The vestibular system in vertebrates, is a part of the inner ear. It is the
sensory system that provides the leading contribution to the sense of balance
and spatial orientation for the purpose of coordinating movement with balance.
Together with cochlea, a part of auditory system, it constitutes the labyrinth of
the inner ear.
In brief: as movements consist of rotations and translations, the vestibular
system comprises two components: the semi-circular canals which indicate
rotational movements and the otoliths which indicate linear acceleration. The
vestibular system sends signals primarily to the neural structures that control
eye movement; these provide anatomical basis of the vestibulo-ocular reflex;
which is required for clear vision. Signals are also sent to the muscles that keep
an animal upright and in general control posture; these provide anatomical means
required to enable an animal to maintain its desired position in space.
In order for the body to maintain a gaze stabilization, orientation, navigation,
balance etc. (these are outputs), the brain analyses the inputs in form of
equilibrium system (ear), visual input (eye is a very dominant input) etc.
,Your own vestibular organ is able to detect high frequency of head movement.
You cannot see clearly when finger moves >1Hz but head can move up to 6Hz.
Basically, everything that moves have a vestibular system. It is important to
understand that the visual input analysis is slow and auditory input is much
faster. That is why we often hear something before we can see it (ex. Thunder).
If there is a disease, then there might be an issue with either input or output.
What constitutes the vestibular system?
- Labyrinths (3 semi-circular canals and otoliths)
- Vestibulo-cochlear nerve (8th)
- Vestibular nuclei in the brain stem
- Vestibular connections with cerebellum
- Vestibular connections with cortex
- Vestibulo ocular reflex
o This reflex elicits compensatory eye-movements during head
movements.
- Vestibulo spinal reflex (jerk is a third derivative of acceleration)
- Perception of self in space
- Vestibulo autonomic system
The main functions of the vestibular system is to generate information to the
central nervous system with a fourfold purpose:
1) Provide general orientation of the body with respect to gravity
2) Enable balanced locomotion and body position
3) Readjust autonomic functions after body reorientation
4) Ensure gaze stabilization (this is final goal of vestibular system function)
, Other ocular mobility and stability systems include:
Smooth pursuit = maintains image of a small moving target on the fovea
(the centre of the retina, and the region of the highest visual acuity). You
cannot do smooth pursuit on the imaginary target.
Saccades = rapid eye movement that brings the object of interest onto
the fovea. Basically just movement left to right or up down.
Optokinetic = stabilizes large moving images on the fovea
Visual fixation = maintains image of a stationary object on the fovea
Vergence = moves the eyes in opposite directions so that the images are
held simultaneously on both foveae.
The orientations of the canals cause a different canal to be stimulated by
movement of the head in different planes, and more than one canal is
stimulated at once if the movement is off those planes. The lateral canal
detects angular acceleration of the head when the head is turned, and the
superior and posterior canals detect vertical head movements when the head
is moved up or down.
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