Anatomy and physiology Of The ear

The Sense of Hearing

Sound
It is mechanical waves transmitted through a medium( air, water, and other matters). The waves are formed of positive and negative components which comprise a cycle. There are 2 parameters which identify sound, the frequency and intensity.
The frequency is the number of cycles per second, and is measured by Hertz(Hz) , one cycle per second is one Hz, and 1000 CPS is 1000 Hz. The other parameter is the intensity which is measured by the decibel(dB).
Hearing is a pure sensory science linking man to the world around him. Human ear can generally hear sounds with frequencies between 20 Hz and 20,000Hz .
Anatomy and Physiology of the Ear
The ear is the organ that detects sound and it is an energy transducer, which converts acoustic energy into electrochemical energy. It changes sound pressure waves from the outside world into a signal of nerve impulses sent to the brain. It does not only receive the sound, but also aids in balance and body position.
For descriptive purpose, the ear is divided into three parts: The external ear, the middle ear and inner ear
I-The external ear or Outer ear, the outer ear is composed of:
1-The pinna (Auricle), which is a plate of cartilage covered with skin on either side of the head. It serves to collect the sound to be processed at deeper levels.
2-The external auditory meatus (EAM): It is approximately 7mm in diameter and 2.5 cm long. It is S shaped . The ear canal is very important, unless the canal is open, hearing will be dampened. Ear wax (cerumen) is produced by glands in the skin of the outer portion of the ear canal. The outer ear ends at the most superficial layer of the tympanic membrane fig.( 1).


Fig (1) Anatomy of the ear.
The tympanic membrane (TM) or (ear drum): it is a window of the middle ear. It is thin, semitransparent membrane, and makes the boundary between outer and middle ears. It is 55 mm2 in surface area and is slightly concave .The tip end of the handle of the malleus is attached to the center of the tympanic membrane, and this point of attachment is constantly pulled by the tensor tympani muscle, which keeps the tympanic membrane tensed as shown in fig (2).


Fig.(2) Anatomical landmark of the left tympanic membrane
II- The middle ear
It is an air-filled cavity behind the tympanic membrane. It is a small space occupied by three of the smallest bones of the body, known as the ossicles, which include the malleus, incus, and stapes. These ossicles convert the sound waves striking the eardrum into mechanical vibrations. The opening of the Eustachian tube is also within the middle ear. The Eustachian tube connects from the chamber of the middle ear to the back of the nasopharynx. It is usually closed and opened during swallowing, yawning to equalize the pressure in middle ear.
The malleus: It consists of a head ,neck, and three processes: the long process (the manubrium, or handle) is attached to the mobile portion of the eardrum ,shown in fig.( 3).
The incus: is the bridge between the malleus and stapes.It consists of a body and two processes.
The stapes: is the smallest named bone in the human body. The head (caput) articulates with the incus, and the neck bifurcates to become the crura of stapes which converges on the footplate (the leg-like portion) of stapes. The footplate of the stapes helps in hearing and it rests on the oval window.


Fig. (3) Articulated ossicular chain of the right ear in medial view
The ossicles of the middle ear are suspended by ligaments in such a way that the combined malleus and incus act as a single lever, having its fulcrum approximately at the border of the tympanic membrane.(Fig 4) Fig(4) SHAPE \* MERGEFORMAT 



