Central Auditory System

The central auditory system or the central auditory pathway consists of chains or series of nuclei (groups of nerve cell bodies in the central nervous system) connected by fiber tracts made up of axons that extend from the medulla to the cerebral cortex. This system helps in processing and relaying auditory information in the form of nerve impulses into the brain’s highest levels of the cerebral cortex.

Basically, in the central auditory system, the vestibulocochlear nerve (CN VIII) carries information from the receptors in the Organ of Corti in the inner ear (cochlear hair cells) to the central nervous system. Shortly after, the pathway reaches the primary auditory cortex wherein conscious perception occurs.

In the previous chapter, we have discussed the role of the Organ of Corti as the portion responsible for transducing mechanical sounds into electrical signals. Basically, the process is that the middle ear transforms acoustic energy into mechanical energy. This mechanical energy is then converted by the Organ of Corti into electrical neural information.

Cochlear hair cells have afferent or sensory and efferent or motor innervations. Both types are essential in the process of hearing.

More than 30,000 nerve fibers innervate the hair cells. These nerve fibers connect to the outer hair cells through the tunnel of Corti before joining with the fibers of the inner hair cells. Then, they move through the habenula perforata. These nerve fibers aim to reach the spiral ganglia wherein they would give out information along with their peripheral processes. It is important to note that in the region between the endings of the hair cells, these nerve fibers are unmyelinated (not covered, bare). The inner hair cells send 95% of the afferent information, while the puter hair cells, despite having a larger quantity, sends only 5% of the afferent information. Moreover, the neural information of the inner hair cells come from the turbulence in the endolymph’s flow, while the neural information of the outer hair cells are products of the tectorial membrane’s shearing action.

Transduction is the process of transforming mechanical vibrations from the basilar membrane into neural information. This occurs at the level of the hair cells.

Transduction is primarily dependent on the hair cells’ intracellular resting potential. This quantity is approximately -40 mV for the inner hair cells and -70 mV for the outer hair cells. Moreover, the hair cells’ positive resting potential and negative resting potential yield a voltage potential difference within the range of 120 to 150 mV.

Whenever there is a stimulus in the mechanical gate, the stereocilia opens and the tip links reach or stretch from the topmost portion of the shorter cilia to the adjacent taller cilia’s side. The opening of this “gate” a higher concentration of potassium in the endolymph flows toward the lower concentration. It is important to note as well that the endolymphs are rich with the positively charged ions, potassium and calcium.