> Auditory Brain : overview

The final neuron of the primary auditory pathway links the thalamus to the auditory cortex, where the message, already largely decoded during its passage through the previous neurons in the pathway, is recognised, memorised and perhaps integrated into a voluntary response.

A last relay, before the cortex, occurs in the thalamus (median geniculate body); genouillé médian)it's here that an important integration occurs: preparation of a motor response (eg vocal response).

Leaving this relay, a third neuron carries the mesage up to the level of the mesencephalus (superior colliculus). These two relays play an essential rôle in the localisation of sound.

The second major relay in the brain stem is in the superior olivary complex: the majority of the auditory fibres synapse there having already crossed the midline.

The first relay of the primary auditory pathway occurs in the cochlear nuclei in the brain stem, which receive Type I spiral ganglion axons (auditory nerve); at this level an important decoding of the basic signal occurs: duration, intensity and frequency.

After the reticular formation, the non-primary pathway leads to the non-specific thalamus, then to the polysensory cortex.
Note: connections are also made with the hypothalamus and the vegetative centres (not shown on the diagram)

In the reticular pathway of the brainstem and the mesencephalus, several synapses occur. It's here that the auditory information is integrated with all the other sensory modalities to be 'triaged' into which has the highest priority at any given moment. In other words, the reticular pathways participate with the wake and the motivation centres in selectioning the information that should be treated as priority by the brain.

The first relay, in common with the primary auditory pathway, locates in the cochlear nuclei (brainstem). From there, the small fibers rejoin the ascending reticular pathway.

Conscious perception requires the integrity of both types of pathways. For instance, during sleep the primary auditory pathway functions normally, but no conscious perception is possible because the link between reticular pathways and the wake and motivation centers is inactive. Conversely, trauma affecting the cortex may suppress the conscious perception, while the continuing integrity of the non-primary pathways may result in vegetative reflex reactions to a sound. Also, because the brain stem and midbrain are intact, startle reflexes are still present.