PSYC2274 Chapter Notes - Chapter 12: Angular Acceleration, Linear Motion, Vestibular System
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- The vestibular organs are a set of five specialized sense organs, and these are located in
the inner ear right next to the cochlea.
- The vestibular organs sense head motion and orientation with respect to gravity.
- They also make predominant contributions to the sense of tilt and self-motion.
- Equilibrium is comprised of spatial orientation perception (that is, perception of linear
motion, angular motion, and tilt) combined with reflexive vestibular responses like
posture, vestibulo-autonomic reflexes, and vestibulo-ocular reflexes.
- Spatial orientation is a sense consisting of 3 modalities that interact.
- Perception of linear motion, angular motion, and tile.
- More on this later!!
- The vestibular “sixth sense” is often overlooked, but we don’t know why!
- It contributes to clear vision when we move our heads, helps us maintain balance
when we stand, and so much more…
- Probably not recognized because it has no one centralized location.
- However, loss of vestibular function/malfunction is debilitating! Affects posture,
walking, eye control, etc.
- The fundamental nature of this vestibular system is shown by the fact that the organs
appeared early on in evolutionary history and haven’t really changed since then.
- Also, the system is largely automatic. In fact, vestibular perception is often regulated to
the attentional background. In addition, many of its responses are reflexive.
- An example of this is the vestibulo-ocular reflex (VOR), which helps us see clearly by
reflexively rotating the eyeballs in the sockets to compensate for head rotation. This helps
keep images stable on the retina.
So…what are the vestibular contributions to equilibrium?
- Well, the vestibular system provides the sensory foundation for spatial orientation, which
includes perception of translation, rotation and tilt.
- The vestibular system also makes important contributions to balance by helping us stay
upright and stable
- Through the VOR, it also helps stabilize eyes during head motion and makes crucial
contributions to clarity of sight.
- Thus, the sense of equilibrium due to vestibular contributions is ACTIVE, not PASSIVE!
- Our sense of equilibrium combines info flowing from our brain to our muscles
with info flowing in to the brain from various sensory systems, especially from
visual, vestibular, and kinesthetic systems.
- Basically, kinesthetic refers to the perception of the position and movement of our limbs
in space.
- The term active sensing includes self-generated probing of the environment, and
balances info from efferent commands (info flowing from CNS out to periphery, such as
motor commands regulating muscle contraction) with info from afferent signals (info
flowing in to CNS from sensors in the periphery).
- In summary…the vestibular system contributes to our sense of equilibrium, but it does
not exclusively provide the sensory foundation for any perceptual modality.
Modalities and qualities of spatial orientation
- As mentioned above, the perception of spatial orientation includes 3 modalities. The
senses of:
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- Angular motion → rotational motion like that of a spinning top or swinging
saloon doors that rotate back and forth.
- Linear motion → translational motion like the predominant movement of a
train car or bobblehead doll.
- Tilt → to attain a sloped position like that of the Leaning Tower of Pisa.
- But why do we call these modalities instead of qualities? Color and brightness are
different qualities…why not these?
- Well, the key lies in the energy being transduced.
- Transduction is to convert from one form of energy to another.
- Color and brightness are different interpretations of the exact same energy (light),
which is why they were both considered qualities.
- On the other hand, seeing and hearing involve different types of energy (light and
pressure waves).
- Similarly, perceiving angular motion, linear motion, and tilt requires that 3
different stimuli be transduced.
How is angular motion, linear motion, and tilt sensed?
- These 3 stimulation energies are all sensed by two types of vestibular sense organs.
- These are…
- The semicircular canals → any of 3 toroidal tubes that sense angular
acceleration, which is the rate of change of angular velocity. This signal makes a
predominant contribution to our sense of angular motion.
- The otolith organs → any of 2 mechanical structures (called the utricle and the
saccule) that sense both linear acceleration (the rate of change of linear velocity)
and gravity (a force that attracts a body towards the center of the Earth).
- The 2 types of sense organs establish at least 2 sensory modalities…but then how come
there are 3 modalities???
- Well, the perception of tilt comes from the brain’s estimate of orientation with
respect to gravity, and the perception of linear motion results from the brain’s
estimate of linear acceleration.
