Demonstration of the motor control of the brain
Motor control system of the brain is studied by removal of a particular portion and observing the alterations in the somatic motor activity.
Decorticate preparation
Decorticate preparation is demonstrated in cat or dog. In such preparations cerebral cortex with underlined white matter will be removed. Such animal will exhibit normal standing posture.
When the animals are suspended in space they extend their limbs. This extension disappears when they contact the supporting surface. In such preparations tactile placing and visual placing reflexes are absent. Vestibular placing, tonic reflexes and body righting reflexes are present.
Righting reflex
To perform righting reflex and progressive movement certain brain structures are necessary. They are sub thalamic nucleus, Red nucleus, substantia nigra with inter connecting fiber tracts, basal ganglia, reticular formation and cerebellum.
These enters responsible for righting reflexes are located within the rostral mid brain. This can be demonstrated in amphibians/animals when it is put on its back it immediately rights itself. When it is placed in water it swims. Frog response to various stimuli by jumping and crawling.
Decerebrate preparation
Decerebrate preparation preparation emphasis the importance of the motor activity of structures or parts of the brain anterior to the pons and lower mid brain. This preparation is done by transection of the brain stem at the level of mid brain.
Such animal exhibit pronounced antigravity rigidity that is depended on hyper excitability of the segmental myotatic reflex. This is known as “Decerebrate rigidity”.
This is primarily due to hyper excitability of gamma motor neurons. Hence, otherwise known as “GAMMA RIGIDITY”. Transecting the dorsal root supply to the particular limb will be abolished by this rigidity.
The higher centre responsible for the development of rigidity is located within pontine reticular formation. During this preparation pontine reticular formation is relieved from inhibitory influences of cerebral cortex.
Other portion of brain stem reticular formation is also renowned of inhibitory influences from brain stem. These causes increase in reticulo spinal excitation on gamma motor neurons supplying intra fusal fibres.
To maintain the decerebrate rigidily cerebellum and verticular system play a major role. In the decerebrate preparation, tonic labyrinth and tonic neck reflexes are elicited.
Vestibulo spinal tracts are major system contributed to decerebrate rigidly. Righting reflexes like vertibular lighting reflexes are absent in these preparation. They stand when placed in a standing position but they cannot be moved.
Decrebrated animal will have increase in muscle tone of both flexors and extensors of the fore limbs. The limbs become pillars while standing and tail become erect and horizontal.
Head is extended and body shows hyper extension (hobby/cortoon/caricature animal) Increase in muscle tone is due to intact vestibulo spinal tract but, removal of inhibitory reticulo spinal tract.
Normal standing posture is exaggerated. Stretching postural reflexes like stretch reflex, supporting reactions, tonic reflexes labyrinth reflexes are present.