1.5. The central auditory pathway
The nerves that carry information from the cochlea to the brain centres are called afferent nerves. There is also feedback from the brain centres to the cochlea, referred to as efferent nerve pathways.
The afferent auditory pathways from the cochlea to the cerebral cortex have a complex course.
The auditory nervous system is functionally crossed, so that information from the right ear is primarily transmitted to the left cortex. The efferent component of the auditory system has multiple functions, including regulation of the OHCs and general inhibitory action throughout the central nervous system.
At various places in this extensive network, there are ‘cores’ where certain forms of processing occur. Furthermore, there are ‘cross connections’ in which bundles of neurons activate contralateral nuclei.
The eighth cranial nerve (Nerve VIII) referred to as the cochlear nerve, is a combination of the vestibular nerve and the acoustic nerve. The cochlear nerve enters the cochlear nucleus based within the brainstem from both sides, just below the pons.
Information processing within the nervous system is structured hierarchically. Things happen in order, in four steps:
- The first processing takes place in the brainstem. It concerns the distribution of information. The sound information enters via the cochlear nerve (Nerve VIII) to the cochlear nucleus on the same side (ipsilateral). From there, the majority of the information travels on the primary pathway to the superior olivary complex on the opposite (contralateral) side, and some of the information travels on a secondary pathway to the superior olivary complex on the same side.
- The second level of information processing occurs in the midbrain (mesencephalon). The sound information continues to be relayed via the lateral lemniscus to the inferior colliculus in the midbrain. From the inferior colliculus, the information is relayed both from the same side (ipsilateral) and the opposite side (contralateral) to the thalamus (inner brain).
- The third level of processing occurs in the thalamus, which is a paired, grey-level matter situated within the diencephalon. This area can be referred to as the inner brain. The sound information received at this level is processed within the medial geniculate nucleus that is based within the thalamus. This area can be referred to as the inner brain.
- The fourth and final level is at a cortical level where the information is then sent to the auditory cortex. The auditory cortex is located in the temporal lobe. This is where the analysis of the information takes place and is processed. The processing of sound information is done tonotopically in the auditory cortex, similar to the way in which sound is processed tonotopically in the inner ear.
The following video provides a clear and concise explanation of the auditory pathway:
Below is a more detailed video of auditory processing, which includes a visual explanation of the tonotopical arrangement of sound in the basilar membrane and how this corresponds to sound being processed in the auditory cortex:
Section 2 – Aetiology and types of deafness
Previous pages in section 1
1.2 The outer ear
1.3 The middle ear
1.4 The inner ear