Anatomy and Physiology of the Ear: Outer, Middle, and Inner Structures

Outer Ear

Two main parts: Auricle and External Auditory Meatus.

Auricle

Also known as Pinna; made of cartilage and skin.

Auricle Landmarks

Lobe, Helix, Antihelix, Tragus, Concha.

Auricle Functions

Collects all sounds, giving an extra boost in intensity to high-frequency sounds and helps to localize sound.

External Auditory Canal

Also known as ear canal; starts at concha and ends at Tympanic Membrane (TM).

External Auditory Canal Sections

Divided into Outer (lateral) with cerumen-producing glands and hair, and Inner (medial) with skin over bone.

External Auditory Canal Functions

Protects TM (ear drum) and transfers all sounds to TM, giving an extra boost to high-frequency sounds.

Resonant Frequency of External Auditory Canal

2000 to 4000 Hz.

Resonant Frequency of Concha

Closer to ~5000 Hz.

Gain in Sound Intensity

Together, they produce a gain in the 1500 - 7000 Hz range.

Tympanic Membrane

Barrier between outer ear and middle ear; concave disk and very thin.

Landmarks of a Healthy Tympanic Membrane

Cone of Light, Handle of the Malleus, Color.

Pars Flaccida

Not as strong (flaccid); part of the Tympanic Membrane.

Pars Tensa

Stronger (more tense/stiff); tubes are often placed here because it is stronger and able to hold the tubes in place.

Eardrum Characteristics

Eardrum is very red, not flat, not shiny; indicates a sick ear.

Middle Ear Functions

Carries sound from outer ear to inner ear and overcomes loss of energy from outer ear to inner ear.

Middle Ear Volume

2 cm3.

Middle Ear Lining

Lined with mucous membrane.

Eustachian Tube

Connects the middle ear to the nasal pharynx.

Ossicles

Smallest bones in the body; consists of Malleus, Incus, and Stapes.

Ossicular Chain

All three ossicles together.

Stapes

Connected to ear drum and the next ossicle, the Incus

Malleus

aka "hammer"

Incus

aka "Anvil"

Sound Transfer through middle ear

Ear drum moves in and out; Rigid connection of Malleus and Incus causes rotation; Force transferred to Stapes; Oval Window moves in and out

Impedance Mismatch

Air and fluid have difference impedances; Cochlear (inner ear) fluids are less compressible than air; Cochlear fluids have more density, therefore more impedance to the transmission of sound than when it travels through air

Otoscopy

First part of ear exam; Visual inspection; Equipment used: Otoscope; Observe the state of the canal and ear drum

Cochlear Anatomy

The Cochlea is very small, but very complex; Long tube coiled into a spiral; 2 ¾ turns

Cochlear Fluids

Perilymph found in Scala Vestibuli and Scala Tympani; Endolymph found in Scala Media

Scala Media

Completely closed; Triangular shape; Contains the Organ of Corti

Labyrinth

Includes Bony Labyrinth and Membranous Labyrinth

Cochlea - Scala Media

Borders include Reissner's Membrane and Basilar Membrane

Cochlear Anatomy - Reciprocal relationship

Reciprocal relationship between the oval window and round window

Cochlear Fluids - Composition

Perilymph composed primarily of Sodium; Endolymph composed primarily of Potassium

Inner Ear

Housed in the petrous part of the temporal bone; Consists of Cochlear (hearing) components and Vestibular (balance) components

Function of Inner Ear

Main function: transducer of energy; Hearing: changes the mechanical energy from middle ear into a form of information that the brain can interpret; Balance: changes body/head movements into a form of information that the brain can interpret

Middle Ear Ligaments

Malleus Ligament and Incus Ligament; Function: suspensory ligaments

Middle Ear Muscles

Tensor tympani and Stapedius; Function: contract in response to loud sounds

Stria Vascularis

A structure in the cochlea that plays a crucial role in the production of endolymph.

Organ of Corti

Sits on the basilar membrane and is the site of transduction of energy, containing sensory cells.

Hair Cells

Sensory cells in the cochlea that detect sound; includes inner hair cells (1 row) and outer hair cells (3 rows).

Cochlear Physiology

The study of how sound waves are transformed into neural signals in the cochlea.

Sound Wave Movement

Vibration transferred from stapes to scala vestibuli through the oval window, causing a traveling wave.

Tonotopic Organization

The arrangement of the cochlea and auditory pathways where different frequencies are processed in specific locations.

Depolarization of Hair Cells

The process that occurs when hair cells detect sound, leading to the generation of nerve impulses.

Inner Hair Cells

Hair cells that function as sound detectors.

Outer Hair Cells

Hair cells that act as amplifiers in the cochlea.

Afferent Fibers

Nerve fibers that carry signals from the ear to the brain.

Efferent Fibers

Nerve fibers that carry signals from the brain to the ear, regulating cochlear sensitivity.

VIIIth Cranial Nerve

Also known as the vestibulocochlear nerve, it forms when the auditory and vestibular nerve fibers converge.

Internal Auditory Canal (IAC)

A passage in the temporal bone that carries the auditory and vestibular nerves.

Retrocochlear Pathway

The auditory pathway that extends beyond the cochlea to the central auditory system.

Crossover

The process by which most structures in the central auditory system receive input from both ears.

Auditory Cortex

Located in the temporal lobes, it is responsible for the reception of auditory signals.

Central Auditory System

The part of the auditory system that processes sound information after it leaves the cochlea.

Asymmetries of Cerebral Auditory Areas

Differences in the physical and functional characteristics of auditory processing areas in the brain.

Pathway of Sound

The route sound travels from the outer ear through the auditory nerve to the auditory cortex.

Cochlear Portion

The part of the VIIIth cranial nerve that is specifically responsible for carrying auditory information.

Basilar Membrane Vibration

The movement of the basilar membrane that leads to the activation of hair cells.

Shearing of Hair Cell Stereocilia

The bending of the stereocilia on hair cells that occurs due to basilar membrane movement.

Cerumen Production

Location in the external auditory canal where cerumen is produced.

Main Job of the Outer Ear

The primary function and acoustic purposes served by the outer ear.

Sound Transduction in Outer Ear

Description of how sound is transduced through the outer ear.

Middle Ear Structures

Structures that make up the middle ear and their functions.

Purpose of the Middle Ear

Functions served by the middle ear.

Sound Transduction in Middle Ear

Description of how sound is transduced through the middle ear.

Cochlea Arrangement

Description of the cochlea's arrangement, including compartments and barriers, and the type of fluid in each.

Tonotopic

Meaning of the term 'tonotopic' in reference to the cochlea.

Sound Waves in Cochlea

How sound waves travel through the cochlea and how the inner ear creates interpretable sound signals.

Innervation of Hair Cells

Description of how nerves are attached to inner hair cells versus outer hair cells.

Roles of Hair Cells

Difference in the roles of inner hair cells and outer hair cells.

Sound Transduction in Inner Ear

Description of sound transduction from the inner ear to the auditory nerve.

Auditory Nerve Fiber Arrangement

Arrangement of nerve fibers in the auditory nerve and distinction from the Internal Auditory Canal.

Internal Auditory Canal Structures

Anatomical structures contained in the Internal Auditory Canal (IAC).