Chapter 12: Spatial Orientation and the Vestibular System Flashcards
Vestibular organs
Set of 5 organs
— 3 semicircular canals
— 2 otolith organs
Located in each inner ear.
Sense head motion and head orientation with respect to gravity
Also called the “vestibular labyrinth” or the “vestibular system”
An often overlooked sense:
The vestibular “sixth sense”
Evolutionarily very old
The vestibular organs help us in many ways:
Provide a sense of spatial
Allow for the vestibulo-ocular reflex
Stabilizes visual input by counter rotating the eyes to compensate for head movement
Provide a sense of spatial orientation, consisting of
- Linear motion
- Angular motion
- Tilt
Allows for the Vestibulo-Ocular Reflex:
Stabilizes visual input by counter rotating the eyes to compensate for head movement
vestibulo-ocular reflex
Stabilizes visual input by counter rotating the eyes to compensate for head movement
Semicircular canals
The 3 toroidal tubes in the vestibular system that sense angular acceleration, a change in angular velocity
Otolith organs
The mechanical structures in the vestibular system that sense both linear acceleration and gravity.
Source of our sense of linear velocity and gravity
The vestibular organs do not respond to constant velocity
They only respond to changes in velocity, aka acceleration
Push-pull symmetry
Hair cells in opposite ears respond in a complementary fashion to each other
When hair cells in the left ear depolarize, those in the analogous structure in the right ear hyperpolarize
Coding of direction in the semicircular canals
3 semicircular canals in each ear
Each canal is oriented in a different plane
Each canal is maximally sensitive to rotations perpendicular to the canal plane
Hair cell responses
In the absence of stimulation, hair cells release neurotransmitter at a constant rate
When hair cell bundles bend, change in hair cell voltage is proportional to the amount of deflection
Bending toward tallest stereocilia: Depolarization
Bending away from tallest stereocilia: Hyperpolarization
Hair cells increase firing to rotation in one direction and decrease firing to rotation in the opposite direction
Semicircular canals
Each one is about 3/4 of a toroid (donut) shape, measuring 15mm long and 1.5mm in diameter
Canals are filled with a fluid called perilymph
A second, smaller toroid is found inside the larger toroid, measuring 0.3mm in diameter
Formed by a membrane filled with fluid called endolymph
Cross section of each canal swells substantially near where the canals join the vestibule: Ampulla
Ampulla
Cross section of each canal swells substantially near where the canals join the vestibule
Semicircular canals (cont’d)
Within the endolymph space of each ampulla is the crista
When the head rotates, the inertia of the endolymph causes it to lag behind, leading to tiny deflections of the hair cells
Cristae
The specialized detectors of angular motion located in each semicircular canal in a swelling called the ampulla
Each crista has about 7000 hair cells, associated supporting cells, and nerve fibers
Cilia of hair cells project into jellylike cupula which forms an elastic dam extending to the opposite ampulla wall, with endolymph on both sides of dam.
Semicircular canal dynamics
Canal afferent neurons are sensitive to back and forth rotations of the head, as well
Greatest sensitivity to rotations at 1 Hz or less
Faster rotations than 1 Hz would be dangerous
Firing rate goes up and down as the head rotates back and forth
The overall normalized amplitude of the canal neuron response scales with head rotation frequency
Otolith
Otolith organs sense acceleration and tilt
There are 2 otolith organs in each ear:
Utricle: Contains about 30,000 hair cells
Saccule: Contains about 16,000 hair cells
Each organ contains a macula: A specialized detector of linear acceleration and gravity
Utricle
Contains about 30,000 hair cells
Saccule
Contains about 16,000 hair cells
macula
Each Otolith organ contains a macula:
A specialized detector of linear acceleration and gravity.
Each macula is roughly planar and sensitive primarily to shear forces
Hair cells
encased in a gelatinous structure that contains calcium carbonate crystals called otoconia (“ear stones” in Greek)
Coding of amplitude in the otolith organs
Larger accelerations (or larger gravitational shear forces) move the otolith organ’s otoconia more
This leads to greater deflection of the hair cell bundles
Change in receptor potential is proportional to magnitude of linear acceleration or gravitational shear
3 experimental paradigms are typically used to investigate spatial orientation perception:
Threshold estimation:
What is the minimum motion needed to correctly perceive motion direction?
Magnitude estimation: Participants report how much (e.g., how many degrees) they think they tilted, rotated, or translated.
Matching:
Participants are tilted and then orient a line with the direction of gravity.
This is done in a dark room with only the line visible to avoid any visual cues to orientation.
Threshold estimation:
What is the minimum motion needed to correctly perceive motion direction?
Magnitude estimation:
Participants report how much (e.g., how many degrees) they think they tilted, rotated, or translated.
Matching:
Participants are tilted and then orient a line with the direction of gravity.
This is done in a dark room with only the line visible to avoid any visual cues to orientation.
Rotation perception
At first, constant rotation (in the dark) is perceived accurately
Soon, however, subjects feel as if they are slowing down
After 30 seconds, they no longer feel as if they are rotating
Time course of habituation for perceived velocity is slower than time course of habituation for velocity neurons:
“Velocity storage”
When rotation stops, subjects feel as if they are rotating in the opposite direction
Vestibulo-ocular reflexes (VORs):
Counter-rotating the eyes to counteract head movements and maintain fixation on a target
VORs are accomplished by 6 oculomotor muscles that rotate the eyeball
Angular VOR:
The most well-studied VOR
Example: When the head turns to the left, the eyeballs are rotated to the right to partially counteract this motion
Torsional eye movements:
When the head is rolled about the x-axis, the eyeballs can be rotated a few degrees in the opposite direction to compensate
“The Spins”
Ingesting a great deal of alcohol in a short amount of time causes alcohol to quickly enter the bloodstream
The alcohol causes the cupula to become lighter than the surrounding endolymph.
This causes the system to become sensitive to gravity in addition to rotational acceleration.
Visual–vestibular integration
Vection: An illusory sense of self motion produced when you are not, in fact, moving
Example: The feeling of flying while watching an IMAX movie
Example: Being stopped in your car at a light next to a semi. The semi begins to roll forward and you press on the brake because you feel as if you are rolling backwards
Sometimes a mismatch between visual and vestibular information can cause motion sickness
Since the vestibular system has such a widespread influence, what happens when it fails?
Possible problems:
Spatial disorientation Imbalance Distorted vision unless head is held perfectly still Motion sickness Cognitive problems
