The ear

The ear is the organ of hearing and balance. It is divided into three sections: the outer, the middle, and the inner ear. The outer ear comprises the auricle (pinna), external auditory meatus (auditory canal), and tympanic membrane (eardrum), which separates the outer ear from the middle ear. The middle ear is a hollow structure that comprises the tympanic cavity, the ossicles, and the eustachian tube. The eustachian tube connects the middle ear to the nasopharynx, equalizing the pressure between the middle ear and the atmosphere. The auricle captures sound waves and directs them through the external auditory canal towards the tympanic membrane, thus setting it in motion. The ossicles amplify the resulting vibrations and transmit them to the inner ear via the oval window. The inner ear is a fluid-filled cavity that contains the organ of Corti and the vestibular system. The organ of Corti is responsible for sound detection, and it transmits auditory information to the brain via the cochlear nerve, while the vestibular system is responsible for the registration of body movement and spatial orientation. It transmits information to the brain via the vestibular nerve.

• Definition: the external portion of the ear
• Parts
  • Auricle (pinna)
    • Structure
      • The only visible part of the ear
      • Consists of elastic cartilage
    • Function: collects sound waves; important for directional hearing
  • External auditory meatus (auditory canal)
    • Structure
      • The outer third is formed by cartilage, the inner two thirds are formed by bone.
      • Lined by thin keratinized stratified squamous epithelium along the entire canal; also covers the external tympanic membrane.
      • Epithelium contains ceruminous glands that produce cerumen (ear wax): yellow-brownish, waxy, bactericidal secretion.
    • Function: transmission of sound waves to the tympanic membrane
  • Tympanic membrane (eardrum)
    • Structure
      • Thin, cone-shaped membrane
      • Cutaneous outer, fibrous middle, and mucous inner layer
      • Umbo: the point where ossicles attach to the tympanic membrane
      • Cone of light: a cone-shaped light reflection of the otoscope light in the anterior inferior quadrant, which helps with orientation
    • Function: Sound waves cause vibration of the tympanic membrane, which in turn transmits these vibrations to the ossicles of the middle ear (i.e., the malleus, incus, and stapes).

The area behind the auricle and the external auditory canal are innervated by the glossopharyngeal and vagus nerve. Mechanical cleansing of the external auditory canal can lead to nausea and coughing!

• Definition: : The middle portion of the ear that is located internal to the tympanic membrane and external to the oval window of the inner ear.
• Consists of: tympanic cavity, epitympanic recess, mastoid process, and eustachian tube
• Function: transmits sound waves from the air to the ossicles and then to the inner ear

Parts

Tympanic cavity

• Air-filled space that is located within the petrous portion of the temporal bone
• Contains ossicles, muscles, and nerves
• Connected to the nasopharyngeal cavity via the eustachian tube
• Oval window
  • An opening located in the middle ear at the base of the cochlea through which sound waves are transmitted to the inner ear
  • Covered by the footplate of the stapes
• Round window
  • A membrane-covered opening in the middle ear that lies below the stapes-covered oval window.
  • Vibrations of the footplate of the stapes are transmitted to the round window through the perilymph, which causes it to vibrate in the opposite phase of the oval window.
  • Injury to the round window (e.g., violent nose blowing, barotrauma while diving) causes sensorineural hearing loss.

Auditory ossicles

• Three small bones in the middle ear that form a chain of bones connecting the tympanic membrane laterally to the oval window of the inner ear medially
  • Malleus: inserts laterally into the tympanic membrane; the most lateral ossicle
  • Incus: connects to the malleus laterally and to the stapes medially
  • Stapes: inserts medially into the membrane of the oval window; the most medial ossicle
• Function: : The ossicles receive the acoustic vibrations from the tympanic membrane and amplify them; . They then transmit the amplified acoustic vibrations to the fluid of the inner ear via the oval window with minimal energy loss.

