Physiology of Posture

Prof. Vajira Weerasinghe

Dept of Physiology

Dynamic vs static nature of motor control

• Static stability – is dependent on the position of the centre of

gravity with respect to the base of support

• whereas dynamic stability – is dependent more on the moment of inertia of

the body

Adult vs child

• In normal standing, a tall adult will have a much larger moment of inertia than a toddler

• Once the centre of gravity moves outside the base of support the body will begin to fall– The adult with the large moment of

inertia will fall much more slowly and will therefore have a longer time to react to prevent the fall

– This is one of the reasons that young children fall more often than adults.

Postural control

• Maintaining static nature of the body

maintenance of posture• mainly to maintain the static posture

• necessary for the stability of movements

• involve a set of reflexes

• integrated at spinal cord, brain stem and cortical level

normal postural control

• three inputs are required– Vision– Proprioception (joint position sense)– Vestibular Mechanism (balance mechanisms)

• these reflexes are under higher centre inhibition

• transection of spinal cord or brain stem at different levels release this inhibition

• then the relevant reflexes are seen

spinal cord level

Spinal cord level• stretch reflex• positive supporting reaction (magnet reaction)

– Stimulus: contact with palm or sole– Response: foot extended to support the body– receptors: proprioceptors in distal flexors, tactile afferents

• negative supporting reaction– stretch – release of positive supporting reaction

Video

spinal cord transection

• initially a period of spinal shock (2 weeks)

• then followed by appearance of a upper motor neuron lesion features– spasticity

midbrain

pons

medulla

Spinal shock

• all spinal reflexes below the level of lesion are completely depressed

– may be due to the sudden cessation of tonic bombardment of spinal cord interneuron pool by descending influences

after the spinal shock

• reflexes will reappear, mostly exaggerated• bladder become reflex

• mass reflex– afferent stimuli irradiate to several reflex centres– noxious stimulus causes: withdrawal

response,evacuation of bladder, rectum, sweating, pallor

medulla levelmidbrain

pons

medulla

medulla level• tonic labyrinthine reflex

• Stimulus: gravitational pull• Response: contraction of limb extensors• receptors: vestibular organs• (work through vestibulospinal tract)

• tonic neck reflexes• Stimulus: turning of the head• Responses:

– turning to a side: extension of ipsilateral limb– turning up: lower limbs flex– turning down: upper limbs flex

• receptors: neck proprioceptors

Video

Video

tonic neck reflexes

tonic neck reflexes

tonic neck reflexes

tonic neck reflexes

tonic neck reflexes

transection of brain stem at superior border of pons

• decerebrate rigidity

midbrain

pons

medulla

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

midbrain levelmidbrain

pons

medulla

midbrain level

• righting reflexes are seen

video

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

video

midbrain level• body-on-head righting reflex

• Stimulus: pressure on side of body• Response: righting of head• receptors: cutaneous receptors

• body-on-body righting reflex• Stimulus: pressure on side of body• Response: righting of body• receptors: cutaneous receptors

transection of brain stem at superior border of midbrain

• righting reflexes

midbrain

pons

medulla

• rigidity is seen only when animal lies on its back

• otherwise animal will try to right itself

• righting reflexes (labyrinthine, neck & body)

• grasp reflex may be seen

• pupillary light reflexes are present

• nystagmus is seen

cortical level

midbrain

pons

medulla

cortex

cortical level• optical righting reflex

• Stimulus: visual cues• Response: righting of head• receptors: visual system

• placing reactions• Stimulus: visual, cutaneous, proprioceptive cues• Response: foot placed on a surface to support body• receptors: various

cortical level• hopping reaction

• Stimulus: lateral displacement while standing• Response: hops• receptors: muscle spindles

removal of cortex

• decortication

decortication

• righting reflexes are present

• temperature regulation & other visceral homeostatic mechanisms integrated at the hypothalamus are present

• rigidity is seen (decorticate rigidity)

postural adjustments

vestibular nucleicerebellum

pressure& otherreceptors

neckreceptors

Retina Occulomotor system vestibularsystem

complex pathways