Vestibular Anatomy & Physiology

What is the Primary Function of the Vestibular System?

To help maintain balance and orientation.

What are the movements in 3-dimension spaces?

Roll, Yaw, and Pitch

• Each canal codes for one of these direction of movement

What is Roll?

Movement when we roll our ear toward our shoulder (ie cartwheel)

What is Yaw?

Movement when we shake our head side to side (ie shaking head no)

What is Pitch?

Movement when we tilt nose up and down (ie nodding head)

What are the Three Semicircular Canals and what is the overall type of movement?

Semicircular canals help with angular (rotational) acceleration

• Horizontal

• Anterior

• Posterior

Which canal controls for Yaw movement?

Horizontal/Lateral → Yaw (head shaking)

Think yee haw

Which canal controls for Pitch movement?

Anterior → Pitch (head nodding)

Think AP

Which canal controls for Roll movement?

Posterior → Roll (turning head to shoulder)

Think PR

What is the Cupula?

A gelatinous structure that sits on the hair cells, which are similar to the ones in the cochlea

• Physical movement of the head triggers movement of the fluid in the cochlea, which moves the cupula and its cilium detects this movement which sends the signal to the vestibular nerve, allowing our brain to register

• Translating physical movement of the head to neuron signals that the brain can interpret

Movement of Endolymph

• Head movement in one direction causes the endolymph fluid to move the opposite way

• This fluid pushes the cupula toward the direction the head is moving

• Therefore, stereocilia bend in the direction the head is moving

• Just like in the cochlea, beding of the stereocilia can open up ion channels and depolarize the hair cells

Which Direction do the Stereocilia Go to Open Ion Channels?

• If the other stereocilia are pointed towards the kinocilium, the ion channels will open

• If the other stereocilia are pointed away from the kinocilium, the ion channels will close

What occurs in frequency of nerve firing for each position of the ion channels?

• Ion channels open = depolarization = increase in frequency of nerve firing

• Ion channels close = hyperpolarization = decrease in frequency of nerve firing

How does the Endolymph Motion Work?

Inertia → The tendency of fluid to resist motion

(Explains why you’ll feel dizzy after spinning even when you stop)

What is the Vestibulo-Ocular Reflex (VOR)?

• The eyes move in the opposite direction of the head motion to maintain clear vision/stabilize gaze

• Helps stabilize the image on the retina during head movement

• Ex). Head → right = endolymph → left = right canal excited/left inhibited → eyes move left

What are the Otolith Organs & Their Physiology? 

• Utricle & Saccule

• They detect linear acceleration and head position relative to gravity (car motion/elevator)

• Otoliths move in response to gravity, pulling down the otolithic membrane and bending the stereocilia

What is the movement for the utricle?

Horizontal movement (forward and back)

What is the movement for the saccule?

Vertical movement (up and down)

What is the Macula?

Hair cells + gelatinous otolithic membrane + otoconia (CaCO crystals)

Why Should Otoliths Not Go Into the Semicircular Canals?

If the otoliths (CaCO structures) go into the semicircular canals, it will cause vertigo (the sensation of spinning even while being still)

What Does the Change in Nerve Firing Frequency Depend on?

The orientation of hair cells/kinocilium (there are different orientations throughout the organ)