Human Anatomy Part 2 - K. A. Dyubenko A. K. Kolomiytsev Yu. B. Chaykovsky 2008

Special Part
Sense organs, organa sensuum
Vestibulocochlear organ - Pathway of the auditory analyzer

The Cell bodies of the first-order bipolar Neurons of the Auditory pathway are located in the spiral ganglion (ganglion spirale), situated at the Base of the modiolus. Their peripheral processes pass through the perforations of the osseous spiral lamina to reach the spiral organ (organ of Corti), terminating on its receptor Cells.

The central processes of the spiral ganglion neurons converge within the internal acoustic meatus to form the cochlear nerve. The nerve then passes through the internal acoustic opening and enters the Brainstem within the recess between the Medulla Oblongata, the Pons, and the Cerebellum. These central processes terminate in the dorsal and ventral cochlear nuclei (nucl. cochlearis dorsalis et ventralis). Each of these nuclei contains a complete somatotopic representation of the entire spiral organ: fibers originating from the base of the cochlea terminate in the dorsomedial parts, whereas those from the apex terminate in the ventromedial PARTS OF THE nuclei. Research has shown that Hearing preservation in humans requires the integrity of at least 68% of the cells in the dorsal cochlear Nucleus and 38% in the ventral cochlear nucleus. The processes of the second-order neurons from the dorsal cochlear nucleus course across the floor of the Rhomboid fossa to form the medullary striae (striae medullares). At the midline, they dip into the Brain tissue of the contralateral side (decussation) and then ascend, becoming part of the lateral lemniscus (lemniscus lateralis). A small portion of the fibers from the dorsal cochlear nucleus remains uncrossed and ascends as part of the ipsilateral lateral lemniscus.

The processes of the second-order neurons arising from the ventral cochlear nucleus decussate within the substance of the pons and ascend to join the lateral lemniscus. Within the pons, the auditory fiber system interlaces with its intrinsic nuclei and fibers. This Structure, located between the ventral and dorsal parts of the pons, is known as the trapezoid body (corpus trapezoideum). It should be noted that only a minor fraction of the fibers from the ventral cochlear nucleus ascends directly to the subcortical auditory centers without synapsing; the majority Relay in the anterior and posterior nuclei of the trapezoid body. Furthermore, a portion of the fibers from the ventral cochlear nucleus (similar to those from the dorsal nucleus) does not cross the midline and ascends within the ipsilateral lateral lemniscus.

In humans, the ventral cochlear nucleus is more developed than the dorsal nucleus, the latter having a sparse cortical representation. Lesions of the rhomboid fossa in the region of the medullary striae result in virtually no noticeable hearing impairment.

Both crossed and uncrossed fibers of the lateral lemniscus project to the subcortical auditory centers: the inferior colliculus and the medial geniculate body. Neuronal processes from the inferior colliculus give rise to the tectospinal tract (tractus tectospinalis), connecting the auditory pathway with the red nucleus (extrapyramidal system), the rubrospinal tract (tractus rubro-spinalis), and the nuclei of Cranial Nerves III, IV, and VI. These connections mediate a wide array of reflex Movements of the HEAD, eyes, trunk, and vocal cords in response to auditory stimulation. The processes of neurons originating in the medial geniculate body pass through the posterior limb of the internal capsule to the middle section of the superior temporal gyrus, which houses the primary auditory cortex (Brodmann areas 41 and 42).