The vestibulospinal reflex is a brainstem reflex whose circuitry involves the vestibular organ and nuclei vestibulares. Activation of the reflex causes contraction of the extensor muscles while inhibiting the flexor muscles of the extremities. In decerebration rigidity, the reflex becomes prominent.
What is the vestibulospinal reflex?
A brainstem reflex is known as the vestibulospinal reflex, and its circuitry involves the vestibular organ and the nuclei vestibulares. Reflexes are the invariable motor responses of organisms to a specific stimulus. True reflexes cannot be suppressed and are completely beyond voluntary control. The reflex-triggering stimulus is registered by sensory cells and transported in the form of excitation along afferent nerve pathways into the central nervous system, where it is switched to efferent motor nerves and reaches the involved effectors or muscles at the end of the reflex arc. The vestibulospinal reflex, or VSR, follows this pattern. The VSR is a brainstem reflex that, similar to the vestibulo-ocular reflex, is wired through the nuclei vestibulares and the vestibular organ. The motor response of the reflex is in a contraction of the extensors. These are the muscles of the extremities that realize the extension of the limbs. In contrast, the flexors correspond to the muscles for realizing flexion. While the extensors are contracted by the vestibulospinal reflex, the flexors are simultaneously inhibited by the VSR. The reflex occurs in response to stimuli from the vesibular organ. When this vestibular organ sends stimuli that signal an imbalance to the central nervous system, the nervous system stabilizes the posture of the body via activation of the VSR. The reflex is one of the vestibular reflexes, which serve to control eye, head, and body position at rest.
Function and task
The vestibulo-spinal reflex corresponds to an automatic, involuntary response to stabilize body posture. The first site of the reflex arc is the stimulation of the organ of equilibrium, so mainly by a head movement. The so-called vestibular nuclei with afferents from the organ of equilibrium play a major role in the reflex. These nerve nuclei are in close connection with the motoneurons in the spinal cord. When a person falls forward, this close connection allows a reflexive compensatory step forward to intercept the impending fall. Vestibular reflexes are a condition for standing and walking, but beyond these functions they also influence the neck muscles and head position. For example, turning the body triggers a reflex that results in a compensatory head movement in the opposite direction. In this way, the visual axis is automatically stabilized. The position of the head in relation to the body is determined by proprioceptors in the neck, which become active together with the labyrinth as body position receptors. The neck reflexes can be triggered by passive head rotations and, once triggered, act on the limb muscles and trunk muscles. In this way, vestibular reflexes regulate body balance in the form of an interaction of vestibulo-spinal responses and a neck reflex, which in turn affects the muscles of the extremities. The vestibulo-spinal reflex is wired through a total of four neurons. When the body or a single side of the body suddenly drops, the macular organs utriculus and sacculus in the vestibular organ are stimulated. This increases the discharge rate of the hair cells. This increased discharge rate is associated with the release of glutamate into the synaptic cleft between the afferents of the vestibulocochlear nerve and the hair cells. The afferent fibers of the first neuron project to the four vestibular nuclei. For the vestibulospinal reflex, the vestibular nucleus vestibularis lateralis, which corresponds to the second neuron in the reflex arc, is of particular importance. From here, the reflex is connected to the first motoneuron of the tractus vestibulospinalis, which corresponds to the third neuron in the reflex arc. This neuron draws as an extrapyramidal spinal cord tract to the individual spinal cord segments and projects in the anterior horn to the second motoneuron and fourth neuron of the reflex arc, which draws to the extensors of the limbs. The vestibulospinal pathway has an uncrossed course.In this way, the unilateral sinking of the vestibular organ during stumbling causes a contraction of the mutual extensors. On the other hand, when the entire body sinks, because the ground sinks fully, the extensors are activated on both sides of the body. The tractus vestibulospinalis causes inhibition of alpha-motoneurons at the same time. The vestibulospinal reflex does not depend on the cerebral cortex.
Diseases and disorders
Decerebration is the medical term for a functional uncoupling of the brainstem from the so-called telencephalon, which can occur as a result of direct traumatic injury or an increase in intracranial pressure after cerebral ischemia, cerebral hemorrhage, and tumors. At the onset of decerebration, the vestibulospinal reflex appears with all distinctness. This phenomenon thus characterizes, for example, the dying process. The extensors of the dying person’s extremities contract and the dying person falls into the so-called decerebration rigidity. Decerebration is usually accompanied by fixed pupils and impaired consciousness. In decerebration rigidity, these phenomena are associated with spastic extensor posture of the extremities, which is caused by an interruption of the brainstem in the four mound plate area. In the described phenomenon, the interrupted area lies below the nucleus ruber and at the same time above the nucleus vestibularis lateralis. Due to the interruption, the nucleus ruber no longer exerts an inhibitory influence on the motoneurons of the individual extensors. In a consequence, there is an excessive, uninhibited activity of the extensors, which is realized by the tractus vestibulospinalis. In addition to massive tone of the extensor muscles, those affected by decerebration show a loss of balance.
