The ears are a pair of special sense organs that are each divided into three regions—from external to internal, the outer ear, the middle ear, and the inner ear. The ear houses sensory receptors for hearing and equilibrium. Hearing is the detection and interpretation of sound waves, and equilibrium is the sense of balance, allowing the brain to know the orientation of the head.
The outer ear—or external ear—functions to funnel sound waves into the ear canal, where they are received by the middle ear. The outer ear region that is visible is called the auricle or pinna. The auricle is the skin-covered cartilaginous structure that protrudes from the lateral sides of the head. The stiff, outer ring of the auricle that folds in is the helix, and the dangling earlobe is called the lobule. The hole of the auricle leads to the ear canal, which enters the skull through the external acoustic meatus of the temporal bone. The external acoustic meatus is lined with skin and contains ceruminous glands that secrete cerumen, or earwax, into the canal. The cerumen acts to trap any contaminants. The canal ends at the tympanic membrane (eardrum), which vibrates from sound waves and separates the outer and middle ears.The middle ear, or tympanic cavity, is an air-filled space housed in the petrous part of the temporal bone. The middle ear is lined with mucus. The lateral wall contains the tympanic membrane, the medial wall contains the oval window and round window, and the anterior wall has a hole leading to the pharyngotympanic tube (also known as the auditory tube or eustachian tube). This tube ends in the pharynx and is important for allowing the air pressure in the middle ear to equalize with the pressure in the external environment. Three small bones called auditory ossicles span the middle ear from the tympanic membrane to the oval window. From lateral to medial, the ossicles are the malleus, incus, and stapes. The malleus attaches to the tympanic membrane, and the stapes attaches to the oval window. The incus forms synovial joints with the other two ossicles, and they are all held together by ligaments. The middle ear is separated from the inner ear by the oval and round windows. The oval window receives vibrations from the stapes, and the round window is flexible to allow the fluid in the inner ear to move in response to the vibrations of the oval window. The inner ear is fluid filled and contains the sensory receptors for hearing and equilibrium. It is contained within the temporal bone with one open cavity called the vestibule and two structures with tubes running through the bone—the cochlea and the semicircular canals. The vestibule contains two membrane sacs, the saccule and the utricle. These house receptors that inform the brain when the head tilts. The semicircular canals are three bony, circular tubes oriented in different directions, and they detect changes in head position. The cochlea is a spiral-shaped organ resembling a snail shell, and it transmits signals from sound waves. The inner ear is filled with a perilymph solution, which is cerebrospinal fluid. Within the membranous structures of the inner ear is endolymph, synonymous with intracellular fluid.
The Auditory Sense
Cross-section of the Cochlear Tube
Longitudinal Section of the Cochlea
Besides the input received from the middle ear, the spiral organ can be further modified to adjust the way sound is perceived. The basilar membrane contains both sensory neurons and motor neurons. The motor neurons act on cochlear hair cells located on the outer region of the membrane, signaling them to either increase or decrease the movement of the basilar membrane. By increasing the motion, the sounds can be fine-tuned. A decrease in motion is a protective mechanism against loud noises. The middle ear can also help mute loud noises by contracting local muscles in the middle ear to limit vibrations of the auditory ossicles. These muscles also contract during vocalizations so the sound of a person's own voice is not too loud.