Different species, from mollusk to man, actively maintain a basic body posture (that is a particular orientation of their body in space) due to the activity of postural control system. For example, marine mollusk Clione and man maintain the vertical (headup orientation), the fish and terrestrial quadrupeds maintain the dorsal side-up body orientation. Deviations in any plane from this orientation evoke corrective movements, which lead to a restoration of the initial orientation. The stabile posture also presents a basis on which voluntary movements of different parts of the body can be superimposed. Maintenance of body posture is a non-volitional activity that is based, in many species, on innate neural mechanisms. Postural systems differ from those for movement control in their behavioral goals. The systems for movement control cause a movement of the whole body or its segments from one position in space to the other, as in walking or reaching. The systems for postural control prevent movements. they stabilize a position (or orientation) of the body in space, or orientation of its segments in space and in relation to each other.
Two principal modes of postural activity can be distinguished: (1) The feedback mode is compensation for the deviation from the desired posture (see Figure 1A). (2) The feedforward mode is anticipatory postural adjustments aimed at counteracting the destabilizing consequences of voluntary movements (see Figure 1B). In this lecture I will focus on the feedback mode of postural activity.