The ear is the organ of both hearing and equilibrium.
Hearing is the transduction of sound waves into a neural signal that relies on the structures of the ear. The ear is subdivided into 3 major parts: the external ear, middle ear, and internal ear. The outwardly visible structure that is often referred to as the ear is more correctly referred to as the outer ear (external ear), or the auricle. The C-shaped curves of the auricle direct sound waves towards the ear canal, which enters into the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal (sometimes caused external acoustic meatus) is the tympanic membrane, or ear drum, which vibrates with the movement of air in sound waves.
Along the length of the auditory canal are ceruminous glands that contribute to the production of cerumen (earwax). Because cerumen is sticky it can help prevent small particles from finding their way to the tympanic membrane. Cerumen also helps prevent bacterial growth, waterproofs the auditory canal and tympanic membrane, and may be a deterrent to small insects.
The middle ear consists of a space spanned by three small bones, the ossicles, which amplify the movements of the tympanic membrane. These small bones are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup (Fig. 8.39). The malleus is attached to the tympanic membrane and articulates with the incus, which articulates with the stapes. The stapes is then attached to the inner ear at the oval window where the sound waves will be transferred to the inner ear.
The middle ear is also connected to the pharynx through the auditory tube (Eustachian tube) that helps equilize air pressure across the tympanic membrane. When flying, you may have experienced what happens when the pressures across the tympanic membrane are not equal. As the plane climbs, pressure on the outside of the membrane decreases. If there is not a corresponding decrease in pressure in the middle ear, the pressure difference will cause the eardrum to push outward, causing pain and muffled hearing. The auditory tube is normally closed, but will typically open when muscles of the pharynx contract during swallowing or yawning. For this reason, chewing gum or drinking as the plane climbs will often relieve these symptoms. The auditory tube also provides a pathway of drainage for fluids that accumulate during middle ear infections (otitis media). Unfortunately, it is also the auditory tubes that play a role in causing otitis media, as microorganisms can use this path to move from the pharynx into the middle ear.
The inner ear is entirely enclosed within the temporal bone. It has three separate regions: the cochlea, which is responsible for hearing and the vestibule and semicircular canals, which are responsible for balance and equilibrium. The neural signals from the regions of the inner ear are relayed to the brainstem through separate fiber bundles, but which run together as the vestibulocochlear nerve, cranial nerve VIII.
The connection between the middle ear and inner ear is at the oval window, a membranous area at the entrance of the snail-shaped cochlea. The vibrations transmitted through the ossicles pass into the cochlea by way of the oval window. The cochlea is composed of 3 chambers separated from one other by membranes. The scala vestibuli (upper chamber) and the scala tympani (lower chamber) extend the length of the cochlea and are continuous with a connection at the helicotrema (Fig. 8.40). The cochlear duct is the third, middle chamber positioned between the scala vestibuli and scala tympani. As the oval window is pushed in by sound waves vibrating from the ossicles, fluid within this tube is pushed along its length and the round window at its other end bulges out as a result of that movement.
The organ for hearing, which contains the sensory receptors is known as the spiral organ of Corti and is located throughout the cochlear duct. The organ of Corti is composed of a lower basilar membrane against the scala tympani and an upper tectorial membrane within the cochlear duct (Fig. 8.41). The receptors for hearing are hair cells with stereocilia that are sandwiched between the basilar membrane below and tectorial membrane above. The vibration of the stapes is transferred into the cochlea by way of the oval window, and fluids within the scala vestibuli and scala tympani begin to move. The movement of these fluids causes vibrations in the basilar membrane. As the basilar membrane begins to vibrate, the hair cells come in contact with the tectorial membrane positioned above (image a boat rising and falling with the waves – as the basilar membrane rises, the hair cells hit the tectorial membrane). The movement of the stereocilia on the hair cells, stimulates the formation of a nerve impulse. The impulse created by the spiral organ of Corti is transmitted into the brain via the vestibulocochlear nerve (cranial nerve VIII).
Along with hearing, the inner ear is responsible for encoding information about equilibrium (the sense of balance), which it does in the vestibule and semicircular canals, structures that are sometimes collectively referred to as the vestibular apparatus (Fig. 8.42). Several types of sensory receptors provide information to the brain for the maintenance of equilibrium. The eyes and proproceptors in joints, tendons, and muscles are important in informing the brain about equilibrium. However unique receptors within the inner ear play a crucial role in monitoring equilibrium. There are two types of equilibrium: static (gravitational) equilibrium, which involves the movement of the head with respect to gravity and dynamic (rotational) equilibrium, which involves acceleration of the head in rotation, horizontal, and vertical movements. Similar to the cochlea, the both the vestibule and semicircular canals use hair cells with stereocilia to detect movement of fluid, in this case, in response to changes in head position or acceleration.
