In this article we will discuss about the structure and function of human ear with its suitable diagram.
Ear has two important functional components:
1. Cochlea the hearing part containing receptor for hearing is located here.
2. The vestibular part having semicircular canals, the utricle, and the saccule are present here.
The receptor in these is responsible for the maintenance of equilibrium and posture.
Function of ear in general for hearing and also act as a direction detector:
a. Important protective role.
b. It modulates once own voice.
Ear has three parts the outer, the middle and the inner ear. The outer ear has the pinna, in lower animals this can move which helps in detecting the direction of sound waves. Sound waves which are captured by the pinna pass through the external auditory canal and vibrate the tympanic membrane.
The auditory tube is directed medially, downwards and forwards. The skin around the tube has lots of ceruminous glands which on exposure forms the ear wax. The direction of the external auditory tube as well as the ear wax protects the tympanic membrane from injuries.
The tympanic membrane is a fibrous structure. Its main function is to act as a resonator. Sound waves make the membrane vibrate. Tympanic membrane has a surface area of 68 sq mm. When the frequency of the sound wave is less than 2000 cps the entire membrane vibrates. If the frequency is more than 2000 cps, the membrane vibrates in segments; approximately 75% of the membrane vibrates.
Figure 10.23 (graphical representation) showing the relationship between the frequency of sound waves and the intensity of the sound. It shows that the sound frequencies between 2000 and 4000 cps are heard with the lowest intensities.
Middle Ear:
Contents of the Middle Ear:
The middle ear contains three bony ossicles namely the malleus, the incus and the stapes. These ossicles articulate with one another. The long process of the malleus articulates with the short process of the incus and forms a lever system. The handle of malleus is attached to the tympanic membrane and the foot plate of the stapes is attached to the oval window.
Through this mechanism, the vibrations of the tympanic membrane are conducted to the inner ear. The middle ear is also connected to the pharynx through the pharyngotympanic tube (Eustachian tube/auditory tube).
There are two small muscles in the middle ear. They are the tensor tympani and the stapedius. The tensor tympani when contracts make the tympanic membrane tense. The contraction of the stapedius pulls the foot plate of the stapes outwards. Both of these actions decrease the conduction of sound waves into the inner ear.
Functions of the middle ear:
1. Impedance matching
2. Static pressure equilibration
3. Protective function—acoustic reflex (attenuation reflex)
4. Acts as a physiological filter.
5. Because of the impedance matching, it forms the preferential route of conduction.
1. Impedance matching:
As the sound waves are passing through the air medium, through the ear ossicles into the fluid medium of the internal ear, because it has to vibrate the fluid, a certain amount of sound energy is lost. This will give rise to a decrease in the sound intensity and the significance of the sound may be lost. The mechanism involved in minimizing the loss of sound energy is known as impedance matching.
The mechanisms involved are:
a. When the frequency of the sound wave is more than 2000 cps, only 75% of the tympanic membrane is thrown into vibration which is about 58 mm2. The foot plate of the stapes is about 3.2 mm2. The pressure applied over a larger surface area of the tympanic membrane is getting converged on to a much smaller area in the oval window. This magnifies the pressure acting on the oval window by about 14 to 17 times.
b. The handle of the malleus is longer than the short process of the incus and they articulate with each other forming a lever system. Because of this lever mechanism, there is an additional magnification by about 1.3 times. Therefore, the total magnification increased is by about 17 to 21 folds. Thus the loss of sound energy is minimized. If this mechanism fails, the person will have a hearing deficit of approximately 10 to 20 dB.
2. Static pressure equilibration:
For the proper functioning of the tympanic membrane as a vibrator, the pressure on either side of the membrane must be kept equal. Atmospheric pressure is the one which acts on the tympanic membrane from outside. Since the middle ear is connected to the pharynx, the pressure in the middle ear is also made equal to the atmospheric pressure.
Normally, the pharyngotympanic tube is kept closed. Whenever the pressure in the middle ear falls, the tube opens up connecting the middle ear to the pharynx and the pressure is equalized.
If the fall in the pressure in the middle ear is too much as it can happen when an unconscious person is brought to the sea level, there is a possibility that the tympanic membrane may rupture. This results in a loud noise being followed by signs and symptoms of shock.
