Structures of the vestibular system

Answer 1

The vestibular system detects angular and linear acceleration. This is made possible by two related parts of the peripheral vestibular system: 1) the otoliths (utricle and saccule), which are sensitive to accelerative forces in a single plane (e.g. gravity); and 2) the semicircular canals - of which there are three at roughly orthogonal planes to each other - that are sensitive to accelerative forces in a circular plane (e.g. if you were to sit in the middle of a merry-go-round).

There are peripheral vestibular organs located on each side of the head behind the ear. Technically they are referred to 'inner ear organs'.

The primary function of the vestibular system (collectively: the peripheral organs + VIII cranial nerve + brainstem vestibular nuclei) is to detect accelerative forces and generate nerve impulses to the brain for integration with other senses and motor control. For simplicity we refer to the vestibular system as providing 'balance-sense' - however, 'balance' is how the brain interprets the signals. The organs themselves sense acceleration.

The brain needs these signals in order to stabilize the eye muscles during head movements, allowing us to maintain gaze on a target. Vestibular signals are also used for axial-muscle motor control which allow us to maintain a center of gravity whilst upright. Blood-pressure regulation is also influenced by the vestibular system during changes in posture from a supine to an upright position.

Vestibular signals are extensively integrated throughout the brain. In fact, they can even influence mood, the ability to think (calculate), and our memory systems - particularly spatial memory (i.e., 'cognitive mapping' of our environment). Interestingly, mood-centers of the brain also feed back into the vestibular system, which is why people afraid of heights (acute anxiety) may get a sudden attack of vertigo (perceived movement of the world, in the absence of real movement) if high up on the top of a building, say. The opposite is also true - people with balance disorders often develop DSM-IV psychiatric comorbidities, like agoraphobia, generalised anxiety disorder, and depression. The same people also have excruciating difficulty remembering where they are in relation to where they want to go - and an unfortunate consequence of this is that they often get lost. Even in familiar places.

In terms of evolution, the vestibular system is the oldest sensory system. This may explain the intricate dependency the brain has for information about bodily acceleration.

Answer 2

The system consists of the peripheral organs, the VIIIth cranial nerve, and the vestibular nuclei in the brainstem. The peripheral organs include the semicircular canals (3 per vestibular organ, also referred to as the vestibular labyrinth), and the otoliths (the saccule and utricle).