Neuroscience 13c – Control of Posture
Anil Chopra
- List the different types of physical information used in postural control systems.
- State which biological sensors produce which kind of information
- Understand that different senses interact and that weighting may occur in cases of sensory conflict.
- Be able to distinguish automatic (reflex) and voluntary movements and posture
- Understand the influence of cognitive factors on postural behaviour.
Sensory information about the environment comes from a number of different sources:
Vision - Position, speed, direction, motion of other objects
Otoliths - Gravity (tilt), linear acceleration
Semicircular canals - Rotational velocity and acceleration
Proprioception - Force, pressure, motion within the body.
If the task is simple, then all the sources are not needed.
If there is sensory loss then control is adaptive and there is some degree of redundancy, therefore people lacking information from one sense can perform remarkably well if the motor task is not too difficult.
Causes for sensory loss can include:
Illness - Menière's disease, cataract, neuropathy
Injury - Physical injury, poisoning, drug toxicity
Ageing - Visual, proprioceptive and vestibular performance is worse in older people
Some of the compensatory mechanisms that people adopt when they have sensory loss:
Stance - Wide base of stance front to back and side to side to increase stability
Stance - Knee hyperextends to provide passive stability by locking the joint
Gait - Trunk and arms move little to reduce the need for control of moving masses
Gait - Knee again hyperextends to provide stability
Patients often find it difficult to keep balance when they have their eyes closed.
Conscious Action
Most postural tasks do not require conscious control:
-simple tasks such as maintaining a stance does not requite conscious control
-tasks such as orienting, control of head altitude and movement of support can be carried out without conscious control.
Autonomic reactions are triggered by particular combinations of sensory stimuli and are appropriate for particular situations. Voluntary compensation may be required if the autonomic action is not sufficient
Conscious anticipation produces a postural set – i.e. a tendency to respond in a particular way to an expected perturbation
Sensory Weighting
Most information sent by the various sources is concurrent i.e. it is interpreted in the same way. When it is not, the brain can give more weight to certain senses e.g. the illusion of movement when stationary if an adjacent train is moving.
If this is prolonged or severe, then it can produce motion sickness.
Voluntary Control
Automatic reactions are stimulated when a certain set of information is received, however if the response is inappropriate, then voluntary compensation can occur.
Conscious anticipation produces a postural set, i.e. a tendency to respond in a specific way, appropriate to an expected perturbation.