Special Senses – Anatomy of the Ear

Special Senses – Anatomy of the Ear

Topics Covered

Introduction

Anatomy of the ear

Cochlea

Semicircular canals

Introduction

Sound/Hearing

·      Sound is any audible vibration of molecules

·      Vibrating object pushes air molecules into eardrum making it vibrate

·      Signals sent to brain by cochlear nerve via the auditory (CN VIII) nerve.

·      somatic afferents and special visceral afferents.

Equilibrium/Balance

·      Two types of equilibrium:

o   Static: determines nonmoving position. i.e. standing upright or lying down. 

o   Dynamic: motion is detected. i.e. sudden acceleration, spinning.

- Signals sent to brain by vestibular nerve via the auditory (CN VIII) nerve.

Anatomy of the ear

(Link to figure 14.5)

Outer ear

•       Fleshy auricle (pinna=helix and lobule) directing air vibrations down auditory canal (external auditory meatus)

–      S-shaped tunnel within temporal bone ending at eardrum

•       Eardrum, also known as the tympanic membrane, separates the outer and middle ear.

Middle ear

•       Tympanic membrane: 1 cm in diameter, slightly concave, freely vibrating membrane

•       Tympanic cavity: air-filled cavity in temporal bone separated from air outside the head by tympanic membrane

•       Ear ossicles span tympanic cavity

–      Tympanic membrane -> malleus ->incus ->stapes ->oval window of inner ear

•       Tympanic cavity filled with air by auditory tube connected to nasopharynx

–      opens during swallowing or yawning to equalize air pressure on both sides of eardrum

Inner ear

–      Cochlea

–      Vestibular apparatus

•       vestibule (saccule and utricle)

•       semicircular ducts (anterior, posterior, lateral)

Transmission of sound

(Figure 14.6)

•       Tympanic membrane vibrates quite easily

–      Sends vibrations through ossicles to oval window of inner ear

•       Due to size difference between large tympanic membrane and small oval window, magnifies vibrations in inner ear (18X).

•       Tympanic Reflex – Protection of inner ear by muscle contraction in response to loud noises

–      tensor tympani muscle pulls tympanic membrane inward, tightening it

–      stapedius muscle reduces mobility of stapes to lessen vibrations

–      designed for slowly building noises like thunder not gunshots

•       does not protect us from sustained loud noises such as music

–      muscles also contract while speaking; enables us to hear others

Cochlea

(Linked to Figure 14.11 and Figure 14.12)

•       Three chambers within the cochlea:

•       the middle chamber is part of the membranous labyrinth and is filled with endolymph

•       the surrounding two are part of the bony labyrinth and are filled with perilymph.

•       Endolymph in the scala media carries vibrations from oval window to Organ of Corti.

•       Endolymph vibrates gelatinous tectorial membrane, causing the vibration of the stereocilia on hair cells.

•       Two types of hair cells:

–      Inner hair cells: responsible for hearing

–      Outer hair cells: increase precision

–      Sends signals along cochlear nerve

•       Loudness produces more vigorous vibrations & excites more hair cells over a larger area

–      triggers higher frequency of action potentials along the cochlear nerve; brain interprets this as louder

•       Determination of pitch depends on which part of cochlea receives vibrations.

–      Stimulation closer to the middle ear results from sounds of shorter wavelength, which are high-pitched

–      Stimulation further from the middle ear results from sounds of longer wavelength, which are low-pitched

Semicircular ducts/ Vestibule

•       Saccule: located next to cochlea

•       Utricle: located next to semicircular ducts

•       Both chambers contain macula

–      patch of hair cells with stereocilia buried in a gelatinous otolithic membrane weighted with granules called otoliths

–      otoliths add to the density & inertia and enhance the sense of gravity and motion

•       With the head erect, stimulation is minimal, but when the head is tilted, weight of membrane bends the stereocilia (static equilibrium)

•       When car begins to move at green light, linear acceleration is detected since heavy otolith lags behind (one type of dynamic equilibrium)

•       Ampullae of semicircular ducts contain crista ampullaris (one in each ampulla), which consists of hair cells buried in a mound of gelatinous membrane called the cupula

•       Orientation of ducts causes different ducts to be stimulated by rotation in different planes

•       Crista Ampullaris: As head turns, the endolymph lags behind pushing the cupula and stimulating its hair cells