Decerebration

For decerebrate response, see abnormal posturing.

Decerebration is the elimination of cerebral brain function in an animal by removing the cerebrum, cutting across the brain stem, or severing certain arteries in the brain stem.

Decerebration describes the ligation along the neural axis in distinct parts of the brain in experimental animals. Generally we distuingish between lower decerebration (the cut is made above the upper border of the pons), middle decerebration (cut is made through the red nucleus) and upper decerebration (cut is made so the cortical area is removed). As a result the animal abolishes certain reflexes which are integrated in different parts of the brain. Furthermore the reflexes which are functional will be hyperreactive (and therefore very accentuated) due to the removal of inhibiting higher brain centers (e.g. the facilitatory area of the reticular formation will not receive regulating input from cerebellum, basal ganglia and the cortex).

Lower decerebration results in a "Bulbospinal" animal: Reflexes which are integrated within the spinal cord and medulla oblongata are functional, reflexes integrated in midbrain and cortex are absent. Types of reflexes present in bulbospinal animal

Reflex	Stimulus	Response	Receptor
Stretch reflex	Stretch	Contraction of muscle	Muscle spindles
Positive supporting reaction (magnet reaction)	Contact with sole or palm	Foot extended to support body	Proprioceptors in distal flexors
Negative supporting reaction	Stretch	Release of positive supporting reaction	Proprioceptors in extensors
Tonic labyrinthine reflexes	Gravity	Contraction of limb extensor muscles	Otolithic organs
Tonic neck reflexes	Head turned (1) To side, (2) Up, (3) Down	Change in pattern of contraction (1) Extension of limbs on side to which head is turned, (2) Hind flex leg, (3) Foreleg flex	Neck proprioceptors

The most obvious accentuation is seen in the tonic labyrinthine reflexes, the otholitic organ mediates input about the gravitational force exerted on the body and the labyrinthine reflex acts on the extensor muscles in order to resist this gravitational force. In an animal where the cortical areas or the midbrain have been "cut off" from the neural axis, this reflex is hyperactive and the animal will maximally extend all four limbs. This phenomenon is known as decerebrate rigidity. In humans true decerebrate rigidty is rare since the damage to the brain centers it might be caused by usually are lethal. However decorticate rigidity can be caused by e.g. bleeding in the internal capsule which causes damage to upper motor neurons. The symptoms of decorticate rigidity are flexion in the upper limbs and extension in the lower limbs.

References

• Lange "Review of Medical Physiology", 22nd edition, pp. 202,ff