The information for static equilibrium and linear acceleration (dynamic) comes from the utricle and saccule within the vestibule. The saccule and utricle each contain a sense organ, called the macula, where stereocilia and their supporting cells are found. These maculae (plural) are oriented 90 degrees to one another so that they respond to positions in different planes.
The organs can respond to changes in position and acceleration because the tips of their stereocilia project into a dense otolithic membrane made up of a mixture containing granules of calcium and protein, called otoliths, (translated in medical terminology – ear stones). When the head moves, gravity causes the stones to move. The movement of the stones within the membrane causes the stereocilia to bend, initiating action potentials in the vestibular nerve fibers that innervate them. Bundles of stereocilia are arranged in various directions, so that any direction of inclination will depolarize a subset of the hair cells. How the body senses head position and the linear (horizontal or vertical) direction of acceleration is determined by the specific pattern of hair-cell activity across the maculae.
The semicircular canals are three ring-like extensions from the vestibule and are mostly responsible for dynamic equilibrium. One ring is oriented in the horizontal plane and two others are in the vertical plane. At the base of each semicircular canal, where it meets with the vestibule is an enlarged region known as the ampulla, which contains a hair-cell containing structure, called the crista ampullaris that responds to rotational movement. The stereocilia of the hair cells extend into the cupula, a membrane that attaches to the top of the ampulla.
When the head rotates in a plane parallel to the semicircular canal, the fluid in the canal does not move as quickly as the head is moving. This pushes the cupula in the opposite direction, deflecting the stereocilia and creating a nerve impulse. Considering the semicircular canals on either side of the head, three orthogonal planes are defined, the horizontal plane with both horizontal canals, and two vertical planes 90o to each other with the anterior canal from one side and the posterior canal from the other. In each pair, deflection of the cupula on one side of the body causes depolarization of the hair cells while the same movement causes hyperpolarization of the hair cells on the other side of the body. By comparing the relative movements of all six semicircular canals, the vestibular system can establish movement in any direction within three-dimensional space.
Key Points
• The human ear can be divided into three functional segments: the outer ear, the middle ear, and the inner ear.
• Sound waves are collected by the pinna, travel through the auditory canal, and cause a vibration of the tympanum (eardrum).
• The three ossicles of the middle ear (malleus, incus, and stapes) transfer energy from the vibrating eardrum to the inner ear.
• The incus connects the malleus to the stapes, which allows vibrations to reach the inner ear.
• The inner ear exists on the other side of the oval window from the middle ear, by the temple of the human head, and consists of three parts: the semicircular canals, the vestibule, and the cochlea.
• Within the cochlea, the inner hair cells are most important for conveying auditory information to the brain.
Key Terms
Hearing: The transduction of sound waves into a neural signal that relies on the structures of the ear
Ossicles: very small bone, especially one of those in the middle ear.
Oval window: An oval opening at the head of the cochlea, connecting the middle and inner ear, through which sound vibrations of the stapes are transmitted.
Round window: A membrane-covered opening in the inner wall of the middle ear that compensates for changes in cochlear pressure.
Spiral organ of Corti: The receptor organ for hearing and is located in the mammalian cochlea.
Static (gravitational) equilibrium: Involves the movement of the head with respect to gravity.
Dynamic (rotational) equilibrium: Involves acceleration of the head in rotation, horizontal, and vertical movements.
Utricle: The part of the membranous labyrinth of the ear into which the semicircular canals open and that contains the macula utriculi.
Saccule: A little sac; specifically, the smaller chamber of the membranous labyrinth of the ear.
Auditory tube: The tube that runs from the middle ear to the pharynx, also known as the Eustachian tube.
Macula: The vestibule is a region of the inner ear which contains the saccule and the utricle, each of which contain a macula to detect linear acceleration.
Otoliths: Ear stones.
Semicircular canals: Three tiny, fluid-filled tubes in your inner ear that help you keep your balance.
Ampulla: A part of the inner ear that surrounds sensory receptors that are responsible for movement related sensory experiences like spatial awareness and pressure change.
Crista ampullaris: The sensory organ of rotation.
Cupula: A membrane that attaches to the top of the ampulla
Cochlea: the complex, spirally coiled, the tapered cavity of the inner ear in which sound vibrations are converted into nerve impulses.
Malleus: Small hammer-shaped bone of the middle ear.
Incus: small anvil-shaped bone in the middle ear; connects the malleus to the stapes
Stapes: small stirrup-shaped bone of the middle ear
Pinna: the visible, cartilaginous part of the ear that resides outside of the head and collects sound waves
Tympanum: the innermost part of the outer ear; the eardrum
Transduce: to convert energy from one form to another
