Notes

Vestibular Canals, Organs & Cells

The vestibular system plays a central role in the maintenance of equilibrium and gaze stability. (It is also involved in keeping eye position). The vestibular system, by means of its receptors for the perception of linear and angular acceleration, plays a central role in orientation. It seems to be designed to answer two basic questions:

• Which way is up?
• In which direction am I moving?

It is very elusive to test, as it is difficult to reach and stick-in electrodes to study. It presents five peripheral "receptors" (three Semicircular Canals, Utricule, Saccule).

• Semicircular Canals: respond to rotation acceleration of the head.
• Otolith Organs: respond to linear acceleration and static position.
  • Utricle
  • Saccule

The Semicircular Canals

• Posterior canal shares plane with contralateral anterior canal.
• Horizontal canals share plane.
• These canals contain two types of hair cells

Vestibular Hair Cells

• Type I (Inner)
• Type II (Outer)

Responses of the Cristae All Kinocilia are oriented in the same direction. Crista in each pair of canals respond inversely to each other. The Cupula presents a gelatinous structure, while the Endolymph is liquid. Once the liquid move, the Cupula bends in one or the other direction depending on the movement of the liquid, which in turn makes the cilia bending causing spikes elicitation. Hence, the cilia transform mechanical bending into electrical signals.

The Otolithic Organs

• Saccule: roughly vertical orientation, responds to acceleration components within the saggital plane.
• Utricle: horizontal (+ 30 degrees) orientation, responds to left-right acceleration.

Excitation Patterns in the Utricle

The Otoliths register linear acceleration and static tilt

Vestibular System Diagram

Vestibular Nuclei (VN)

Vestibular signals originating in the two labyrinths first interact with signals from other sensory systems in the VN. Only one fraction of the neurons in the VN receive direct vestibular input, and most neurons receive afferent input from other sensory systems (visual or proprioceptive) or regions of the CNS (cerebellum, reticular formation, spinal cord and contralateral VN). Consequently, the output of neurons from the VN reflects the interaction of many systems.

Vestibular Cortex

Vestibulo - Ocular Reflex (VOR)

To hold images of the seen world steady on the retina during brief head rotations.

Dizziness - Vertigo - Disequilibrium