Vestibular system

Question 1

the inner ear

Answer 1

Question 2

List the essential parts of the The membranous labyrinth of the inner ear.

Answer 2

three semicircular ducts (horizontal, anterior and posterior), two otolith organs (saccule and utricle), and the cochlea

Question 3

Describe the main parts and action of the semicircular ducts.

Answer 3

As the head rotates in one direction, inertia of the fluid causes it to lag, and hence generate relative motion in the semicircular duct in the direction opposite that of the head movement. This moving fluid bends the broad vane of the cupula. The stereocilia of the hair cells are bent because they are embedded in the gelatinous cupula. Shearing of the hair cells opens potassium channels

Question 4

Describe the transduction process of the semicircular duct.

Answer 4

The basic transduction mechanism is the same in the auditory and vestibular systems (See Figure 12.1). A mechanical stimulus bends the cilia of the hair cells. Fine thread-like tip links connect to trap doors in the adjacent cilium. Bending the hair cells stretches the tip link, causing an influx of K+ ions and the generation of neural impulses in the VIIIth cranial nerve.

Question 5

What are the name and characteristic of the vestibular hair cells?

Answer 5

Hair cells in the vestibular system are slightly different from those in the auditory system, in that vestibular hair cells have one tallest cilium, termed the kinocilium. Bending the stereocilia toward the kinocilium depolarizes the cell and results in increased afferent activity. Bending the stereocilia away from the kinocilium hyperpolarizes the cell and results in a decrease in afferent activity.

Question 6

Name the vestibular structures and their function

Answer 6

Sacculus–linear acceleration (vertical)

Utricule–linear acceleration (horizontal)

Semicircular Canals –rotational (x,y,z)

Question 7

macula

Answer 7

The sensory epithelium of the otolith organs, comprising hair cells and associated supporting cells.

Receptive organ with hair cells protruding into a flat gelatin matrix studded with otoliths.

Question 8

Describe the overall structure of the vestibular macula.

Answer 8

Question 9

Describe the vestibular ocular reflex

Answer 9

Coordination of head and eye movements

Eye movements counter head movements

Purpose: A reflex eye movement that stabilizes images on the retina during head movement.

Stabilization occurs by producing eye movements in the direction opposite to the head movement, thus preserving the image on the center of the visual field.

Question 10

nystagmus

Answer 10

Repetitive rotational movements of the eyes nor- mally elicited by large-scale motion of the visual field (opto- kinetic nystagmus), with each cycle involving a slower phase driven by central circuits in the brainstem and higher brain centers and a faster, reflexive phase resetting the position of the eye in the orbit; in the absence of physiological visual or vestibular stimuli, nystagmus may indicate brainstem or cerebellar pathology.

Question 11

List the vestibular CNS connections

Answer 11

Spinal Cord: alpha and gamma motor neurons (extensor muscles)

Cranial Nerves: 3rd, 4th, and 6th (Oculomotor, Trochlear, Abducens) eye movements

Cerebral Cortex: Motor output

Cerebellum: vestibular afferent innervation

Reticular Formation: sensory (alert cortex) and motor functions (help regulate muscle tone)

Autonomic system: sweating, vomiting

Question 12

List the most common ways of examining vestibular function.

Answer 12

Electronystagmography/Videonystagmography (ENG/VNG)

Vestibular Evoked Myogenic Potential (VEMP)

Rotary Chair Test

Question 13

Describe Electronystagmography/Videonystagmography (ENG/VNG)

Answer 13

Patients are asked to follow a moving light with their eyes and sit and lie in different positions while their eye movements are monitored with infrared goggles. Each ear canal is also irrigated with small amounts of warm and cool water (caloric test) as the patient lies on an examination table. The water causes a temperature change that creates eye movements (nystagmus) that can be measured and compared for each ear.

Question 14

Bony labyrinth

Answer 14

Hollow cavity in the petrous temporal bone consisting of vestibule, semicircular canals and cochlea.

