There are two important muscles of the middle ear attached to the ossicles i.e. the Tensor Stapedius and Tensor Tympani. These are the smallest muscles of the human body. The Stapedial muscle is 6mm long and is embedded in the posterior wall of the middle ear. The muscle inserts into the posterior neck of the stapes, so that when it contracts, the stapes is rotated posteriorly. The Tensor Tympani is approximately 25mm in length, arising from the anterior wall of the middle ear space. Contraction of this muscle pulls the malleus antero-medially, thereby reducing the range of movement of the tympanic membrane by placing indirect tension on it. Indeed, both the tensor tympani and the stapedial muscles stiffen the middle ear transmission system, thereby reducing transmission of acoustical information in the lower
frequencies. That is, contraction of these muscles reduces the strength of the signal reaching the cochlea, potentially protecting it from damage due to high signal intensity. This reaction is known as the acoustic or aural reflex. Sometimes it is also known as the stapedial reflex. It is triggered by loud sounds, typically greater than 85dB SPL. The neural circuit for the acoustic reflex is such that stimulation of either ear results in response by both the ears, although attenuation of the signal by the ipsilateral ear is stronger than the attenuation in the contralateral ear. Unfortunately, the protective function is compromised, in that the stiffening of the ossicular chain provides little barrier to transmission of the high-frequency sound so dominant in modern industrial societies. The aural reflex will not provide protection for instantaneous sounds (peak levels), such as gunshot blasts, as it takes a fraction of a second for activation.
3.4 Inner Ear
The inner ear (also called the labyrinth due to its complicated shape) is comprised of a complex system of fluid filled cavities positioned deep in the temporal bone (Figure 3.3) and located behind the eye socket. The bony labyrinth is a system of channels winding their way through the bone with distinct regions or cavities including the cochlea and three semicircular canals positioned at right angles to each other. The semicircular canals are part of the body’s balancing system. The bony labyrinth is filled with perilymph, which is connected to and similar in composition to the cerebrospinal fluid. The membranous labyrinth is a continuous series of membrane sacs contained within the bony labyrinth and floats in the perilymph. It bounds an extracellular fluid space, called the endolymphatic space (ELS), which contains the endolymph, a fluid of unique ionic composition. Unlike some other non-mammalian species, the endolymphatic space is entirely bounded by cells, with no form of direct communication with the perilymph. The cochlea is a snail-like configuration with 2½ turns.
Figure: 3-3 Bony labyrinth
Source: Marieb & Hoehn (2007)
In the cochlea the scala vestibuli is part of the bony labyrinth and filled with perilymph. It lies superior to the scala media and the oval window is located at its base. The scala media is also known as the cochlear duct and endolymphatic space as it is filled with endolymph. The two chambers are separated by Reissner’s (or the vestibular) membrane. The external wall of the scala media, the stria vascularis, is made up of rich vascularised mucosa that secretes endolymph. The scala media contains the organ of Corti. The scala tympani are part of the bony labyrinth (like the scala vestibuli). It contains perilymph and lies inferior to the cochlear duct. The scala tympani and scala vestibuli are in contact (or in communication) through the region known as the helicotrema, which is located at the cochlear apex. The basilar membrane of the organ of Corti separates this chamber from the scala media. (Figure 3.4)
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Figure 3-4: Organ of Corti
Source: Marieb & Hoehn (2007)
The membranous labyrinth of the cochlea, the cochlea duct, resides between the scala vestibuli and tympani, making up the intermediate scala media. This structure houses the sensory apparatus for hearing. The basilar membrane forms the floor of the scala media, separating the scala media and scala tympani. It is on this membrane that the organ of hearing is found. The organ of Corti (Figure 3-4) is grossly similar to the design of the vestibular organs (described later). There are four rows of hair cells resting on a bed of Deiters’ cells for support. The outer three rows of hair cells, known as outer hair cells, are separated from single row of inner hair cells by the tunnel of Corti. The superior surface of the outer hair cells and the phalangeal processes of Deiters’ cells form a matrix termed the reticular lamina, through which the cilia protrude. The tectorial membrane overlays the hair cells, and has functional significance in the processing of acoustic stimuli. The outer hair cells are clearly embedded in this membrane, but the inner hair cells do not make physical contact with tectorial membrane, although its proximity to the hair cell is an important contributor to hair cell excitation.
