The Vestibular System

Question 1

What are the functions of the vestibular system?

Answer 1

Contributes to perception of self motion, head position, and spatial orientation relative to gravity. Also important for stabilizing gaze and posture.

Only unusual or novel movements are PERCEIVED, most activity occurs that the subconscious level.

Question 2

Where is the vestibular apparatus located relative to the cochlea?

Answer 2

Posterolateral; fluid filled space in the petrous portion of the temporal bone

Question 3

Otolith Organs - Utricle

What is the name of the receptor region? what is its orientation? What kind of motion is it most sensitive to?

Answer 3

It is called the Macula.

It is horizontally placed in the floor of the utricle.

It is primarily concerned with movement in the horizontal plane.

Question 4

Otolith Organs - Saccule

What is the name of the receptor region? what is its orientation? What kind of motion is it most sensitive to?

Answer 4

It is called the Macula.

It is vertically placed in the medial wall of the saccule.

It is primarily concerned with movement in the vertical plane.

Question 5

Maculae and Membranous Ampullae

What is the structure of the hair cells in these receptor regions?

Answer 5

Apical surface has 60 to 100 stereocillia and a single, longer KINOCILIUM.

Stereocillia are arranged in rows of graded height with the tallest closest to the kinocilium.

Tip links connect stereocilia to their next tallest neighbor

Question 6

Maculae and Membranous Ampullae

What are the types of hair cells in these receptor regions?

Answer 6

Type I - Pear Shaped
These are innervated by chalice like terminals of afferent vestibular cells

Type 2 - Columnar shaped
Innervated by simple bouton terminals of afferent vestibular cells

Question 7

Maculae and Membranous Ampullae

What is the name of the gelatinous layer associated with the maculae? What is its structure?

Answer 7

Otolithic membrane

Gelatinous coating of the otolithic organs covered by calcium carbonate crystals that are very dense are not displaced by endolymph movements.

Envelops the stereocilia and kinocilium of the macculae

Question 8

Maculae and Membranous Ampullae

What is the name of the gelatinous layer associated with the membranous ampullae? What is its structure?

Answer 8

Cupula

It is a gelatinous structure attached to the floor, roof, and walls of the membranous ampulae, forming a fluid-tight partition that has the same specific density as endolymph

Envelops the stereocilia and kinocilium of the membranous ampullae

Question 9

Utricular macula - organization of hair cells

Answer 9

The hair cells are polarized so that the kinocilium is on the side CLOSEST to the striola

Question 10

Saccular macula - organization of hair cells

Answer 10

The hair cells are polarized so that the kinocilium is on the side FARTHEST to the striola

Question 11

How are the hair cells in the membranous ampullae organized?

Answer 11

The hair cells are embedded in a ridge called the crista, which is perpendicular to the long axis of the semicircular duct.

Hair cells on the crista of the HORIZONTAL semicircular duct are all arranged with their kinocilium on the side CLOSER to the UTRICLE

Hair cells on the cristae of the ANTERIOR and POSTERIOR semicircular ducts are all arranged with their kinocilium on the side FARTHER from the UTRICLE

Question 12

How are movement signals encoded in the membranous ampullae?

Answer 12

Hair cells int he membranous ampullae are sensitive to ROTATIONAL MOVEMENTS of the head

When the head turns in the plane of the corresponding semicircular duct, the semicircular duct moves with it, but the endolymph lags because of inertia, causing a drag between the endolymph and the duct wall.

this drag moves the CUPULA, deflecting the hair cells.

The semicircular canals in the left and right ears form functional pairs that always respond oppositely to head movements that affect them

Question 13

How are movement signals encoded in the maculae?

Answer 13

hair cells in the maculae are responsive to linear movements and tilting of the head in response to gravity.

hair cells on different sides of the striola in each of the macculae will have opposite excitatory patterns so that when hair cells on one side are depolarized, the hair cells on the other side are hyperpolarized.

