VERTIGO and DIZZINESS

Subtitle

What is vestibulo-ocular reflex and how important it is for us?

Vestibulo-ocular reflex The vestibulo-ocular reflex is  a reflex which helps us to bring the image we are trying to fixate, on our fovea. The fovea is the center most part of the macula. This tiny area is responsible for our central, sharpest vision.  A healthy fovea is key for reading, watching television, driving, and other activities that require the ability to see detail.  Unlike the peripheral retina, it has no blood vessels.  Instead, it has a very high concentration of cones (photoreceptors responsible for color vision), allowing us to appreciate color. To be clearly perceived, images of the outside world have to slide over retina at a speed of no more than few degrees per second. The images otherwise would appear fuzzy and blurred like a photograph taken with a low shutter speed while camera is moving. If the vestibulo-ocular reflex would not have been in existence, the eyes would have remained anchored in the orbit with every disruptive head movement. In this arbitrary situation, because of inability to fixate vision on a target, the images intended to be seen would appear blurred/fuzzy. The vestibulo-ocular reflex drives the eye with same velocity but in the opposite direction in the plane of disruptive head movement. The sensory arc of the reflex begins at the semicircular canals of the inner ear. Each of the three (horizontal, anterior and posterior) semicircular canals is stimulated by movements in its plane and also induces eye movements in its plane (Flouren’s law). The corresponding semicircular canals of both ears are yoked in such a way that when the head rotates, one canal increases its rate of firing while the corresponding canal in the opposite ear slows its rate. The impulses travel by the way of the vestibular nerve to the ipsilateral vestibular nuclear complex in the pontomedullary junction. Fibers from the ampullae of the semicircular canals terminate primarily in the superior nucleus and rostral part of the medial nucleus, whereas fibers from the utricular macula end predominantly in the lateral nucleus, with some fibers also going to the medial and descending nuclei. When a vestibular nucleus is excited, it tends to deviates the eyes toward the contralateral side. However, each vestibular nuclear complex has neurons that increase the rate of firing with ipsilateral head rotations and others that increase their discharge rate with contralateral rotations. This feature coupled with the existence of the vestibular commissure in the vicinity of the vestibular nuclei, may explain why vestibular function recovers when one side is damaged. From the vestibular nucleus the signal for horizontal eye movements is relayed to the abducens nucleus in the contralateral side of the lower pons. Vestibular nucleus neurons project not only to motor neurons but also send collaterals to the nucleus propositus hypoglossi and the cell groups of the paramedian tracts. The nucleus propositus hypoglossi and adjacent medial vestibular nucleus make up the neural integrator, which is crucial for gaze holding; the paramedian tract cells relay information about eye movements to the cerebellar flocculus.

What is anatomy of peripheral vestibular system?

Anatomy of Peripheral Vestibular System  

Osseous (grey/white) and membranous (lavender) labyrinth of the left inner ear. Perilymph fills the osseous labyrinth external to the membranous labyrinth, whereas endolymph fills the membranous labyrinth.

The important structures are three semicircular canals and its constituents-

• Semicircular Canals (SCC)

  • Horizontal

  • Anterior

  • Posterior

• Cupula

• End organ receptors

• Endolymph

The spatial orientation of the semicircular canals is such that the posterior canal on one side is in the same plane as the contralateral anterior canal and both horizontal canals are in the same plane, 30º above the horizontal as depicted below-

The utricle is  

• Connected to SCC

• Contains endolymph

• Otoliths (otoconia)

   • Calcium carbonate

   • Attached to hair cells

   •  Macule (end organ)

Turning the head causes the endolymph fluid within the semicircular duct to move. This action bends the cupula and the kinocilia and stereocilia of the hair cells. The hair cells are stimulated to release calcium, leading to nerve impulses with a firing rate measurable in the vestibular nerve that is proportional to the angular acceleration of the head. A more rapid head turn produces a stronger impulse than a slow turn.