Vestibular+Occular+Reflex

Vestibulo-ocular Reflex (VOR)
by Brittany Smith & Evan Smith


 * Overview**

One of three vestibular reflexes, the vestibulo-ocular reflex (VOR) is responsible for maintaining eye fixation during head rotation by moving the eyes in the opposite direction of the head's rotation, at approximately the same speed. As mentioned in the name, the VOR incorporates both the visual and the vestibular systems, tying together information from the semicircular canals and eye fixation. This reflex serves to allow gaze to be fixed during movement, though often declines or becomes limited in old age.


 * Functional Anatomy**

Vestibulo-ocular reflex consists of:
 * The vestibulo-ocular reflex involves a 3-neuron arc, consisting of the oculomotor nuclei, vestibular nuclei, and vestibular ganglion, sometimes referred to as Scarpa’s ganglia (which is the location of the cell bodies of the vestibular nerves) . This three part arc is the framework to the VOR and the pathways that generate the reflex are extremely complex in nature.
 * The 3-neuron arc, as the simplest VOR, consists of the following:
 * Primary Afferent Fibers: from the cristae of the semicircular ducts.
 * Vestibular Nuclei: neurons send their axons to the nuclei of the extraocular muscles (passing in the medial longitudinal fasciculus).
 * Motor Neurons: send their axons to the extraocular muscles.
 * Abducens nuclei otherwise known as Cranial Nerve VI (CN VI) which innervates the lateral rectus muscle is responsible for rotating the eyes laterally.
 * Oculomotor nuclei also known as Cranial Nerve III (CN III) which is primarily responsible for controlling medial rectus (causing eyes to pull in)

The actions involved in a leftward head rotation:
 * Head rotates left
 * Eyes turn to the right (at pretty much at the same speed) but in order to do that, have to
 * Contract right lateral rectus & left medial rectus muscle
 * Need to get motor nuclei and abducens to fire to get right eye to rotate out
 * Need to get motor nuclei and oculomotor to fire to get left eye to rotate in
 * Ampulopetal flow (or flow toward the utricle) – left lateral/horizontal canal which causes depolarization
 * Leaving via the Vestibulocochlear nuclei or Cranial Nerve VIII (CN VIII) to the left medial vestibular nuclei
 * Then with a neuron that projects its neuron, synapses in the medial lateral abducens - has output exciting right lateral rectus
 * Abducens nuclei (CN VI) will excite //contralateral// oculomotor nucleus (CN III) which results in //ipsilateral// excitement of initial vestibular nuclei firing
 * When the eyes are rotating – don’t want the antagonist muscles contracting therefore out of the vestibular nuclei (going to ipsilateral abducens, which inhibits lateral rectus) and projecting to oculomotor nucleus (which inhibits contralateral medial rectus).

If all of a sudden, you take out the left vestibular nerve, the right side is firing more and the eyes will begin a slow drift to the left - then realize they are not supposed to be there and will jump back (nystagmus) as if your head is turning (VOR).

Figure 1.1- Muscles of the eye and pathways (Purves)


 * Input & Output Pathways**

Beginning in the vestibular system - the semicircular canals are activated by rotation of the head. Each canal sends impulses through the vestibulocochlear nerve (CN VIII) through Scarpa's ganglia ending in the vestibular nuclei in the brainstem. === ===

Stemming from these nuclei, fibers will decussate to the contralateral cranial nerve VI (abducens) nuclei synapsing with either the pathway projecting directly to the lateral rectus of the eye (via abducens nerve) or a nerve tract projecting from the abducens nucleus (via the abducens internuclear interneurons or abducens interneurons) to the ocularmotor nuclei containing motorneurons that drive eye muscle activity, specifically activating the medial rectus muscles of the eye through the oculomotor nerve.

As head rotates to the left, the discharge of the vestibular fibers supplying the left horizontal canal is increased while the discharge on the right is decreased. Resulting in excitation of the right abducens nerve fibers innervating the right lateral rectus muscle and the left oculomotor nerve fibers innervating the left medial rectus muscle. All of this will result in turning the eye to the right through ensuing muscle contractions.

** Figure 1.2- This image represents and illustrates the function and pathways of the VOR. **

 * Function in Control of Normal Movement**

The vestibular system is predominantly involved in monitoring head motion and positioning. Fixating on an object and moving the head from side to side gives rise to the action of the vestibulo-ocular reflex. During this motion the eyes automatically compensate for movement of the head. This is done by moving the eyes in the opposite direction while at the same distance. Allowing the image to be kept on the same area of the retina, and providing a constant picture.

