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Sense Organs

Hearing and Equilibrium

The ear is a special sense organ that receives sound stimuli and information about the orientation of the head, which influences equilibrium.

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 ear houses the sensory receptors for hearing and equilibrium. There are three regions of the ear-the outer ear, the middle ear, and the inner ear. Sound enters the outer ear and causes the tympanic membrane (eardrum) to vibrate. Sound vibrations then travel through the ossicles, or bones, of the middle ear to the cochlea, where perceived sound is transmitted to the brain.
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

Hearing is the ability to receive and interpret sound waves.
Sound waves travel through air and other matter. The outer ear, middle ear, and inner ear are all involved in hearing, with the inner ear containing the complex sensory organ—the cochlea. The cochlea has three compartments, all following the spiral channel through the structure. Two compartments, the scala vestibuli and the scala tympani, contain perilymph. These compartments are continuous at the terminal end of the cochlea. The scala vestibuli is in contact with the oval window, and the scala tympani abuts the round window (or secondary tympanic membrane). In between the scala vestibuli and scala tympani is the cochlear duct, the third compartment of the cochlea, which is filled with endolymph and contains the sensory region called the spiral organ (or organ of Corti). The cochlear duct has two membranes. On the scala tympani side is the basilar membrane, and on the scala vestibuli side is the vestibular membrane. The basilar membrane contains cochlear hair cells and supporting cells. A cochlear hair cell, a cell in the cochlea that moves in response to incoming sound waves and alters generation of impulses in the auditory nerve, bends in response to incoming sound waves. They contain microvilli called stereocilia, which are attached to a gelatinous membrane called the tectorial membrane.

Cross-section of the Cochlear Tube

The cochlea is the organ used to detect sound waves. The cochlea is fluid filled and divided into three main compartments, the scala vestibuli, the scala tympani, and the cochlear duct.
When sound waves enter the outer ear, the waves travel down the ear canal and hit the tympanic membrane. The tympanic membrane vibrates according to the amplitude and frequency of the sound waves. These vibrations are transferred and amplified through the auditory ossicles of the middle ear. The stapes transfers this energy to the oval window. When the oval window pushes into the cochlea, the perilymph inside the scala vestibuli is pushed away. The resulting change in pressure moves the vestibular membrane, which pushes the basilar membrane down, which then causes the perilymph in the scala tympani to push on the round window. The oval window recoils and sets in motion the reverse scenario. The frequency and degree of this back-and-forth movement that occurs is dependent on the sound wave. As the basilar membrane moves up and down, the hair cells push on the tectorial membrane, causing stereocilia to bend. This physical change stimulates the hair cells to send signals to the cochlear nerve.

Longitudinal Section of the Cochlea

Sound waves transfer from the outer ear, through the middle ear, and to the oval window of the inner ear at the cochlea. In the cochlea, the sound wave displaces the inner structures and stimulates the sensory neurons to send signals to the brain about pitch and loudness.
The location of movement on the basilar membrane affects how the sound is interpreted. The frequency determines the pitch, and the amplitude determines the loudness. High-frequency sounds resonate at the beginning of the cochlear duct, and low frequencies vibrate the more pliable ends. The loudness is interpreted based on how much the stereocilia bend from the higher amplitude of the waves. Most auditory processing occurs at the primary auditory cortex in the cerebral cortex.

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.

The Sense of Balance

Equilibrium is the detection of changes in position or movements of the head.
There are two locations within the inner ear that contain mechanoreceptors to detect a change in head position. The saccule and utricle each contain a sensory region called a macula. Each macula is a sheet of epithelial tissue covered with a gelatinous layer (the otolith membrane) that contains calcium carbonate crystals called otoliths. The macula's epithelium is made up of hair cells and supporting cells. Neurons surround the hair cells and receive equilibrium signals to be transmitted to the brain via the vestibular nerve. The hair cells have stereocilia, sensing organelles in the inner ear, that project into the otolith membrane. Each hair cell contains a specialized cilium called a kinocilium. The location of the kinocilium is important for directionality. The macula in the saccule is oriented vertically, and the macula in the utricle is oriented horizontally. When the head tilts or moves, gravitational forces pull the fluid, called endolymph, over the surface of the maculae. The otoliths move in response, causing the hair cells to bend. For example, if the head tilts forward, the stereocilia in the utricle bend forward. If a person stands up quickly, the stereocilia in the saccules bend downward. Once the movement stops, the signals are no longer relayed until the head changes position again. The direction the kinocilium moves determines if the hair cells send more signals or fewer signals to the vestibular neurons. The utricle provides information about forward and backward acceleration and head tilting. The saccule provides information about the upward and downward accelerations of the head.
Otoliths are crystals of calcium carbonate found in the inner ear that bend sensory receptors to detect a change in head position.
The semicircular canals are bony canals that contain membranous semicircular ducts. There are three ducts that are oriented in different planes. They are the anterior, posterior, and lateral ducts. The endolymph flows freely through the ducts. The direction of flow is determined by the rotation of the head. Each semicircular duct contains a sensory region called an ampulla, which contains the sensory receptors called the crista ampullaris. Like the macula, the ampulla contains hair cells that are capped with a gelatinous membrane, in this case called the ampullary cupula. Neurons from the vestibular nerve are in close contact with the hair cells of the crista ampullaris. When the head rotates, the cells of the crista ampulla move with the body, but the endolymph begins to flow in the opposite direction as it "catches up" to the body. This flow of endolymph bends the cupula, stimulating the hair cells and activating the underlying neurons. If the head continues to rotate at the same velocity, the stimulation ceases. When the head stops, the fluid continues to flow, once again activating the hair cells until the spinning of the endolymph stops. This can be visualized by spinning a bottle of water and then stopping it, or by thinking of the sensation of dizziness after spinning.
The ampulla located at the base of each semicircular duct detects accelerations of the head. When the head begins to move or rotate, the fluid in the ducts (endolymph) moves over the ampulla, bending the sensory receptors.
The structures of the ear are instrumental in maintaining a sense of equilibrium. The receptors in the vestibule and semicircular ducts are important for preventing falls. The information received by the vestibular nerve is processed in the brain stem and cerebellum. The responses generated here are both unconscious and reflexive. If the head unintentionally moves, the body can respond almost immediately to correct the change. An example is a person nodding off while sitting up. When the head falls, the vestibular nerve and information received by proprioceptors stimulate muscles to pick the head back up, waking the person in the process.