Central Auditory Pathways

The central pathway of the auditory system contains a large number of relay nuclei within the brain stem (Fig. 6). Central fibers from primary sensory neurons in the spiral ganglia project along with vestibular fibers in cranial nerve VIII and synapse first within the dorsal and ventral cochlear nuclei located near the pontomedullary junction. Ascending fibers from these nuclei project to both the oval window into scala vestibuli

Auditory Pathway Action Potential Nerves

FIGURE 4 Structure and functional relationships of the basilar membrane. The cochlea is shown rolled out with the scala vestibuli dissected away, leaving only the basal portion with the oval window. The scala media and the organ of Corti have also been removed, exposing the basilar membrane, with the underlying scala tympani. The basilar membrane (shown in blue) is narrow and stiff at the base and is wide and floppy near the apex, near the helicotrema. Low pitches vibrate the wide part of the membrane; high pitches, the narrow part. Note that the perilymph in the scala vestibuli and tympani are continuous at the opening provided by the helicotrema, allowing perilymph from the two scalae to mix. The endolymph in the scala media is completely isolated from these.

helicotrema

FIGURE 4 Structure and functional relationships of the basilar membrane. The cochlea is shown rolled out with the scala vestibuli dissected away, leaving only the basal portion with the oval window. The scala media and the organ of Corti have also been removed, exposing the basilar membrane, with the underlying scala tympani. The basilar membrane (shown in blue) is narrow and stiff at the base and is wide and floppy near the apex, near the helicotrema. Low pitches vibrate the wide part of the membrane; high pitches, the narrow part. Note that the perilymph in the scala vestibuli and tympani are continuous at the opening provided by the helicotrema, allowing perilymph from the two scalae to mix. The endolymph in the scala media is completely isolated from these.

ipsilateral and contralateral superior olivary nuclei. Axons from cells in the superior olive travel along with other sensory fibers within the lateral lemniscus to reach the inferior colliculus. Collicular fibers synapse in the medial geniculate nucleus of the thalamus. Geniculate fibers represent the final projection to the primary auditory cortex located along the superior lip of the temporal lobe. Auditory fibers are represented bilaterally within the central pathway because of partial decussation of projections at several of the relay points within the brain stem. Thus, central auditory lesions rarely cause deafness in only one ear.

Like the visual cortex, the primary auditory cortex (Brodmann's areas 41 and 42) is composed of six layers: layer IV contains input fibers from the medial geniculate body; cells in layer IV project to small pyramidal cells in layers II and III before projecting to other cortical association areas and to layers V and VI before projecting back to lower brain stem structures. In comparison with the visual system, much less is known about how central pathways process auditory information. In general, we have a basic understanding of only three fundamental attributes of sound: intensity, pitch, and location in space (Table 1).

Table 1 Coding of Sound Intensity, Pitch, and Location

Intensity

Loud sound Soft sound Pitch

High pitch

Low pitch

Location

Vertical plane Horizontal plane

Large amplitude deflection in basilar membrane Small amplitude deflection in basilar membrane

Vibrates base of basilar membrane

Hair cells fire at phase-locked, high frequency

Vibrates apex of basilar membrane

Hair cells fire at phase-locked, low frequency

Determined by echo patterns created by pinna at sound onset Change in location of a low-frequency continuous tone Change in location of a high-frequency continuous tone

More action potentials per neuron and more neurons firing Fewer action potentials per neuron and more neurons firing

Activates appropriate neurons in tonotopic map

Cortical cells in isofrequency bands fire at phase-locked high frequency Activates appropriate neurons in tonotopic map

Cortical cells in isofrequency bands fire at phase-locked low frequency

Interaural onset delay Interaural phase delay

Interaural intensity difference

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