The semicircular canals house sensory information (the semicircular canals) that directs the eyes to move in opposition to the movement of the head, the vestibular system will detect brief, transient changes in the positioning of the head and then rapidly correct eye movements. Distal processes of Scarpa's ganglia innervate semicircular canals and otolithic organs, as the central processes project to vestibular nuclei through the vestibular portion of cranial nerve VIII.

Vestibular nuclei serve as integration centers, receiving information and input from the contralateral vestibular nuclei, cerebellum, somatosensory and visual systems. The vestibular system is responsible for three different classes of reflexes by way of the central projections. Cranial nerve VIII is the primary link between the brainstem and the cerebellum, with information coming from the end organs of the vestibular system.

Processing occurs within the brainstem and cerebellum. The cerebellum is involved in a feedback loop that monitors the accuracy of movement perception.


 * Signs of Dysfunction**

The human body is a remarkable creation which is taken for granted for on a daily basis, unless a mechanism or system malfunctions. Myriad of people are unaware of the systems in their body, such as the vestibulo-ocular reflex, until a dysfunction occurs and does not allow for proper function and control. Minor dysfunction of the vestibulo-ocular reflex can manifest itself in many ways, depending on the location and size of the lesion, and possibly result in a number of disorders.

Listed below, in this chart are some lesions or disorders which can result in impairment in the VOR.

Table 1.1 - Vestibular-ocular Reflex Dysfunctions: Manifestations (link-http://emedicine.medscape.com/article/1836134-overview)
 * < **Lesion or Disorder** ||< **Clinical Signs and Symptoms** ||< **Gain** ||< **Phase (Time Constant)** ||< **Symmetry** ||
 * < Unilateral peripheral lesion, acute ||< Nystagmus in plane of the affected canal; spontaneous nystagmus ||< Decreased toward side of lesion ||< Decreased vestibuloocular reflex time constant ||< Asymmetric (directional preponderance) ||
 * < Unilateral peripheral lesion, compensated ||< ... ||< Still decreased (but closer to 1) ||< Time constant rises slightly compared with acute ||< Asymmetric ||
 * < Bilateral peripheral lesion, acute ||< Oscillopsia, visual impairment ||< ... ||< Vestibuloocular reflex time constant <6 s ||< ... ||
 * < Bilateral peripheral lesion, compensated ||< Some oscillopsia with rapid head movement ||< Near normal ||< Decreased time constant ||< ... ||
 * < Cerebellar ||< Oscillopsia, down-beat nystagmus ||< Little change (increased or decreased) ||< ... ||< Asymmetric ||
 * < Central vestibular dysfunction ||< Oscillopsia, purely vertical or torsional nystagmus, balance disorders, spontaneous nystagmus ||< Decreased or increased (variable) ||< Prolonged or shortened time constant (variable) ||< Asymmetric ||


 * Dizziness and Loss of Vestibular Receptors****:**

As mentioned in the overview, the integrity of the vestibulo-ocular reflex diminishes with age. As people get older, the vestibular system begins to break down showing itself in a variety of ways, from a loss of balance, increased likelihood of falls, benign prolonged positional vertigo, etc. Generally after the age of 75, people begin to report a less accurate vestibular reflex, meaning that they are less able to focus, or fixate, while their head is rotating, or that their eyes have a tendency of going with their head's rotation and then jumping back. Sometimes this includes suppression, which is necessary when the head rotates in the same direction as the visual scene which for some would result in dizziness. The VOR insures that the gaze is kept fixed at one point in the environment when the head rotates in order to keep the retinal image stable, but if it is damaged or diminished it can result in a slew of problems. An injury to the vestibular system generally results in impaired VOR, this reflex can also be impaired via systemic disease processes.


 * Summary / Conclusion**

Overall, the VOR is one of the many body systems or reflexes that is overlooked until something goes wrong with it. It is one of the many body functions that we simply take for granted. Imagine if we shook our head no and our eyes went with us, just think of the dizziness that would occur every time you disagreed with someone. Of course, if you are the one attempting to convince someone to agree then an aversion to dizziness might work in your favor, but for the most part it is crucial to be able to turn one's head without the eyes following along blindly, no pun intended. A multitude of lesions and diseases can impact the VOR as well as the gradual breakdown of body systems that comes with old age. As of yet, there is no way to reverse these signs of aging, but that is one direction research could begin to travel. The VOR is a link between the vestibular and ocular systems, it links the proprioception with the visual input. It could be shown to demonstrate how important vision is to balance, the perception of motion, and other vestibular constructs. In the meantime, it functions to provide the ability to fixate while the head is rotating, and the complexity for such a seemingly simple task is astounding.


