The generation of the decerebrate state through vessel occlusion leads to hyperactivity in the lateral vestibulospinal tract due to damage to Purkinje cells in the anterior vermis of the cerebellum. These Purkinje cells provide the primary inhibitory projection to the lateral vestibular nucleus. Even after transection of the dorsal roots, the hypertonia caused by the lateral vestibulospinal tract remains, indicating its direct influence on α motor neurons.

Observations of decerebrate animals reveal extensor hypertonus and increased phasic stretch reflexes, highlighting the significance of motor control pathways originating in the brainstem. Certain brainstem systems play a crucial role in posture and locomotion. Eye movement control also heavily relies on brainstem circuits.

Head movement or neck bending triggers several reflex mechanisms, including vestibular reflexes, tonic neck reflexes, and righting reflexes. These reflexes involve sensory receptors such as the vestibular apparatus, which responds to head movement, and stretch receptors in the neck.

Vestibular reflexes are one type of postural reflex. The semicircular canals are stimulated by head rotation, and in addition to generating eye movement, they lead to postural adjustments. These adjustments are mediated by commands transmitted through the lateral and medial vestibulospinal tracts and the reticulospinal tracts. The lateral vestibulospinal tract activates extensor muscles for posture support. For example, if the head is rotated to the left, postural support increases on the left side to prevent falling. A person lacking labyrinthine function in the left ear tends to fall to the left, while irritation of the left labyrinth causes a tendency to fall to the right. The medial vestibulospinal tract causes contractions of neck muscles opposing the induced movement, known as the vestibulocollic reflex.

Tilting the head affects the linear acceleration on hair cells of the otolith organs in the vestibular apparatus. This can produce eye movement and postural adjustments. For instance, when a quadruped tilts its head and body forward, extension of the forelimbs and flexion of the hind limbs occur to restore the body to its original orientation. Conversely, tilting the head and body backward results in flexion of the forelimbs and extension of the hind limbs. The otolithic organs also contribute to the vestibular placing reaction. In preparation for landing, stimulation of the utricles leads to extension of the forelimbs.

Tonic neck reflexes represent another type of positional reflex, activated by muscle spindles in the neck muscles. Neck muscles contain a high concentration of muscle spindles. Bending the neck evokes tonic neck reflexes without involvement of the vestibular system. Neck extension leads to extension of the forelimbs and flexion of the hind limbs, while neck flexion produces the opposite effects. Bending the neck to the left results in greater contraction of extensor muscles in the left limbs and relaxation of flexor muscles in the right limbs.

Chapter 9: Organization of Motor Function.
When vessel occlusion is used to generate the decerebrate state, the lateral vestibulospinal tract becomes hyperactive because of damage to Purkinje cells in the anterior vermis of the cerebellum, which provide the major inhibitory projection to the lateral vestibular nucleus. This hypertonia is actually not lost after transection of the dorsal roots, which implies that the lateral vestibulospinal tract is acting to a significant extent directly on α motor neurons (either monosynaptically or via interneurons).
Brainstem Control of Posture and Movement
The importance of motor control pathways that originate in the brainstem is evident from observations of the extensor hypertonus and increased phasic stretch reflexes that occur in decerebrate animals. Particular brainstem systems have been identified as influencing posture and locomotion. Brainstem circuits are also critically involved in the control of eye movement; these circuits are discussed in a separate section at the end of the chapter.
Several reflex mechanisms are evoked when the head is moved or the neck is bent. There are three types of postural reflexes: vestibular reflexes, tonic neck reflexes, and righting reflexes. The sensory receptors responsible for these reflexes include the vestibular apparatus (see ), which is stimulated by head movement, and stretch receptors in the neck.
The vestibular reflexes constitute one class of postural reflex. Rotation of the head activates sensory receptors of the semicircular canals (see ). In addition to generating eye movement, the sensory input to the vestibular nuclei results in postural adjustments. Such adjustments are mediated by commands transmitted to the spinal cord through the lateral and medial vestibulospinal tracts and the reticulospinal tracts. The lateral vestibulospinal tract activates extensor muscles that support posture. For instance, if the head is rotated to the left, postural support is increased on the left side. This increased support prevents the person from falling to the left as the head rotation continues. A person who has any disease that eliminates labyrinthine function in the left ear tends to fall to the left. Conversely, a person with a disease that irritates (stimulates) the left labyrinth tends to fall to the right. The medial vestibulospinal tract causes contractions of neck muscles that oppose the induced movement (vestibulocollic reflex).
Tilting the head also changes the linear acceleration on individual hair cells of the otolith organs of the vestibular apparatus. The resulting changes in hair cell activity can produce eye movement and postural adjustment. For example, when a quadruped, such as a cat, tilts the head and body forward (without bending the neck and consequently without evoking the tonic neck reflexes), the result is extension of the forelimbs and flexion of the hind limbs. This vestibular action tends to restore the body toward its original orientation. Conversely, if the quadruped tilts the head and body backward (without bending the neck), the forelimbs flex and the hind limbs extend. Otolithic organs also contribute to the vestibular placing reaction. If an animal, such as a cat, is dropped, stimulation of the utricles leads to extension of the forelimbs in preparation for landing.
The tonic neck reflexes are another type of positional reflex. These reflexes are activated by the muscle spindles found in neck muscles. These muscles contain the largest concentration of muscle spindles of any muscle in the body. If the neck is bent (without tilting of the head), the neck muscle spindles evoke tonic neck reflexes without interference from the vestibular system. When the neck is extended, the forelimbs extend and the hind limbs flex. The opposite effects occur when the neck is flexed. Note that these effects are opposite those evoked by the vestibular system. Furthermore, if the neck is bent to the left, the extensor muscles in the limbs on the left contract more, and the flexor muscles in the limbs on the right side relax.
CHAPTER 9 Organization of Motor Function