Anatomy and Physiology of the Auditory System

Keypoints

• 1. The auditory system consists of four anatomically separate structures:

  • (a) those that conduct the stimulus to the receptors

  • (b) the receptors

  • (c) the auditory nerve

  • (d) the central auditory nervous system

• 2. The most important part regarding tinnitus is the auditory nervous system.

• 3. The auditory nervous system consists of two parallel ascending pathways that project to auditory cortices and two (reciprocal) descending pathways that project to nuclei of the auditory pathways.

• 4. The nuclei in the ascending auditory pathways process information in a serial hierarchical fashion, and processing occurs in modules with specific functions.

• 5. Two separate ascending sensory pathways have been identified in the auditory pathways: classical pathways and the non-classical pathways. Also, the somatosensory and visual pathways have two different ascending tracts.

• 6. The classical pathways are also known as the lemniscal system, or the specific system, and the non-classical pathways are also known as the extralemniscal system, or the unspecific system. The non-classical pathways have been divided into the defuse system and the polysensory pathways.

• 7. The classical and non-classical pathways process information differently and have different central targets, especially regarding connections to the thalamus and the cerebral cortex.

• 8. The non-classical ascending auditory pathways branch off the classical pathways at several levels, the most prominent being the central nucleus of the inferior colliculus.

• 9. The auditory pathways receive input from the somatosensory system at the external nucleus of the inferior colliculus and from the dorsal cochlear nucleus as well.

• 10. The auditory pathways are mainly crossed, but there are extensive connections between nuclei at the two sides at two levels: the pontine nuclei (superior ­olivary complex) and the midbrain level (inferior colliculus). There are also extensive ­connections between the two sides at the cerebral cortical level.

• 11. The auditory nerve sends collaterals to cells in all these divisions of the cochlear nucleus. That is the earliest sign of the anatomical basis for parallel processing of information. Parallel processing occurs throughout the ascending pathways by axons branching to connect to more than one group of nerve cells.

• 12. Descending auditory pathways are abundant, in particular, the cortico-thalamic pathways, but little is known about their function. The descending pathways are largely reciprocal to the ascending pathways. The descending pathways reach as far caudal as the receptors in the cochlea.

• 13. The classical sensory pathways are interrupted by synaptic contacts with neurons in the ventral parts of the thalamus, which project to the primary ­sensory cortices.

• 14. The non-classical sensory pathways use the dorsal and medial thalamus as relay, the neurons of which project to secondary and association cortices thus bypassing the primary sensory cortices.

• 15. Neurons in the dorsal and medial thalamus make direct (subcortical) connections with other parts of the CNS, such as structures of the limbic system, while the classical sensory systems connect to other parts of the CNS, mainly via association cortices.

• 16. There are anatomical connections between the upper spinal cord and the dorsal cochlear nucleus and between the caudal trigeminal nucleus and the dorsal cochlear nucleus. There are anatomical connections between the somatosensory system and midbrain nuclei of the non-classical auditory system.

• 17. Neurons in the nuclei of the classical pathways respond distinctly to specific sensory stimuli and have distinct frequency selectivity.

• 18. Sound stimulation may increase the firing rate of auditory nerve fibers, but saturation occurs for most fibers at low sound intensities.

• 19. Periodic sounds cause many nerve fibers to become locked to the waveform of the sound, and consequently, the firing of such fibers becomes time locked to each other. It subsequently causes the discharge of many neurons in the ascending auditory pathways, which then become time locked to each other.

• 20. Stream segregation implies that different types of information (for example, spatial and object information) are processed in anatomically different parts of the sensory nervous system.

• 21. Parallel processing allows the same information to be processed in anatomically different parts of the nervous system, while stream segregation implies that different kinds of information are processed in anatomically different structures.

• 22. Much less is known about the functional role of the non-classical pathways compared to the classical pathways, but neurons of the nuclei of the non-classical pathways respond less distinctly and are broader tuned than cells in the classical pathways and respond to a broad range of stimuli. They also integrate information on wider spatial scales than the classical pathways.

• 23. Neurons in the nuclei of the classical auditory pathways, up to and including the primary auditory cortex, respond only to one sensory modality (sound) while neurons of higher order cortices (secondary and association cortices) integrate information from several sensory systems and respond to different sensory modalities. This response can be modulated by input from non-sensory brain areas such as the amygdala.

• 24. Some neurons in the ascending non-classical pathways respond to more than one sensory modality. Their response to sound can be modulated by other sensory input.

• 25. The non-classical pathways make direct (subcortical) connections from the thalamus to other parts of the CNS, such as structures of the limbic s­ystem, while the classical sensory systems connect to other such parts of the CNS mainly via association cortices.

• 26. Stimulation of the somatosensory system affects perception of sounds in children, indicating involvement of the non-classical auditory system in children.

• 27. There are no signs of cross-modal interaction in adults, except with some forms of tinnitus and in autistic individuals, indicating that the non-classical auditory pathways are not normally active in adults.

• 28. Sensory systems connect to motor systems, the limbic system, reticular activating system, and the autonomic nervous system through subcortical and cortical routes.

• 29. There is considerable interaction between different systems in the brain, such as between different sensory systems and between sensory systems and non-sensory systems.