 * Glossary**

//__gaze__//: to look steadily and intently.  //__nystagmus__:// involuntary eye movements. //__semicircular canals__//: three fluid-filled bony channels in the inner ear, situated at right angles communicating with the brain regarding orientation and balance. //__vestibular system__:// a complex system of the inner ear that functions in mediating the vestibular sense and consists of the saccule, utricle, and semicircular canals, called also vestibular apparatus.
 * // __vestibulo-ocular reflex (VOR)__: // A reflexive movement of the eyes stimulated by rotational movements of the head, stabilizes the visual image on the retinas. **


 * Links / Suggested Readings**

1. __Video:__ http://www.youtube.com/watch?v=j_R0LcPnZ_w&feature=related //The following URL opens a video describing and demonstrating horizontal and vertical vestibulo-ocular reflex movements.//

2. __Research Article__: http://www.springerlink.com/content/k06u8aaglqmq2kfh/fulltext.pdf //This article refers to the VOR as a model for motor system learning because of it's simple and well documented behaviors.//

3. __Research Article:__ http://jn.physiology.org/cgi/content/full/81/5/2415 //This article investigates the dependence of the VOR upon varying angular changes.//

4. __Research Article__: [] //This article tests subjects VOR response to changing head and bodily movements.//

5. __Research Article__: [] //This article discovers that semicircular canal occusion can cause permenant VOR changes.//


 * Quiz Questions & Answers**


 * True/False**

1. The human body does not have three vestibular reflexes, but rather two specialized arcs.

2. The 3-neuron arc consists of Primary Afferent Fibers, Vestibular Nuclei, and Motor Neurons.

3. The Abducens nuclei otherwise known as Cranial Nerve VI, innervates the medial rectus muscle and is responsible for rotating the eyes medially.

4. Serving as integration centers, Vestibular nuclei receive information and input from the cerebellum, contralateral vestibular nuclei, visual and somatosensory systems.

5. The VOR is a link between only the occular systems, obtaining sensory information from the cerebellum.


 * Multiple Choice**

6. Fixating on an object and moving the head from side to side gives rise to the action of: a) Smooth Pursuit b) Optokinetic Reflex c) Vestibulo-ocular Reflex d) Dizziness e) None of the above

7. Which Cranial nerve(s) is the primary link between the brainstem and the cerebellum? a) I b) VI c) III d) VIII e) IV f) I & III g) III & IV h) None of the above

8. What are the clinical signs and symptoms of Central Vestibular Dysfunction? a) Oscillopsia b) Purely vertical or torsional nystagmus c) Balance disorders d) Spontaneous nystagmus e) A & C f) All of the above g) None of the above

9. Semicircular canals will send impulses through the vestibulocochlear nerve by way of Scarpa's ganglia, ending in the vestibular nuclei in the __. a) Brain b) Cerebellum c) Basal ganglia d) Brainstem e) Vestibular System f) A & D g) C & D

10. The Vestibulo-ocular Reflex mainly serves to allow for: a) Stabilization of the visual image on the retinas. b) Stabilization of the head motions during movement. c) Helpful proprioceptive information. d) None of the above.


 * Short Answer:**

1.) Describe the mechanisms and structures involved in the VOR - using both words and diagrams.

2.) How would an older person experience a problem in the VOR? Detail the structures and/or mechanisms that would be most affected.


 * Essay:**

1.) You are at a hockey game and a focused on what is going on out on the ice, and the person you are sitting with is trying to carry out a conversation with you at the same time. In order to offer the respect necessary to the person you are talking to, but to not miss a second of the play you turn your head quickly toward your conversing neighbor while keeping both eyes focused on the game. Explain how this is even possible.


 * Answers:**

1. F 2. T 3. F 4. T 5. F 6. C 7. D 8. F 9. D 10. A


 * References**

1. Brodel, Per. //The Central Nervous System: Structure and Function // . Oxford: Oxford UP, 2004.

2. Manali, Amin. "Vestibuloocular Reflex Testing: EMedicine Clinical Procedures." EMedicine-Medical Reference. Medscape, 2010. Web. 01 May 2010. .

3. "Neural Learning Rules for the Vestibulo-Ocular Reflex -- Raymond and Lisberger 18 (21): 9112 -- Journal of Neuroscience." //The Journal of Neuroscience Online//. Web. 5 May 2010. .  4. Purves, Dale. //Neuroscience //. Sunderland, Mass.: Sinauer Associates, 2004.

5. Squire, Larry R. //Fundamental Neuroscience //. San Diego, Calif.: Academic, 2003.