Hensen S Stripe

+ 120

Loud thunder

1,000,000

2,000

+ 140

Pain and damage

10,000,000

Modified from Gulick WL, Gescheider GA, Frisina RD. Hearing: Physiological Acoustics, Neural Coding, and Psychoacoustics. New York: Oxford University Press, 1989, Table 2.2, p. 51.

Modified from Gulick WL, Gescheider GA, Frisina RD. Hearing: Physiological Acoustics, Neural Coding, and Psychoacoustics. New York: Oxford University Press, 1989, Table 2.2, p. 51.

External auditory canal

Semicircular Superior canals Posterior Lateral

Vestibule Vestibular nerve Facial nerve

Cochlear nerve

Cochlear nerve

External auditory canal

Semicircular Superior canals Posterior Lateral

Pinna

Structure Ear Facial Nerve

Vestibule Vestibular nerve Facial nerve

Outer ear i i Middle ear

Inner ear

Pinna

Outer ear i i Middle ear

Inner ear

The overall structure of the human ear. The

"structures of the middle and inner ear are encased in the temporal bone of the skull.

slightly emphasize frequencies in the range of 1,500 to 7,000 Hz and aids in the localization of sources of sound. The external auditory canal extends inward through the temporal bone. Wax-secreting glands line the canal, and its inner end is sealed by the tympanic membrane or eardrum, a thin, oval, slightly conical, flexible membrane that is anchored around its edges to a ring of bone. An incoming pressure wave traveling down the external auditory canal causes the eardrum to vibrate back and forth in step with the compressions and rarefactions of the sound wave. This is the first mechanical step in the transduction of sound. The overall acoustic effect of the outer ear structures is to produce an amplification of 10 to 15 dB in the frequency range broadly centered around 3,000 Hz.

The Middle Ear. The next portion of the auditory system is an air-filled cavity (volume about 2 mL) in the mas-toid region of the temporal bone. The middle ear is connected to the pharynx by the eustachian tube. The tube opens briefly during swallowing, allowing equalization of the pressures on either side of the eardrum. During rapid external pressure changes (such as in an elevator ride or during takeoff or descent in an airplane), the unequal forces displace the eardrum; such physical deformation may cause discomfort or pain and, by restricting the motion of the tympanic membrane, may impair hearing. Blockages of the eustachian tube or fluid accumulation in the middle ear (as a result of an infection) can also lead to difficulties with hearing.

Bridging the gap between the tympanic membrane and the inner ear is a chain of three very small bones, the ossicles (Fig. 4.18). The malleus (hammer) is attached to the eardrum in such a way that the back-and-forth movement of the eardrum causes a rocking movement of the malleus. The incus (anvil) connects the head of the malleus to the third bone, the stapes (stirrup). This last bone, through its oval footplate, connects to the oval window of the inner ear and is anchored there by an annular ligament.

Four separate suspensory ligaments hold the ossicles in position in the middle ear cavity. The superior and lateral ligaments lie roughly in the plane of the ossicular chain and anchor the head and shaft of the malleus. The anterior ligament attaches the head of the malleus to the anterior wall of the middle ear cavity, and the posterior ligament runs from the head of the incus to the posterior wall of the cavity. The suspensory ligaments allow the ossicles sufficient freedom to function as a lever system to transmit the vibrations of the tympanic membrane to the oval window. This mechanism is especially important because, although the eardrum is suspended in air, the oval window seals off a fluid-filled chamber. Transmission of sound from air to liquid is inefficient; if sound waves were to strike the oval window directly, 99.9% of the energy would be reflected away and lost.

Two mechanisms work to compensate for this loss. Although it varies with frequency, the ossicular chain has a lever ratio of about 1.3:1, producing a slight gain in force. In addition, the relatively large area of the tympanic membrane is coupled to the smaller area of the oval window (approximately a 17:1 ratio). These conditions result in a pressure gain of around 25 dB, largely compensating for the potential loss. Although the efficiency depends on the frequency, approximately 60% of the sound energy that strikes the eardrum is transmitted to the oval window.

