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I ANATOMY                 

III CLINICAL EXAMINATION  

IV FIXATION INSTABILITY

V SUPRANUCLEAR to NUCLEAR   

VI VESTIBULAR SYSTEM  

VII CEREBELLAR EYE MOVEMENTS    

VIII VISUAL FIELDS, PUPIL & THE EYE

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An excellent summary of this material is found here: The Vestibular System. Angelaki D, Dickman JD.

The vestibular system acts to stabilize vision and balance, and is involved in the control of compensatory eye movements in response to local head movements in order to maintain visual fixation.  To do this, it transforms forces associated with head acceleration and gravity into biological signals that travel directly to motor centres for postural and ocular stability, as well as to the cortex to help with orientation. 

The system also possesses the ability to detect motion of the head and body in space, which is fundamental for navigation and spatial orientation, and contributes to equilibrium, in particular while walking on uncertain or slippery terrains1.

Remarkably, the vestibular system is constantly active:
" The messages never stop and cannot be turned off. Even when we are completely motionless, they signal the relentless pull of gravity. Perhaps because of their constant monologue, the vestibular sensation is different to the other senses. There is no overt, readily recognizable, localisable, conscious sensation from these organs. They provide a silent sense"2.

The vestibular system thus transduces the motion and positions of the head into signals that result in these two important reflexes3

  1. The vestibular-ocular reflex ensures vision is maintained during head motion by stabilizing the line of sight (i.e. gaze)
  2. The vestibulospinal reflex helps keep the head and body upright.

Control of eye movement is the clearest example of how the vestibular system creates a stable representation of external space, in this case visual space. The vestibular system profoundly influences eye movements via the vestibuloocular reflex to stabilize the visual image on the retina in the presence of head motion2.
The vestibular nuclei represent the final premotor center for all types of slow eye movements, including not only the vestibuloocular reflexes, but also optokinetic nystagmus, and pursuit. Many neurons in the vestibular nuclei also show a burst or pause in activity during saccades and fast phases of vestibular nystagmus. Several types of vestibular nuclei neurons have been described, differing in their response properties to rotation, saccades, and smooth pursuit eye movements. In particular, the position-vestibular-pause (PVP) cells are thought to represent the second-order neuron in the three-neuron arc pathway, shown below.:

 

Figure 1. The three-neuron pathway connecting the vestibular system to eye movements

 


All terrestrial and aquatic animals need to know which way is up and, therefore, which way gravity acts, so it is not surprising that special systems appear early in evolutionary history for the detection of gravity.
A sense of the force of gravity and which way is up is with us at all times. This internal construction is based on multiple sensory sources, important among which are the vestibular organs2.

Figure 2. The importance of knowing which way is up (See: Subjective Visual Vertical)

 

Figure 3. Vestibular mechanisms influence postural tone and walking through the descending vestibulospinal tracts:

The vestibular system does not operate in isolation, but in concert with all of the other available sensory information. The other senses provide additional and different views of the state of the body.
The vestibular system's intergration with visual, proprioceptive and other extra-vestibular information is such that the combined experience leads to a sense of motion. As with other senses, vestibular afferents show a great deal of convergence: each primary afferent innervates many hair cells, and secondary vestibular neurons of the vestibular nuclei receive input from many primaries.

We are usually unaware of a distinct sensation arising from vestibular activity; gradual vestibular loss, as opposed to acute, may even go unnoticed, as compared with other senses.  Unlike most forms of sensation, vestibular system afferents are noteworthy, since both spike timing and rate codes coexist at the sensory periphery, and not centrally5.
Note that there is no primary vestibular cortex that receives only vestibular signals, and that all cortical neurons receiving vestibular signals also receive other sensory signals, especially visual and somatosensory.

 

The vestibular system encodes self-motion information by detecting the motion of the head-in-space. It provides us with our subjective sense of self-motion and orientation, and therefore plays a vital role in:

The vestibular apparatus responds to changes in the body’s position in space. 

 

Figure 4. The vestibular organs are fixed in the skull so that their signals are always referenced to the coordinate frame of the head:

For the brain to control the eyes is straightforward since the vestibular organs are referenced to the head, and the eyes are locked in the head. However, the head can adopt a wide range of positions with respect to the body, and to function correctly, the brain needs to transform vestibular signals from a head-fixed to an earth-fixed reference frame, the brain needs to know the relative orientation of all body segments between the feet and the head. This knowledge it gets from the non-vestibular senses2

Note that inputs these two systems, vestibular and non-vestibular, are complementary but provide different information: for example, when standing and maintaing balance, there may be different  head-on-body positions (e.g., head upright vs facing down) which will activate different populations of afferents from otolith and semicircular canal end organs, but non-vestibular whole-body postural corrections will be identical. Naturally, the converse also applies, constant vestibular signals of head motion may need whole-body movement to take place in different directions, depending on which way the head is facing.

Figure 5. The difficulties of determining body position during weightlessness:

The vestibular system must integrate multiple internal representations of head and body movement obtained from several different sensory systems into a single internal coding of space that provides a frame of reference for encoding motor commands5.
For example, reaching to a target involves a series of sensory and sensorimotor processes;localization of the target with respect to the subject’s body is the initial source of the motor response.
Spatial encoding for movement needs:

 

References

  1. Laurens J, Angelaki DE. The functional significance of velocity storage and its dependence on gravity. Exp Brain Res. 2011;210(3-4):407-22.
  2. Day BL, Fitzpatrick RC. The vestibular system. Curr Biol. 2005;15(15):R583-6.
  3. Frohman EM, Solomon D, Zee DS. Vestibular Dysfunction and Nystagmus in Multiple Sclerosis. Int MSJ 1995 3(3):87-99.
  4. Cullen KE. The vestibular system: multimodal integration and encoding of self-motion for motor control. Trends Neurosci. 2012;35(3):185-96.
  5. Baloh RW, Kerber, K. Baloh and Honrubia's Clinical Neurophysiology of the Vestibular System. Oxford University Press; 2010.