Page 73 - Handbook of Electronic Assistive Technology

60 HANDBOOK OF ELECTRONIC ASSISTIVE TECHNOLOGY

Tonic labyrinthine reflex. This is a primitive and pathological reflex that is seen in
totally involved individuals due to abnormal simultaneous contraction of extensors
and flexors in the whole body. A flexed posture is exaggerated when the person is
placed in the prone position, in which contraction of the flexors is predominant. An
extended posture is exaggerated in the supine posture, in which contraction of the
extensors is predominant (Shumway-Cook and Woollacott, 2001).

The Musculoskeletal System

The musculoskeletal system provides form, support, stability and movement to the body,
which are all inherently required for postural control. It is made up of bones, muscles,
joints, cartilage, ligaments and other connective tissue; the integrity of these components
is essential for their successful functioning. Problems with the musculoskeletal system
could be congenital (i.e., present from birth) such as is seen in a condition called arthro-
gryposis multiplex congenita, a condition where contractures of the limbs are present
from birth. Otherwise, it could be as a result of an acquired or developmental condition or
disease such as multiple sclerosis, brain injury or muscular dystrophy, where neurological
damage may cause the muscle to become denervated (loss of nerve supply) and immo-
bile. Conversely, the nerve supply to the muscle might be overexcited and cause a limb to
become immobile due to being in a constant state of hypertonia or spasticity.
The long-term immobility of a joint or muscle can lead to muscle wasting, loss of mus-
cle bulk, shortening of muscle tissue and eventually contracture of the muscle or joint.
This loss of muscle strength, joint range of movement or muscle length will all negatively
impact on the body’s ability to sustain balance and postural control.

The Sensory System
The sensory system provides information to the CNS. The key sensory systems involved in
postural control are:
The somatosensory system: Proprioceptive, cutaneous and joint receptors provide
information on the position of the body and the forces and pressures acting on the
body in relation to the supporting surfaces. They also provide information about the
relationship of body segments to one another.
Proprioceptive sensory organs are located in muscles and tendons, or within the
connective tissues (ligaments and capsules) of joints. These feed into the postural
control system information regarding the status and function of the musculoskeletal
system, with a constant flow of information to the spinal cord, the cerebellum and
the brain. The most important sensory nerve endings for controlling the muscular
system are the muscle spindle fibres and the Golgi tendon organs. Muscle spindles are
found in the belly of skeletal muscles and provide the CNS with information regarding
the length of the muscle and whether it is in a state of stretch. The Golgi tendon
organs are found within the tendons that attach the muscle to bone and provide the