Let’s back way up, start from the beginning and remember why we teach/coach/train/practice. That won’t be the same answer for everyone, but for me it was multilayered- I loved how physical pursuits like trail running, dance and yoga could quell some of the busyness of my thoughts helping me be less identified with them and the chemical change I felt: relief, embodiment, euphoria, a sense of well being. I loved the focus required to stay still in a precise shape of a posture, and the concentration required to learn something, meditation and intention to grow able to contribute to the world.

As I worked with people to help them move in more ways with more ease, contending with nagging patterns of disfunction, pain, injury, chronic heightened arousal wreaking all kind of havoc on their lives, too much time sitting in traffic and at desk jobs in between hundreds of weekend miles cycling,

The three parts of the ear are the inner, outer, and middle ear

The outer and middle ear are involved with hearing

The inner ear functions in both hearing and equilibrium

Receptors for hearing and balance:

Respond to separate stimuli

Are activated independently

Where am I going?

Which way is up?

Vestibular organs: The set of five organs—three semicircular canals and two otolith organs—located in each inner ear that 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

Spatial orientation: A sense comprised of three interacting sensory modalities: Our senses of linear motion, angular motion, and tilt

1. Angular motion: Can be sensed when rotating head from side to side as if to say “no”

2. Linear motion: Sensed when accelerating or decelerating in a car

3. Tilt: Can be sensed when nodding head up and down as if to say “yes”

Why considered different “modalities”?

Sensing linear motion, angular motion, and tilt involves different receptors and/or different stimulation energy

Semicircular canals: The three toroidal tubes in the vestibular system that sense angular acceleration, a change in angular velocity

Source of our sense of angular motion

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 Provide a sense of spatial orientation, consisting of

Linear, angular and tilting motion, coordinating with vision for the vestibulo-ocular reflex

which Stabilizes visual input by counter rotating the eyes to compensate for head movement

The vestibular organs sense head motion: canals sense rotation; otoliths sense linear acceleration (including gravity).

The central vestibular system distributes this signal to oculomotor, head movement, and postural systems for gaze, head, and limb stabilization..

The visual system complements the vestibular system.

Visuo-vestibular conflict causes acute discomfort.

Vertigo: A sensation of rotation or spinning

Imbalance

Blurred vision

Illusory self-motion

Spatial orientation: A sense comprised of three interacting sensory modalities: Our senses of linear motion, angular motion, and tilt

1. Angular motion: Can be sensed when rotating head from side to side as if to say “no”

2. Linear motion: Sensed when accelerating or decelerating in a car

3. Tilt: Can be sensed when nodding head up and down as if to say “yes”

Why considered different “modalities”?

Sensing linear motion, angular motion, and tilt involves different receptors and/or different stimulation energy

Semicircular canals: The three toroidal tubes in the vestibular system that sense angular acceleration, a change in angular velocity

Source of our sense of angular motion

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

Coordinate system for classifying direction:

x-axis: Points forward, in the direction the person is facing

y-axis: Points laterally, out of the person’s left ear

z-axis: Points vertically, out of the top of the head

Axes are defined relative to the person, not relative to gravity

Linear motion

Movements represented in terms of changes in the x-, y-, and z-axes

Any arbitrary linear motion can be represented as a change along these three axes

The vestibular organs do not respond to constant velocity

They only respond to changes in velocity—acceleration

Gravity and acceleration share a deep connection and can be considered equivalent

Hair cells: Support the stereocilia that transduce mechanical movement in the vestibular labyrinth into neural activity sent to the brain stem

Mechanoreceptors: Sensory receptors that are responsive to mechanical stimulation (pressure, vibration, movement)

Like the hair cells involved in hearing, hair cells act as the mechanoreceptors in each of the five vestibular organs

Head motion causes hair cell stereocilia to deflect, causing a change in hair cell voltage and altering neurotransmitter release

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 three-fourths of a toroid (donut) shape, measuring 15 mm long and 1.5 mm in diameter

Canals are filled with a fluid called perilymph

A second, smaller toroid is found inside the larger toroid, measuring 0.3 mm 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

Semicircular canals (cont’d)

Within the endolymph space of each ampulla is the crista

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

When the head rotates, the inertia of the endolymph causes it to lag behind, leading to tiny deflections of the hair cells

