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