Question 15

Perilymph

Answer 15

Fluid in the bony labyrinth, but outside the membranous labyrinth. In communion with CSF.

Question 16

Membranous labyrinth

Answer 16

Watertight sac inside boney labyrinth consisting of cochlear duct, saccule, utricle and semicircular canals.

Question 17

Endolymph

Answer 17

Fluid inside membranous labyrinth.

Question 18

Hair cells

Answer 18

Specialized cells of inner ear for hearing (cochlea) or balance (macula or christae).

Question 19

Stereocilia

Answer 19

Processes from apex of hair cells that detect distortion.

Question 20

Kinocillium

Answer 20

Single process from apex of hair cell that polarizes the cell.

Question 21

Ampula

Answer 21

Dilated part of semicircular canal containing cristae.

Question 22

Cristae

Answer 22

Receptive organ containing hair cells in a flame-shaped gelatin (cupula)

Question 23

Otoliths

Answer 23

Calcium carbonate crystals in the gelatin of the macula designed to add weight to the gel.

Question 24

Vestibular complex

Answer 24

Collection of nuclei in the dorsolateral medulla all concerned with vestibular function.

Question 25

Flocculos

Answer 25

The oldest part of the cerebellum. Involved with regulating vestibular function.

Question 26

What are the differences between endolymph and perilymph?

Answer 26

Bony labyrinth contains perilymph (like CSF) while membranous labyrinth contains endolymph that has high potassium and very low in sodium and calcium. There is also +85mV of potential differnce between the endolymph and perilymph.

Question 27

What does it mean that hair cells are “polarized”, and what is the significance?

Answer 27

Each hair cell is polarized, with movment of the stereocillia toward the kinocillium resulting in depolarization and away from the kinocillium resulting in hyperpolarization.

Question 28

Where are the macula a located?

Answer 28

There is one macula in the saccule and one in the utricle

Question 29

What are macula specialized to detect?

Answer 29

Macula are specialized to detect head position related to gravity as well as linear acceleration.

Question 30

What is the structure and orientation of the semicircular ducts?

Answer 30

Each ear has 3 semicircular canals (and ducts) that are each at right angles to one another.

Question 31

Where are cristae located, what is their structure, and what are they specialized to detect?

Answer 31

There is a dilation (ampulla) in the semicircular ducts that contain cristae. Cristae have hair cells in a flame-shaped gelatinous mass. Each christae contains hair cells that are all polarized in the same direction. The chriatae are specialized for perception of angular acceleration (spinning, pitching, tumbling) since this kind of motion is necessary to get the fluid to remain still when the head moves

Question 32

What is the relationship between hair cells and the vestibulocochlear nerve?

Answer 32

Hair cells synapse on peripheral processes of the vestibular nerve. The vestibular part of the vestibulocochlear nerve (CNVIII) consists of axons of bipolar cells in the vestibular ganglion.

Question 33

Where do central processes of the vestibulocochlear nerve terminate?

Answer 33

The central process of these vestibular nerve fibers terminate in the vestibular complex of the dorsolateral medulla and the vestibulocerebellum (flocculonodular lobe)

Question 34

What are the symptoms of damage to the vestibular system?

Answer 34

Damage to the vestibular system results in vertigo, ataxia (trouble walking straight) and nystagmus.

Question 35

What kind of eye movements occur as a result of vestibular system activation?

Answer 35

Nystagmus due to vestibular damage or stimulation consists of a slow drift of the eyes due to a vestibular input. If the person is attempting to focus on a target, the eyes will then “snap back” to the target due to a separate reflex (fixation reflex). This is called “jerk nystagmus”.

Question 36

What is a normal response to injection of ice water into the right ear canal of a patient who is supine with her head slightly propped up?

Answer 36

Caloric testing results in convection currents in the endolymph that trick the inner ear into believing there is movement. Therefore, there is both vertigo and nystagmus. Ice water in this position results in convenction currents that make the patient feel as if she is spinning away from the ice water and, therefore there is slow movement toward the icewater and a rapid jerk in away from the icewater.