The inner and outer hair cells differ markedly in number. The 3,500 inner hair cells form a single row stretching from the base to apex. The upper surface of each hair cell is graced with the series of approximately 50 stereocilia forming a slight ‘U’ pattern. There are three rows of inner hair cells, broadening to four rows in the apical end, and numbering approximately 12,000. As with the inner cells, the stereocilia protrude from the surface of each outer hair cell, but with a W or V
shaped pattern formed by 150 stereocilia. The Celia of a hair cell are connected by thin, filamentous link.
Shorter cilia are connected to the taller celia by “tip link” and celia are also linked laterally, thus ensuring that movement of one celia involves disturbance of adjacent celia on a hair cell. Stereocilia found in the apex are longer than those found in the base.
The morphology of the hair cells differs markedly. The inner hair cells are teardrop or gourd shaped, with the broad base and narrowed neck. The outer hair cells, in contrast, are shaped like a test tube. One simply must be awed by the cochlea; the structure would neatly fit on the eraser of a pencil, and the fluid within it would be a drop on your table top.
The structures are astoundingly small and delicate. The inner ear is responsible for performing spectral and temporal acoustic analyses of the incoming signal. Spectral analysis refers to the process of extracting or defining the various frequency components of a given signal. The frequency and intensity of vibration define the psychological correlates of pitch and loudness.
Figure 3-5: Cross section of the Cochlea
Source: Marieb & Hoehn (2007)
A cross section (Figure 3-5) through one of the turns of the cochlea (inset) showing the scala tympani and scala vestibuli, which contain perilymph, and the cochlear duct, which is filled with endolymph.
Figure 3-5 shows the cochlear duct and spiral lamina divide the cavity of the bony cochlea into three separate scala (chambers): the scala vestibuli, the scala media and scala tympani.
The cochlea is specially designed to sort out the frequency components of an incoming signal, determine their amplitude, and even identify basic temporal aspects of that signal. Subsequently, processing occurs as the signal works its way rapidly along the auditory pathway, ultimately reaching the brain.
The outer hair cells in the cochlea are able to produce low intensity sounds called otoacoustic emissions (OAEs). Otoacoustic emissions are acoustic signals generated by the normal inner ear, either in the absence of acoustic stimulation (spontaneous emissions) or in response to acoustic stimulation (acoustically-evoked emissions) or
electrical stimulation (electrically-evoked emissions). There are three types of Otoacoustic emission testing: Spontaneous, Transient, and Distortion Product. The Spontaneous oto-acoustic emissions (SOAE) are sounds emitted without an acoustic stimulus. The Transient (evoked) otoacoustic emissions (TOAE) are sounds emitted due to acoustic stimuli of very short duration. Distortion product otoacoustic emissions (DPOAE) are the sounds emitted due to a stimulus by sounds of different frequencies. While the response is emitted from the cochlea, the middle and outer ear must be able to transmit the sound (in reverse) back to the OAE recording microphone, and so OAEs can only measure the peripheral auditory system (outer, middle and inner ear). OAEs are often used as a neonatal screening tool to test the presence/absence of cochlear function. In addition, this technique is used for children too young to participate in conventional hearing tests, and also, otoacoustic emission testing is used routinely in an audiology clinic for early identification of noise induced hearing loss.
Figure 3-6 Basilar membrane: analysis of sound frequencies
Source: Marieb & Hoehn (2007)
The analysis of sound frequencies by the basilar membrane reveals that:
(A) The fibres of the basilar membrane become progressively wider and more flexible from the base of the cochlea to the apex. As a result, each area of the basilar membrane vibrates preferentially to a particular sound frequency.
(B) High-frequency sound waves cause maximum vibration of the area of the basilar membrane nearest to the base of the cochlea.
(C) Medium-frequency waves affect the centre of the membrane.
(D) Low-frequency waves preferentially stimulate the apex of the basilar membrane. The locations of cochlear frequencies along the basilar membrane shown are a composite drawn from different sources. (Figure 3-6)