The ear is the organ of hearing and balance. It is classically divided into three components: the outer, the middle, and the inner ear. The outer ear is air-filled and comprises the auricle, the external auditory meatus, and the tympanic membrane, which separates the external ear from the middle ear. The middle ear is air-filled and comprises the tympanic cavity, the ossicles, and eustachian tube. The inner ear is fluid-filled and contains the organ of Corti (which is responsible for sound detection and connects to the brain via the cochlear nerve) and the vestibular system (which is responsible for balance and spatial orientation and connects to the brain via the vestibular nerve).
- Definition: the external portion of the ear
- The only visible part of the ear
- Consists of elastic cartilage
- Function: collects sound waves; important for directional hearing
External auditory meatus (auditory canal)
- 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)
- 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).
- Auricle (pinna)
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, mastoid process, and eustachian tube
- Function: transmits sound waves from the air to the ossicles and then to the inner ear
- Air-filled space that is located within the petrous portion of the temporal bone
- Contains ossicles, muscles, and nerves (e.g., )
- Connected to the nasopharyngeal cavity via the eustachian tube
- Oval 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.
- 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
- Definition: connects the middle ear cavity with the nasopharynx
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.
- 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)
- The smallest skeletal muscle in the human body; attaches to the stapes
- Origin: cartilaginous and bony margins of the eustachian tube
- 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
- Process of the temporal bone behind the ear
- Becomes pneumatized (aerated) with age
- 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.
- It is subdivided into
- Osseous labyrinth (bony labyrinth)
- Lodged within the osseous labyrinth in perilymph (low potassium, high sodium)
- 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 to controlspiral organ of Corti and auditory hair cells
- Tympanic duct (scala tympani): filled with perilymph; ends at round window
- Fluid-filled, snail-shaped cavity in the labyrinth
- Layers of the cochlea
- 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.
- Vibrations lead to stimulation of a specific location on the basilar membrane depending on the sound frequency.
- Auditory pathway : auditory neural signals detected by the hair cells of the organ of Corti → bipolar neurons of the cochlear or spiral ganglion via the cochlear part of the vestibulocochlear nerve → ventral and dorsal nuclei (in the medullopontine junction) → superior olivary nucleus (from the ventral nucleus to the trapezoid body) → bilaterally to the lateral lemniscus → inferior colliculus → medial geniculate body → auditory radiations → primary auditory cortex (transverse temporal gyri of Heschl) and amygdala → 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.
- The system of balance and spatial orientation
- Consists of
- Otolith organs: fluid-filled pouches lined with sensory epithelium (maculae acusticae)
- Three semicircular canals (horizontal, superior, and posterior semicircular canals): sense rotary movements (i.e., angular accelerations) at their dilated ends (i.e., ampullas)
- 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 ).
- Outer ear
- Middle ear
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:
- Otic capsule: A mesenchymal cartilaginous shell around the otic vesicle that develops into the bony labyrinth.