Chapter 14 - The Vestibular System

Question 1

The major structure in the Vestibular System is the ..

Answer 1

Labyrinth.

Question 2

The Labyrinth is buried deep in the temporal bone and consists of the …

Answer 2

otolith organs and the semicircular canals.

Question 3

What are the constituents of the otolith organs?

Answer 3

The utricle and the saccule.

Question 4

How many semicircular canals do we have?

Answer 4

Three.

Question 5

The utricle and saccule are specialised primarily to respond to …

Answer 5

linear accelerations of the head and static head position relative to the gravitational axis.

Question 6

The utricle and saccule are specialised primarily to respond to linear accelerations of the head and static head position relative to the gravitational axis, whereas the semicircular canals are specialized for responding to …

Answer 6

Rotational accelerations, as their shapes suggests.

Question 7

What rotation is a Pitch?

Answer 7

Pitch is a rotation around the y-axis. This means it’s a nod.

Question 8

What rotation is a Roll?

Answer 8

Roll is a rotation around the x-axis. This means it’s a tilt of the head.

Question 9

What rotation is a Yaw?

Answer 9

Yaw is a rotation around the z-axis. This means it’s a head shake.

Question 10

The vestibular hair cells are located …

Answer 10

in the utricle and saccule, as well as three jug like swellings called ampullae, located at the base of the semicircular canals next to the utricle.

Question 11

Vestibular hair cells are very much alike auditory hair cells. How do they differ?

Answer 11

In the utricle and saccule, a specialised area called the stroll divides the hair cells into two populations having opposing polarities.

Question 12

Overlaing the hair cells and their hair bunnies is a what?

Answer 12

A gelatinous layer, and above this layer is a fibrous structure, the otolithic membrane, in which are embedded crystals of calcium carbonate called otoconia.

Question 13

What is the function of the otoconia?

Answer 13

They make the otolithic membrane heavier than the structures and fluids surrounding it; thus, when the head tilts, gravity causes the membrane to shift relative to the sensory epithelium. The resulting heading motion between the otolithic membrane aunt eh macula displaces the hair bundles, which generates a receptor potential in the hair cells.

Question 14

When the head tilts, gravity causes the membrane to shift relative to the sensory epithelium. The resulting heading motion between the otolithic membrane aunt eh macula displaces the hair bundles, which generates a receptor potential in the hair cells. Explain why this also happens during acceleration.

Answer 14

When the head undergoes linear accelerations; the greater relative mass of the otolithic membrane causes it to lag behind the macula temporarily, leading to transient displacement of the hair bundle.

Question 15

How does the otolith organs convey the difference between linear accelerations and head tilts?

Answer 15

They don’t. They cannot distinguish between those two stimuli. However, together with the information from the semicircular canals the brain is able to distinguish between the two. (no vision needed)

Question 16

Each of the three semicircular canals has at its base what?

Answer 16

A bulbous expansion - the ampulla.

Question 17

The ampulla is structured how?

Answer 17

The ampulla houses the sensory epithelium, or crista, that contains the hair cells. The hair bundles extend out of the crista into a gelatinous mass, the cupula.

Question 18

What is the cupula?

Answer 18

The cupula is a gelatinous mass in the ampulla in which the hair bundles lie. The cupula bridges the width of the ampulla, forming a viscous barrier through which endolymph cannot circulate. As a result, the relatively compliant cupula is distorted by movements of the endolymphatic fluid.

Question 19

How are the hair cells in the cupula organised?

Answer 19

They all have the kinocilia in the same direction.

Question 20

The vestibular system is a special sensory system. What makes it so special?

Answer 20

1. It is multi sensory. Many neurons in the vestibular nuclei - the earliest point in the processing - receive visual input
2. Many neurons in the vestibular nuclei function as premotor neurons it addition to giving rise to ascending sensory projections.

Question 21

The information from the vestibular system is projected centrally, first coming to the …

Answer 21

Vestibular nucleus of the medulla.

Question 22

The central projections of the vestibular system participate in two major classes of reflexes:

Answer 22

1. Those responsible for maintaining equilibrium and gaze during movement
2. Those responsible for maintaining posture.

Question 23

Mention one important reflex mediated by the vestibular system.

Answer 23

The vestibulo-ocular reflex (VOR) which is responsible for producing eye movements that counter head movements, thus permitting the gaze to remain fixed on a particular point.

Question 24

What is oscillopsia?

Answer 24

A condition in which patients find it impossible or difficult to fixate on visual targets while the head is moving. This is probably caused by vestibular damage, which causes the vestibulo-ocular reflex (VOR) to fail. If the damage is unilateral the patient can often regain the ability to fixate, but bilateral damage has the persistent and disturbing sense that the world is moving when the head moves.