How does vestibular apparatus work

Image sourced and used with permission from Centre for Neuro Skills. The vestibular system in each inner ear is made up of three semi-circular canals and two pockets, called the otolith organs, which together provide constant feedback to the cerebellum about head movement.

Each semi-circular canal has a different orientation to detect a variety of movements such as nodding or rotating. Movement of fluid inside the canals caused by head movement stimulates tiny hairs that send messages via the vestibular nerve to the cerebellum.

These organs contain small crystals that are displaced during these movements to stimulate tiny hairs, which transmit the message via the vestibular, or balance nerve to the cerebellum. During translational motion, the eyes will change their vergence angle as the visual object moves from close to farther away or vice versa. These responses are a result of activation of the otolith receptors, with connections to the oculomotor nuclei similar to those described above for the rotational vestibuloocular reflex.

With tilts of the head, the resulting eye movement is termed torsion , and consists of a rotational eye movement around the line of sight that is in the direction opposite to the head tilt. As mentioned above, there are major reciprocal connections between the vestibular nuclei and the cerebellum. There are two vestibular descending pathways that regulate body muscle responses to motion and gravity, consisting of the lateral vestibulo-spinal tract LVST and the medial vestibulo-spinal tract MVST.

Reflexive control of head and neck muscles arises through the neurons in the medial vestibulospinal tract MVST. The MVST neurons receive input from vestibular receptors and the cerebellum, and somatosensory information from the spinal cord. MVST neurons carry both excitatory and inhibitory signals to innervate neck flexor and extensor motor neurons in the spinal cord. For example, if one trips over a crack in the pavement while walking, MVST neurons will receive downward and forward linear acceleration signals from the otolith receptors and forward rotation acceleration signals from the vertical semicircular canals.

The VCR will compensate by providing excitatory signals to the dorsal neck flexor muscles and inhibitory signals to the ventral neck extensor muscles, which moves the head upward and opposite to the falling motion to protect it from impact. The LVST comprises a topographic organization of vestibular nuclei cells that receive substantial input from the cerebellum, proprioceptive inputs from the spinal cord, and convergent afferent signals from vestibular receptors.

LVST neurons contain either acetylcholine or glutamate as a neurotransmitter and exert an excitatory influence upon extensor muscle motor neurons. For example, LVST fibers produce extension of the contralateral axial and limb musculature when the body is tilted sideways. Some vestibular nucleus neurons send projections to the reticular formation, dorsal pontine nuclei, and nucleus of the solitary tract.

These connections regulate breathing and circulation through compensatory vestibular autonomic responses that stabilize respiration and blood pressure during body motion and changes relative to gravity. They may also be important for induction of motion sickness and emesis. The cognitive perception of motion, spatial orientation, and navigation through space arises through multisensory information from vestibular, visual, and somatosensory signals in the thalamus and cortex Figure 6A.

Vestibular nuclei neurons project bilaterally to the several thalamic regions. The posterior nuclear group PO , near the medial geniculate body, receives both vestibular and auditory signals as well as inputs from the superior colliculus and spinal cord, indicating an integration of multiple sensory signals. Some anterior pulvinar neurons also respond to motion stimuli and project to cortical area 3a, the posterior insula, and the temporo-parietal cortex PIVC.

In humans, electrical stimulation of the thalamic areas produces sensations of movement and sometimes dizziness. Area 2v cells respond to motion, and electrical stimulation of this area in humans produces sensations of moving, spinning, or dizziness. PIVC and areas 3a and 2v are heavily interconnected. Vestibular neurons also have been observed in the posterior parietal cortex; in area 7, in the ventral intraparietal area VIP , the medial intraparietal area MIP , and the medial superior temporal area MST.

VIP contains multimodal neurons involved in spatial coding. Lesions of the parietal cortical areas can result in confusions in spatial awareness. How these different cortical regions contribute to our perception of motion and spatial orientation is still not well understood.

Our ability to know where we are and to navigate different spatial locations is essential for survival. Cells in the limbic system and the hippocampus that contribute to these functions have been identified, including place cells, grid cells, and head direction cells Figure 6B. Head direction cells in the anterior-dorsal thalamus encode heading direction, independent of spatial location Taube, It is thought that these cell types work together to provide for spatial orientation, spatial memory, and our ability to navigate.

The pathway by which vestibular signals reach the navigation network is not well understood; however, damage to the vestibular system, hippocampus, and dorsal thalamus regions often disrupts our ability to orient in familiar environments, navigate from place to place, or even to find our way home. For example, reading a book in a car on a winding road can produce motion sickness, whereby the accelerations experienced by the vestibular system do not match the visual input. However, if one looks out the window at the scenery going by during the same travel, no sickness occurs because the visual and vestibular cues are in alignment.

Sea sickness, a form of motion sickness, appears to be a special case and arises from unusual vertical oscillatory and roll motion. The vestibular nuclei then pass the information on to a variety of targets, ranging from the muscles of the eye to the cerebral cortex. The vestibular system is a sensory system that is responsible for providing our brain with information about motion, head position, and spatial orientation; it also is involved with motor functions that allow us to keep our balance, stabilize our head and body during movement, and maintain posture.

Thus, the vestibular system is essential for normal movement and equilibrium. Vestibular sensations begin in the inner ear in the vestibular labyrinth, a series of interconnected chambers that are continuous with the cochlea. The most recognizable components of the vestibular labyrinth are the semicircular canals. These consist of three tubes, positioned approximately at right angles to one another, that are each situated in a plane in which the head can rotate.

This design allows each of the canals to detect one of the following head movements: nodding up and down, shaking side to side, or tilting left and right.

These movements of the head around an axis are referred to as rotational acceleration, and can be contrasted with linear acceleration, which involves movement forward or backward. The semicircular canals are filled with a fluid called endolymph, which is similar in composition to the intracellular fluid found within neurons.

When the head is rotated, it causes the movement of endolymph through the canal that corresponds to the plane of the movement. The endolymph in that semicircular canal flows into an expansion of the canal called the ampulla.

Within the ampulla is a sensory organ called the crista ampullaris that contains hair cells , the sensory receptors of the vestibular system. Hair cells get their name because there is a collection of small "hairs" called stereocilia extending from the top of each cell. Hair cell stereocilia have fine fibers, known as tip links, that run between their tips; tip links are also attached to ion channels.

When the stereocilia of hair cells are moved, the tip links pull associated ion channels open for a fraction of a millisecond.