The three bones are arranged so that the movement of the tympanic membrane causes movement of the malleus, which causes the movement of the incus, which causes the movement of the stapes. When the stapes footplate pushes on the oval window, it causes the movement of the fluid within the cochlea. The articulation of the incus with the stapes causes the stapes to push forward on the oval window every time the tympanic membrane moves inward, and to pull backward every time the malleus and tympanic membrane moves outward.
Impedance Matching
The middle ear acts as an impedance matching device to transfer sound energy efficiently from air to a fluid medium in the cochlea. Sound pressure is amplified through the middle portion of the ear and is passed from the medium of air into a liquid medium. Therefore, the ossicular lever system increases the force of movement this is done by several simple mechanisms :
The first mechanism is the "lever principle". There is a difference in the dimensions of the articulating ear ossicles. The length of manubrium is approximately 9 mm, while that of the long process of incus is about 7 mm. This leads to an increase in the force applied to the stapes footplate compared with that applied to the malleus by about 1.3 times .
A second mechanism is the surface area of the tympanic membrane is many times that of the stapes footplate. The surface area of the tympanic membrane is about 55 mm2, whereas the surface area of the stapes footplate averages 3.2 mm2. The sound energy strikes the tympanic membrane and is concentrated to the smaller footplate. This 17-fold difference in area times the 1.3-fold ratio of the lever system causes about 22 times as much total force to be exerted on the fluid of the cochlea as is exerted by the sound waves against the tympanic membrane.
Combination of all these results in a gain about 25- 27 dB. Because fluid has far greater inertia than air does, it is easily understood that increased amounts of force are needed to cause vibration in the fluid.
The round window is a membrane-covered round opening between the cochlea and the middle ear. The round window is actually situated below and a little to the back of the oval window, from which it is separated by a rounded elevation, the promontory .
Attenuation of Sound by Contraction of the Tensor Tympani and Stapedius Muscles:
When loud sounds are transmitted through the ossicular system and from there into the central nervous system, a reflex occurs after a latent period of few milliseconds to cause contraction of the stapedius muscle and the tensor tympani muscle. The tensor tympani muscle pulls the handle of the malleus inward while the stapedius muscle pulls the stapes outward. These two forces oppose each other and thereby cause the entire ossicular system to develop increased rigidity, thus greatly reducing the ossicular conduction of low frequency sound .
The function of this mechanism is to:
1-To protect the cochlea from damaging vibrations caused by excessively loud sound.
2-To mask low-frequency sounds in loud environments.
3-To decrease a persons hearing sensitivity to his or her own speech.
III-The inner ear
The inner ear it is called labyrinth and it is made of two parts one within the other the bony labyrinth is encased in the hardest bone of the body, the petrous bone. Within this ivory hard bone, there is a membranous labyrinth is surrounded by perilymph, which by itself contains a fluid called endolymph.
There are three major sections of the inner ear
1-The front portion is the snail-shaped cochlea, which functions in hearing.
2-The posterior portion, the semicircular canals helps maintain balance.
3-Interconnecting the cochlea and the semicircular canals is the vestibule. It contains the sense organs responsible for balance, the utricle and saccule.
The cochlea: it is approximately 35 mm in length and coild 2.5 times into a snail shape about the size of large pea.Within the cochlea are three fluid filled spaces (1)the scala vestibuli, (2) the scala media, and (3) the scala tympani. The scala vestibuli and scala media are separated from each other by Reissners membrane (also called the vestibular membrane); the scala tympani and scala media are separated from each other by the basilar membrane. On the surface of the basilar membrane lies the organ of Corti, which contains the hair cells as shown in fig.( 5). The scala tympani and the scala vestibuli are filled with perilymph and communicate with each other at the helicotrema. At the base of the cochlea, the scala vestibule ends at the oval window which is closed by the foot plate of the stapes. The scala tympani end at the round window that is closed by secondary TM. The scala media is continuous with the membranous labyrinth and does not communicates with the other two scala. It is filled with endolymph and there is no communication between endolymph and perilymph. Fig (6)


Fig (5) Section through the cochlea.
 SHAPE \* MERGEFORMAT 

Fig (6)


The Basilar Membrane: is a fibrous membrane that separates the scala media from the scala tympani. These fibers are stiff, elastic reed like structures that are fixed at their basal ends but are free at their distal ends, so they can vibrate like the reeds of a harmonica. If the cochlea is unwounded and stretched straight, we can see that it tapers from base to apex. The basilar membrane tapers in an opposite direction. It is wider at the apex and narrower at the base (resemble a harp). More important is the basal end which is about 100 fold stiffer than its apical end. At the basal end, it has short and taut fibers that vibrate best at a very high frequency, while the long, looser fibers near the tip of the cochlea vibrate best at a low frequency.
The organ of Corti lies on the surface of the basilar membrane. The actual sensory receptors in the organ of Corti are two specialized types of cells called hair cells .
The hair cells are mechanoreceptors that release a chemical neurotransmitter when stimulate by mechanical pressure or distortion, two type of hair cell:
A single row of internal (or inner) hair cells and three or four rows of external (or outer) hair cells. Their cilia (steriocilia) project into the tectorial membrane.
The bases and sides of the hair cells synapse with a network of cochlear nerve endings. Between 90 and 95 per cent of these endings terminate on the inner hair cells. This emphasizes their special importance for the detection of sound
The outer hair cells, in some way, control the sensitivity of the inner hair cells at different sound pitches.

Traveling Wave

The initial effect of a sound wave entering at the oval window is to cause the basilar membrane at the base of the cochlea to bend in the direction of the round window. However, the elastic tension that is built up in the basilar fibers as they bend toward the round window initiates a fluid wave that travels along the basilar membrane toward the helicotrema, and is called Traveling Wave

Endocochlear potential

The scala vestibuli and scala tympani contain perilymph, so that the perilymph is almost identical with cerebrospinal fluid and has a relatively low potassium
The scala media is filled with endolymph. The endolymph that fills the scala media is an entirely different fluid secreted by the stria vascularis, and contains a high concentration of potassium and a low concentration of sodium, which is exactly opposite to the contents of perilymph. An electrical potential of about +80 millivolts exists all the time between endolymph and perilymph, with the positivity inside the scala media and the negativity outside. This is called the endocochlear potential.
The importance of the endocochlear potential is that the tops of the hair cells project through the reticular lamina are bathed by the endolymph of the scala media, whereas perilymph bathes the lower bodies of the hair cells. Furthermore, the hair cells have a negative intracellular potential of 70 millivolts with respect to the perilymph but 150 millivolts with respect to the endolymph at their upper surfaces where the hairs project. It is believed that this high electrical potential at the tips of the stereocilia sensitizes the cell by increasing its ability to respond to the slightest sound.

