In this note, we are going to discuss the anatomy and function of the ear, as well as the vestibular and auditory systems.
The ear is anatomically divided into three regions: the external ear, the middle ear, and the internal ear.
- The external ear is the visible portion and directs sound waves to the tympanic membrane.
- The middle ear is a chamber within the petrous part of the temporal bone.
Structures in the middle ear amplify sound waves and transmit them to the appropriate part of the internal ear.
- The internal ear contains the sensory organs responsible for hearing and equilibrium (balance).
Let’s discuss each part in greater detail.
External Ear
The external ear consists of the outer visible part, called the auricle,
which leads to the tube, called the external acoustic meatus.
The auricle protects the meatus and directs sound waves towards the middle ear, in particular, towards the tympanic membrane (also known as eardrum).
It’s a thin, transparent membrane separating the external ear from the middle ear.
The external acoustic meatus is lined with numerous hairs and modified sweat glands known as ceruminous glands. Cerumen, the waxy secretion of the ceruminous glands, inhibits the growth of microorganisms and reduces the likelihood of infection.
Middle Ear
The middle ear
consists of the tympanic cavity, which contains the auditory ossicles.
The middle ear connects the tympanic membrane with the receptor complex of the internal ear.
The tympanic cavity is connected to the nasopharynx via the auditory tube (also known as the Eustachian tube)
and has connections with the mastoid air cells, which vary in size and quantity.
The auditory tube is responsible for equalizing the pressure between the middle ear and the external environment. This pressure balance is essential for protecting the tympanic membrane because any pressure gradient could lead to its damage.
Auditory Ossicles
The three smallest bones in the body, called the auditory ossicles, are located within the tympanic cavity. They transfer sound vibrations from the tympanic membrane to a fluid-filled chamber within the internal ear.
These ossicles are called the malleus,
incus,
and stapes.
The malleus, the outermost of the three middle ear bones, is connected to the tympanic membrane. The incus, the middle bone, connects the medial surface of the malleus to the stapes, the innermost bone. The base of the stapes almost completely fills the oval window, a hole in the bony wall of the middle ear cavity.
Vibration of the tympanic membrane converts sound waves into mechanical movements. These movements are transferred sequentially from the malleus to the incus and then to the stapes. The stapes’ movement creates vibrations in the fluid-filled inner ear. Notably, an in-out movement of the eardrum results in a powerful rocking motion at the stapes, amplifying the initial movement of the eardrum.
However, this degree of magnification can present a problem when exposed to loud noises. To protect the eardrum and the ossicles from forceful movements under such conditions, there are two small muscles within the tympanic cavity.
The tensor tympani muscle originates from the petrous portion of the temporal bone and attaches to the handle of the malleus.
When the tensor tympani contracts, it pulls the malleus medially, resulting in the tightening of the tympanic membrane. This reduces the range of its possible movement, leading to a protective effect. The tensor tympani is supplied by motor fibers from the mandibular branch of the trigeminal nerve (CN5).
The stapedius muscle arises from the posterior wall of the tympanic cavity and inserts to the stapes.
Its contraction pulls the stapes posteriorly, restricting its movement at the oval window. This muscle is innervated by the facial nerve (CN7).
Internal Ear
The sensory receptors for equilibrium and hearing are located within the internal ear. These receptors are situated within an intricate system of interconnected fluid-filled tubes and chambers, collectively known as the membranous labyrinth.
The membranous labyrinth is encircled by the bony labyrinth, a protective structure formed by the temporal bone.
The membranous labyrinth is filled with a fluid called endolymph. This fluid’s distinct ionic composition is essential for the proper functioning of the sensory receptors in the internal ear. In contrast, a fluid known as perilymph fills the space between the membranous labyrinth and the bony labyrinth.
The bony labyrinth is subdivided into the vestibule,
the semicircular canals,
and the cochlea.
Equilibrium
The vestibular complex, a part of the internal ear, provides equilibrium sensations by detecting rotation, gravity, and acceleration. It consists of the semicircular canals,
the utricle,
and the saccule.
The latter two structures are the membranous sacs within the vestibular cavity.
The anterior, posterior, and lateral semicircular canals originate from the vestibule and envelop three semicircular ducts of the same name. These ducts are all connected to the utricle.
Within each duct, there is an expanded region called the ampulla.
This ampulla houses the sensory receptors, called the hair cells, which are sensitive to rotational movement of the head.
The part of the ampulla’s wall where these hair cells are located is known as the ampullary crest.
Each hair cell has numerous processes called stereocilia and a single longer process called kinocilium. The kinocilia and stereocilia are situated within a gelatinous structure termed the ampullary cupula.
As the head rotates in the plane of the duct, the endolymph shifts along the duct’s axis.
This movement of the endolymph pushes the ampullary cupula. Fluid movement in one direction stimulates the hair cells, triggering the release of neurotransmitters, and movement in the opposite direction inhibits this process.
Let’s move back to the utricle and saccule. These structures are the membranous sacs within the vestibular cavity, connected by a narrow, tube-like passageway called the endolymphatic duct.
