The auditory pathway consists of special-somatosensitive fibers that transmit recorded impulses from the organ of Corti to the primary and secondary auditory cortex of the cerebrum. First instant of the auditory pathway are the sensory cells of the auditory sense, which transform sound into electrical impulses. Hearing loss may be due to impaired conduction within the auditory pathways.
What is the auditory pathway?
The organ of Corti forms the seat of the sense of hearing. Located in the cochlea of the human inner ear, the organ corresponds to a complex system of receptors, supporting cells and nerve fibers. Special somatosensitive fibers within the sense of hearing are known to physicians as the auditory pathway. They run from the organ of Corti in the inner ear to the primary and secondary auditory cortex in the cerebrum. Auditory impressions are received here and interconnected via multiple neurons. The first neuron of the auditory pathway is located in the ganglion spirale cochleae. Its central projections target the nuclei cochleares of the medulla oblongata. The fifth neuron targets the primary auditory cortex in the gyri temporales transversi of the temporal lobe, reaching the auditory cortex. Central hearing takes place in the auditory pathways. This is purely neural hearing, also known as auditory perception. Often, a direct part is distinguished from an indirect part in the second neuron of the auditory pathway. The auditory pathway contains both ascending (afferent)and descending (efferent) neural pathways with intercalated nuclei called auditory nuclei. The central structure begins with the sensory cells of the inner ear.
Anatomy and structure
The first neuron of the auditory pathway corresponds to a bipolar neuron in the ganglion spirale cochleae, whose central projections project to the nuclei cochleares of the medulla oblongata. Sensory input is switched at this point to the second neuron, the direct part of which travels from the posterior cochlear nucleus, unconnected, through the superior olive complex and across the lemniscus lateralis of the opposite side to advance to the inferior colliculus and be switched to the third neuron. The indirect part of the auditory pathway runs from the nucleus cochlearis anterior to the opposite side at this point and includes circuitry such as the nuclei olivares superiores and the nucleus corporis trapezoidei. This indirect part is known as the corpus trapezoideum. In the third neuron, auditory pathway fibers in the form of the lemniscus lateralis travel to the inferior colliculus, where they are partially interconnected to the fourth neuron. From the inferior colliculus, the fibers reach the medial corpus geniculatum via the brachium colliculi inferioris and project to the fifth neuron. At this point, the auditory pathway fibers run sublenticularly and cross the internal capsule. The fifth neuron projects to the primary auditory cortex.
Function and tasks
As part of the auditory system, the auditory pathway is one of the sensory systems and plays a role in auditory perception. In land creatures such as humans, airborne sound is transmitted to the fluid-filled inner ear during hearing. The mechanical energy of the sound waves is converted into electrical energy by the inner hair cells by means of mechano-electrical signal transduction. In the axons of the auditory nerve, this energy travels to the brain in the form of action potentials. In humans and other mammals, the auditory pathway ultimately begins with the sensory cells of the inner ear, which use glutamatergic synapses to excite individual neurons with cell bodies in the spiral ganglion. The excited nerve cells belong to the auditory nerve, which leads fiber systems to the medulla oblongata nuclei. In the superior olivary nucleus complex, transit time differences and intensity differences between the two ears are evaluated, among other things, in order to be able to assign the direction of sound sources. Side crossings and side couplings of auditory fibers enable directional hearing. Incomplete perceived information from the individual ears can also be completed thanks to the side couplings. The auditory pathway plays a major role specifically for central hearing. This form of neuronal hearing comprises two stages: processing on an unconscious level and subsequent conscious perception. Central hearing as unconscious processing is a permanent process that also takes place during sleep. Conscious perception, on the other hand, remains limited to the waking state. The importance of central hearing compared to peripheral hearing has only recently been recognized for humans.
Diseases
For a long time, age-physiological deficits in auditory processing were equated with a general impairment of hearing. In the meantime, medical science has recognized that age-related hearing loss is not only due to hair cell damage of the inner ear but, in addition, to changes in central neural auditory processing. Central hearing loss can, for example, be due to Alzheimer’s disease, which leads to incorrect evaluation of what is heard. This phenomenon occurs not only in the context of age-related dementia, but may also be related to inflammation or stroke. Nerve conduction-induced hearing loss also occurs with growths on the auditory nerve. Sound conduction through the hearing organ in the inner ear proceeds properly in such growths. However, the growth can compress nerves of the auditory pathway so that electrical potentials do not reach the brain properly. This type of hearing loss is also referred to as neural hearing loss. Complicated tone sequences such as speech are only partially recognized as a consequence. Patients with neural hearing loss hear that something is being said, but cannot understand what is being said. Inner ear diseases with auditory nerve involvement also impede the neuronal transmission of impulses. The result is sensorineural hearing loss, which may be associated with damage to the auditory pathways. Even with standard auditory perception, these associations can result in auditory sensory disturbance that is related to neuronal disturbances in auditory pathway conduction.