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Summary 2.4 Problem 9: Balance
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This is a detailed summary of problem 9 of course 2.4 Perception. It includes notes from a youtube video that helped me understand some concepts better.
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Problem 9: BalanceCase 1: LG
1. What parts of the vestibular system are involved in balance and how do they work?
2. What happens when there is a deficit in the vestibular organs?
Source: Yantis, 2nd ed, Chapter 13, pp. 471-474
The Vestibular System: Perceiving balance and Acceleration
Acceleration: is part of any motion that involves a change in speed or direction. e.g. you undergo
linear acceleration when you speed up or slow down in a car that’s moving in a straight line, and you
undergo acceleration around a turn (even if you don’t change speed) because of the change in
direction. In each case, you feel the acceleration in your body, which is bend backward as you
accelerate (speed up), forward as you decelerate (slow down), and to the side as you go around a
turn. The only motion that does not involve acceleration is motion in a straight line at a constant
speed.
Without information about head and body position and acceleration, you would constantly be in
danger of tilting, losing your balance, and falling to the ground. Simply standing or sitting upright
requires constant sensing of balance, so you can reflexively adjust your body if you start to lead on
one side.
The neural signals carrying information about balance and acceleration are produced by hair calls
within a specialized collection of sensory organs in the inner ear, adjacent to the cochlea on both sides
of the head and collectively referred to as the vestibular system.
Vestibular system: the sense organs used to produce neural signals carrying information about
balance and acceleration; includes the semi-circular canals and the otolith organs.
Semi-circular canals: part of the vestibular system; three mutually perpendicular hollow curved tubes
in the skull filled with endolymph; responsible for signalling head rotation. (parts of the nearby
cochlea are also filled with endolymph).
- At the base of each canal is a small chamber called an ampulla (plural ampullae), also filled with
endolymph and containing a specialized structure called a crista (plural cristae). The cristae contain
the hair cells, with stereocilia enclosed within a gelatinous cap called a cupula.
- The otolith organs which are responsible for signalling when the head is undergoing linear
acceleration or being held in a tilted position, consist of the utricle and the saccule, each of which
contains a specialized structure with hair cells called a macula. Within each macula, the hair cells are
oriented in different directions and are embedded in a gelatinous layer topped by tiny crystals called
otoconia/otoliths, this fibrous structure filled with otoconia is called the otolithic membrane.
Rotation of the head causes the endolymph within one or more of the semi-circular canals to move;
the movement of the endolymph displaces the capula, which causes the stereocilia of the hair cells to
bend, which in turn causes the hair cells rate of neurotransmitter release to increase above baseline
or decrease below baseline, depending on the direction of the head rotation. Because the three
canals are perpendicular to each other, each different head rotation evokes a different pattern of
relative neurotransmitter release by the hair cells in the three canals; thus, each such pattern serves
as a neural code indicating how the head is rotating.
, When the head is held in a tilted position
e.g. looking at shoes, gravity pulls on the
otoconia in a different direction than
when the head is held upright, and the
force of this gravitation pull causes the
otoconia to drag on the gelatinous layer
in the macula, with resultant bending on
the hair cells’ stereocilia and changes in
neurotransmitter release corresponding
with the particular tilt of the head. The
stereocilia in the maculae are oriented in
different directions, so each different
acceleration or tilt of the head results in
different amounts and directions of
bending of the stereocilia. Thus, just as
with the hair cells of the semi-circular
canals, the different patterns of
neurotransmitter release by the hair cells
of the otolith organs function as a code
indicating the degree and direction of
acceleration and tilt of the head. (video
notes page 13.)
Brain & auditory nerve
Nerve fibers carrying neural signals
from the hair cells in the semi-circular
canals and the otolith organs bundle
together to form the vestibular nerve,
which carries these signals to the
vestibular complex in the brain stem. The
vestibular complex contains multiple
vestibular nuclei, which receive signals
not only from the vestibular system, but
Other regions receiving signals from the vestibular nuclei e.g. the frontal also from the visual system, the system
eye field are involved in controlling eye movements in order to maintain that controls eye movements, and the
a stable gaze, which is among the most important functions of the motor systems responsible for neck
vestibular system. Typically, when redirecting the gaze, the first movements. Signals from the vestibular
movement is a saccade, a rapid movement of the eyes only, to point the nuclei go to several brain regions,
eyes at a new target. The saccade is often followed by a turn and/or a tilt including the parietal insular vestibular
of the head, so the eyes and head end up aligned with one another, both cortex (PIVC), deep in the lateral sulcus
pointing at the new target. The hair cells in the semi-circular canals which also receives signals from virtually
produce signals in response to this head movement, which in turn cause every other cortical region that receives
signals to flow from the vestibular complex to the oculomotor system, signals from the vestibular nuclei.
which moves the eyes in a direction opposite to the direction of the head The PIVC is thought to provide a
movement in order to keep the eyes pointed at the target while the head representation of the position and
is moving. This unconscious compensating movement of the eyes during orientation of the head, which can be
head movement is known as the vestibulo-ocular reflex. As the basis for used as a basis for maintaining balance
the remarkable stability of visual experience despite the nearly constant during complex movements.
movement of the head, this reflex must occur almost instantaneously— The brain combines signals from the
the elapsed time from the beginning of a head movement to the vestibular system with signals from other
beginning of the compensating eye movement is just 5 msec. Damage to parts of the body to produce a unified
the vestibular system can lead to vertigo, a false sensation in which the perception of whole-body position,
individual or the individual’s surroundings seem to move or spin. One of balance, and movement.
the most common causes of vertigo is otoconia crystals becoming
dislodged from a macula and accumulating in one of the semi-circular
canals: when the head is turned or tilted to one side, the crystals produce
abnormal forces on the capula, stimulating the hair cells in a way that
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