- But, classic physics teaches us that gravity and linear acceleration are very
different from one another, and the brain actually does its best to separate the
signals from the otolith organs into signals representative of gravity and those of
linear acceleration.
- In summary, there are 3 interacting sensory modalities (sense of angular motion, sense of
linear motion, and sense of tilt) that parallel 3 sources of stimulation energy (angular
acceleration, linear acceleration, and gravity).
- NOTE: the vestibular system only detects ACCELERATION, not movement!
What about the qualities of spatial orientation?
- Each of the 3 spatial orientation modalities include 2 qualities:
- Amplitude → this is the size increase/decrease of a head movement.
- Direction → the line you move along/face, with reference to the point or region you
move toward/face.
Amplitude in depth…
- In respect to linear motion, translation can have a high velocity (like a jet before takeoff)
or low velocity (a car inching forward in traffic).
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- We can perceive rotational velocity with a high (vigorously shaking head) or low (slow
rotation of the minute hand on a clock) amplitude.
- In addition, tilt amplitudes can be small (gently nodding your head) or large (when you
lie down or hang upside down).
Direction in depth…
- Let’s first define the Cartesian coordinate system that moves with the head.
- X-axis points forward, y-axis points out the left ear, and z-axis points out the top
of the head.
- Also, 3 directions define our sense of angular motion (aka our head can rotate in 3
independent ways)...
- Roll angular velocity, pitch angular velocity (nodding head yes), and yaw angular
velocity (shaking head no).
- These 3 angular motions can be combined to represent any 3D head rotation.
- Also, 3 directions define our sense of linear motion.
- Stepping forward/back along x-axis, sliding from right/left along y-axis,
translating up/down along z-axis.
- Likewise, these 3 linear motions can be combined to represent any 3D linear
motion.
- There are also 2 tilt directions for each orientation.
- When you’re upright, you may experience a pitch tilt forward/backward.
- You also might experience a roll tilt to the left or right.
- But…what happened to the third dimension????????
- When head rotations align with gravity, there is no change in head tilt.
- THUS, there are 3 directions for angular velocity/translation, but only 2
for tilt!!!!!!!!
So…what about the mammalian vestibular system?
- Our vestibular organs are about the size of a large pea and can be found in the inner ear
next to the cochlea. They respond primarily to head motion (linear and angular) and head
tilt with respect to gravity.
- Each inner ear contains one vestibular labyrinth, and each one includes 5 sense organs (3
semicircular canals [rotational motion] and 2 otolith organs [gravity and linear
acceleration] ).
- Neither of the sense organs (semicircular canals / otolith organs) respond to constant
velocity, which is the speed and direction in which something moves.
- Instead, they respond to CHANGES in velocity → acceleration!
What about the hair cells?
- Hair cells refer to any cell that has stereocilia for transducing mechanical movement in
the inner ear into neural activity sent to the brain. Some hair cells also receive inputs
from the brain. Thus, these are mechanical transducers!
- Hair cells act as mechanoreceptors in each of the 5 vestibular organs and respond to
mechanical stimulation (pressure, vibration, or movement).
- Any head motion causes hair cell stereocilia to deflect, and this causes a change in hair
cell voltage, which alters NT release and evokes APs in vestibular-nerve fibers that have
synapses on the hair cell. Then, these AP are carried to the brain by afferent neurons.
- In the absence of a stimulation, hair cells have a negative voltage and release NT at a
constant rate, which evokes a constant rate of APs in the afferent neurons.
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Document Summary
The vestibular organs are a set of five specialized sense organs, and these are located in the inner ear right next to the cochlea. The vestibular organs sense head motion and orientation with respect to gravity. They also make predominant contributions to the sense of tilt and self-motion. Equilibrium is comprised of spatial orientation perception (that is, perception of linear motion, angular motion, and tilt) combined with reflexive vestibular responses like posture, vestibulo-autonomic reflexes, and vestibulo-ocular reflexes. Spatial orientation is a sense consisting of 3 modalities that interact. Perception of linear motion, angular motion, and tile. The vestibular sixth sense is often overlooked, but we don"t know why! It contributes to clear vision when we move our heads, helps us maintain balance when we stand, and so much more . Probably not recognized because it has no one centralized location. However, loss of vestibular function/malfunction is debilitating!
