(Rigidity occurs in all muscles of the body) (210-11 Ganong 22nd) (692- Guyton 11th) (247- Ganong 23rd)DECEREBRATE - RIGIDITY -* When the brain stem of an animal is sectioned below the mid level of the mesencephalon. But the pontine and medullary reticular system as well as the vestibular system are left intact, the animal develops a condition called decerebrate rigidity* This rigidity does not occur in all muscles of the body but does occur in the antigravity muscles - the muscles of the neck and trunk and the extensors of the leg.* The cause of decerebrate rigidity is blockage of normally strong input to the medullary reticular nuclei from the cerebral cortex, the red nucleus and the basal ganglia. Lacking this input, the medullary reticular inhibitor system become non functional; Full overactivity of the pontine excitatory system occurs, and rigidity develops.* Decerebrate rigidity is found to be spasticity due to diffuse facilitation of stretch reflexes* The facilitation is due to two factors: increased general excitability of the motor neuron pool and increased in the rate of discharge in the y' efferent neuron* Decerebration produces no phenomenon akin to spinal shock. And the rigidity develops as soon as the brain stem is transected