Ear Anatomy-Histology Correlate By: Michael Lu, Class of ‘07

The external ear consists of the auricle and the external acoustic meatus. Sound waves in the air are transmitted to the eardrum of the middle ear. The middle ear starts from the tympanic membrane, the epitympanic recess, and the ossicles – malleus, incus, and stapes. The ossicles sense the vibrations of the sound waves on the tympanic membrane, converting and amplifying the energy into mechanical energy, which travels into the inner ear. Note the auditory, or Eustachian, tube connecting the middle ear to the nasopharynx, which allows the equilibration of pressure. The inner ear consists of 1) the cochlea for auditory sense and 2) the semicircular ducts, ampullae, utricle, and saccule for the sense of balance and position. The membranous labyrinth is found within the bony labyrinth.

Note the handle, or manubrium, of the malleus (commonly known as the hammer) firmly attaches to the upper half of the tympanic membrane. The head of the malleus contacts the incus. The incus, or anvil, body articulates with the head of the malleus. The long process or limb of the incus descends vertically, parallel to the handle of the malleus, and contacts the stapes. The stapes, or commonly known as the stirrup, head articulates with the incus. The two crura or limbs diverge and join at the flattened oval base, which attaches to the oval window. Via the three ossicles, vibrations are transmitted from the tympanic membrane to the oval window and to the inner ear, which will be discussed later. Note the other structures within the tympanic cavity of the middle ear. The tensor tympani muscle inserts into the handle of the malleus and dampens vibrations of the tympanic membrane. The stapedius muscle may not be visible, but its tendon attaching to the stapes may be seen. It acts to dampen vibrations of the stapes. Note the facial nerve (CN VII) within the facial canal, making an almost 90 degree turn to exit the stylomastoid foramen. It gives off the greater petrosal nerve from the geniculate ganglion. Note also the chorda tympani coming off the facial nerve and traveling between the long limb of the incus and malleus handle.

Shown here is the view of the tympanic membrane through an otoscope Shown here is the view of the tympanic membrane through an otoscope. Note most of the eardrum has a strong fibrous layer that provides rigidity; this area is called the pars tensa. The upper one-sixth of the tympanic membrane lacks this fibrous layer and is termed the pars flaccida. The manubrium of the malleus attaches firmly on the tympanic membrane and can be seen. The cone of light from the otoscope can be seen inferior and anterior to the manubrium. Together, they look like an arm bending at the elbow. As mentioned before, the inner ear specializes in auditory sense and sense of balance and equilibrium. The cochlear nerve provides auditory sense from the cochlea, the spiral-shaped organ that receives the vibrations from the oval window and converts them to nerve impulses. The vestibular nerve receives input from the semicircular ducts, the utricle, and the saccule. The semicircular ducts detect angular acceleration. The utricle and saccule detect linear acceleration. The cochlear and vestibular nerves join together as CN VIII, the vestibulocochlear nerve. Note once again the facial nerve (CN VII) traveling in close proximity to CN VIII and making a sharp bend near the tympanic cavity. Note also the geniculate ganglion, the greater petrosal nerve, and the chorda tympani.

The inner ear is located in the petrous part of the temporal bone The inner ear is located in the petrous part of the temporal bone. Both the tympanic cavity and bony labyrinth are found here. Within the bony labyrinth is the membranous labyrinth. The bony labyrinth contains perilymph (high Na+, low K+), while the membranous labyrinth has endolymph (high K+, low Na+). The inner ear is composed of the following: semicircular ducts (anterior, posterior, and lateral) with associated ampullae the utricle the saccule the cochlea Review: The tympanic membrane transmits vibrations through the ossicles to the oval window. The vibrations then enter the inner ear and are detected by specialized cells in the cochlea. The round window acts as a release valve for the vibrations. Note in the bottom panel the organization of the cochlea. The cochlear duct contains the organ of Corti and three compartments – the scala vestibuli, the scala media, and the scala tympani – all of which will be detailed in the following slide. The cell bodies of cochlear neurons, or spiral ganglia, wrap around the modiolus of the cochlea. Their axons come together as the cochlear nerve found in the center of the modiolus.

The 3 membranous tubes of the cochlea are shown at higher magnification: scala vestibuli – perilymph scala media (cochlear duct) – endolymph scala tympani – perilymph The scala vestibuli and scala media are separated by the vestibular (Reissner’s) membrane. The scala media and scala tympani are separated by the basilar membrane, which is not a basement membrane but instead includes the organ of Corti. The basilar membrane oscillates in response to various frequencies transmitted from the oval window. Low frequency sounds vibrate near the apex of the cochlea, while high frequency is sensed near the base. Note the 3 outer hair cells (labeled 1, 2, 3) with apical stereocilia displaced by the tectorial membrane when the basilar membrane vibrates. The reticular lamina is a rigid plate of cytoplasm that prevents abnormal displacement of the stereocilia and subsequent depolarization. The hair cells are supported by phalangeal cells. The inner and outer pillar cells are filled with microtubules to provide a rigid, triangular inner tunnel. This remains rigid to allow the organ of Corti to rock back and forth, instead of collapsing on itself by the vibrations. Inner hair cells are found as a single row near the inner pillar cells.

Within the bony vestibule, we find the utricle and the saccule of the membranous labyrinth. It is very difficult to identify one from the other without seeing surrounding structures. The utricle is closer to the semicircular ducts, while the saccule is closer to the cochlea. The utricle and saccule are filled with endolymph, while the surrounding vestibule with loose connective tissue contains perilymph. Both the utricle and saccule contain a neurosensory area known as the macula. This consists of hair and supporting cells overlaid by a gelatinous otolithic membrane covered with otoliths or otoconia. Displacement of the otolithic membrane moves the stereocilia of the hair cells, allowing the maculae to sense linear acceleration.

The three semicircular ducts are oriented in 3 perpendicular planes The three semicircular ducts are oriented in 3 perpendicular planes. This allows detection of angular acceleration. At the origin of each semicircular duct near the utricle, there are ampullae. Each one of these dilations has a crista ampullaris projecting into the lumen. The thickened sensory epithelium at the tip of the crista contain many neurosensory cells with stereocilia, which is similar to the macula. The hair cells insert into the cupula, which is a gelatinous material similar to the otolith membrane but lacking any otoliths. The cupula shown in the bottom right appears dehydrated. Like other parts of the inner ear, the membranous part of the semicircular duct contains endolymph while the bony part contains perilymph.