3. Protective function:
Explosive noises may damage the very fine structures of the inner ear. Within a matter of 15 to 17 milliseconds (the latent period), the two small muscles in the middle ear contract. The tympanic membrane is pulled inwards and the foot plate of the stapes is drawn outwards. This results in decreased amount of sound waves reaching the inner ear.
This protects the finer structures present in the cochlea. This reflex is known as the tympanic reflex. This reflex can be initiated even by the ticking sounds of a time piece. In paralysis of the facial nerve, the stapedius muscle is paralyzed. Hence, the protective mechanism is lost and these patients complain of painful hearing—hyperacusis.
4. It acts as a physiological filter:
It allows the transmission of speech frequency and prevents the transmission of noise frequency. The axis of rotation of the foot plate of the stapes gets changed and it prevents the transmission of noises.
5. Preferential route of conduction:
There are two routes through which the sound waves can be conducted to the inner ear. One of the routes will be through the bone conduction and the other being the ossicular conduction (air conduction). Since, impedance matching is available only for ossicular conduction, this route of conduction forms the preferential route of conduction.
The Inner Ear:
This part lodges two important structures, namely the cochlea and the vestibular apparatus. The cochlea is the hearing part of the inner ear (Fig. 10.24).
The cochlea is a coiled structure about two and a half circle. The cochlea is divided into three compartments by two membranes namely the basilar membrane and the Reissner’s membrane.
The upper compartment is scala vestibuli, the middle is scala media and the lower scala tympani. The scala vestibuli and scala tympani contain perilymph, the composition of this fluid resembles that of ECF and the scala media contains endolymph, the composition of which resembles that of ICF.
The receptors for hearing are the organ of Corti (hair cells) present on the basilar membrane. There are two types of hair cells namely, the outer row of hair cells, arranged in three rows and a single row of inner hair cells. The outer row of hair cells is test tubelike, whereas the inner row of cells is flask-like (Fig. 10. 25).
Signals produced by these receptors are carried by the cochlear division of the eighth cranial nerve. These receptors also receive efferent nerve supply. These fibers take origin from the olivary nucleus (olivocochlear bundle of nerve fibers). Overlying the hair cells is the tectorial membrane. The hairs on the hair cells are actually embedded in the substance of the tectorial membrane.
Cochlea:
The cochlea is the hearing part of the inner ear. The cochlea is a coiled structure about two and a half circle. The cochlea is divided into three compartments by two membranes, namely the basilar membrane and the Reissner’s membrane.
The upper compartment is scala vestibuli, the middle scala media and the lower scala tympani (Fig. 10.26). The scala vestibuli and scala tympani contain perilymph, the composition of this fluid resembles that of extracellular fluid and the scala media contains endolymph, the composition of which resembles that of intracellular fluid.
Resistance offered by Reissner’s membrane is extremely small as it is a thin delicate membrane. Reissner’s membrane stretches from the upper surface of the spiral lamina to the bony wall of the canal a little above the attachment of the basilar membrane.
Basilar Membrane:
Basilar membrane is attached to the spinal lamina to the outer wall of the canal. There is no tension in the fibers maintaining the basilar membrane.
1. If a cut is made in the basilar membrane, no gaping is seen in the membrane showing the fibres are not taut or kept under tension.
2. Basal part of basilar membrane is narrow and width is gradually increased upwards to the apex. Basilar membrane is about 32 mm long.
3. Rods of Corti form the supporting pillars. The height of these rods are increased from base to apex, and the rods of Corti are present on the basement membrane.
There are certain differences between the base and apical part of cochlea (Fig. 10.27).
They are with respect to:
a. Breadth
b. Thickness of membrane
c. Response to frequencies
The receptors for hearing are the organ of Corti (hair cells) present on the basilar membrane. There are two types of hair cells namely, the outer row of hair cells, arranged in three rows and a single row of inner hair cells. The outer row of hair cells is test tubelike, whereas the inner row of cells is flask-like.
Signals produced by these receptors are carried by the cochlear division of the 8th cranial nerve (Fig. 10.28). These receptors also receive efferent nerve supply. These fibers take origin from the olivary nucleus (olivocochlear bundle of nerve fibers).
