VPB 121: Veterinary Physiology - II (2+1)

MOTOR CONTROL OF THE BRAIN

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

• 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

• This 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.