Anatomy, Physiology, and Pathology of the Organs of Hearing, Vision, and Speech - Shvetsov A.G. 2006

Auditory Analyzer
Anatomy of the Auditory Analyzer

The human Organ of Hearing (Fig. 7) detects (outer ear), amplifies (Middle ear), and perceives (Inner ear) sound vibrations, essentially acting as a distance analyzer whose peripheral (sensory) division is located in the pyramid of the Temporal bone (cochlea).

The outer ear includes the auricle and the external auditory canal, which ends at a dense fibrous membrane—the tympanic membrane (eardrum), serving as the boundary between the outer and middle ear. The auricle acts as a collector of sound waves and helps determine the direction of the sound source during binaural hearing (listening with both ears). Both ears perform the same function but do not communicate, which helps gather more complete information. The auditory canal is not only a conductor of sound but also a resonator in the speech frequency range of 2,000 to 2,500 Hz. Sound at these frequencies is amplified by 5 to 10 dB. Longitudinal air vibrations carrying sound cause mechanical vibrations of the tympanic membrane; however, to be transmitted to the membrane of the cochlear window, which separates the middle ear from the inner ear, and subsequently to the endolymph of the inner ear, these vibrations must be significantly amplified.

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Fig. 7. Ear Structure

Outer ear: 1 — auricle; 2 — auditory canal; 3 — tympanic membrane.

Middle ear: 4 — middle ear cavity; 5 — auditory tube; auditory ossicles: malleus (a), incus (b), stapes (c);

Inner ear: 6 — cochlea; 7 — auditory nerve.

Vestibular apparatus: 8 — vestibule with sacs; 9 — semicircular canals.

The middle ear acts as an amplifier of sound vibrations captured by the ear. The human sound-conducting apparatus is a highly sophisticated mechanical system. It is capable of responding to minimal air vibrations and conducting them to the sound-perceiving system, where the primary Analysis of the sound wave takes place. Vibrations of the tympanic membrane, which converts airborne sound waves into mechanical vibrations, are transmitted to the articulating auditory ossicles located in the middle ear cavity—the malleus, incus, and stapes (Fig. 7). According to the latest data, this system of auditory ossicles amplifies the sound coming from the tympanic membrane by 20–25 times, which helps overcome the resistance of the oval window membrane separating the middle ear cavity from the inner ear cavity, and transmits the vibrations to the endolymph of the inner ear. The Role of the tympanic membrane and auditory ossicles is to transform high-amplitude, relatively low-force air vibrations into vibrations of the ear's endolymph with relatively low amplitude but high pressure. In the presence of high-intensity sounds, the articulating system of auditory ossicles takes on a protective, Shock-absorbing function. The primary pathway for sound delivery to the cochlea is air conduction, while the secondary pathway is bone conduction, where the sound wave acts directly on the BONES OF THE Skull.

One of the crucial conditions for normal air conduction of sound is the absence of a pressure difference on both sides of the tympanic membrane, which is maintained by the ventilating capacity of the auditory (Eustachian) tube. The latter is 3.5 cm long and only 2 mm wide, connecting the tympanic cavity to the nasopharynx as a canal. During swallowing, this passage opens, ventilating the middle ear and equalizing its pressure with atmospheric pressure.

The inner ear has the most complex structure. Located in the petrous part of the temporal bone, it consists of a bony labyrinth containing a membranous labyrinth made of Connective Tissue. The membranous labyrinth is nested within the bony labyrinth and generally mirrors its shape. Perilymph is located between the bony and membranous labyrinths, while endolymph is inside the membranous labyrinth. The inner ear is divided into three parts: the cochlea, the vestibule, and the semicircular canals, but only the cochlea serves as the sensory organ of hearing. The other two structures belong to the vestibular analyzer system.

The Organ of hearing is located in the cochlea, which is a spiral bony canal that winds spirally around a cone-shaped bony core (modiolus) for 2.5 to 2.75 turns, ending blindly at the apex of the pyramid.

Fig. 8. Spiral organ in the cochlea

A — dissected cochlea: 1 — position of the spiral organ in the cochlea; 2 — basilar membrane; 3 — auditory nerve.

B — spiral organ: 1 - tectorial membrane; 2 - reticular membrane; 3 — outer and inner Hair Cells; 4 - supporting cells; 5 — cochlear nerve fibers (in cross-section); 6 - outer and inner pillars; 7 — cochlear nerve.

The spiral canal of the cochlea is 28–30 mm long. In diameter, the spiral canal is wide at its base (6 mm) and gradually narrows to 2 mm as it approaches the apex of the cochlea. A bony spiral lamina projects from the modiolus, around which this canal winds, into the lumen of the canal, extending toward the outer wall of the spiral canal but ending halfway across. From the free edge of the bony spiral lamina to the opposite wall of the cochlea, the basilar membrane is stretched along its entire length, forming part of the membranous cochlea. Thus, the spiral canal of the cochlea is divided by longitudinal partitions into the upper (scala vestibuli), middle (spiral organ), and lower (scala tympani) chambers, filled with endolymph. The auditory receptors are located on the basilar membrane of the spiral organ, situated in the middle part of the canal (Fig. 8A).

The basilar membrane consists of approximately 20,000 thin, elastic fibers stretched like strings of varying lengths between the bony spiral lamina and the outer wall of the cochlea (resembling a musical harp). Near the basal turn of the cochlea, the fibers are shorter and thinner, while at the apical turn, they are longer and thicker. The tension of the fibers gradually decreases from the base to the apex of the cochlea. The connection between the fibers is very weak, allowing for the isolated vibration of individual sections of the membrane. Only those fibers whose natural frequencies match the incoming signal are set into vibration (similar to The phenomenon of Resonance). The fewer vibrating fibers there are, and the closer they are located to the oval window, the lower the frequency of the sound.

Fig. 9. Auditory Analyzer

The dendrites of the bipolar sensory cells, which make up the spiral ganglion located right there in the central part of the cochlea, innervate the auditory hair cells. The axons of these bipolar Cells of the spiral (cochlear) ganglion form the cochlear branch of the Vestibulocochlear nerve (cranial nerve VIII), which travels to the nuclei of the auditory analyzer located in the Pons (second auditory neuron), the subcortical auditory centers in the corpora quadrigemina (third auditory neuron), and the auditory cortex in the temporal lobe of each hemisphere (Fig. 9), where auditory sensations are formed. In total, there are approximately 30,000–40,000 afferent fibers in the auditory nerve. Vibrating hair cells trigger excitation only in strictly specific fibers of the auditory nerve, and thus in strictly specific Nerve Cells of the Cerebral Cortex. Each hemisphere receives information from both ears (binaural hearing), making it possible to determine the sound source and its direction. If a sounding object is on the left, the impulses from the left ear reach the Brain earlier than those from the right ear. This slight time difference allows us not only to determine the direction but also to perceive sound sources from different areas of space. This type of sound perception is called spatial or stereophonic.