Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Regulation of Posture and Movement
System for Posture Regulation - Role of the Cortex

Effects of Decortication

Extirpation of the Cerebral Cortex (decortication) does not cause significant motor impairments in many mammalian species. In primates, the consequences of this intervention are more pronounced, yet purposeful movements remain possible. Decorticate animals exhibit all the reactions characteristic of mesencephalic animals. Furthermore, they are easier to maintain than mesencephalic ones because their thermoregulation and other visceral homeostatic mechanisms, integrated within the Hypothalamus, remain unimpaired (see Chapter 14). The most striking deficit is the inability to respond using previously acquired (learned) behaviors. In animals with an extirpated cortex, conditioned Reflexes can only be restored through specialized training, though achieving this under standard laboratory conditions is extremely difficult.

Decorticate Rigidity

Moderate rigidity develops in decorticate animals following the extirpation of the cortical area from which gamma-efferent activity is inhibited via the reticular formation. Similar to the rigidity resulting from a transection of the CNS rostral to the superior border of the Midbrain, decorticate rigidity is masked by phase postural reflexes and becomes apparent only when the animal is completely at rest; it is observed on the affected side in humans experiencing hemiplegia due to Hemorrhage or thrombosis in the internal capsule. Due to Anatomical Features, the small Arteries of the internal capsule are presumably particularly susceptible to rupture or thrombosis. Consequently, decorticate rigidity of this origin is quite common: 60% of intracerebral hemorrhages occur in the internal capsule, 10

- in the cerebral cortex, 10% in the Pons, 10% in the thalamus, and 10%

- in the Cerebellum.

The exact localization within the cerebral cortex of the fibers that inhibit stretch reflexes remains a subject of debate. Evidence suggests that under certain experimental conditions, stimulation of the anterior border of the precentral gyrus inhibits both stretch reflexes and cortically induced movements. This region, adjacent to the basal nuclei, is referred to as area 4s, or the suppressor strip.

Hopping and Placing Reactions

Two Types of postural reactions—hopping and placing—are severely impaired following decerebration. The first of these consists of hopping movements that adjust THE POSITION OF the limbs to support the body when a lateral shove is applied. The second ensures firm contact between the FOOT and the supporting surface. These reactions are elicited in blindfolded animals suspended in the air when any part of the foot touches a supporting surface. Similarly, if an animal's muzzle or even its facial vibrissae Touch the edge of a table, the animal immediately places both forelimbs onto the table; furthermore, if a limb of a standing animal is displaced from the support surface, it is promptly repositioned. Vestibular placing reactions were discussed previously. Cats, dogs, and primates, when lowered toward a surface, extend their limbs toward it provided they can see it.