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the Semicircular Canal s, which indicate rotational movements; and the Otoliths , which indicate linear translations. The vestibular system sends signals primarily to the neural structures that control our eye movements, and to the muscles that keep us upright. The projections to the former provide the anatomical basis of the Vestibulo-ocular Reflex , which is required for clear vision; and the projections to the muscles that control our posture are necessary to keep us upright.


SEMICIRCULAR CANALS


Our world has three spatial dimensions. Accordingly, our vestibular system contains three Semicircular Canals in each Labyrinth . They are approximately Orthogonal to each other, and are called


Push-pull systems


The canals are cleverly arranged in such a way that each canal on the left side has an almost parallel counterpart on the right side. Each of these three pairs works in a ''push-pull'' fashion: when one canal is stimulated, its corresponding partner on the other side is inhibited, and vice versa.

This push-pull system allows us to sense all directions of rotation: while the ''right horizontal canal'' gets stimulated during head rotations to the right (Fig 2), the ''left horizontal canal'' gets stimulated (and thus predominantly signals) by head rotations to the left.


Vestibulo-ocular reflex (VOR)


The vestibular system needs to be fast: if we want clear vision, head movements need to be compensated almost immediately. Otherwise our vision corresponds to a photograph taken with a shaky hand. To achieve clear vision, signals from the semicircular canals are sent as directly as possible to the eye muscles. This direct connection involves only three neurons, and is correspondingly called
''Three-neuron-arc'' (Fig 3). Using these direct connections, eye movements lag the head movements by less than 10 ms, one of the fastest reflexes in the human body. The automatic generation of eye movements from movements of the head is called Vestibulo-ocular Reflex , or short ''VOR''.