Approximate axis of rotation

Stapedius Superior muscle ligament

Temporal bone,

Approximate axis of rotation

Stapedius Superior muscle ligament

Temporal bone,

Ligament The Middle Ear

A model of the middle ear. Vibrations from the eardrum are transmitted by the lever system formed by the ossicular chain to the oval window of the scala vestibuli. The anterior and posterior ligaments, part of the suspensory system for the ossicles, are not shown. The combination of the four suspensory ligaments produces a virtual pivot point (marked by a cross); its position varies with the frequency and intensity of the sound. The stapedius and tensor tympani muscles modify the lever function of the ossicular chain.

Sound transmission through the middle ear is also affected by the action of two small muscles that attach to the ossicular chain and help hold the bones in position and modify their function (see Fig. 4.18). The tensor tympani muscle inserts on the malleus (near the center of the eardrum), passes diagonally through the middle ear cavity, and enters the tensor canal, in which it is anchored. Contraction of this muscle limits the vibration amplitude of the eardrum and makes sound transmission less efficient. The stapedius muscle attaches to the stapes near its connection to the incus and runs posteriorly to the mastoid bone. Its contraction changes the axis of oscillation of the ossicular chain and causes dissipation of excess movement before it reaches the oval window. These muscles are activated by a reflex (simultaneously in both ears) in response to moderate and loud sounds, they act to reduce the transmission of sound to the inner ear and, thus, to protect its delicate structures. Because this acoustic reflex requires up to 150 msec to operate (depending on the loudness of the stimulus), it cannot provide protection from sharp or sudden bursts of sound.

The process of sound transmission can bypass the ossicular chain entirely. If a vibrating object, such as a tuning fork, is placed against a bone of the skull (typically the mastoid), the vibrations are transmitted mechanically to the fluid of the inner ear, where the normal processes act to complete the hearing process. Bone conduction is used as a means of diagnosing hearing disorders that may arise because of lesions in the ossicular chain. Some hearing aids employ bone conduction to overcome such deficits.

The Inner Ear. The actual process of sound transduction takes place in the inner ear, where the sensory receptors and their neural connections are located. The relationship between its structure and function is a close and complex one. The following discussion includes the most significant aspects of this relationship.

Overall Structure. The auditory structures are located in the cochlea (Fig. 4.19), part of a cavity in the temporal bone called the bony labyrinth. The cochlea (meaning "snail shell") is a fluid-filled spiral tube that arises from a

Oval window

Vestibule-

Stapes

Oval window

Vestibule-

Stapes

Stria Vascularis

A Reissner's membrane

B Stria vascularis C Spiral ligament D Basilar membrane

E Osseous spiral lamina

F Hensen's stripe G Inner phalangeal cells H Tectorial membrane I Inner hair cell J Reticular lamina

K Stereocilia L Outer hair cells M Cells of Hensen N Arborized cuticular rods O Cells of Claudius

P Cells of Böttcher

Q Deiters' cells

R Arch of Corti

S Internal sulcus

T Inner sulcus cells

U Spiral limbus V Tunnel i The cochlea and the organ of Corti. Left: An overview of the membranous labyrinth of the cochlea. Upper right: A cross section through one turn of the cochlea, showing the canals and membranes that make up the structures involved in the final processes of auditory sensation.

Lower right: An enlargement of a cross section of the organ of Corti, showing the relationships among the hair cells and the membranes. (Modified from Gulick WL, Gescheider GA, Frisina RD. Hearing: Physiological Acoustics, Neural Coding, and Psy-choacoustics. New York: Oxford University Press, 1989.)

cavity called the vestibule, with which the organs of balance also communicate. In the human ear, the cochlea is about 35 mm long and makes about 23/4 turns. Together with the vestibule it contains a total fluid volume of 0.1 mL. It is partitioned longitudinally into three divisions (canals) called the scala vestibuli (into which the oval window opens), the scala tympani (sealed off from the middle ear by the round window), and the scala media (in which the sensory cells are located). Arising from the bony center axis of the spiral (the modiolus) is a winding shelf called the osseous spiral lamina; opposite it on the outer wall of the spiral is the spiral ligament, and connecting these two structures is a highly flexible connective tissue sheet, the basilar membrane, that runs for almost the entire length of the cochlea. The basilar membrane separates the scala tympani (below) from the scala media (above). The hair cells, which are the actual sensory receptors, are located on the upper surface of the basilar membrane. They are called hair cells because each has a bundle of hair-like cilia at the end that projects away from the basilar membrane.

