I ANATOMY                 

III EYE MOVEMENTS &  NYSTAGMUS

IV FIXATION INSTABILITY

V SUPRANUCLEAR to NUCLEAR   

VI VESTIBULAR SYSTEM  

VII CEREBELLAR EYE MOVEMENTS    

VIII CN PALSIES, VISUAL FIELDS, PUPIL & THE EYE


 

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

 

The vestibular output from semicircular canals and the otolith organs travels via the vestibular branch of the vestibulo-cochlear nerve to the brainstem and the cerebellum in order to carry out the necessary computations for determining head position and motion.  The vestibular neurons are bipolar and their cell bodies lie in the vestibular nerve ganglion (Scarpa's ganglion). The distal processes of these cells carry signals from the vestibular apparatus, and the central processes project via the vestibular portion of the 8th cranial nerve to the vestibular nuclei (and also directly to the cerebellum). The vestibular nuclei are important centers of integration, receiving input from the vestibular nuclei of the opposite side, as well as from the cerebellum and the visual and somatic sensory systems.

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 to navigation and spatial orientation, and contributes to equilibrium, especially when walking on uncertain or slippery terrains, where the vestibulospinal input leads to appropriate adjustsments of muscle activity and body position in order to maintain posture1.

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.  Since we are usually unaware of a distinct sensation arising from vestibular activity, gradual vestibular loss may go unnoticed.

Additional reasons for our lack of awareness of a distinct sensation arising from vestibular activity are that vestibular input is integrated with visual, proprioceptive and other sensory information and it is this combined experience which leads to a sense of motion3,4. Note that there is no primary vestibular cortex that receives only vestibular signals, and that all cortical neurons which do receive vestibular signals also receive other sensory signals, especially visual and somatosensory. 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 body2.  As with other senses, vestibular afferents show a great deal of convergence: each primary afferent receives information from many vestibular hair cells, and secondary vestibular neurons of the vestibular nuclei receive input from many primaries.

The vestibular system provides us with our subjective sense of self-motion and orientation, and therefore plays a vital role in the stabilization of gaze and the control of balance and posture. The vestibular system achieves this by transducing the motion and positions of the head into signals which result in two important reflexes5

  1. The vestibular-ocular reflex ensures vision is maintained during head motion by stabilizing the line of sight (ie 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 in order 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 tracts6:

The semicircular canals contribute to postural reflexes, via projections to the medial vestibular nucleus, which gives rise to the medial vestibulospinal tract. 
- projects to cervical cord via the medial longitudinal fasciculus, and keeps the head still in space, mediating the vestibulo-colic reflex.

The utricle and saccule contribute to postural reflexes, via projections to the lateral vestibular nucleus, which gives rise to the lateral vestibulospinal tract (LVST). 
- projects to entire cord 
- allows the legs to adjust for head movements.
- provides excitatory tone to extensor muscles.
- decerebrate rigidity is the loss of inhibition from cerebral cortex and cerebellum on the LVST, and exagerates the effect of the tonic signal in the LVST

 

From: Vestibulospinal Tracts. Retrieved from: http://www.learnneurosurgery.com/vestibulospinal-tract.html

 

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, since 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.  Since the vestibular apparatus can provide information about change in orientation with respect to the head only, it cannot accomplish postural adjustments on its own. Sensors in the neck and other postural muscles are important in signaling to the central nervous system changes in the relationship between the head and the body7. These two systems function in conjunction with the eyes to form a remarkable control system that maintains us in an erect position in a wide variety of stable and unstable postures.

Note that inputs from 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 (eg, 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:

On earth, the otoliths help the brain interpret the position of the head in space by detecting head tilt relative to gravity. In the absence of a gravitational reference during spaceflight, the static otolith signals are no longer effective, and visual and proprioceptive cues are primarily used to interpret the position of the head8.

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 commands9.
For example, reaching to a target involves a series of sensory and sensorimotor processes, and 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. Cullen KE, Sadeghi S(2008) Vestibular system. Scholarpedia, 3(1):3013.
  4. Cullen KE. The vestibular system: multimodal integration and encoding of self-motion for motor control. Trends Neurosci. 2012;35(3):185-96.
  5. Frohman EM, Solomon D, Zee DS. Vestibular Dysfunction and Nystagmus in Multiple Sclerosis. Int MSJ 1995 3(3):87-99.
  6. Goldberg J. The vestibular system. Retrieved from: http://www.columbia.edu/itc/hs/medical/neuralsci/2004/slides/32_LectureSlides.pdf
  7. Mann M. The Nervous System In Action. Retrieved from: http://michaeldmann.net/mann9.html 
  8. Reschke MF, Kolev OI, Clément G. Eye-Head Coordination in 31 Space Shuttle Astronauts during Visual Target Acquisition. Sci Rep. 2017;7(1):14283.
  9. Baloh RW, Kerber, K. Baloh and Honrubia's Clinical Neurophysiology of the Vestibular System. Oxford University Press; 2010.