Overlying the hair cells is the tectorial membrane. The hairs on the hair cells are embedded in the substance of the tectorial membrane. The hairs of the hair cells are bathed in endolymph present in scala media.
When the sound vibrations are transmitted through the foot plate of the stapes to the inner ear, the fluid medium is set into motion (Fig. 10.29). This in turn moves the basilar membrane, which later on moves the tectorial membrane. The shearing motion of the tectorial membrane bends the hairs of the receptor cells.
Mechanism of Stimulation of Receptors in Cochlea:
1. Movement of oval window.
2. Disturbance of fluid in scala vestibuli.
3. Movement of Reissner’s membrane.
4. Disturbance of fluid in scala media.
5. Movement of tectorial membrane.
6. Shear motion on the hair of hair cells due to movement of tectorial membrane
7. Stimulation of receptor cells (Fig. 10.29).
This brings about the production of receptor potentials known as cochlear microphonic potentials. The amplitude of the microphonic potentials depends on the intensity of the impinging sound waves. Greater the intensity, greater is the amplitude of the microphonic potentials.
The cochlear microphonic potentials are nothing but the local potentials and hence have almost all the properties of local potential. These cochlear microphonic potentials in turn bring about the development of action potentials in the auditory nerve fibers.
Further:
1. The disturbance of fluid in the scala media also brings about movement of basilar membrane.
2. Leads to disturbance of fluid present in scala tympani
3. Movement of round window
There should be movement of the round window in an appropriate direction when the oval window moves. This is essential because, in the cochlea the fluid is present and this fluid is incompressible. If fluid is unable to get disturbed, there will not be scope for the stimulation of receptors since the receptors for hearing are nothing but mechanoceptors.
Theories of Hearing:
1. Resonance theory.
2. Traveling wave theory
3. Place theory.
The basilar membrane is about 31 mm long and its width increases gradually from the base to the apex. Depending on the frequency of the sound waves, different parts of the membrane is displaced to varying extent. For low frequency, the apical portion of the membrane gets displaced to a greater extent stimulating those receptors.
For higher frequency sounds, the basal part of the membrane gets displaced stimulating those receptors. Whenever there is disturbance in the fluid medium of cochlea, a wave of disturbance originates from the base of cochlea irrespective of the pitch of the sound.
This wave as it traverses from the base towards the apex, the amplitude of wave goes on increasing till it comes across a point on the basilar membrane which is tuned to respond maximally for that particular frequency (Fig. 10.30).
Beyond the area of maximal disturbance, the wave dies out. Hence the receptors present at the site of maximal disturbance get stimulated. This fact is proved by recording microphonic potentials from different parts of the basilar membrane and also directly observing the movement of the membrane.
Frequency analysis of the sound waves is, therefore, partly made at this level itself. Further analysis is made by the auditory cortex when these impulses reach the cortex.
Auditory Pathway (Fig. 10.31):
The cochlear afferent nerve fibers from the receptors reach the spiral ganglion. From the ganglia, the fibers reach the anterior and posterior cochlear nuclei present in the brainstem and synapse. From the posterior and anterior cochlear nuclei, nerve fibers take origin and synapse in the superior olivary nucleus and posterior nucleus of trapezoid body of same side as well as on the opposite side.
From these structures, nerve fibers taking origin reach the medial geniculate body through any of the following pathways:
a. Some of the fibers directly reach the medial geniculate body and synapse.
b. Some fibers synapse in the inferior colliculus and from there reaches the medial geniculate body. The crossing of the fibers to the opposite side can occur even at inferior colliculus.
c. Some other fibers synapse in the nucleus of lateral leminscus. From here, the fibers reach the inferior colliculus and synapse and finally reach medial geniculate body.
The whole bundle of nerve fibers taking origin from the superior olivary nucleus and posterior nucleus of trapezoid body is known as lateral lemniscus. The lateral lemniscus gives out collaterals that feed information to the reticular formation present in the brainstem.
From the medial geniculate body, fibers taking origin are called as auditory radiation fibers. Auditory radiation fibers pass through the posterior limb of internal capsule to reach the auditory cortex present in the superior temporal gyrus.