Reissner's membrane, a delicate sheet only two cell layers thick, divides the scala media (below) from the scala vestibuli (above) (see Fig. 4.19). The scala vestibuli communicates with the scala tympani at the apical (distal) end of the cochlea via the helicotrema, a small opening where a portion of the basilar membrane is missing. The scala vestibuli and scala tympani are filled with perilymph, a fluid high in sodium and low in potassium. The scala media contains endolymph, a fluid high in potassium and low in sodium. The endolymph is secreted by the stria vascularis, a layer of fibrous vascular tissue along the outer wall of the scala media. Because the cochlea is filled with incompressible fluid and is encased in hard bone, pressure changes caused by the in-and-out motion at the oval window (driven by the stapes) are relieved by an out-and-in motion of the flexible round window membrane.

Sensory Structures. The organ of Corti is formed by structures located on the upper surface of the basilar membrane and runs the length of the scala media (see Fig. 4.19). It contains one row of some 3,000 inner hair cells; the arch of Corti and other specialized supporting cells separate the inner hair cells from the three or four rows of outer hair cells (about 12,000) located on the stria vascularis side. The rows of inner and outer hair cells are inclined slightly toward each other and covered by the tectorial membrane, which arises from the spiral limbus, a projection on the upper surface of the osseous spiral lamina.

Nerve fibers from cell bodies located in the spiral ganglia form radial bundles on their way to synapse with the inner hair cells. Each nerve fiber makes synaptic connection with only one hair cell, but each hair cell is served by 8 to 30 fibers. While the inner hair cells comprise only 20% of the hair cell population, they receive 95% of the afferent fibers. In contrast, many outer hair cells are each served by a single external spiral nerve fiber. The collected afferent fibers are bundled in the cochlear nerve, which exits the inner ear via the internal auditory meatus. Some efferent fibers also innervate the cochlea. They may serve to enhance pitch discrimination and the ability to distinguish sounds in the presence of noise. Recent evidence suggests that efferent fibers to the outer hair cells may cause them to shorten (contract), altering the mechanical properties of the cochlea.

The Hair Cells. The hair cells of the inner and the outer rows are similar anatomically. Both sets are supported and anchored to the basilar membrane by Deiters' cells and extend upward into the scala media toward the tectorial membrane. Extensions of the outer hair cells actually touch the tectorial membrane, while those of the inner hair cells appear to stop just short of contact. The hair cells make synaptic contact with afferent neurons that run through channels between Deiters' cells. A chemical transmitter of unknown identity is contained in synaptic vesicles near the base of the hair cells,- as in other synaptic systems, the entry of calcium ions (associated with cell membrane depolarization) is necessary for the migration and fusion of the synaptic vesicles with the cell membrane prior to transmitter release.

At the apical end of each inner hair cell is a projecting bundle of about 50 stereocilia, rod-like structures packed in three, parallel, slightly curved rows. Minute strands link the free ends of the stereocilia together, so the bundle tends to move as a unit. The height of the individual stereocilia increases toward the outer edge of the cell (toward the stria vascularis), giving a sloping appearance to the bundle. Along the cochlea, the inner hair cells remain constant in size, while the stereocilia increase in height from about 4 |xm at the basal end to 7 |xm at the apical end. The outer hair cells are more elongated than the inner cells, and their size increases along the cochlea from base to apex. Their stere-ocilia (about 100 per hair cell) are also arranged in three rows that form an exaggerated W figure. The height of the stereocilia also increases along the length of the cochlea, and they are embedded in the tectorial membrane. The stereocilia of both types of hair cells extend from the cuticular plate at the apex of the cell. The diameter of an individual stereocilium is uniform (about 0.2 |xm) except at the base, where it decreases significantly. Each stereocilium contains cross-linked and closely packed actin filaments, and, near the tip, is a cation-selective transduction channel.

Mechanical transduction in hair cells is shown in Figure 4.20. When a hair bundle is deflected slightly (the threshold is less than 0.5 nm) toward the stria vascularis, minute mechanical forces open the transduction channels, and cations (mostly potassium) enter the cells. The resulting depolarization, roughly proportional to the deflection, causes the release of transmitter molecules, generating afferent nerve action potentials. Approximately 15% of the transduction channels are open in the absence of any deflection, and bending in the direction of the modiolus of the cochlea results in hyperpolarization, increasing the range of motion that can be sensed. Hair cells are quite insensitive to movements of the stereocilia bundles at right angles to their preferred direction.

