PHYSIOLOGYOF

POSTURE

Dr. PADMAJA HARI MD

Posture is the attitude taken by the body in any particular situation like standing posture, sitting posture, etc. even during movement, there is a continuously changing posture

The basis of posture is the ability to keep certain group of muscles in sustained contraction. Variation in the degree of contraction and tone in different groups of muscle decides the posture of the individual. The muscles which maintain posture should have the ability to remain contracted for long periods.

MECHANISM •Posture-maintaining muscles contain more of red muscle fibres, which are slowly contracting and not easily fatigued. All muscles in the body are a mixture of red and pale(white) muscles. Muscles of hand, eye etc., has a preponderance of white muscle fibres which are easily fatigued.•Only a fraction of the muscle fibres are active at any given time due to asynchronous contraction.

POSTURAL REFLEXES

The postural reflexes help to maintain the body in upright and balanced position. They also provide adjustments necessary to maintain a stable posture during voluntary activity.

REFLEX ARC of postural reflexes is as follows:

•Afferent pathways of reflex arc come from the eyes, the vestibular apparatus and the proprioceptors.

•Integrating centers are formed by neuronal networks in brain stem and spinal cord.

•Efferent pathways consist of alpha-motor neurons supplying the various skeletal muscles which form the effector organs.

TYPES OF POSTURAL REFLEXESBroadly they are of two types:

•Static reflexes: These are elicited by gravitational pull and involve sustained contraction of muscles.

•Statokinetic reflexes: These reflexes, also called phasic reflexes, are elicited by acceleratory displacement of the body. They maintain a stable postural background for voluntary activity.

•Both these types of postural reflexes are integrated at various levels in the CNS from the spinal cord to cerebral cortex and are affected largely by pyramidal pathways.

The basic reflex operating in maintaining posture is stretch reflex and the receptor involved is the muscle spindle. Any posture can be voluntarily assumed, but maintenance of posture is reflexly done. The degree of contraction of any muscle depends on the degree of firing from α-motor neuron. Motor neuron is influenced by pyramidal and ex-trapyramidal tracts coming down directly or through gamma motor neurons or interneurons. 

MAINTANENANCE OF POSTURE Decision for a particular posture may occur in the cortex, planning and programming occur in the basal ganglia and cerebellum, and the information comes down through the pyramidal tract to the motor neurons supplying muscles.

In a standing posture, the centre of gravity is acting in such a way that the body falls forwards. So the antigravity muscles like extensors of neck, back,hip,legs etc., should be in a contracted state. In the normal standing posture of humans, the upper limbs are slightly flexed, and the flexor group of muscles are the antigravity muscles in the upper limb. The vestibular receptors, proprioreceptors, visual receptors etc., play important role in maintaining posture. In the standing posture, impulse coming through vestibulospinal tract and reticulospinal tract also play an important role. If there is a change in the position of head, the receptors in the utricle and saccule are stimulated and some group of muscles contract and the head is held erect or in a particular posture. If an animal is pushed to one side, the limbs of that side extend or there will be a hopping movement

REFLEX STIMULUS RESPONSE RECEPTORS INTEGRATING CENTRE IN CNS

A.Static reflexes1.Local static reflexesStretch reflex  2.PositiveSupportingreflex 

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

REFLEX STIMULUS RESPONSE RECEPTORS

INTEGRATING CENTRE IN CNS

2.Segmental static reflexesCrossed extensor reflex   3.General static reflexesAttitudinal reflexes1. Tonic

labyrinthine reflex

2. 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

REFLEX STIMULUS RESPONSE RECEPTORS INTEGRATING CENTRE IN CNS

2.Long loop stretch reflex   3.Righting reflexes

• Labyrinthinerighting reflex

•Body righting reflex

•Neck righting reflex

•Body on body righting reflex

•Limbs righting reflex

• opticalrighting 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 pelvisRighting 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 

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

ROLE OF DIFFERENT REGIONS OF NERVOUS SYSTEMS IN MAINTENANCE OF POSTURE

Role of spinal cord: spinal animal Effects of spinal cord transaction:The effects produced by complete spinal cord transaction occur in three stages:•Stage of spinal shock,•Stage of reflex activity, and•Stage of reflex failure.

ROLE OF BRAIN STEMM: BULBOSPINAL ANIMAL OR DECEREBRATE ANIMALDecerebrate animalDecerebrate animal is one in whom the brain stem is transected at intercollicular level (between superior and inferior colliculi).

ROLE OF CEREBELLUM

Spinocerebellum regulates the postural reflexes by modifying muscle tone .It facilitates the gamma motor neurons in the spinal cord via cerebello-vestibulo-spinal neurons in the spinal and Cerebello-reticulo-spinal tracts .The gamma motor neurons reflexly modify the activity of alpha motor neurons and thus regulate the muscle tone. Thus, cerebellum forms an important site of linkage of alpha-gamma systems responsible for muscle tone

ROLE OF BASAL GANGLIA: DECORTICATE ANIMAL

Decorticate animal is one in whom the whole cerebral cortex is removed but the basal ganglia and brain stem are left intact.

DECEBRATE PREPARATION DECORTICATE PREPARATION

• Extensive extensor rigidity moderate rigidity as in

And decerebrate posture hemiplegia

• Righting reflexes absent rightining reflexes present

• Temperature regulation temperature regulation Absent normal

• Only a standing posture can stand and walk reflexly, Can be maintained but placing and hopping Reactions are absent

MECHANISM OF STANDING IN MAN

Tall human being has to stand over a narrow base of feet,therefore,maintenance of erect posture is more difficult than the quadruped animals. Mechanisms which play important role in erect standing posture are.Reflex adjustment in muscle tone of antigravity musclesUndoubtedly plays most important role in making the man stand erect. From this statement,it may be presumed that a continued contraction of most of the trunk and leg muscles keeps the posture upright. However, electromyographic studies have revealed very little muscle activity in a person standing quietly in upright position.Configuration of hip and knee joints is such that they are kept extended by the gravity itself. However, a little activity of the antigravity muscles is required to maintain the very precarious balance. This explains the little muscle activity revealed by electromyographic studies.

The effect of gravity has to be opposed by reflex contraction of the antigravity muscles all the time, otherwise a standing man may fall in any direction(forwards,backwards or sideways).The different antigravity muscles which oppose the fall under various circumstances are:Extensors of the trunk and flexors of legs contract sufficiently to restore the balance when the body sways forward Recti abdominis and leg extensors contract to restore the balance when the body sways backward Contralateral external oblique abdominal muscles maintain the balance when the body leans sidewaysHead has a tendency to sway more than the trunk: since the centre of gravity of head passes in front of the centre of gravity of atlanto-occipital joint, therefore, head always has got a tendency to roll forwards.To hold the head in erect position the cervico-occipital muscles are to be maintained in a state of constant tension. 

Reflex changes in antigravity muscles described above are induced by:

•Stretch receptors in the trunk and leg muscles,

•Visual afferents also play important role in reflex maintenance of upright posture in man. This is why, when the eyes are closed, the upright posture is less steady and there occurs more swaying(bending) of the trunk.

•Vestibular afferents help in maintaining the erect position of head.

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