Eustachian tube (pharyngotympanic tube)

• Definition: connects the middle ear cavity with the nasopharynx
• Function:
  • Controls the pressure within the middle ear and allows the pressure to be equalized on both sides of the tympanic membrane (e.g., important for altitude compensation)
  • Usually closed but opens when chewing, swallowing, or yawning (ear clearing)

During and after a cold, the eustachian tube can become blocked by swelling of the mucosa and mucus impaction. This often results in dull hearing, pain, and a feeling of pressure or fullness in the affected ear.

Tensor tympani muscle

• A skeletal muscle that attaches to the malleus
  • Origin: cartilaginous and bony margins of the eustachian tube
  • Insertion: handle of the malleus
  • Action: : Contraction of the muscle pulls the malleus medially. This action tenses the tympanic membrane and damps vibration in the ear ossicles, resulting in a reduction of the perceived amplitude of sound.
  • Innervation: medial pterygoid nerve, a branch from the mandibular nerve (V3)

Stapedius muscle

• The smallest skeletal muscle in the human body; attaches to the stapes
  • Origin: pyramidal eminence (hollow protrusion from the posterior wall of the tympanic membrane)
  • Insertion: neck of the stapes
  • Action: : contraction of the muscle pulls the neck of the stapes laterally, thereby damping the vibrations of the stapes; and allowing it to control the amplitude of sound waves being transmitted to the inner ear.
  • Innervation: : the nerve to stapedius, a branch from the facial nerve

The stapedius muscle dampens transmission of loud noises to the inner ear (acoustic or stapedial reflex). Stapedius weakness (e.g., due to Bell palsy) can result in hyperacusis (increased sensitivity to environmental sounds, which can also result from ear trauma or middle/inner ear infection)

Mastoid process

• Process of the temporal bone behind the ear
• Becomes pneumatized (aerated) with age, with mastoid air cells formation

A middle ear infection can spread to neighboring structures and result in perforation of the tympanic membrane, labyrinthitis, mastoiditis, cerebral venous sinus thrombosis, and meningitis.

• Definition: : The innermost portion of the ear that contains organs of hearing (i.e., cochlea) and equilibrium (i.e., vestibule) and is situated within the petrous part of the temporal bone.

Parts

Labyrinth

• It is subdivided into
  • Osseous labyrinth (bony labyrinth)
    • The bony wall of the inner ear in the temporal bone
    • Contains the vestibule, the three semicircular canals, and the cochlea
    • Lined with periosteum
  • Membranous labyrinth
    • Lodged within the osseous labyrinth in perilymph (low potassium, high sodium; transmits sound vibrations from the stapes-covered oval window to the round window)
    • Filled with endolymph (high potassium, low sodium)
    • Contains sensory organs
• Contains complex series of fluid-filled spaces
  • Vestibular duct (scala vestibuli): filled with perilymph; begins at oval window; connected with tympanic duct at helicotrema (passage at apical end of cochlea)
  • Cochlear duct (scala media): filled with endolymph; contains spiral organ of Corti and auditory hair cells
  • Tympanic duct (scala tympani): filled with perilymph; ends at round window