The endolymphatic duct terminates in a closed space called the endolymphatic sac.
In the utricle and saccule, specialized hair cells are clustered within the so-called maculae. The macula of the utricle is sensitive to horizontal movement, whereas the macula of the saccule detects vertical movement.
Within the ampullae, hair cell processes are encased in a gelatinous structure known as the otolithic membrane.
The surface of this membrane is covered in densely packed calcium carbonate crystals, referred to as otoliths.
In an upright head position, the otoliths are located on top of the otolithic membrane in the macula of the utricle.
When you tilt your head, the otoliths are pulled to the side by gravity. This distortion causes the hair cell processes in the macula to bend, which leads to a release of neurotransmitters.
Sensory neurons of the adjacent vestibular ganglion are responsible for monitoring the hair cells located within the vestibule and semicircular ducts.
The collection of sensory fibers emerging from the ganglion collectively forms the vestibular nerve.
Information conveyed by these fibers terminates at neurons located in the vestibular nuclei, a region situated at the boundary between the pons and the medulla oblongata.
These fibers transmit information to the cerebellum, the motor nuclei of extra-ocular muscles, and the cerebral cortex.
Eventually, the motor commands that modify peripheral muscle tone in relation to reflexive head or neck movement are transmitted by the vestibulospinal tracts.
Cochlear Duct and Hearing
The cochlea
contains the cochlear duct, a thin, extended section of the membranous labyrinth.
The cochlear duct is situated between two chambers, or scalae, filled with perilymph. These are the scala vestibuli, known as the vestibular duct,
and the scala tympani, or tympanic duct.
All three ducts are encased by the bony labyrinth, excluding the area of the oval window (which is the base of the scala vestibuli)
and the round window (which is the base of the scala tympani).
The scalae are interconnected at the tip of the spiral-shaped cochlea, forming a single, long, and continuous chamber filled with perilymph.
The entire complex loops around a central bony core called modiolus.
There are usually 2.5 turns in the cochlear spiral. Within the modiolus we can find the spiral ganglion that contains the cell bodies of the sensory neurons that monitor the receptors in the cochlear duct.
When a sound vibrates, the tympanic membrane, the malleus and incus transfer these vibrations to the stapes. The stapes’ movements cause vibrations in the oval window that trigger perilymph and eventually lead to the stimulation of receptors within the cochlear duct.
These receptors are called hair cells, and they are arranged within the so-called spiral organ, or organ of Corti.
These hair cells have stereocilia that reach the overlying tectorial membrane, firmly connected to the inner cochlear duct wall.
The vibration of a portion of the basilar membrane causes the stereocilia to bend, resulting in stimulation of the sensory neurons within the spiral ganglion.
Their afferent fibers form the cochlear nerve,
which enters the medulla oblongata and synapses with the cochlear nuclei.
From there, information travels to the superior olivary nuclei of the pons
and the inferior colliculi of the midbrain.
Deep within the midbrain lies a specialized processing center responsible for coordinating involuntary motor responses upon encountering acoustic stimuli. These reflexes, known as auditory reflexes, automatically change the position of your head in response to abrupt loud noises.
The medial geniculate body in the thalamus serves as the relay station for the rest of ascending auditory stimuli from the cochlear nuclei.
Subsequently, projection fibers carry the auditory information to the auditory cortex within the temporal lobe.
Vestibular and auditory systems
- ear
- auris
- external ear
- auris externa
- middle ear
- auris media
- inner ear
- auris interna
- auricle
- auricula
- external acoustic meatus
- meatus acusticus externus
- tympanic membrane
- membrana tympani
- ceruminous glands
- glandulae ceruminosae
- tympanic cavity
- cavitas tympani
- malleus
- malleus
- incus
- incus
- stapes
- stapes
- tensor tympani muscle
- musculus tensor tympani
- stapedius muscle
- musculus stapedius
- membranous labyrinth
- labyrinthus membranaceus
- osseous labyrinth
- labyrinthus osseus
- endolymph
- endolympha
- perilymph
- perilympha
- vestibule
- vestibulum
- semicircular canals
- canales semicirculares
- cochlea
- cochlea
- utricle
- utriculus
- saccule
- sacculus
- ampulla
- ampulla
- ampullary crest
- crista ampullaris
- ampullary cupula
- cupula ampullaris
- otolithic membrane
- membrana statoconiorum
- vestibular ganglion
- ganglion vestibulare
- vestibular nerve
- nervus vestibularis
- cochlear duct
- ductus cochlearis
- scala vestibuli
- scala vestibuli
- scala tympani
- scala tympani
- oval window
- fenestra vestibuli
- round window
- fenestra cochleae
- modiolus
- modiolus
- spiral ganglion
- ganglion spirale
- organ of Corti
- organum spirale
- basilar membrane
- membrana basilaris
- tectorial membrane
- membrana tectoria
- cochlear nerve
- nervus cochlearis
- cochlear nuclei
- nuclei cochleares
- superior olivary nuclei
- nuclei olivares superiores
- inferior colliculi
- colliculi inferiores
- medial geniculate body
- corpus geniculatum mediale
