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The central auditory pathway of the brain

Hearing, like any other sensory modality, has a primary channel and centres, i.e. totally dedicated to this function, and so-called non-primary channels where all the modalities converge.

Schematically, this pathway is short (3 or 4 relays), fast (large myelinated fibres) and leads to the primary auditory cortex (on the right).
It carries the information coded by the cochlea, each of the relays performing a specific decoding and interpretation work which is then transmitted to the higher relays.see details below.

Primary auditory pathways

Schematically, this pathway is short (3 or 4 relays), fast (large myelinated fibres) and leads to the primary auditory cortex (right).
It carries information encoded by the cochlea, with each of the relays performing specific decoding and interpretation work, which is then transmitted to the higher relays.

Auditory cortex (3) in the temporal area (2) in humans. In fact, the auditory area at the bottom of the lateral sulcus (1 = Sylvius sulcus) is represented only by transparency.

The first relay of the primary auditory pathway is constituted by the cochlear nuclei of the brain stem (medulla oblongata) which receive the axons of the type I neurons of the spiral ganglion (auditory nerve); at this level an important work is carried out in the basic decoding of the message: duration, intensity, frequency.
A second major relay of the brain stem is the upper olivary complex: most of the auditory fibres synapse there after crossing the median line.

At the start of this relay, a third neuron brings the message up to the level of the lower colliculus (mesencephalon). These two relays play an essential role in the localization of sound.

A last relay, before the cortex, is carried out in the medial knee body (thalamus); this is where an important integration work is done: preparation of a motor response (vocal for example).

The last neuron of the primary auditory pathway connects the thalamus to the auditory cortex where the message, already largely decoded by the work of the underlying neurons, is recognized and memorized and can be integrated into a voluntary response.

NON-PRIMARY PATHWAYS
After the first relay (cochlear nuclei), which is common to all auditory pathways, small fibres join the ascending reticular pathway common to all sensory modalities.

After several relays in the reticular formation, then in the non-specific thalamus, this pathway leads to the polysensory cortex. The role of this pathway, which brings together the different sensory messages sent simultaneously to the brain, is to allow selection of the type of information to be processed in priority; it is connected to the motivational and awakening centres, as well as to the centres of vegetative life. For example, when reading a book while listening to a record, this system allows the attention to focus on the most captivating and/or important task.

See details below

Reticular pathway
The first relay, common with the primary pathway, consists of the cochlear nuclei (brain stem). From these nuclei, small fibres join the ascending reticular pathway.

In the reticular formation of the brain stem and the mesencephalon several relays are performed. This is where auditory information is integrated with all other sensory modalities to participate in the “selective sorting” of the “priority” modality at a given moment: that is to say, the reticular pathways participate with the awakening systems and motivations in the selection of the information to be processed in priority by the brain.

After reticularization, the non-primary pathway leads to the non-specific thalamus and then to the polysensory cortex.
NB: at the passage of the connections (not shown on the diagram) are also carried out with the hypothalamus and the vegetative centres.

In case of Cure For Tinnitus this central nervous system plaus crucial role.

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