More importantly, cerebral interaction with body regions is both reversed and inverted with regard to the pre- and postcentral gyri. It is reversed in that the left hemisphere controls the right side of the body and vice versa. The inverted aspect describes how lower body regions are integrated in the uppermost aspects of the gyri and vice versa. Thus, damage to the lower left precentral gyrus would cause loss of motor ability on the right side of the face or head. Below the cerebral hemispheres are areas of the brain often collectively referred to as the brainstem .
Activities integrated in the brainstem are involuntary and below the realm of consciousness.
The first brainstem region is the diencephalon containing a number of important areas. The thalami , which direct information to and from the various cord tracts and brain regions, occur there. The floor of this brain region, the hypothalamus , contains the centers for thirst, sleep, temperature regulation, and osmoregulation via secretion of ADH from the related posterior pituitary. Sympathetic nerve tracts supplying the iris of the eye originate in the hypothalamus. When active, radial muscles of the iris are stimulated to contract causing the pupil to dilate.