Lecture 19: Hearing and Balance

Vestibulocochlear (VIII)

Organs for hearing and balance are both innervated by the ______ nerve.

Inner Ear

Where are the organs of hearing and balance located?

Hearing: Cochlea

- the organ that transduces sound into nerve impulses
- part of the labyrinth.

Labyrinth: Inner Ear

- the structure comprised of the cochlea, the vestibule (utricle & saccule) and semicircular canals

Balance: Semicircular Canals

- organs that transduce angular acceleration (i.e. rotation) into nerve impulses
- three per side
- three are oriented perpendicularly to each other; sense rotation in x, y and z axes

Balance: Vestibule

- these organs sense linear acceleration, including gravity

Utricle

any acceleration in the horizontal plane; part of vestibule

Saccule

acceleration in the sagittal plane; part of vestibule

Bony Labyrinth

passageways in temporal bone

Membranous Labyrinth

membrane-covered tubes inside the bony labyrinth

External Ear Function

collects sound

Tympanic Membrane Function

vibrates in response to sound waves

Ossicles Function

maleus (hammer), incus (anvil), stapes (stirrup), which translate motion of tympanic membrane to oval window of cochlea

Cochlea Function

transduction of sound

External Ear

- air-filled; open chamber
- pressure from sound moves the ear drum

Middle Ear

- ossicles transmit pressure exerted by sound to the cochlea

Inner Ear (Cochlea)

- sound vibrations are transmitted to liquid-filled, closed chamber
- floor of chamber ("basilar membrane") vibrates in response to sound

Cochlea

on the anterior-medial side of the labyrinth

Middle Ear and Ossicles

What is structure #1?

Cochlea

What is structure #2?

Auditory Tube

What is structure #3?

Tympanic Membrane

What is structure #4?

External Ear Canal

What is structure #5?

External Ear

What is structure #6?

Cochlear Nerve

What is structure #1?

Vestibular Nerve

What is structure #2?

Saccule

What is structure #3?

Utricle

What is structure #4?

Semicircular Canals

What is structure #5?

Ampulla

What is structure #6?

Oval Window

What is structure #7?

Round Window

What is structure #8?

Hair Cells

What is structure #1?

Hair Cells Function

where sound transduction occurs

Basiliar Membrane

What is structure #2?

Basiliar Membrane Function

membrane transmitting sound vibrations to hair cells

Spiral Ganglion

What is structure #3?

Spiral Ganglion Function

primary afferent neurons for hearing

Wide, thin, flaccid

What is the width and tension of the basiliar membrane at the apex?

Narrow, thick, tense

What is the width and tension of the basiliar membrane at its base?

Low

The apex of the basiliar membrane has ____ resonant frequency.

High

The base of the basiliar membrane has ____ resonant frequency.

100 Hz

low frequency

1000 Hz

medium frequency

10,000 Hz

high frequency

Basiliar

Every frequency resonates at a unique point along the _____ membrane.

Excitation of Hair Cells

- depolarization
- increased impulse frequency

Inhibition if Hair Cells

- hyperpolarization
- decreased impulse frequency

Spinal Ganglion Neuron

bipolar neuron found in the inner ear whose axons form the auditory nerve

Cochlear Nuclei

sound localization & identification: medulla

Olivary Nuclei

sound localization and volume control: pons

Inferior Colliculus

identification of and reflexive responses to sounds: midbrain

Medial Geniculate Nucleus

thalamus relay nucleus

Auditory Cortex

tonotopic organization, sound identification

Pitches

Sounds usually consist of multiple ____.

Single

A pitch is a _____ frequency.

Deafness

loss of the ability to hear

Conduction Deafness

- ear wax
- issues w/ ossicles

Sensorineural

- most common form of deafness
- can be result from exposure to loud noises
- age-related loss of high-frequency hearing
- treated w/ hearing aids

Tinnitus

- ringing in the ears
- associated w/ hearing loss
- difficult to treat

Rotation

Semicircular canals sense ____.

Endolymph

fluid within the membranous labyrinth of the inner ear

Ampulla

the connection of the semicircular canals

Cupula

a gelatinous mass found in the ampulla of the semicircular canals; moves in response to the flow of the fluid in the canals

Cupula

The flow of fluid causes the ____ to move.

Angular

Semicircular canals sense ____ acceleration.

Linear

Utricle and saccule sense ____ acceleration.

Horizontal

The utricle is hair cells in the ____ sheet.

Vertical

The saccule is hair cells in the ___ sheet.

Vestibular; Flocculonodular

The vestibular apparatus projects its axons to:
- the _____ nuclei in dorsal brainstem
- part of the cerebellum involved in balance (the
_____ lobe)

Cerebellum; Eye Movements

The vestibular nuclei project axons to:
- spinal cord
- ______
- Cranial nerve nuclei controlling _____ (i.e. oculomotor, trochlear, and abducens nuclei--CN 3, 4, 6).

Functions of Vestibular Nuclei

Projections to spinal cord & cerebellum:

- Coordinate eye movements with head & neck

movements

- Coordinate vestibular input with “anti-gravity” muscles

(i.e. reflexively catch yourself if you trip)

Projections to nuclei innervating extraocular

muscles:

- Track objects while turning your head

3

The vestibular system provides only one of __ inputs
that tell us how our body is oriented.

Proprioception

uses gravity, e.g., the force exerted between our feet and the floor

Vision

orients us relative to the earth (down), the sky (up), and what's horizontal

False

(True/False) People can typically function with only one of the three senses.

Vertigo

sensation of turning in the absence of motion

Benign Paroxysmal Positional Vertigo

the sensation of spinning with certain head movements as a result of a problem in the inner ear (fancier medical term)

Ménière's Disease

results in a degeneration of inner ear structures that can lead to hearing loss, tinnitus, vertigo, and an increase in pressure within the inner ear

Nystagmus

repetitive rhythmic movements of one or both eyes in the absence of movement

Motion Sickness

effect when visual and/or motor feedback (autonomic centers) is inconsistent with vestibular input to reticular formation