILIBRIUM:

POSTURE AND EQUILIBRIUM Speaker: Dr Anita teli PG Chairperson: Dr Shrilaxmi Bagali Asst Prof Date & time : 8 th june 2011,10.30 am BLDEU’s Shri B.M.Patil Medical College Bijapur

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Introduction Role of muscle tone Postural reflexes Experimental evidence Equilibrium Applied aspect

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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. 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.

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

Role of muscle tone:

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.

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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.

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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.

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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.

Maintenance of muscle tone :

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.

Supraspinal control on muscle tone:

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.

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.:

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.

Postural Reflexes: :

Postural Reflexes: They help to maintain the body in upright & balanced position Provide adjustments necessary to maintain a stable posture during voluntary activity

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Reflex arc of postural reflexes : Afferent pathways – eyes , vestibular apparatus, proprioceptors. Integrating centre – neuronal networks in brainstem & sp.cord Efferent pathways – alpha motor neurons supplying various skeletal muscles.

Types of postural reflexes:

Types of postural reflexes Static reflexes: These are elicited by gravitational pull & involve sustained contraction of muscles. Statokinetic reflexes (phasic reflexes):Elicited acceleratory displacement of the body . Maintain stable postural background for voluntary activity.

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Static reflexes: these of 3 types Local static reflexes Segmental static reflexes General static reflexes

Local static 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. Positive supporting reaction. Negative supporting reaction.

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Reflex control of antigravity muscle tone: Stretch reflex controls tone in extensor muscles which keep the body upright (antigravity muscles).

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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).

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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.

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Segmental static reflexes : The centre is located in the spinal cord. Characterized by bilateral reflex response when stimulus is applied to one limb. Ex : 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.

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General static reflexes : 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.

Attitudinal 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.

Tonic labyrinthine reflex::

Tonic labyrinthine reflex : Produced in response to alteration in position of head relative to horizontal plane. E.g. while standing on an inclined plane .

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Stimulus : Gravity Receptors : otolith organs present in labyrinthine apparatus. Afferents : impulses generated in the receptors travel along vestibular nerves. Centre : vestibular & reticular nuclei present in medulla oblongata. Efferents : vestibulospinal & reticulospinal tracts. Reflex response : particularly effective in extensors muscles.

Reflex response::

Reflex response: Depending upon the position of head in relation to horizontal plane the reflex response produced is:

Tonic neck reflex::

Tonic neck reflex: Are produced in response to alteration in position of head relative to body.

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Stimulus : stretch of neck muscles. Receptors : pacinian corpuscles in the ligaments of cervical joints particularly atlanto-occipital joint & muscle spindle of neck muscles. Centre : medulla oblongata. Efferent : corticospinal tracts. Reflex response : depends on position of head in relation to body.

Reflex response::

Reflex response: Ventroflexion (turning down) of head causes flexion of forelimbs & extension of hind limbs. Dorsiflexion (turning up) of head causes extension of forelimbs & flexion of hind limbs. Turning of head sideways i.e. towards right or left produces flexion of ipsilateral limbs & extension of contra lateral limbs.

Combined response of tonic & neck labyrinthine reflexes:

Combined response of tonic & neck labyrinthine reflexes On dorsiflexion of head, labyrinthine impulses produce increased tone in extensors of all 4 limbs while impulses from neck extend forelimbs & flex hindlimbs. Actual result is extension of forelimbs & little change in hind limbs. On ventroflexion of head: labyrinthine impulses produce increased tone in flexors of all 4 limbs, while impulses from neck flex forelimbs & extend hind limbs. Actual result is flexion of forelimbs & little change in hindlimbs

Importance :

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 .

Long loop stretch reflexes::

Long loop stretch 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.

Righting reflexes: :

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.

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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. Limbs righting reflex Optical righting reflex.

Head righting reflex.:

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.

Body righting reflex:

Body righting 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. The head can even be righted after bilateral labyrinthectomy.

Neck righting reflex:

Neck righting reflex 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.

Body-on-body righting reflex:

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.

Limbs righting reflex:

Limbs righting reflex Impulses arising from limb muscles are responsible for attainment of appropriate posture of limbs.

