Chapter 20: Ear anatomy

State the structures of the outer (external) ear

Portion of the ear that is open to the air around your head:

• Auricle

• auditory canal

• tympanic membrane (internal border of the auditory canal) —> commonly called the ear drum

State the structures of the middle ear

Hollow, air-filled chamber that houses the three smallest bones in your body:

• malleus

• incus

• stapes

• auditory (eustachian) tube

• oval window

State the structures of the inner ear

Houses sensory organs of both hearing and equilibrium embedded in the temporal bone:

• cochlea

• vestibule

• vestibular nerve

• cochlea nerve

• semicircular canals

Describe the outer parts of the external ear

Auricle: the outer flap of elastic cartilage, adipose tissue and skin

Auditory canal: The opening into the temporal bone is lined with skin

Tympanic membrane: Thin membrane separating the outer and middle ear

Describe the middle parts of the ear

Auditory ossicles: Three small bones that amplify

• Malleus: Hammer-shaped bone in contact with the tympanic membrane

• Incus: Anvil-shaped bone in between the other ossicles

• Stapes: Stirrup-shaped ossicle that touches the oval window

Auditory tube: Connecting tube to the upper throat

Oval window: Membrane that separates the inner and middle ear

Describe the inner parts of the ear

Cochlea: The spiral-shaped organ of hearing

Vestibule: Hollow organ that houses two organs of equilibrium, the utricle and the saccule

Vestibular nerve: Small nerve branches that innervate the organs of equilibrium

Cochlear nerve: Large nerve that innervates the cochlea and carries information about sound

Semicircular canals: Three tubular organs that sense rotational acceleration

Label the ear

For a vibration to be hear what part of the ear must the sound reach? What are the two pathways

The cochlea and there are two pathways to get it to travel to the cochlea: sound through air, sound through bone

Describe how sound travels through the sound through air pathway

• sound travels through the air as vibrations

• vibrations hit the tympanic membrane (eardrum) —> it vibrates

• vibrations are passed to the auditory ossicles

• ossicles amplify force by moving vibrations from a large surface (eardrum) to a smaller one (oval window)

• increased force cuases fluid in the cochlea to move

• fluid movement bends the cilia on hair cells

• bending causes hair cells to depolarize

• hair cells release neurotransmitters

• neurotransmitters trigger action potentials in the cochlear nerve

• signals are sent to the brain

• sound reaches the cochlea through:

  • air conduction

  • bone conduction

• This is why your voice sounds different in recordings

How does sound travel through bone, and why does it affect how you hear your own voice?

• Sound can travel through bone conduction (via the skeleton)

• Vibrations from your voice move through your bones

• These vibrations bypass:

  • Tympanic membrane (eardrum)

  • Auditory ossicles

  • Oval window

• The cochlea is vibrated directly by the temporal bone

• Bone conduction adds extra richness to your voice

• This is why your voice sounds different compared to recordings (which only use air conduction)

What are the two types of hearing tests?

• Weber test

• Rinne test

Describe the weber test

• Allows you to determine whether both ears have the same ability to hear

• does not allow you to determine what sort of hearing deficit is present and it does not allow you to identify a problem if both ears are equally impaired

If sound is uneven during the weber test, what are two possibilities?

• Sensorineural deafness: The cochlea, nerves, or brain do not receive or interpret sound correctly and the sound will be softer/absent in the impaired ear

• Conductive deafness: the auditory canal, tympanic membrane, or ossicles are damaged/blocked. The background noise of the room, carried through the air, is absent in this ear, so the tuning fork sound, carried through the bone, seems louder in the impaired ear

Describe the rinne test

• can be used to determine whether the defect in the amplifying system (conductive deafness) or in the cochlea or nervous system (sensorineural deafness)

• if the amount of time the subject can hear the tuning fork through bone and air is equal for each ear, but different from ear to ear, then the individual has sensorineural deafness in the ear with the shorter times. This should be the ear that heard the sound as quieter in the weber test

• If the amount of time the individual hears the sound through bone is much longer than through the air, then the individual has conductive deafness. This should be in the ear that heard the sound as louder during the Weber test.

• Other combinations of results either indicate complex deficits that cannot be assessed with the Rinne and Weber tests or the limitations of doing these tests in a noisy building.

How is loudness (volume) perceived, and how can loud sounds damage hearing?

• Loudness is the perception of the amplitude (height) of a sound wave

• Vibrations in the cochlea bend the cilia on hair cells

• Bending opens mechanically-gated ion channels

• Hair cells depolarize and release neurotransmitters

• Greater amplitude → more bending of cilia

• More bending → more action potentials sent to the brain

• Higher firing rate = perceived as louder sound

• Very loud or prolonged sound can damage hair cell cilia

• Damage can lead to permanent hearing loss

How does the brain determine the direction of a sound?

• The brain uses both ears (paired ears) to detect sound direction

• Compares differences in:

  • Volume (loudness) between ears

  • Time of arrival of sound at each ear

• These differences help locate where the sound is coming from

• This process is called sound localization

pitch