Auditory Cortex:
In the auditory cortex (superior transverse temporal gyrus), there are two important areas:
i. Primary auditory area (area no. 41, 42)
ii. Association auditory area (area no. 21, 22)
The primary auditory area is connected to medial geniculate body. The association area is connected to the primary auditory area. Fibers from primary auditory area convey information to the association area. The association area also receives fibers directly from the thalamus. The individual tone and frequency is represented in the auditory cortex that has tonotopic representation.
Intensity of sound discrimination:
It is similar to intensity discrimination in general sensory physiology.
Intensity of sound discrimination can be explained by:
1. Recruitment of receptors
2. Weber-Fechner law
Direction Analysis:
The laterality of the sound can be discriminated by:
1. Time lag in the stimulation of receptors present in two different ears. In the ear which is directed towards the source of sound, there will be stimulation of receptors few milliseconds earlier than the stimulation of receptors present in the opposite ear.
2. Decrease in the amplitude of the sound in the opposite ear as the sound waves while reaching the opposite ear will strike against the hard bones of the cranium and would lose some amount of sound energy because of this.
Types of Deafness:
1. Conductive type
2. Perceptive type
3. Central type
1. Conductive type—due to:
a. Accumulation of wax in the auditory meatus.
b. Damage to tympanic membrane.
c. Damage to ear ossicles.
2. Perceptive type—due to:
a. Site of lesion mainly the receptors, e.g. prolonged listening of rock music.
b. May be due to tumor arising from the auditory nerve fibers compressing the other fibers.
c. Toxicity of certain drugs (anti-malarial drugs), quinine and streptomycin (anti-TB drugs).
3. Central type—very rare.
Tests Employed to Detect Hearing Impairment:
Audiometry:
The recording is called audiogram.
Ear phones are placed over the subject’s ear and one ear is tested at a time. Subject is connected to instrument. Gradually, there will be increased frequency of sound. The intensity of the sound applied corresponds to the standard intensity this is reported as normal or represented as 0 db.
If the findings of the study are graphically represented and is around zero line, the subject is supposed to be normal.
Conductive and perceptive types of deafness can be differentiated by the audiometry.
Gross difference between bone conduction and ossicular conduction:
If ossicular conduction is affected to a greater extent, it means that it is a conductive type of deafness and in such person bone conduction is better than ossicular conduction. In perceptive deafness, both bone and ossicular conduction are affected to the same extent.
Audiometry enables to ascertain the:
1. Type of deafness—conductive or perceptive
2. Extent of the loss
Tests Conducted to Ascertain the Type of Deafness:
1. Rinne’s test:
Place the vibrating tuning fork on the mastoid process and ask the subject if he can hear. For accurate result, do not allow the subject to move. Subject is asked to tell when he is unable to hear. When he is unable to hear, transfer the tuning fork from mastoid process to the front of the ear and if subject is able to hear it means that ossicular conduction is better than bone conduction.
2. Weber’s test:
Strike a tuning fork and place the vibrating tuning fork on the forehead of the patient. Subject must be able to hear equally in both the ears.
If he hears better in the right ear, it may be due to:
a. Conductive type of deafness in right ear
b. Perceptive type of deafness in left ear
In conductive type of deafness, when Weber’s test performed, the subject is able to hear better on the affected side. In perceptive type of deafness, subject is able to hear better on the normal side.
Presbyacusis is the hearing loss that is due to old age. In aged people, the ability to hear higher frequencies decline.
Chemical Senses:
Taste Receptors and Olfactory Receptors:
Activity in these receptors concerned with visceral function, i.e. concerned with food intake thus they are classified under visceral receptors. They can be also termed as chemoreceptors as they respond to chemical changes.
Differences between Taste and Smell Sensations:
1. The pathway involved in olfaction does not pass through the thalamus. All the other sensory pathways pass through the thalamus.
2. The olfaction sensation has no neocortical projection—it is a very primitive type of sensation. These two sensations play a vital role in food intake.
In lower animals, the olfactory receptors also play other important roles in:
i. Sexual instinct
ii. Detection of enemies—protective role