Optical righting reflex:

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.

Centers of righting reflex:

Centers of righting reflex Red nucleus lying in mid brain (except optical righting reflex- visual cortex) It controls them by following tracts: Rubro spinal tract Rubro reticular tract

Statotonic reflexes: :

Statotonic reflexes : Elicited by angular (rotatory), linear acceleratory (progressive) stimuli to labyrinthine receptors of vestibular apparatus. These are programmed reflexes that depend on motor cortex . Mediated by lateral vestibulospinal tracts.

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Vestibular placing reaction Visual placing reaction Hopping reaction

Vestibular placing reaction::

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.

Visual placing reaction::

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

Hopping reaction::

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

Experimental evidence::

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

Role of sp.cord: spinal animal:

Role of sp.cord: spinal animal Transection in sp.cord at cervical region. Respiration is maintained artificially by respiratory pump. If transected in mid thoracic region – diaphragmatic respiration continues.

Posture of spinal animal::

Posture of spinal animal: Stretch reflex , supporting reaction present. All other postural reflexes absent. Stretch reflex, supporting reactions though present are very weak & cannot maintain the wt of the animal . So animal cant stand on its legs. Muscle tone returns first in flexors – paraplegia in flexion.

Role of brain stem: decerebrate animal :

Role of brain stem : decerebrate animal Brain stem is transected at the intercollicular level Here cerebral cortex & basal nuclei are removed.

Characteristic features :

Characteristic features Decerebrate rigidity No spinal shock Postural reflexes present are those that have their integration centre in sp.cord, medulla or pons. Stretch reflexes: strongly positive. Positive supporting reaction: elicited by applying pressure on pads of fingers & toes . reflex contraction of both extensors of limb.

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3. Negative supporting reaction: elicited by passive plantar flexion. 4. Crossed extensor reflex: when 1 fore limb is flexed, other forelimb is extended. 5. Tonic neck & tonic labyrinthine reflex 6. Righting reflexes are absent. Therefore a decerebrate animal can stand in upright position but cannot resume upright position once it falls down.

Decerebrate rigidity::

Decerebrate rigidity: Decerebrate rigidity refers to marked increase in tone of extensors i.e. antigravity muscles occurring immediately after decerebration. the increased tone is a constant feature confined to postural muscles. In dog, cat –in extensors Frogs-flexors as frog’s posture is squatting Sloth- flexors Birds- muscles responsible for keeping the wings flexed, are involved.

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Characteristic features : Hyperextension of all four limbs Dorsiflexion of tail & head Extreme hyperextension of spine (opisthotonus) produces concave configuration of back The animal can be made to stand on four limbs but is easily toppled by slight push.

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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.

Mechanism of classical decerebrate rigidity::

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

Ischemic decerebrate rigidity:

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.

Mechanism of ischemic decerebrate rigidity::

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

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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

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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.

Decerebrate rigidity in man::

Decerebrate rigidity in man :

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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.

Role of midbrain : high decerebrate animal:

Role of midbrain : high decerebrate animal Animal in whom brain stem is transected at rostral border of mid brain. Characteristic features : decerebrate rigidity is present but it disappears on performing reflex activity. Animal can stand & typical quadrupedal walking movements can be reflexely performed. Righting reflexes having their integration centre in the midbrain are present Pupillary light reflexes is present.

Role of cerebellum::

Role of cerebellum: Spinocerebellum regulates postural reflexes by modifying muscle tone. It facilitates gamma motor neuron via cerebello-vestibulo-spinal & cerebello-reticulo-spinal tracts. Unilateral cerebellar disease: Atonia or hypotonia in skeletal muscles of same side Attitude changes include: a) rotation of face towards opp side b) lowering of shoulder on affected side c) outward rotation & abduction of leg on involved side.

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Trunk is bent with concavity towards affected side because wt of the body is thrown on unaffected leg Deviation movement: arm held straight out in front of body, deviates laterally when eyes are closed. Deep or tendon reflexes become weak & pendular.