The response time of hair cells is remarkable, they can detect repetitive motions of up to 100,000 times per second. They can, therefore, provide information throughout the course of a single cycle of a sound wave. Such rapid response is also necessary for the accurate localization of sound sources. When a sound comes from directly in front of a listener, the waves arrive simultaneously at both ears. If the sound originates off to one side, it reaches one ear

Transduction Auditory Hair Cells

Mechanical transduction in the hair cells of the ear. A, Deflection of the stereocilia opens apical K+ channels. B, The resulting depolarization allows the entry of Ca2+ at the basal end of the cell. This causes the release of the neurotransmitter, thereby exciting the afferent nerve. (Adapted from Hudspeth AJ. The hair cells of the inner ear. Sci Am 1983;248(1):54-64.)

sooner than the other and is slightly more intense at the nearer ear. The difference in arrival time is on the order of tenths of a millisecond, and the rapid response of the hair cells allows them to provide temporal input to the auditory cortex. The timing and intensity information are processed in the auditory cortex into an accurate perception of the location of the sound source.

Integrated Function of the Organ of Corti. The actual transduction of sound requires an interaction among the tectorial membrane, the arches of Corti, the hair cells, and the basilar membrane. When a sound wave is transmitted to the oval window by the ossicular chain, a pressure wave travels up the scala vestibuli and down the scala tympani (Fig. 4.21). The canals of the cochlea, being encased in bone, are not deformed, and movements of the round window allow the small volume change needed for the transmission of the pressure wave. Resulting eddy currents in the cochlear fluids produce an undulating distortion in the basilar membrane. Because the stiffness and width of the membrane vary with its length (it is wider and less stiff at the apex than at the base), the membrane deformation takes the form of a "traveling wave," which has its maximal amplitude at a position along the membrane corresponding to the particular frequency of the sound wave (Fig. 4.22). Low-frequency sounds cause a maximal displacement of the membrane near its apical end (near the helicotrema), whereas high-frequency sounds produce their maximal effect at the basal end (near the oval window). As the basilar membrane moves, the arches of Corti transmit the move ment to the tectorial membrane, the stereocilia of the outer hair cells (embedded in the tectorial membrane) are subjected to lateral shearing forces that stimulate the cells, and action potentials arise in the afferent neurons.

Because of the tuning effect of the basilar membrane, only hair cells located at a particular place along the membrane are maximally stimulated by a given frequency (pitch). This localization is the essence of the place theory of pitch discrimination, and the mapping of specific tones (pitches) to specific areas is called tonotopic organization. As the signals from the cochlea ascend through the complex pathways of the auditory system in the brain, the tono-topic organization of the neural elements is at least partially preserved, and pitch can be spatially localized throughout the system. The sense of pitch is further sharpened by the resonant characteristics of the different-length stereocilia along the length of the cochlea and by the frequency-response selectivity of neurons in the auditory pathway. The cochlea acts as both a transducer for sound waves and a frequency analyzer that sorts out the different pitches so they

Ossicles Lever Action

The mechanics of the cochlea, showing the action of the structures responsible for pitch discrimination (with only the basilar membrane of the organ of Corti shown). When the compression phase of a sound wave arrives at the eardrum, the ossicles transmit it to the oval window, which is pushed inward. A pressure wave travels up the scala vestibuli and (via the helicotrema) down the scala tym-pani. To relieve the pressure, the round window membrane bulges outward. Associated with the pressure waves are small eddy currents that cause a traveling wave of displacement to move along the basilar membrane from base to apex. The arrival of the next rarefaction phase reverses these processes. The frequency of the sound wave, interacting with the differences in the mass, width, and stiffness of the basilar membrane along its length, determines the characteristic position at which the membrane displacement is maximal. This localization is further detailed in Figure 4.22.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • catalina
    What are the inner and outer structures of the ear?
    7 years ago
  • Natsnet
    What is facial nerve connected to in ear?
    7 years ago
  • mustafa temesgen
    What is above cochlear nerve in the human ear?
    7 years ago

Post a comment