Vestibulo-Ocular Reflex

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Introduction

The position of the head in space and quick changes in the direction of head movement are sensed accurately by the vestibular system.  When the head is in motion or while walking on the streets, we are able to see the surrounding clearly. The Vestibulo-ocular reflex very well contributes to the maintenance of clear vision. When we move our head to the sides, to maintain a clear vision the eyeballs don’t move along with the head but moves towards the opposite side. These movements of the eyeballs help to fix the gaze on the object and thus we see it clearly.

The Vestibulo-ocular reflex (VOR) is one among the reflex of vestibular system. Others are Vestibulospinal reflex (VSR) and Vestibulo-colic Reflex (VCR). The VOR generates eye movements that enable clear vision while the head is in motion. The VCR acts on the neck musculature to stabilize the head. The VSR generates compensatory body movement to maintain head and postural stability and thereby prevent falls.

Abnormalities of vestibular system result in sensations of dizziness or unsteadiness, which do reach our awareness, as well as problems with focusing our eyes and keeping our balance.

Peripheral processing

The vestibular portion of the labyrinth Contains five receptors: Three semicircular canals and two otolith organs

Semi Circular canals

• Anterior Semicircular Canal
• Posterior semicircular canal
• Horizontal Semicircular Canal

Otolith Organs

• Utricle
• Saccule

Semicircular Canals

The SCCs register the angular acceleration and they are at a right angle to each other. It delivers sensory input about head velocity, and that aids the VOR to generate an eye movement which is equivalent to the velocity of the head movement. Sensory receptors of SCC are situated in the ampulla, and the corresponding ampullary crest which contains vestibular hair cells is projected into the cupula which enables the VOR to generate an eye movement that matches the velocity of the head movement.      The angular motion of the head either vertical or horizontal marks in either increase or decrease in hair cell activity and an opposite change in neuronal activity in paired canals. The firing in the neurons returns to its normal state in prolonged motion of the head. Otolith Organs

The utricle and saccule register forces related to linear acceleration. They respond to linear head motion and static tilt with respect to the gravitational axis. Unlike the SCCs, the Otoliths doesn’t need hydrodynamic systems to complete its works, it is embedded with calcium carbonate crystals ‘Otoconia’. In an upright individual, the saccule is vertical (parasagittal), whereas the utricle is horizontally oriented (near the plane of the lateral SCC).

The saccule senses linear acceleration in the sagittal plane, associated with a forward pitch of the head. The utricle senses acceleration in horizontal plane, provoked by a roll (lateral tilt) of the head.

Central Processing

The neurons from both SCCs and Otolith organs joins to eighth cranial nerve and their cell bodies are in the vestibular ganglion also known as Scarpa’s ganglion. The axons then enter into the pons and most of them goes to floor of the medulla, where the vestibular nuclei are located. A portion of vestibular sensory receptors travels to the cerebellum, reticular formation, thalamus and cerebral cortex.

There are four vestibular nuclei

• Medial vestibular nuclei
• Lateral vestibular nuclei
• Superior vestibular nuclei
• Inferior vestibular nuclei

The medial and superior vestibular nuclei receive input from the SCCs. The output of the medial nucleus is to the medial vestibulo spinal tract for controlling neck muscles by its connections to cervical spinal cord. It also plays a very crucial role in coordinating the interaction between head and eye movement. Neurons from medial and superior vestibular nuclei has its ascending connections to motor nuclei of the ocular muscles and thus helps in stabilizing the vision during head motion.

The input to the lateral vestibular nuclei is from SCCs, utricle, cerebellum and also from the spinal cord. The output is to the vestibulo-ocular tracts and lateral vestibulo spinal tracts which activates the antigravity muscles in neck, trunk and limbs.

The inferior vestibular nucleus receives neuron from SCCs, Otolith organs and cerebellar vermis. The output is to vestibulospinal tract and vestibulo reticular tract.

In summary, vestibular system consists of both static and dynamic functions. The static functions are controlled by otolith organs, they monitor the absolute position of the head in the space which is very important in postural control. the dynamic functions are predominantly by the SCCs. It helps in monitoring the head rotations and angular acceleration thereby fixing the gaze through VOR.