The curve in the striola makes it so that only certain hair cells will be responsive to a given linear movement

Question 14

Processing Vestibular Information.

1) What are the afferent cells of the vestibular system called?
2) Where are their cell bodies located?
3) What is their distribution?
4) How do they encode stimulus attributes?

Answer 14

1) Vestibular Ganglion Cells
2) Internal acoustic meatus
3) They terminate on all hair cells
4) They transmit information by increasing or decreasing their firing rate. The change in the firing rate can be either sustained or transient, which would signal wither absolute head position or acceleration.

Question 15

Processing Vestibular Information - Efferent cells

1) Where are their cell bodies located?
2) What is their distribution?
3) What is their possible function?

Answer 15

1) Cell bodies are in the brainstem rostral to the vestibular nuclei
2) the majority of the terminate on the TYPE 2 HAIR CELLS (columnar) and a few terminate on the afferent fibers of the TYPE 1 hair cells (pear shaped)
3) Thought to control the sensitivity of the hair cells

Question 16

What are VORs?

Answer 16

Vestibulo-ocular reflexes

Question 17

What do VORs occur in response to?

Answer 17

Linear and rotational movements

Question 18

What is the pathway of the VOR (in the example of turning the head to the Left)?

Answer 18

1) Left horizontal semicircular canal excites the left vestibular nuclei neurons.
2) Projections from the LEFT vestibular nuclei stimulate the RIGHT abducens nuclei neurons and interneurons
3a) The right abducens nuceli send motor signals to produce a contraction the RIGHT lateral rectus to ABDUCT the right eye
3b) The right abducens nuclei INTERNEURONS excite LEFT oculomotor nuceli, which produce contraction of the LEFT medial rectus muscle, ADDUCTING the left eye
4) Matching bilateral connections inhibit contraction of the LEFT LR and RIGHT MR
5) the resulting rightward eye movement keeps the object of interest on the fovea

Question 19

Can the VOR be suppressed? If so, when do we suppress it?

Answer 19

The VOR can be suppressed at will, and we do this all the time when we look at a moving object while turning our head in the same direction.

Question 20

When does Nystagmus Occur?

Answer 20

In response to large head rotations, such as with a 360 body turn.

Question 21

What are the phases that occur in nystagmus? What occurs at the eyes in each of these stages?

How are nystagmus movements named?

Answer 21

Slow phase - VOR directs the eyes in the direction opposite the head movement

Fast Phase - when the eye reaches the limit of how far it can turn in the orbit, it goes back rapidly to a central position, moving in the same direction as the head.

Named for the direction of movement in the fast phase

Question 22

What part of the spinal cord is associated with the vestibulospinal reflex?

Answer 22

The lateral vestibulospinal tract in the ventral funiculus of the spinal cord

Question 23

What are the types of neurons in the spinal cord involved in the Vestibulospinal and vestibulocolic reflexes?

Answer 23

Alpha and gamma motor neurons and interneurons.

Question 24

Describe what happens in the Vestibulospinal reflex with a tilt to the left.

Answer 24

if the body tilts to the left, left LVST neurons excite alpha and gamma neurons in the axial and limb extensor muscles, causing EXTENSION on the side towards which we are falling

Question 25

What part of the spinal cord is associated with the vestibulocolic reflex?

Answer 25

The medial vestibulospinal tract in the medial longitidunal fasciculus of the cervical spinal cord

Question 26

Describe what happens in the vestibulocolic reflex if you trip (and are falling forward)

Answer 26

Bilateral MVST neurons excite alpha and gamma motor neurons activating the neck extensor muscles and inhibiting the neck flexor muscles.

Question 27

What is the funciton of the vestibulospinal and vestibulocolic reflexes?

Answer 27

produce reflexive postural adjustments of the body and head

Question 28

What is the role of the vestibulo-thalamo-cortical network?

Answer 28

Perception of motion and spatial orientation.