Posture is maintained through a balance of muscle contractions regulated by reflexes. The key reflex is the stretch reflex, where muscle spindles detect changes in muscle length and signal the spinal cord to contract or relax muscles. There are two types of postural reflexes - static and statokinetic. Static reflexes maintain posture against gravity, while statokinetic reflexes allow for voluntary movement. Multiple areas of the central nervous system integrate these reflexes, including the spinal cord, brainstem, cerebellum and cerebral cortex. Vision and vestibular signals also provide important inputs. Together this network allows humans to maintain an upright stance through low levels of continuous muscle contraction adjusted by reflexes.

3. 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.

4. 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.

5. 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.

6. TYPES OF POSTURAL REFLEXES
Broadly 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.

7. 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.

8. 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

9. 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

10. REFLEX STIMULUS RESPONSE RECEPTORS INTEGRATING
CENTRE IN CNS
2.Segmental
static reflexes
Crossed
extensor reflex
3.General static
reflexes
Attitudinal
reflexes
1. 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

11. 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(monosyna
ptic reflex) visual
receptor(long loon
reflex)
Otolith organs in
saccules of
labyrinth.exterocep
tors
Exteroceptors
Muscle spindles
Exteroceptors
Muscle spindles
Eyes
Spinal cord
Cerebral cortex
Mid brain
Mid brain
Mid brain
Mid brain
Mid brain
Cerebral cortex

12. 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

14. 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 ANIMAL
Decerebrate animal
Decerebrate animal is one in whom the brain stem is transected at
intercollicular level (between superior and inferior colliculi).

15. 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.

16. 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

17. 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 muscles
Undoubtedly 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.

18. 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 sideways
Head 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.

19. 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.