Posture

Development of Postural Control

Two models of the central nervous system are the neural mechanisms underlying the development of posture: the reflex–hierarchical model and the systems model. The reflex–hierarchical model outlines an order of reflexes from least to most complex that are controlled by components of the central nervous system, accordingly increasing in complexity. Primitive reflexes (e.g., the rooting reflex or turning the head when the cheek is touched) are controlled by the spinal cord. The brain stem controls the asymmetric tonic neck, or fencing, reflex (i.e., when the neck is turned to one side, the arm and leg of that side extend while the other arm and leg flex); the symmetric tonic neck, or crawling, reflex (i.e., when the head and neck are extended, the arms extend and the knees bend); and the tonic labyrinthine reflex (i.e., when the head looks up, the arms and legs fully extend, and when the head is curved, the arms and legs fold in). The righting reactions, which are responsible for maintaining the body’s alignment, are controlled by the midbrain. Finally, equilibrium reactions, a highly complex reflex, are controlled by the cerebral cortex. According to this model, voluntary movement control is achieved either through inhibition of more primitive reflexes or through adding on to each level of reflexes to make them the foundation for the voluntary movement. This theory posits that each postural milestone must master its equilibrium reactions before attaining the next milestone.

The systems or distributed control model of the central nervous system states that the body’s movements change as a function of its continuously changing environment. In essence, our actions and movements differ depending on which position we are in. According to this theory, the systems involved in postural control are postural muscle response synergies; visual, vestibular, and somatosensory systems; motor and sensory systems; and muscle strength, joint range of motion, and body morphology. The synergy of postural muscle responses is used to control balance. Infants’ and toddlers’ strategies of postural control need to shift to accommodate their rapidly changing bodies and therefore their constantly fluctuating centers of gravity. Visual, vestibular, and somatosensory systems detect a loss of balance. Children learning to stand are initially more influenced by visual cues than adults because their bodies transmit less sensory information, whereas adults predominantly use somatosensory inputs. Individuals continue to depend on visual cues for balance through adolescence. Similarly, children and adults alike use somatosensory information from ankle joints to calibrate their bodies. Sensory reweighting, a complex skill, involves taking in all available sensory information and adjusting one’s body accordingly. This skill requires understanding that any change in sensory input affects all other sensory inputs. Mature development combines sensory reweighting with an anticipatory process, a complex internal model, which allows an individual to predict the effects of self-motion. Finally, motor and [Page 1700]sensory systems make modifications to accommodate changes in the environment.

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