Cochlea

• Fluid-filled, snail-shaped cavity in the labyrinth
• Layers of the cochlea
  • Bone
  • Vestibular duct
  • Reissner membrane
  • Cochlear duct
  • Basilar membrane: thin at the basal end and wide at the apical end; supports organ of Corti
  • Tympanic duct
  • Bone
• Function: transformation of airborne vibrations into auditory neural signals
• Stria vascularis: produces potassium-rich endolymph for the scala media, which is essential for the endocochlear potential
• The cochlear duct (scala media) contains hair cells .
  • These are specialized cells situated on a basilar membrane.
  • They have apical modifications called stereocilia.
• Vibrations lead to stimulation of a specific location on the basilar membrane, depending on the sound frequency (tonotopy).
  • High-frequency sounds stimulate hair cells at the base of the cochlea; the basilar membrane in this area is thin and rigid.
  • Low-frequency sounds stimulate hair cells at the apex of the cochlea near an area called helicotrema; the basilar membrane in this area is wide and flexible.
• Auditory pathway :
  1. Auditory neural signals are detected by the hair cells of the organ of Corti, which are innervated by the bipolar neurons of the cochlear or spiral ganglion (first-order neuron).
  2. The signals are transmitted to the ventral and dorsal cochlear nuclei (second-order neuron) via the axons of the bipolar neurons, which constitute the cochlear part of the vestibulocochlear nerve (auditory nerve).
  3. The pathway diverges into the following parts:
     • Direct pathway: Axons of the dorsal cochlear nucleus neurons join the contralateral lateral lemniscus and reach the neurons of the inferior colliculus (third-order neurons) on the other side.
     • Indirect pathway:
       • Axons of the ventral cochlear nucleus neurons reach the superior olivary nucleus (third-order neuron) on the ipsilateral and contralateral sides.
       • Axons of the neurons of the superior olivary nucleus join the lateral lemniscus and synapse on the neurons of the inferior colliculus (fourth-order neurons).
  4. Axons of the neurons of the inferior colliculus synapse on the cells of the medial geniculate body (fourth-order or fifth-order neuron).
  5. Axons of the neurons of the medial geniculate body ascend the posterior limb of the internal capsule, where they form the auditory radiations, after which they reach primary auditory cortex (superior temporal gyrus in the temporal lobe) and amygdala.
  6. The primary auditory cortex connects to auditory association cortex.

Loop diuretics are ototoxic because they act on both the Na/K/2Cl carrier in the ascending loop of Henle of the kidneys and the same transporters in the stria vascularis of the cochlea. This effect can lead to edema of cochlear tissues, a decrease of the cochlear electrical potential, and temporary or permanent sensorineural hearing loss.

Vestibular system

• The system of balance and spatial orientation
• Consists of
  • Otolith organs: fluid-filled pouches lined with sensory epithelium (maculae acusticae)
    • Utricle: senses motions in the horizontal plane (e.g., forward-backward, left-right movements)
    • Saccule: senses motions in the sagittal plane (e.g., up-down movement)
  • Three semicircular canals (horizontal, superior, and posterior semicircular canals): sense rotary movements (i.e., angular accelerations) at their dilated ends (i.e., crista ampullaris or ampullary crest)
• Specialized cells known as vestibular hair cells transduce minute displacements into neural signals that are then transmitted to the brain stem via the vestibular part of the vestibulocochlear nerve (see vestibular pathway).

References:^[1]

• Outer ear
  • Auricle: arises from the fusion of six auricular hillocks
  • External auditory meatus: arises from ectoderm of 1st pharyngeal cleft
• Middle ear
  • Pharyngeal arch 1 (mesoderm): incus, malleus, tensor tympani muscle, CN V
  • Pharyngeal arch 2 (mesoderm): stapes, stapedius muscle, CN VII
  • Pharyngeal pouch 1 (endoderm): eustachian tube and middle ear cavity
  • Tympanic membrane: inner layer (endoderm), middle layer (mesoderm), outer layer (ectoderm)
• Inner ear: develops from the otic placode, a thickening of the ectoderm, which invaginates to form auditory vesicles or otocysts
  • Otocysts form the membranous labyrinth:
    • Utricle and saccule (contain sensory hair cells)
    • Cochlear duct (contains organ of Corti and endolymph), endolymphatic duct (connects the utricle and saccule with the endolymphatic sac), and semicircular canals
    • Vestibular and cochlear ganglions of CN VIII
  • Otic capsule: A mesenchymal cartilaginous shell around the otic vesicle that develops into the bony labyrinth.

References:^[2][3][4]

• Retroauricular hematoma
• Otitis externa
• Acute otitis media
• Otosclerosis
• Chronic otitis media
• Cholesteatoma
• Benign paroxysmal positional vertigo
• Ménière disease
• Vestibular neuritis
• Hearing loss
• Ear barotrauma
• Tinnitus
• Microtia
  • A congenital deformity of the ear (i.e., the auricles) characterized by underdevelopment of the pinna and hearing impairment.
  • Microtia varies in severity and can occur as an isolated birth defect or as part of a syndrome (e.g., Treacher-Collins syndrome).
• Anotia: a congenital ear deformity characterized by the complete absence of the pinna and hearing impairment