Role of basal ganglia: Decorticate animal:

Role of basal ganglia : Decorticate animal Animal in whom whole of cerebral cortex is removed but basal ganglia & brain stem are left intact. Postural characteristics of a decorticate animal: Moderate rigidity due to loss of cortical area that inhibits spinal gamma motor neurons discharge via reticular formation Typical posture in decorticate man- full extension of legs arms lying across chest , semiflexion at elbow slight pronation of forearm & flexion at wrist & fingers.

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Postural reflexes that can be elicited Typical neck reflexes Righting reflexes Postural reflexes that are affected are Hopping reactions Placing reactions

Equilibrium:

Equilibrium Refers to maintenance of line of gravity constant at rest & during movement by adjusting the tone of different muscles. Various parts which help in maintenance of equilibrium are: 1. vestibular apparatus 2. cerebellum 3. brain stem 4. other factors.

Role of Vestibular apparatus:

Role of Vestibular apparatus Semicircular canals & the utricle & saccule collectively form the vestibular apparatus. The receptors for vestibular system are called hair cells which are slowly adapting mechanoreceptors. The hair cells of utricle & saccule are located in mass of tissue called macula.

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Otolith organs detect changes in position of head & help in maintenance of equilibrium under static condition. The otolith organs also detect linear acceleration of the head & help in maintenance of equilibrium during such movements. Semicircular canals detect angular acceleration & help in maintaining equilibrium during dynamic phase. They also predictive function, when person is in dynamic state,they predict ahead of time that the person is likely to fall off balance & help nervous system to do adjustments.

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Vestibular apparatus plays imp role in maintenance of posture through vestibular reflexes which include; 1. vestibular placing reaction 2. righting reflexes 3. vestibulo-ocular reflex 4. vestibulo-otolit reflex

Role of cerebellum:

Role of cerebellum Uvula of cerebellum gets impulses from macula of utricle & saccule & helps in maintaining equilibrium under static conditions. Flocculonodular lobe of cerebellum gets impulses from the semicircular canals & helps in maintaining equilibrium during rapid changes in direction of motion.

Role of brain stem:

Role of brain stem Main role is played by 4 pairs of vestibular nuclei Superior vestibular nuclei : recieve signals from semicircular canals & sends impulses to; - medial longitudinal fasciculus to cause corrective movements of eyes,& - medial vestibular tract to cause appro movements of the neck & head. Medial vestibular nuclei : receive signals from semicircular canals & send signal to ; - medial longitudinal fasciculus to cause corrective movements of eyes,& - vestibulospinal tract to cause appro movements of head & neck

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Lateral vestibular nuclei : receive signals from otolith organs & in turn send: - through lateral vestibulospinal tract to spinal cord for controlling body movements. Inferior vestibular nuclei : receive signals from semicircular canals & utricle & in turn send signals to : - cerebellum ,& - reticular formation of brain stem

Role of other factors :

Role of other factors Neck proprioceptors : transmit information about orientation of head with respect to the body to vestibular & reticular nuclei of brain stem & cerebellum. Neck reflexes : function opposite to vestibular reflexes to maintain equilibrium of the entire body. When vestibular apparatus is destroyed then the bending of head produces muscular reflexes(neck reflexes) in the forelimbs. Body exteroceptors & proprioceptors :transmit information from other parts of the body besides neck. Visual receptors : also play imp role in maintenance of equilibrium.

Applied aspect:

Applied aspect Labyrinthectomy : A).Bilateral labyrinthectomy is characterized by, 1. Equilibrium is maintained by visual sensation, but the individual cannot right himself when blindfolded. 2. postural reflexes are severely affected 3. Muscle tone is decreased but there is no permanent loss. 4. Hearing loss is also there.. B).Unilateral labyrinthectomy characterized by, 1. oblique deviation of eyeballs 2. Nystagmus 3. Rotation & lateral flexion of the head 4. Flexion of the limbs on the side of lesion & rotation of trunk decreases. Motion sickness : due to excessive & repeated stimulation of vestibular apparatus.

References :

References Guyton Arthur C. The Nervous System: Motor and Integrative Physiology. Textbook of Medical Physiology.11 th Edition. W. B. Saunders Co; 2006. Pg 673-713. Khurana Indu. Somatic motor system. Text Book Of Medical Physiology. 1 st Edition, Elsevier; 2006. Pg 1076-1085.

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