Human Anatomy - Kotsan I. Y. 2009

Vestibulocochlear organ (organ of hearing and equilibrium)
Auditory analyzer

The Auditory Analyzer is a complex morphofunctional system responsible for the perception, transmission, analysis, and integration of auditory stimuli. Thanks to the auditory analyzer, we receive acoustic information about Changes in the environment and promptly make appropriate responses to sounds originating from objects, sometimes located at a considerable distance from us. The auditory analyzer allows us to determine the direction and loudness of a sound. Due to our high resolving power and auditory memory, we can identify which objects produce specific sounds.

Hearing is one of the key factors enabling human communication through speech. The Development of oral speech in a child without a functioning auditory analyzer is practically impossible.

Functionally, the auditory analyzer consists of peripheral and central divisions, as well as a neural pathway.

The peripheral division of the auditory analyzer is represented by the receptor Hair Cells of the Cytology/practical/79.html">Spiral organ of Corti.

The central division of the auditory analyzer includes the auditory cortex of the cerebral hemispheres and subcortical structures (the nuclei of the inferior colliculi of the corpora quadrigemina and the medial geniculate bodies).

Nerve impulses are conducted from the specialized Hair cells of the spiral (Corti's) organ to the cortical centers of the cerebral hemispheres via the Auditory pathway (Fig. 274).

The first-order Neurons of this pathway are represented by bipolar cells, whose Cell bodies are located in the spiral ganglion of the cochlea within the spiral canal of the Inner ear. The peripheral processes—the dendrites of the bipolar cells—are connected to the sensory hair cells of the spiral organ of Corti. The central processes—the axons of the bipolar cells—pass through the longitudinal canals of the modiolus and form the cochlear nerve. In the internal acoustic meatus, the cochlear nerve joins the vestibular nerve to form the Vestibulocochlear nerve. Entering the Brain substance as part of the cochlear ROOT of the vestibulocochlear nerve, the axons of the first-order neuron reach the dorsal (posterior) and ventral (anterior) cochlear nuclei located in the region of the Rhomboid fossa. These nuclei contain The Cell bodies of the second-order neurons.

Fig. 274. Diagram of the auditory analyzer pathway

1 — temporal lobe (superior temporal gyrus; areas of the auditory centers); 2 — transverse temporal gyrus of Heschl (zone of primary auditory centers); 3 — auditory radiation of the internal capsule; 4 — medial geniculate body and brachium of the inferior colliculus; 5 — inferior colliculi of the Midbrain; 6 — lateral lemniscus; 7 — dorsal (posterior) cochlear Nucleus; 8 — ventral (anterior) cochlear nucleus; 9 — spiral ganglion of the cochlea; 10 — inferior olives with the olivocochlear bundle of Rasmussen; 11 — trapezoid body; 12 — compendium (obex)

The axons of the dorsal nucleus cells emerge onto The surface of the rhomboid fossa and extend toward the median sulcus of the rhomboid fossa as medullary striae, traversing the floor of the rhomboid fossa at the border between the Pons and the Medulla Oblongata. In the region of the median sulcus, the bulk of the medullary stria fibers plunge into the brain substance and cross to the opposite side, where they pass between the anterior and posterior PARTS OF THE pons as part of the trapezoid body, and subsequently continue within the lateral lemniscus to the subcortical auditory centers. A smaller portion of the medullary stria fibers joins the lateral lemniscus of the same side.

The axons of the cells of the ventral cochlear nucleus terminate on the cells of the superior olivary Nucleus of the same side (a minority) or course deep within the pons to the corresponding nucleus of the opposite side, forming the trapezoid body.

The collection of axons of the third-order neurons (whose cell bodies lie in the region of the superior olivary nucleus), along with fibers from the ventral and dorsal cochlear nuclei that pass through the olivary nuclei in transit and partially synapse in The Nucleus of the lateral lemniscus, constitute the lateral lemniscus. The fibers of the lateral lemniscus terminate in the primary (subcortical) auditory centers (the medial geniculate bodies and the inferior colliculi of the corpora quadrigemina), where the cell bodies of the fourth-order neurons are located.

In the inferior colliculi of the corpora quadrigemina, the tectospinal tract is formed; its fibers project to the motor cells of the anterior horns of the Spinal Cord, while a small number of these fibers travel to the motor cranial nerve nuclei. This pathway mediates involuntary protective motor responses to sudden auditory stimuli (motor Responses of the eyes, HEAD, trunk, and limbs).

The axons of the cells of the medial geniculate bodies pass as a compact bundle through the posterior part of the posterior limb of the internal capsule, and then fan out to form the auditory radiation, ultimately reaching the cortical center of the auditory analyzer located in the superior temporal gyrus (Heschl's gyrus). The cortical auditory center receives auditory stimuli from both sides, though predominantly from the contralateral side. Due to the incomplete decussation of the auditory pathways, unilateral lesions of the lateral lemniscus, subcortical auditory center, or cortical auditory area may not be accompanied by severe hearing impairment.

Damage to the auditory analyzer may be accompanied by hearing loss (hypacusis) or total hearing loss—deafness (anacusis).

Unilateral deafness can occur with a unilateral lesion of the peripheral auditory apparatus, the cochlear nerve, or the dorsal and ventral nuclei. When the lesion is localized in the auditory cortex, auditory hallucinations may occasionally occur.

Numerous connections between the medial geniculate bodies and the motor nuclei of the V and VII cranial nerve pairs make it possible to regulate the function of the Middle ear Muscles. Specifically, via fibers extending from the medial geniculate bodies to the motor nucleus of the Trigeminal nerve, reflex contraction of the tensor tympani Muscle occurs in response to an acoustic stimulus, whereas The connection between the medial geniculate bodies and the motor nucleus of the Facial Nerve causes contraction of the stapedius muscle. All this serves not only to ensure adequate tension of individual elements of the sound-conducting apparatus (accommodation function), which is necessary for precise sound perception, but also to protect the receptor apparatus of the cochlea from excessively loud sounds (protective function).