Preparations Of Animals For Experimental Studies

Preparations Of Animals For Experimental Studies Introduction

The various functions of nervous system, particularly the maintenance of posture and equilibrium are studied by observing the effects of sections or lesions at different levels of the central nervous system in animals. The commonly used animals are monkeys, dogs, and cats.

Decorticate Preparation

• Decorticate animal is the one without cerebral cortex. It is prepared by removing the whole cerebral cortex leaving basal ganglia intact. It is also prepared by removing all the connections of the cerebral cortex.
• The effect of decortication varies with the species and the conditions under which the animal is being examined.

Read And Learn More: Medical Physiology Notes

• In the dog or cat, when the animal is on its feet, posture is normal. The muscle tone is normally distributed and, it is present equally in the flexor and extensor muscles.
• Movements during walking can be performed by reflex activity. If the animal is suspended in the air, there is gQygPQ hyperextension of all the limbs. In decorticate monkey, the tone is gravely affected. Movements of walking cannot occur by reflex activity.

Effects in Man: In man, decorticate condition is caused by intracranial hemorrhage, head injury, brain abscess, or brain tumor. The decorticate condition is called decorticate rigidity.

Decorticate Rigidity: Decorticate rigidity is the abnormal postural changes that involve rigid extension of the lower limbs and flexion of upper limbs at elbow joint across the chest. The amis and fingers are also flexed. The posture may develop on one side or both sides of the body.

Effects on Reflexes: Reflexes at the neck level can be elicited. These neck reflexes affect the body also. When the neck is turned to the right, there is flexion of the lower and upper limbs on the opposite side. But, on the same side, there is extension of the limbs. It may be due to the cutting or lesion of direct corticospinal tract. Some fibers of the corticospinal tract are known to have an inhibitory influence on the extensor muscles.

Decerebrate Preparation

• It is prepared by removing all connections of cerebral hemispheres at the level of midbrain, by sectioning in between the superior colliculus and inferior colliculus.
• This preparation is characterized by a state of stiffness or rigidity, which is known as decerebrate rigidity, resembling the effects of upper motor neuron lesion. This preparation was first done by Nobel laureate Sir Charles Sherrington in cat.

Decerebrate Rigidity

• Decerebrate rigidity is the rigid extension of all the limbs due to decerebration. This type of rigidity is well-pronounced in the extensor muscles. The term decerebrate rigidity was coined by Sherrington in 1897.
• The reason for decerebrate rigidity is the release of the centers, situated below the section from higher inhibitory controls. The inhibitory area is 4S. It is situated in the motor cortex of the cerebrum, just anterior to area 4 and behind area 6. In this area, Betz cells are absent.
• From here, the fibers are projected to spinal cord via reticular formation. So in decerebration, the discharge from neurons of area 4S cannot reach spinal motor neurons. This leads to exaggeration of spina! motor neurons resulting in rigidity.
• Decerebrate rigidity is also produced by stopping the blood flow to the forebrain. It is done by occlusion of the common carotid arteries and the basilar artery at the center of the pons.

Opisthotonos: In decerebrate animals, the caricature or characteristic posture is the extension of all the four limbs, extension of the tail, and arching of the back or hyperextension of the spine. This type of attitude of the animal is called opisthotonos. The animal can stand but, if disturbed, the posture cannot be maintained.

Decerebration in Man: In man, decerebration occurs due to a lesion in the diencephalon or midbrain.

Thalamic (Midbrain) Preparation

• In this preparation, all the connections of thalamus with the cerebral cortex are removed by sectioning at the superior border of midbrain. All the fine sensations such as fine touch, tactile discrimination, and tactile localization are lost. The conscious kinesthetic sensation is also lost. But, the crude touch, pressure, pain, and temperature sensations remain intact.
• The righting reflexes are retained. The muscle tone is not affected. Rigidity is absent. But when the animal is held in the air, extensor rigidity develops. The coordination of the reflex movements is not lost. However, the exaggeration of movements occurs during emotional states. Abnormal involuntary movements are absent.

Spinal Preparation

• The complete transection of the spinal cord is called spinal preparation. The immediate effect of complete ruction of spinal cord is the spinal shock. The recovers from the shock after some time. The tone is returned to the flexor muscles.
• Extensor muscles do not regain the tone or regain it after some time and these muscles attain the flaccidity. It is because the facilitatory impulses from reticular formation are cut off in spinal preparation.