Pathophysiological Essence
When a transverse section of the brainstem is made experimentally between the superior and inferior colliculi, a state of decerebrate rigidity develops instantly.
Primal physiological characteristic of this pathology is a total and massive increase in extensor muscle tone. Muscles contract so intensely that the animal's limbs extend, become rigid, and are practically unbendable.
Due to this muscle tension, the animal can theoretically be placed on all four paws. In normal physiology, this visual phenomenon is aptly termed "a caricature of standing." However, it is important to understand that the animal is completely incapable of standing independently without constant, ideal external balancing. It cannot actively maintain the given posture and instantly falls at the slightest disturbance of equilibrium, because the higher centers of fine spatial regulation are disconnected.
Role of the Midbrain and Red Nuclei
The neural centers responsible for generating and maintaining the state of rigidity are localized within midbrain structures. A key role in controlling muscle tone during this phenomenon is played by the red nuclei (nucleus ruber).
The dissociation of these nuclei from other parts of the central nervous system produces the classic picture of extensor hypertonus. Proof of the exceptional importance of this structure is a simple experiment: if an additional brainstem transection is performed below the level of the red nuclei, the state of rigidity completely disappears. This convincingly confirms that nucleus ruber is a critically important link in the cascade regulating muscle tension.
Cervical Postural Reflexes in Rigidity
Against the background of decerebrate rigidity, researchers can clearly observe the isolated function of cervical postural reflexes. In classic experiments, these are demonstrated in a decerebrate cat whose inner ear labyrinths have been previously destroyed. This is done specifically to completely eliminate the influence of the vestibular apparatus on muscular tone.
The triggering mechanism for these reflexes consists of afferent nerve impulses continuously arriving from neck muscle proprioceptors. When the position of the head is passively altered, limb tone changes in a strictly predictable manner:
- Passive head elevation: leads to a noticeable reflex decrease in the tone of the forelimb extensors.
- Passive head flexion (downward tilt): causes the exact opposite physiological effect — tension in the forelimb extensors increases sharply.
- Head rotation (turning to the side): provokes an asymmetrical motor response. For example, turning the head to the right reflexively increases the contraction of the extensor muscles in the right limbs specifically. Accordingly, turning the muzzle to the left elicits a similar reaction for the left half of the body.