Ear

Outer Ear

• Anthelix (antihelix) is the folded Y-shaped part of the ear. The upper parts of this Y are the superior crux and the inferior crux.
• Antitragus is the lower edge of the conchal bowl just above the fleshy lobule of the ear. It is made of cartilage and covered by skin.
• Auricle (pinna) (commonly referred to as “the flap” of the ear) is also made of cartilage and covered by skin.
• Ceruminous gland is located along the external auditory meatus. It secrets cerumen (ear wax).
• Concha is the hollow bowl-like portion of the outer ear that leads to the canal.
• External auditory meatus (ear canal) is the tunnel leading to the tympanic membrane.
• Helix is the rolled-up edge of the outer frame of the auricle.
• Lobe (lobule) is fleshy lower portion of the ear.
• Scapha is the scooped-out outer depression near the edge of the ear between the antihelix and helix.
• Tragus is the small projection just in front of the ear canal.

Middle Ear

• Eustachian tube (auditory or pharyngotypanic tube) extends from the middle ear to the nasopharynx, permitting air to enter or leave the middle ear cavity. Its function is to equalize air pressure on both sides of the eardrum. Normally the walls of the tube are collapsed. Swallowing and chewing actions open the tube to allow air in or out, as needed for equalization. Equalizing air pressure ensures that the eardrum vibrates maximally when struck by sound waves.The air pressure in the middle ear must be the same as the external atmospheric pressure in order for the eardrum to vibrate properly. “Popping” the ear opens the eustachian tubes, equalizing air pressures. Since the Eustachian tubes of children are short and nearly horizontal, bacteria easily spreads from the pharynx to the middle ear. Otitis media, for example, is a common complication of a strep throat.
• Ossicles consist of three linked and movable bones which convert sound waves striking the eardrum into mechanical vibrations. These three bones are the smallest in the human body and named for their shape.
  • Malleus (hammer) joins the inside of the eardrum.
  • Incus (anvil) is the middle bone which connects the other two.
  • Stapes (stirrup) footplate attaches to the cochlea.
• Tympanic cavity is air-filled and carved out of the temporal bone. It connects to the throat/nasopharynx via the Eustachian tube.
• Tympanic membrane (ear drum) separates the external ear from the middle ear. It is stretched across the end of the ear canal and vibrates when struck by sound waves. These sound waves are transmitted to the three auditory bones: malleus, incus, and stapes. These then transmit the vibrations to the fluid-filled inner ear at the oval window.

Inner Ear

It consists of two inter-connected pathways: vestibular and auditory. Both involve the following structures:

• Bony labyrinth is a maze of fluid-filled tubes that run through the temporal bone of the skull. It is subdivided into smaller compartments by the membranous labyrinth, a second series of cellular tubes contained within the bony labyrinth. Fluid around the membranous labyrinth is called perilymph (similar to normal cerebral spinal fluid) while fluid inside is called endolymph (similar to intracellular fluid: high in potassium and low in sodium). There are three major sections of the bony labyrinth:
  • Cochlea is the snail-like portion concerned with the function of hearing, translating incoming sound waves into electrical signals so that they can be understood by the brain. The cochlea is located at the front of the bony labyrinth and makes two and one-half structural turns. If stretched out, the cochlea would be just over an inch long; but its coiled shape leaves it about the size of a pea. Inside, the cochlea is partitioned into three fluid-filled canals:
    • the scala vestibuli (upper, containing perilymph fluid) connects the vestibule and scala tympani.
    • the scala tympani (lower, containing perilymph fluid) is separated from the tympanic cavity by the round window.
    • the triangular-shaped scala media (center, containing endolymph fluid), also called the cochlear duct. It contains the receptors for hearing in the spiral organ called the Organ of Corti, which contains approximately 30,000 neurons. The organ of Corti is lined by a strip of tissue called the basilar membrane, containing about 16,000 to 20,000 hair cells aligned in rows of four or five cells deep. These hair-like projections (microvilli), called cilia and stereocilia, contain endings of the cochlear branch of the 8th cranial nerve. The tallest cilia are embedded in another membrane called the tectorial membrane.
  • Utricle and Saccule are membranous sacs that lie between the semicircular canals and the cochlea. Inside are hair cells that are moved by gravity as the position of the head changes. The impulses generated by these hair cells are carried by the vestibular portion of the 8th cranial nerve to the cerebellum, midbrain, and the temporal lobes of the cerebrum. The cerebrum and midbrain use this information to subconsciously maintain equilibrium.
  • Semicircular canals, along with the utricle and saccule, are concerned with the function of equilibrium or balance. The three semicircular canals are fluid-filled membranous ovals oriented in three different planes. At the base of each is an enlarged portion called the ampulla, which contains hair cells called the crista, which are affected by movement. As the hair cells respond to movement, the impulses generated are carried by the vestibular branch of the 8th cranial nerve (vestibulocochlear nerve – so named because two nerves are joined together to make one auditory nerve: the cochlear, for hearing, and the vestibular nerve for equilibrium) to the cerebellum, where it divides to function for each ear. The generation of this nerve impulse is the fourth transformation of the acoustical signal into electrical energy and impulses help maintain equilibrium during movement.
• Oval (Vestibular) Window is a membrane that separates the middle ear from the inner ear, opening into the vestibule. It sits immediately behind the base of the stapes and begins vibrating when struck by the stapes.
• Round (Cochlear) Window membrane is located at the end of the cochlea. When the sound energy reaches this point, it is transformed into hydraulic energy. Serving as a pressure valve, the round window bulges outward as pressure rises in the inner ear.
• Vestibular labyrinth is located behind the eardrum and assists with balance. It contains a group of three semicircular canals that come from a common base. These three canals are similar to the cochlea in that both are filled with fluid and contain hair cells sensitive to fluid movement. At the base of the canals is the chamber called the vestibule.
• Vestibular system contains the following structures: vestibule, vestibular labyrinth, semicircular canals, utricle, and saccule. The system is involved with maintaining balance.
• Vestibule is a large area in central part of the bony labyrinth that lies between the cochlea and semicircular canals and adjacent to the utricle and saccule. The oval window opens into this area and all of the inner ear organs branch off from this central chamber.

Hearing Process Summary
The process of hearing involves the transmission of vibrations and the generation of nerve impulses. When sound waves enter the ear canal, vibrations are transmitted by the following sequences of structures: ear drum, malleus, incus, stapes, oval window of the inner ear, perilymph and endolymph within the cochlea, and hair cells of the organ of Corti. When the hair cells bend, they generate impulses that are carried by the 8th cranial nerve to the brain. Sounds are heard and interpreted in the auditory areas of the temporal lobes.