Determination of Sound Frequency

1- Place Principle: low-frequency sounds cause maximal activation of the basilar membrane near the apex of the cochlea, and high-frequency sounds activate the membrane near the base of the cochlea. Intermediate frequency sounds activate the membrane at intermediate distances between the two extremes.
2 -Frequency principle or volley principle, by which, low frequency sounds from 20 to 2000 Hz can cause volleys of nerve impulses synchronized at the same frequencies. These volleys are transmitted by the cochlear nerve into the cochlear nuclei of the brain. It is further suggested that the cochlear nuclei can distinguish the different frequencies of the volleys .

Determination of Loudness

Loudness is determined by three ways.
1-The amplitude of vibration of both the basilar membrane and hair cells increases, so that the hair cells excite the nerve endings at more rapid rates.
2-As the amplitude of vibration increases, it causes more and more of the hair cells to become stimulated; thus causing spatial summation of impulses, i.e transmitted by many nerve fibers.
3-The outer hair cells do not become stimulated significantly until vibration of the basilar membrane reaches high intensity .


Localization of the sound
Human beings localize sound within the central nervous system, by different mechanisms:
1-Comparing arrival-time differences, time lag.
2-The difference between the loudness of the sounds in the two ears.

The principle of hearing and the auditory pathway:
All sound waves funnel down through the ear canal and strike the eardrum, causing it to vibrate. The vibrations are passed to the small bones of the middle ear (ossicles), first, vibrations pass to the malleus , which pushes the incus , which pushes the stapes. The base of the stapes rocks in and out against the oval window, this is the entrance for the vibrations. The stapes agitates the perilymph of the bony labyrinth. At this point, the vibrations become fluid-borne. The perilymph, in turn, transmits the vibrations to the endolymph of the membranous labyrinth, The initial effect of a sound wave entering at the oval window is to cause the basilar membrane at the base of the cochlea to bend in the direction of the round window this bending of basilare membrane cause the cilia to bend results into a mechanical transduction that opens 200 to 300 cation channels, allowing rapid movement of positively charged potassium ions from the surrounding scala media fluid into the stereocilia, and causes depolarization of the hair cell membrane .
This, in turn, stimulates the cochlear nerve endings that synapse with the bases of the hair cells. In this way sound waves are transformed into nerve impulses (It is the movement of these hair cells which converts the vibrations into nerve impulses).
Then, the nerve fibers from the spiral ganglion of Corti enter the cochlear nuclear complex to synapse in the dorsal (higher frequencies) and ventral (lower frequencies) nuclei in the upper medulla. At this point, all the fibers synapse, and second-order neurons pass mainly to the opposite side of the brain stem to terminate in the superior olivary nucleus. A few second order fibers also pass to the superior olivary nucleus on the same side. From the superior olivary nucleus, the auditory pathway passes upward through the lateral lemniscus. Some of the fibers terminate in the nucleus of the lateral lemniscus, but many bypass this nucleus and travel onto the inferior colliculus, where all or almost all the auditory fibers synapse. From the inferior colliculi in the midbrain, fibers ascend to the medial geniculate bodies in the thalamus and subsequently, via auditory radiations, to the transverse temporal gyrus of Heschl.

Several important points should be noted.

First,signals from both ears are transmitted through the pathways of both sides of the brain, with a preponderance of transmission in the contralateral pathway.
Second in many places in the brain stem, crossing over occurs between the two pathways:
(1) In the trapezoid body,
(2) In the commissure between the two nuclei of vermis of the cerebellum

Hearing loss :It is a general term for the complete or partial loss of the ability to hear by one or both ears. It is a major public health problem and it is usually divided into three types:
(1) Sensorineural hearing loss: problem within the inner ear or the auditory nerve and higher centers. This is subdivided into:
(a) deafness for low-frequency sounds caused by excessive and prolonged exposure to very loud sounds (a rock band or a jet airplane engine).
(b) deafness for all frequencies caused by drug sensitivity of the organ of Cortiin particular, sensitivity to some antibiotics such as streptomycin, kanamycin, and chloramphenicol
(c)-high frequency hearing loss occur in old age group people called (Presbycusis).
(2) Conductive hearing loss is a problem in the outer or middle ear.There is a block of sound conduction to the inner ear . However, if the cochlea and nerve are still intact sound waves can still be conducted into the cochlea by means of bone conduction from a sound generator applied to the skull over the ear.
(3)Mixed type of hearing loss
Audiometer.
The audiometer is a device for determining the threshold of hearing and the nature of hearing disabilities .Pure tone audiometry (PTA) is described as the gold standard for assessment of hearing level. Pure tones at various frequencies are generated, and their levels are increased and decreased until thresholds are found.