Coding of direction in the semicircular canals

Three 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

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 amplitude in the semicircular canals

In the absence of any rotation, many afferent neurons from the semicircular canals have a resting firing rate of about 100 spikes/s

This firing rate is high relative to nerve fibers in other sensory systems

High firing rate allows canal neurons to code amplitude by decreasing their firing rate, as well as increasing it

Changes in firing rate are proportional to angular velocity of the head aligned with the canal the neuron is in

Semicircular canal dynamics

Neural activity in semicircular canals is sensitive to changes in rotation velocity

Constant rotation leads to decreased responding from the canal neurons after a few seconds

Semicircular canal dynamics (cont’d)

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 organs sense acceleration and tilt

Two 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

Each macula is roughly planar and sensitive primarily to shear forces

Hair cells are 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

Coding of direction in the otolith organs

Arises in part from the anatomical orientation of the organs

Utricular macula: horizontal plane

Sensitive to horizontal linear acceleration and gravity

Saccular macula: vertical plane

Sensitive to vertical linear acceleration and gravity

Three 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

Yaw rotation thresholds

Humans are so sensitive to yaw rotation that we can detect movements of less than 1 degree per second

At this rate, it would take 6 minutes to turn completely around

As yaw rotation frequency decreases, it takes faster movement to be detected

Translation perception

When people are passively translated in the dark, they are able to use a joystick to reproduce the distance they traveled quite accurately

Interestingly, they also reproduce the velocity of the passive-motion trajectory

This implies that the brain remembers and replicates the velocity trajectory

The otolith organs register acceleration, and our brains mathematically integrate the acceleration and turn it into the perception of linear velocity

Sensory integration: The process of combining different sensory signals

Typically leads to more accurate information than can be obtained from individual senses alone

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

Observers looking at a rotating display report rotational vection

Subjects have the illusion of tilt but do not feel as if they turn upside-down

Why don’t people feel as if they are turning upside down?

The vestibular system’s sense of gravity stops the illusion

Astronauts without gravity feel as if they are tumbling under these circumstances

Thus, vestibular information is combined with visual information to yield a “consensus” about our sense of spatial orientation

Vestibulo-ocular reflexes (VORs): Counter-rotating the eyes to counteract head movements and maintain fixation on a target

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

VORs are accomplished by six oculomotor muscles that rotate the eyeball

Vestibulo-autonomic responses

Autonomic nervous system: The part of the nervous system innervating glands, heart, digestive system, etc., and responsible for regulation of many involuntary actions

Motion sickness: Results when there is a disagreement between the motion and orientation signals provided by the semicircular canals, otolith organs, and vision

Could be an evolutionary response to being poisoned

Blood pressure is regulated by vestibulo-autonomic responses

We have a visual cortex and an auditory cortex; do we have a vestibular cortex? Not really

Areas of cortex respond to vestibular input, but they tend to respond to visual input as well

No need to have cortex for processing vestibular information in isolation if visual information is available also

Vestibular information reaches the cortex via thalamo-cortical pathways

Areas of cortex that receive projections from the vestibular system also project back to the vestibular nuclei

Knowledge and expectations can influence perception of tilt and motion

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The vestibular organs sense head motion: canals sense rotation; otoliths sense linear acceleration (including gravity).

The central vestibular system distributes this signal to oculomotor, head movement, and postural systems for gaze, head, and limb stabilization..

The visual system complements the vestibular system.

Visuo-vestibular conflict causes acute discomfort.

Peripheral and brainstem vestibular dysfunction causes pathological sense of self-motion and visuo-vestibular conflict.

Vestibulo-ocular reflex – keep the eyes still in space when the head moves.

Vestibulo-colic reflex – keeps the head still in space – or on a level plane when you walk.

Vestibular-spinal reflex – adjusts posture for rapid changes in position.

Peripheral and brainstem vestibular dysfunction causes pathological sense of self-motion and visuo-vestibular conflict.

Problems with the vestibular system can lead to peculiar sensations:

Spatial Disorientation: Any impairment of spatial orientation (i.e., our sense of linear motion, angular motion, or tilt)

Dizziness: Nonspecific spatial disorientation

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