hearing and balance

Hearing and balance
Objectives: after you finish this lecture you should know:
? Describe the components and functions of the external, middle, and inner ear.
? Describe the way that sound waves are converted into impulses generated in hair cells in the cochlea.
? Explain the roles of the tympanic membrane & the auditory ossicles (malleus, incus, and stapes) in sound transmission.
? Explain how auditory impulses travel from the cochlear hair cells to the auditory cortex .
? Explain how pitch & loudness are coded in the auditory pathways.
? Describe the various forms of deafness and the tests used to distinguish
between them.
? Explain how the receptors in the semicircular canals detect rotational acceleration and how the receptors in the saccule and utricle detect linear acceleration.
? List the major sensory inputs that provide the information that is synthesized in the brain into the sense of position in space.
Introduction
Our ears not only let us detect sounds, but they also help us maintain balance. Receptors for two sensory modalities (hearing and equilibrium) are housed in the ear. The external ear, the middle ear, and the cochlea of the inner ear are concerned with hearing. The semicircular canals, the utricle, and the saccule of the inner ear are concerned with equilibrium. Both hearing and equilibrium rely on a very specialized type of receptor called a hair cell.
Structure and function of the external & middle ear (figure 1 and 2)
The external ear funnels sound waves to the external auditory meatus. In some animals, the ears can be moved like radar antennas to seek out sound. From the external auditory meatus, sound waves pass inward to the tympanic membrane (eardrum).
The middle ear is an air-filled cavity in the temporal bone that opens via the eustachian (auditory) tube into the nasopharynx and through the nasopharynx to the exterior. The tube is usually closed, but during swallowing, chewing, and yawning it opens, keeping the air pressure on the two sides of the eardrum equalized.
The three auditory ossicles, the malleus, incus, and stapes, are located in the middle ear. The manubrium (handle of the malleus) is attached to the tympanic membrane. Its short process is attached to the incus, which in turn articulates with the head of the stapes. The stapes foot plate is attached to the wall of the oval window (part of the inner ear).
Two small skeletal muscles, the tensor tympani and the stapedius, are also located in the middle ear. Contraction of the former pulls the manubrium of the malleus medially and decreases the vibrations of the tympanic membrane; contraction of the latter pulls the foot plate of the stapes out of the oval window. The functions of the ossicles and the muscles are considered in more detail below.


Figure 1: ossicles of the middle ear (A=stapes, B=incus & C=malleus)


Figure 2: anatomy of the ear
Inner ear (figure 3)
The inner ear (labyrinth) is made up of two parts, one within the other. The bony labyrinth is a series of channels in the temporal bone and is filled with a fluid called perilymph, which has a relatively low concentration of K+, similar to that of plasma. Inside these bony channels, surrounded by the perilymph, is the membranous labyrinth. The membranous labyrinth more or less duplicates the shape of the bony channels and is filled with a K+-rich fluid called endolymph.
The labyrinth has three components: the cochlea (containing receptors for hearing), semicircular canals (containing receptors that respond to head rotation), and the otolith organs –saccule and utricle- (containing receptors that respond to gravity and head tilt).
The cochlea is a coiled tube that, in humans, is 35 mm long and makes two and three quarter turns. At the base of the cochlea, the oval window is present and is closed by the footplate of the stapes. Also another opening of the cochlea called round window closed by secondary tympanic membrane.


Figure 3: inner ear
The organ of Corti (figure 4) on the basilar membrane extends from the apex to the base of the cochlea and thus has a spiral shape. This structure contains the highly specialized auditory receptors (hair cells).
There are 23500 hair cells in each human cochlea. Covering the rows of hair cells is a thin, viscous, but elastic tectorial membrane in which the tips of the hairs cells are embedded. The processes of the hair cells are bathed in endolymph, whereas their bases are bathed in perilymph.

Figure 4: cochlea and organ of corti
The cell bodies of the sensory neurons (figure 5) that arborize around the bases of the hair cells are located in the spiral ganglion within the bony core around which the cochlea is wound. The axons of these neurons form the auditory (cochlear) division of the eighth cranial nerve.

Figure 5: hair cells within the cochlea covered by tectorial membrane
On each side of the head, the semicircular canals are perpendicular to each other, so that they are oriented in the three planes of space. A receptor structure, the crista ampullaris, is located in the expanded end (ampulla) of each of the membranous canals. Each crista consists of hair cells and supporting (sustentacular) cells surmounted by a gelatinous partition (cupula) that closes off the ampulla.
The processes of the hair cells are embedded in the cupula, and the bases of the hair cells are in close contact with the afferent fibers of the vestibular division of the eighth cranial nerve.

Figure 6: semicircular canals and the cupula
A pair of otolith organs, the saccule and utricle (figure 7), are located near the center of the membranous labyrinth. The sensory epithelium of these organs is called the macula. The maculae are vertically oriented in the saccule and horizontally located in the utricle when the head is upright. The maculae contain supporting cells and hair cells, surrounded by an otolithic membrane in which are embedded crystals of calcium carbonate, the otoliths. The otoliths, which are also called otoconia or ear dust, range from 3 to 19 ?m in length in humans. The processes of the hair cells are embedded in the otolithic membrane. The nerve fibers from the hair cells join those from the cristae in the vestibular division of the eighth cranial nerve.

Figure 7: otolith organs
Sensory receptors in the ear: hair cells (figure 8)
The specialized sensory receptors in the ear consist of six patches of hair cells in the membranous labyrinth. These are examples of mechanoreceptors. The hair cells in the organ of Corti signal hearing; the hair cells in the utricle signal horizontal acceleration; the hair cells in the saccule signal vertical acceleration; and a patch in each of the three semi-circular canals signal rotational acceleration.
These hair cells have a common structure. Each is embedded in an epithelium